You are viewing a plain text version of this content. The canonical link for it is here.
Posted to commits@tapestry.apache.org by bu...@apache.org on 2017/09/16 02:22:41 UTC
svn commit: r1018228 [38/41] - in /websites/production/tapestry/content: ./
cache/
Modified: websites/production/tapestry/content/tapestry-ioc-modules.html
==============================================================================
--- websites/production/tapestry/content/tapestry-ioc-modules.html (original)
+++ websites/production/tapestry/content/tapestry-ioc-modules.html Sat Sep 16 02:22:40 2017
@@ -27,14 +27,6 @@
</title>
<link type="text/css" rel="stylesheet" href="/resources/space.css" />
- <link href='/resources/highlighter/styles/shCoreCXF.css' rel='stylesheet' type='text/css' />
- <link href='/resources/highlighter/styles/shThemeCXF.css' rel='stylesheet' type='text/css' />
- <script src='/resources/highlighter/scripts/shCore.js' type='text/javascript'></script>
- <script src='/resources/highlighter/scripts/shBrushJava.js' type='text/javascript'></script>
- <script>
- SyntaxHighlighter.defaults['toolbar'] = false;
- SyntaxHighlighter.all();
- </script>
<link href="/styles/style.css" rel="stylesheet" type="text/css"/>
@@ -44,13 +36,26 @@
<div class="wrapper bs">
- <div id="navigation"><div class="nav"><ul class="alternate"><li><a href="index.html">Home</a></li><li><a href="getting-started.html">Getting Started</a></li><li><a href="documentation.html">Documentation</a></li><li><a href="download.html">Download</a></li><li><a href="about.html">About</a></li><li><a class="external-link" href="http://www.apache.org/licenses/LICENSE-2.0">License</a></li><li><a href="community.html">Community</a></li><li><a class="external-link" href="http://www.apache.org/security/">Security</a></li><li><a class="external-link" href="http://www.apache.org/">Apache</a></li><li><a class="external-link" href="http://www.apache.org/foundation/sponsorship.html">Sponsorship</a></li><li><a class="external-link" href="http://www.apache.org/foundation/thanks.html">Thanks</a></li></ul></div></div>
+ <div id="navigation"><div class="nav"><ul class="alternate"><li><a href="index.html">Home</a></li><li><a href="getting-started.html">Getting Started</a></li><li><a href="documentation.html">Documentation</a></li><li><a href="download.html">Download</a></li><li><a href="about.html">About</a></li><li><a class="external-link" href="http://www.apache.org/licenses/LICENSE-2.0">License</a></li><li><a href="community.html">Community</a></li><li><a class="external-link" href="http://www.apache.org/security/">Security</a></li><li><a class="external-link" href="http://www.apache.org/">Apache</a></li><li><a class="external-link" href="http://www.apache.org/foundation/sponsorship.html">Sponsorship</a></li><li><a class="external-link" href="http://www.apache.org/foundation/thanks.html">Thanks</a></li></ul></div>
+
+</div>
<div id="top">
- <div id="smallbanner"><div class="searchbox" style="float:right;margin: .3em 1em .1em 1em"><span style="color: #999; font-size: 90%">Tapestry docs, issues, wikis & blogs:</span><form enctype="application/x-www-form-urlencoded" method="get" action="http://tapestry.apache.org/search.html">
- <input type="text" name="q">
- <input type="submit" value="Search">
-</form></div><div class="emblem" style="float:left"><p><a href="index.html"><span class="confluence-embedded-file-wrapper"><img class="confluence-embedded-image confluence-external-resource" src="http://tapestry.apache.org/images/tapestry_small.png" data-image-src="http://tapestry.apache.org/images/tapestry_small.png"></span></a></p></div><div class="title" style="float:left; margin: 0 0 0 3em"><h1 id="SmallBanner-PageTitle">Tapestry IoC Modules</h1></div></div>
+ <div id="smallbanner"><div class="searchbox" style="float:right;margin: .3em 1em .1em 1em"><span style="color: #999; font-size: 90%">Tapestry docs, issues, wikis & blogs:</span>
+<form enctype="application/x-www-form-urlencoded" method="get" action="http://tapestry.apache.org/search.html">
+ <input type="text" name="q">
+ <input type="submit" value="Search">
+</form>
+
+</div>
+
+
+<div class="emblem" style="float:left"><p><a href="index.html"><span class="confluence-embedded-file-wrapper"><img class="confluence-embedded-image confluence-external-resource" src="http://tapestry.apache.org/images/tapestry_small.png" data-image-src="http://tapestry.apache.org/images/tapestry_small.png"></span></a></p></div>
+
+
+<div class="title" style="float:left; margin: 0 0 0 3em"><h1 id="SmallBanner-PageTitle">Tapestry IoC Modules</h1></div>
+
+</div>
<div class="clearer"></div>
</div>
@@ -62,8 +67,7 @@
</div>
<div id="content">
- <div id="ConfluenceContent"><p>You inform Tapestry about your services and contributions by providing a <strong>module</strong> class.</p><p>The module class is a plain Java class that you create to inform Tapestry about your services and contributions.</p><p>A system of annotations and naming conventions allow Tapestry to determine what services are provided by the module.</p><p>A module class exists for the following reasons:</p><ul><li>To <em>bind</em> service interfaces to service implementations</li><li>To contribute configuration data <em>into</em> services</li><li>To <em>decorate</em> services by providing <em>interceptors</em> around them</li><li>To provide explicit code for building a service</li><li>To set a default <em>marker</em> for all services defined in the module</li></ul><p>All public methods of a module class must be meaningful to Tapestry (be one of the categories above). Any extra public methods result in startup exceptions (because the method ma
y contain a typo).</p><h1 id="TapestryIoCModules-ServiceBuilderMethods">Service Builder Methods</h1><p>Service builder methods were the original way to define a service and provide the logic to construct it; although this is now more commonly (and succinctly) accomplished using the bind() method, there are still many cases where service builder methods are useful.</p><p>Service builder methods are public methods. They are often static. Here's a trivial example:</p><div class="code panel pdl" style="border-width: 1px;"><div class="codeContent panelContent pdl">
-<pre class="brush: java; gutter: false; theme: Default" style="font-size:12px;">package org.example.myapp.services;
+ <div id="ConfluenceContent"><p>You inform Tapestry about your services and contributions by providing a <strong>module</strong> class.</p><p>The module class is a plain Java class that you create to inform Tapestry about your services and contributions.</p><p>A system of annotations and naming conventions allow Tapestry to determine what services are provided by the module.</p><p>A module class exists for the following reasons:</p><ul><li>To <em>bind</em> service interfaces to service implementations</li><li>To contribute configuration data <em>into</em> services</li><li>To <em>decorate</em> services by providing <em>interceptors</em> around them</li><li>To provide explicit code for building a service</li><li>To set a default <em>marker</em> for all services defined in the module</li></ul><p>All public methods of a module class must be meaningful to Tapestry (be one of the categories above). Any extra public methods result in startup exceptions (because the method ma
y contain a typo).</p><h1 id="TapestryIoCModules-ServiceBuilderMethods">Service Builder Methods</h1><p>Service builder methods were the original way to define a service and provide the logic to construct it; although this is now more commonly (and succinctly) accomplished using the bind() method, there are still many cases where service builder methods are useful.</p><p>Service builder methods are public methods. They are often static. Here's a trivial example:</p><parameter ac:name="">java</parameter><plain-text-body>package org.example.myapp.services;
public class MyAppModule
{
@@ -71,9 +75,7 @@ public class MyAppModule
{
return new IndexerImpl();
}
-}</pre>
-</div></div><p>Any public method (static or instance) whose name starts with "build" is a service builder method, implicitly defining a service within the module.</p><p>Here we're defining a service around the Indexer service interface (presumably also in the org.example.myapp.services package).</p><p>Every service has a unique id, used to identify it throughout the Registry of services (the Registry is the combined sum of all services from all modules). If you don't provide an explicit service id, as in this example, the service id is drawn from the return type; this service has an id of "Indexer".</p><p>You can give a service an explicit id by adding it to the method name: buildIndexer(). This is useful when you do not want the service id to match the service interface name (for example, when you have different services that implement the same interface), or when you need to avoid name collisions on the method name (Java allows only a single method with a given name and set of par
ameters, even if the return types are different, so if you have two different service builder methods that take the same parameters, you should give them explicit service ids in the method name).</p><p>Tapestry IoC is <a href="case-insensitivity.html">case insensitive</a>; later we can refer to this service as "indexer" or "INDEXER" or any variation thereof, and connect to this service.</p><p>Service ids must be unique; if another module contributes a service with the id "Indexer" (or any case variation thereof) a runtime exception will occur when the Registry is created.</p><p>We could extend this example by adding additional service builder methods, or by showing how to inject dependencies. See <a href="defining-tapestry-ioc-services.html">the service documentation</a> for more details.</p><h1 id="TapestryIoCModules-AutobuildingServices">Autobuilding Services</h1><p>Main article: <a href="defining-tapestry-ioc-services.html">Defining Tapestry IOC Services</a></p><p>An alternate
, and usually preferred, way to define a service is via a module's bind() method. The previous example can be rewritten as:</p><div class="code panel pdl" style="border-width: 1px;"><div class="codeContent panelContent pdl">
-<pre class="brush: java; gutter: false; theme: Default" style="font-size:12px;">package org.example.myapp.services;
+}</plain-text-body><p>Any public method (static or instance) whose name starts with "build" is a service builder method, implicitly defining a service within the module.</p><p>Here we're defining a service around the Indexer service interface (presumably also in the org.example.myapp.services package).</p><p>Every service has a unique id, used to identify it throughout the Registry of services (the Registry is the combined sum of all services from all modules). If you don't provide an explicit service id, as in this example, the service id is drawn from the return type; this service has an id of "Indexer".</p><p>You can give a service an explicit id by adding it to the method name: buildIndexer(). This is useful when you do not want the service id to match the service interface name (for example, when you have different services that implement the same interface), or when you need to avoid name collisions on the method name (Java allows only a single method with a given name and set
of parameters, even if the return types are different, so if you have two different service builder methods that take the same parameters, you should give them explicit service ids in the method name).</p><p>Tapestry IoC is <a href="case-insensitivity.html">case insensitive</a>; later we can refer to this service as "indexer" or "INDEXER" or any variation thereof, and connect to this service.</p><p>Service ids must be unique; if another module contributes a service with the id "Indexer" (or any case variation thereof) a runtime exception will occur when the Registry is created.</p><p>We could extend this example by adding additional service builder methods, or by showing how to inject dependencies. See <a href="defining-tapestry-ioc-services.html">the service documentation</a> for more details.</p><h1 id="TapestryIoCModules-AutobuildingServices">Autobuilding Services</h1><p>Main article: <a href="defining-tapestry-ioc-services.html">Defining Tapestry IOC Services</a></p><p>An al
ternate, and usually preferred, way to define a service is via a module's bind() method. The previous example can be rewritten as:</p><parameter ac:name="">java</parameter><plain-text-body>package org.example.myapp.services;
import org.apache.tapestry5.ioc.ServiceBinder;
@@ -83,9 +85,7 @@ public class MyAppModule
{
binder.bind(Indexer.class, IndexerImpl.class);
}
-}</pre>
-</div></div><p>For more details, see see <a href="defining-tapestry-ioc-services.html">Defining Tapestry IOC Services</a>. In most cases, autobuilding is the <em>preferred</em> approach.</p><p>Generally speaking, you should always bind and autobuild your services. The only exceptions are when:</p><ul><li>You wish to do more than just instantiate a class; for example, to register the class as an event listener with some other service.</li><li>There is <em>no implementation class</em>; in some cases, you can create your implementation on the fly using JDK dynamic proxies or bytecode generation.</li></ul><p>The bind() method must be static; an exception is thrown if the bind() method exists but is an instance method.</p><h1 id="TapestryIoCModules-Cacheing_ServicesCachingServices"><span class="confluence-anchor-link" id="TapestryIoCModules-Cacheing_Services"></span>Caching Services</h1><p>You will occasionally find yourself in the position of injecting the same services into your servi
ce builder or service decorator methods repeatedly (this occurs much less often since the introduction of service autobuilding). This can result in quite a bit of redundant typing. Less code is better code, so as an alternative, you may define a <em>constructor</em> for your module that accepts annotated parameters (as with <a href="defining-tapestry-ioc-services.html">service builder injection</a>).</p><p>This gives you a chance to store common services in instance variables for later use inside service builder methods.</p><div class="code panel pdl" style="border-width: 1px;"><div class="codeContent panelContent pdl">
-<pre class="brush: java; gutter: false; theme: Default" style="font-size:12px;">public class MyModule
+}</plain-text-body><p>For more details, see see <a href="defining-tapestry-ioc-services.html">Defining Tapestry IOC Services</a>. In most cases, autobuilding is the <em>preferred</em> approach.</p><p>Generally speaking, you should always bind and autobuild your services. The only exceptions are when:</p><ul><li>You wish to do more than just instantiate a class; for example, to register the class as an event listener with some other service.</li><li>There is <em>no implementation class</em>; in some cases, you can create your implementation on the fly using JDK dynamic proxies or bytecode generation.</li></ul><p>The bind() method must be static; an exception is thrown if the bind() method exists but is an instance method.</p><h1 id="TapestryIoCModules-Cacheing_ServicesCachingServices"><parameter ac:name="">Cacheing_Services</parameter>Caching Services</h1><p>You will occasionally find yourself in the position of injecting the same services into your service builder or service decora
tor methods repeatedly (this occurs much less often since the introduction of service autobuilding). This can result in quite a bit of redundant typing. Less code is better code, so as an alternative, you may define a <em>constructor</em> for your module that accepts annotated parameters (as with <a href="defining-tapestry-ioc-services.html">service builder injection</a>).</p><p>This gives you a chance to store common services in instance variables for later use inside service builder methods.</p><parameter ac:name="">java</parameter><plain-text-body>public class MyModule
{
private final JobScheduler scheduler;
private final FileSystem fileSystem;
@@ -104,23 +104,17 @@ public class MyAppModule
return indexer;
}
-}</pre>
-</div></div><p>Notice that we've switched from <em>static</em> methods to <em>instance</em> methods. Since the builder methods are not static, the MyModule class will be instantiated so that the methods may be invoked. The constructor receives two common dependencies, which are stored into instance fields that may later be used inside service builder methods such as buildIndexer().</p><p>This approach is far from required; all the builder methods of your module can be static if you wish. It is used when you have many common dependencies and wish to avoid defining those dependencies as parameters to multiple methods.</p><p>Tapestry IoC automatically resolves the parameter type (JobScheduler and FileSystem, in the example) to the corresponding services that implement that type. When there's more than one service that implements the service interface, you'll get an error (but additional annotations and configuration can be used to ensure the correct service injected).</p><p>For modules
, there are two additional parameter types that are used to refer to <em>resources</em> that can be provided to the module instance (rather than <em>services</em> which may be injected).</p><ul><li><a class="external-link" href="http://www.slf4j.org/api/org/slf4j/Logger.html" rel="nofollow">org.slf4j.Logger</a>: logger for the module (derived from the module's class name)</li><li><a class="external-link" href="http://tapestry.apache.org/current/apidocs/org/apache/tapestry5/ioc/ObjectLocator.html">ObjectLocator</a>: access to other services<br clear="none"> Note that the fields are final: this is important. Tapestry IoC is thread-safe and you largely never have to think about concurrency issues. But in a busy application, different services may be built by different threads simultaneously. Each module class is a singleton, instantiated at most once, and making these fields final ensures that the values are available across multiple threads. Refer to Brian Goetz's <a class="externa
l-link" href="http://www.javaconcurrencyinpractice.com/" rel="nofollow">Java Concurrency in Practice</a> for a more complete explanation of the relationship between final fields, constructors, and threads ... or just trust us!</li></ul><p>Care should be taken with this approach: in some circumstances, you may force a situation in which the module constructor is dependent on itself. For example, if you invoke a method on any injected services defined within the same module from the module class' constructor, then the service implementation will be needed. Creating service implementations requires the module builder instance ... that's a recursive reference.</p><p>Tapestry detects these scenarios and throws a runtime exception to prevent an endless loop.</p><h1 id="TapestryIoCModules-ModuleClassImplementationNotes">Module Class Implementation Notes</h1><p>Module classes are designed to be very, very simple to implement.</p><p>Again, keep the methods very simple. Use <a href="defining
-tapestry-ioc-services.html">parameter injection</a> to gain access to the dependencies you need.</p><p>Be careful about inheritance. Tapestry will see all <em>public</em> methods, even those inherited from base classes. Tapestry <em>only</em> sees public methods.</p><p>By convention, module class names end in Module and are final classes.</p><p>You don't <em>have</em> to define your methods as static. The use of static methods is only absolutely necessary in a few cases, where the constructor for a module is dependent on contributions from the same module (this creates a chicken-and-the-egg situation that is resolved through static methods).</p><h1 id="TapestryIoCModules-DefaultMarker">Default Marker</h1><p>Services are often referenced by a particular marker interface on the method or constructor parameter. Tapestry will use the intersection of services with that exact marker and assignable by type to find a unique service to inject.</p><p>Often, all services in a module should sh
are a marker, this can be specified with a @Marker annotation on the module class. For example, the TapestryIOCModule:</p><div class="code panel pdl" style="border-width: 1px;"><div class="codeContent panelContent pdl">
-<pre class="brush: java; gutter: false; theme: Default" style="font-size:12px;">@Marker(Builtin.class)
+}</plain-text-body><p>Notice that we've switched from <em>static</em> methods to <em>instance</em> methods. Since the builder methods are not static, the MyModule class will be instantiated so that the methods may be invoked. The constructor receives two common dependencies, which are stored into instance fields that may later be used inside service builder methods such as buildIndexer().</p><p>This approach is far from required; all the builder methods of your module can be static if you wish. It is used when you have many common dependencies and wish to avoid defining those dependencies as parameters to multiple methods.</p><p>Tapestry IoC automatically resolves the parameter type (JobScheduler and FileSystem, in the example) to the corresponding services that implement that type. When there's more than one service that implements the service interface, you'll get an error (but additional annotations and configuration can be used to ensure the correct service injected).</p><p>For
modules, there are two additional parameter types that are used to refer to <em>resources</em> that can be provided to the module instance (rather than <em>services</em> which may be injected).</p><ul><li><a class="external-link" href="http://www.slf4j.org/api/org/slf4j/Logger.html" rel="nofollow">org.slf4j.Logger</a>: logger for the module (derived from the module's class name)</li><li><a class="external-link" href="http://tapestry.apache.org/current/apidocs/org/apache/tapestry5/ioc/ObjectLocator.html">ObjectLocator</a>: access to other services<br clear="none"> Note that the fields are final: this is important. Tapestry IoC is thread-safe and you largely never have to think about concurrency issues. But in a busy application, different services may be built by different threads simultaneously. Each module class is a singleton, instantiated at most once, and making these fields final ensures that the values are available across multiple threads. Refer to Brian Goetz's <a class="
external-link" href="http://www.javaconcurrencyinpractice.com/" rel="nofollow">Java Concurrency in Practice</a> for a more complete explanation of the relationship between final fields, constructors, and threads ... or just trust us!</li></ul><p>Care should be taken with this approach: in some circumstances, you may force a situation in which the module constructor is dependent on itself. For example, if you invoke a method on any injected services defined within the same module from the module class' constructor, then the service implementation will be needed. Creating service implementations requires the module builder instance ... that's a recursive reference.</p><p>Tapestry detects these scenarios and throws a runtime exception to prevent an endless loop.</p><h1 id="TapestryIoCModules-ModuleClassImplementationNotes">Module Class Implementation Notes</h1><p>Module classes are designed to be very, very simple to implement.</p><p>Again, keep the methods very simple. Use <a href="d
efining-tapestry-ioc-services.html">parameter injection</a> to gain access to the dependencies you need.</p><p>Be careful about inheritance. Tapestry will see all <em>public</em> methods, even those inherited from base classes. Tapestry <em>only</em> sees public methods.</p><p>By convention, module class names end in Module and are final classes.</p><p>You don't <em>have</em> to define your methods as static. The use of static methods is only absolutely necessary in a few cases, where the constructor for a module is dependent on contributions from the same module (this creates a chicken-and-the-egg situation that is resolved through static methods).</p><h1 id="TapestryIoCModules-DefaultMarker">Default Marker</h1><p>Services are often referenced by a particular marker interface on the method or constructor parameter. Tapestry will use the intersection of services with that exact marker and assignable by type to find a unique service to inject.</p><p>Often, all services in a module sh
ould share a marker, this can be specified with a @Marker annotation on the module class. For example, the TapestryIOCModule:</p><parameter ac:name="">java</parameter><plain-text-body>@Marker(Builtin.class)
public final class TapestryIOCModule
{
- . . .</pre>
-</div></div><p>This references a particular annotation class, Builtin:</p><div class="code panel pdl" style="border-width: 1px;"><div class="codeContent panelContent pdl">
-<pre class="brush: java; gutter: false; theme: Default" style="font-size:12px;">@Target(
+ . . .</plain-text-body><p>This references a particular annotation class, Builtin:</p><parameter ac:name="">java</parameter><plain-text-body>@Target(
{ PARAMETER, FIELD })
@Retention(RUNTIME)
@Documented
public @interface Builtin
{
-}</pre>
-</div></div><p>The annotation can be applied to method and constructor parameters, for use within the IoC container. It can also be applied to fields, though this is specific to the Tapestry web framework.</p><h1 id="TapestryIoCModules-FieldInjection">Field Injection</h1><p>The @<a class="external-link" href="http://tapestry.apache.org/current/apidocs/org/apache/tapestry5/annotations/Inject.html">Inject</a> and @<a class="external-link" href="http://tapestry.apache.org/current/apidocs/org/apache/tapestry5/annotations/InjectService.html">InjectService</a> annotations may be used on instance fields of a module class, as an alternative to passing dependencies of the module in via the constructor.</p><p>Caution: injection via fields uses reflection to make the fields accessible. In addition, it may not be as thread-safe as using the constructor to assign to final fields.</p><p>Using this style, the previous example of a module class may be rewritten:</p><div class="code panel pdl" sty
le="border-width: 1px;"><div class="codeContent panelContent pdl">
-<pre class="brush: java; gutter: false; theme: Default" style="font-size:12px;">public class MyModule
+}</plain-text-body><p>The annotation can be applied to method and constructor parameters, for use within the IoC container. It can also be applied to fields, though this is specific to the Tapestry web framework.</p><h1 id="TapestryIoCModules-FieldInjection">Field Injection</h1><p>The @<a class="external-link" href="http://tapestry.apache.org/current/apidocs/org/apache/tapestry5/annotations/Inject.html">Inject</a> and @<a class="external-link" href="http://tapestry.apache.org/current/apidocs/org/apache/tapestry5/annotations/InjectService.html">InjectService</a> annotations may be used on instance fields of a module class, as an alternative to passing dependencies of the module in via the constructor.</p><p>Caution: injection via fields uses reflection to make the fields accessible. In addition, it may not be as thread-safe as using the constructor to assign to final fields.</p><p>Using this style, the previous example of a module class may be rewritten:</p><parameter ac:name="">ja
va</parameter><plain-text-body>public class MyModule
{
@Inject
private JobScheduler scheduler;
@@ -136,8 +130,7 @@ public @interface Builtin
return indexer;
}
-}</pre>
-</div></div><p> </p><p></p></div>
+}</plain-text-body><p> </p><p></p></div>
</div>
<div class="clearer"></div>
Modified: websites/production/tapestry/content/tapestry-ioc-overview.html
==============================================================================
--- websites/production/tapestry/content/tapestry-ioc-overview.html (original)
+++ websites/production/tapestry/content/tapestry-ioc-overview.html Sat Sep 16 02:22:40 2017
@@ -27,14 +27,6 @@
</title>
<link type="text/css" rel="stylesheet" href="/resources/space.css" />
- <link href='/resources/highlighter/styles/shCoreCXF.css' rel='stylesheet' type='text/css' />
- <link href='/resources/highlighter/styles/shThemeCXF.css' rel='stylesheet' type='text/css' />
- <script src='/resources/highlighter/scripts/shCore.js' type='text/javascript'></script>
- <script src='/resources/highlighter/scripts/shBrushJava.js' type='text/javascript'></script>
- <script>
- SyntaxHighlighter.defaults['toolbar'] = false;
- SyntaxHighlighter.all();
- </script>
<link href="/styles/style.css" rel="stylesheet" type="text/css"/>
@@ -44,13 +36,26 @@
<div class="wrapper bs">
- <div id="navigation"><div class="nav"><ul class="alternate"><li><a href="index.html">Home</a></li><li><a href="getting-started.html">Getting Started</a></li><li><a href="documentation.html">Documentation</a></li><li><a href="download.html">Download</a></li><li><a href="about.html">About</a></li><li><a class="external-link" href="http://www.apache.org/licenses/LICENSE-2.0">License</a></li><li><a href="community.html">Community</a></li><li><a class="external-link" href="http://www.apache.org/security/">Security</a></li><li><a class="external-link" href="http://www.apache.org/">Apache</a></li><li><a class="external-link" href="http://www.apache.org/foundation/sponsorship.html">Sponsorship</a></li><li><a class="external-link" href="http://www.apache.org/foundation/thanks.html">Thanks</a></li></ul></div></div>
+ <div id="navigation"><div class="nav"><ul class="alternate"><li><a href="index.html">Home</a></li><li><a href="getting-started.html">Getting Started</a></li><li><a href="documentation.html">Documentation</a></li><li><a href="download.html">Download</a></li><li><a href="about.html">About</a></li><li><a class="external-link" href="http://www.apache.org/licenses/LICENSE-2.0">License</a></li><li><a href="community.html">Community</a></li><li><a class="external-link" href="http://www.apache.org/security/">Security</a></li><li><a class="external-link" href="http://www.apache.org/">Apache</a></li><li><a class="external-link" href="http://www.apache.org/foundation/sponsorship.html">Sponsorship</a></li><li><a class="external-link" href="http://www.apache.org/foundation/thanks.html">Thanks</a></li></ul></div>
+
+</div>
<div id="top">
- <div id="smallbanner"><div class="searchbox" style="float:right;margin: .3em 1em .1em 1em"><span style="color: #999; font-size: 90%">Tapestry docs, issues, wikis & blogs:</span><form enctype="application/x-www-form-urlencoded" method="get" action="http://tapestry.apache.org/search.html">
- <input type="text" name="q">
- <input type="submit" value="Search">
-</form></div><div class="emblem" style="float:left"><p><a href="index.html"><span class="confluence-embedded-file-wrapper"><img class="confluence-embedded-image confluence-external-resource" src="http://tapestry.apache.org/images/tapestry_small.png" data-image-src="http://tapestry.apache.org/images/tapestry_small.png"></span></a></p></div><div class="title" style="float:left; margin: 0 0 0 3em"><h1 id="SmallBanner-PageTitle">Tapestry IoC Overview</h1></div></div>
+ <div id="smallbanner"><div class="searchbox" style="float:right;margin: .3em 1em .1em 1em"><span style="color: #999; font-size: 90%">Tapestry docs, issues, wikis & blogs:</span>
+<form enctype="application/x-www-form-urlencoded" method="get" action="http://tapestry.apache.org/search.html">
+ <input type="text" name="q">
+ <input type="submit" value="Search">
+</form>
+
+</div>
+
+
+<div class="emblem" style="float:left"><p><a href="index.html"><span class="confluence-embedded-file-wrapper"><img class="confluence-embedded-image confluence-external-resource" src="http://tapestry.apache.org/images/tapestry_small.png" data-image-src="http://tapestry.apache.org/images/tapestry_small.png"></span></a></p></div>
+
+
+<div class="title" style="float:left; margin: 0 0 0 3em"><h1 id="SmallBanner-PageTitle">Tapestry IoC Overview</h1></div>
+
+</div>
<div class="clearer"></div>
</div>
@@ -62,26 +67,7 @@
</div>
<div id="content">
- <div id="ConfluenceContent"><p>Even today, with the overwhelming success of <a class="external-link" href="http://www.springframework.org" rel="nofollow">Spring</a> and the rise of smaller, simpler approaches to building applications (in contrast to the heavyweight EJB 2.0 approach), many people still have trouble wrapping their heads around Inversion of Control.</p><p>Really understanding IoC is a new step for many developers. If you can remember back to when you made the transition from procedural programming (in C, or BASIC) to object oriented programming, you might remember the point where you "got it". The point where it made sense to have methods on objects, and data inside objects.</p><p>Inversion of Control builds upon those ideas. The goal is to make code more robust (that is, with fewer errors), more reusable and much easier to test.</p><p>Prior to IoC approaches, most developers were used to a more <em>monolithic</em> design, with a few core objects and a
<code>main()</code> method somewhere that starts the ball rolling. <code>main()</code> instantiates the first couple of classes, and those classes end up instantiating and using all the other classes in the system.</p><p>That's an <em>unmanaged</em> system. Most desktop applications are unmanaged, so it's a very familiar pattern, and easy to get your head around.</p><p>By contrast, web applications are a <em>managed</em> environment. You don't write a main(), you don't control startup. You <em>configure</em> the Servlet API to tell it about your servlet classes to be instantiated, and their life cycle is totally controlled by the servlet container.</p><p>Inversion of Control is just a more general application of this approach. The container is ultimately responsible for instantiating and configuring the objects you tell it about, and running their entire life cycle of those objects.</p><p>Web applications are more complicated to write than monolithic applications, largely because o
f <em>multithreading</em>. Your code will be servicing many different users simultaneously across many different threads. This tends to complicate the code you write, since some fundamental aspects of object oriented development get called into question: in particular, the use of <em>internal state</em> (values stored inside instance variables), since in a multithreaded environment, that's no longer the safe place it is in traditional development. Shared objects plus internal state plus multiple threads equals an broken, unpredictable application.</p><p>Frameworks such as Tapestry – both the IoC container, and the web framework itself – exist to help.</p><p>When thinking in terms of IoC, <strong>small is beautiful</strong>. What does that mean? It means small classes and small methods are easier to code than large ones. At one extreme, we have servlets circa 1997 (and Visual Basic before that) with methods a thousand lines long, and no distinction between business logic
and view logic. Everything mixed together into an untestable jumble.</p><p>At the other extreme is IoC: small objects, each with a specific purpose, collaborating with other small objects.</p><p>Using unit tests, in collaboration with tools such as <a class="external-link" href="http://easymock.org/" rel="nofollow">EasyMock</a>, you can have a code base that is easy to maintain, easy to extend, and easy to test. And by factoring out a lot of <em>plumbing</em> code, your code base will not only be easier to work with, it will be smaller.</p><h2 id="TapestryIoCOverview-LivingontheFrontier">Living on the Frontier</h2><p>Coding applications the traditional way is like being a homesteader on the American frontier in the 1800's. You're responsible for every aspect of your house: every board, every nail, every stick of furniture is something you personally created. There <em>is</em> a great comfort in total self reliance. Even if your house is small, the windows are a bit drafty or the fl
oorboards creak a little, you know exactly <em>why</em> things are not-quite perfect.</p><p>Flash forward to modern cities or modern suburbia and it's a whole different story. Houses are built to specification from design plans, made from common materials, by many specializing tradespeople. Construction codes dictate how plumbing, wiring and framing should be performed. A home-owner may not even know how to drive a nail, but can still take comfort in draft-free windows, solid floors and working plumbing.</p><p>To extend the metaphor, a house in a town is not alone and self-reliant the way a frontier house is. The town house is situated on a street, in a neighborhood, within a town. The town provides services (utilities, police, fire control, streets and sewers) to houses in a uniform way. Each house just needs to connect up to those services.</p><h2 id="TapestryIoCOverview-TheWorldoftheContainer">The World of the Container</h2><p>So the IoC container is the "town" and in the world o
f the IoC container, everything has a name, a place, and a relationship to everything else in the container. Tapestry calls this world "The Registry".</p><p><span class="confluence-embedded-file-wrapper"><img class="confluence-embedded-image" src="tapestry-ioc-overview.data/ioc-overview.png"></span></p><p>Here we're seeing a few services from the built-in Tapestry IoC module, and a few of the services from the Tapestry web framework module. In fact, there are over 100 services, all interrelated, in the Registry ... and that's before you add your own to the mix. The IoC Registry treats all the services uniformly, regardless of whether they are part of Tapestry, or part of your application, or part of an add-on library.</p><p>Tapestry IoC's job is to make all of these services available to each other, and to the outside world. The outside world could be a standalone application, or it could be an application built on top of the Tapestry web framework.</p><h2 id="TapestryIoCOverview-Se
rviceLifeCycle">Service Life Cycle</h2><p>Tapestry services are <em>lazy</em>, which means they are not fully instantiated until they are absolutely needed. Often, what looks like a service is really a proxy object ... the first time any method of the proxy is invoked, the actual service is instantiated and initialized (Tapestry uses the term <em>realized</em> for this process). Of course, this is all absolutely thread-safe.</p><p>Initially a service is <em>defined</em>, meaning some module has defined the service. Later, the service will be <em>virtual</em>, meaning a proxy has been created. This occurs most often because some other service <em>depends</em> on it, but hasn't gotten around to invoking methods on it. Finally, a service that is ready to use is <em>realized</em>. What's nice is that your code neither knows nor cares about the life cycle of the service, because of the magic of the proxy.</p><p>In fact, when a Tapestry web application starts up, before it services its fi
rst request, only about 20% of the services have been realized; the remainder are defined or virtual.</p><h2 id="TapestryIoCOverview-Classvs.Service">Class vs. Service</h2><p>A Tapestry service is more than just a class. First of all, it is a combination of an <em>interface</em> that defines the operations of the service, and an <em>implementation class</em> that implements the interface.</p><p>Why this extra division? Having a service interface is what lets Tapestry create proxies and perform other operations. It's also a very good practice to code to an interface, rather than a specific implementation. You'll often be surprised at the kinds of things you can accomplish by substituting one implementation for another.</p><p>Tapestry is also very aware that a service will have dependencies on other services. It may also have other needs ... for example, in Tapestry IoC, the container provides services with access to Loggers.</p><p>Tapestry IoC also has support for other configuration
that may be provided to services when they are realized.</p><h2 id="TapestryIoCOverview-DependencyInjection">Dependency Injection</h2><p>Main Article: <a href="injection.html">Injection</a></p><div class="aui-label" style="float:right" title="Related Articles"><h3>Related Articles</h3><ul class="content-by-label"><li>
- <div>
- <span class="icon aui-icon aui-icon-small aui-iconfont-page-default" title="Page">Page:</span>
- </div>
- <div class="details">
- <a href="injection-in-detail.html">Injection in Detail</a>
- </div> </li><li>
- <div>
- <span class="icon aui-icon aui-icon-small aui-iconfont-page-default" title="Page">Page:</span>
- </div>
- <div class="details">
- <a href="injection-faq.html">Injection FAQ</a>
- </div> </li><li>
- <div>
- <span class="icon aui-icon aui-icon-small aui-iconfont-page-default" title="Page">Page:</span>
- </div>
- <div class="details">
- <a href="injection.html">Injection</a>
- </div> </li></ul></div><p>Inversion of Control refers to the fact that the container, here Tapestry IoC's Registry, instantiates your classes. It decides on when the classes get instantiated.</p><p>Dependency Injection is a key part of <em>realization</em>: this is how a service is provided with the other services it needs to operate. For example, a Data Access Object service may be injected with a ConnectionPool service.</p><p>In Tapestry, injection occurs through constructors, through parameters to service builder methods, or through direct injection into fields. Tapestry prefers constructor injection, as this emphasizes that dependencies should be stored in <strong>final</strong> variables. This is the best approach towards ensuring thread safety.</p><p>In any case, injection "just happens". Tapestry finds the constructor of your class and analyzes the parameters to determine what to pass in. In some cases, it uses just the parameter type to find a match, in other cases, annota
tions on the parameters may also be used. It also scans through the fields of your service implementation class to identify which should have injected values written into them.</p><h2 id="TapestryIoCOverview-Whycan'tIjustusenew?">Why can't I just use <code>new</code>?</h2><p>That's a common question. All these concepts seem alien at first. What's wrong with <code>new</code>?</p><p>The problem with new is that it rigidly connects one implementation to another implementation. Let's follow a progression that reflects how a lot of projects get written. It will show that in the real world, <code>new</code> is not as simple as it first seems.</p><p>This example is built around some real-world work that involves a Java Messaging Service queue, part of an application performance monitoring subsystem for a large application. Code inside each server collects performance data of various types and sends it, via a shared JMS queue, to a central server for collection and reporting.</p><p>This cod
e is for a metric that periodically counts the number of rows in a key database table. Other implementations of MetricProducer will be responsible for measuring CPU utilization, available disk space, number of requests per second, and so forth.</p><div class="code panel pdl" style="border-width: 1px;"><div class="codeContent panelContent pdl">
-<pre class="brush: java; gutter: false; theme: Default" style="font-size:12px;">public class TableMetricProducer implements MetricProducer
+ <div id="ConfluenceContent"><p>Even today, with the overwhelming success of <a class="external-link" href="http://www.springframework.org" rel="nofollow">Spring</a> and the rise of smaller, simpler approaches to building applications (in contrast to the heavyweight EJB 2.0 approach), many people still have trouble wrapping their heads around Inversion of Control.</p><p>Really understanding IoC is a new step for many developers. If you can remember back to when you made the transition from procedural programming (in C, or BASIC) to object oriented programming, you might remember the point where you "got it". The point where it made sense to have methods on objects, and data inside objects.</p><p>Inversion of Control builds upon those ideas. The goal is to make code more robust (that is, with fewer errors), more reusable and much easier to test.</p><p>Prior to IoC approaches, most developers were used to a more <em>monolithic</em> design, with a few core objects and a
<code>main()</code> method somewhere that starts the ball rolling. <code>main()</code> instantiates the first couple of classes, and those classes end up instantiating and using all the other classes in the system.</p><p>That's an <em>unmanaged</em> system. Most desktop applications are unmanaged, so it's a very familiar pattern, and easy to get your head around.</p><p>By contrast, web applications are a <em>managed</em> environment. You don't write a main(), you don't control startup. You <em>configure</em> the Servlet API to tell it about your servlet classes to be instantiated, and their life cycle is totally controlled by the servlet container.</p><p>Inversion of Control is just a more general application of this approach. The container is ultimately responsible for instantiating and configuring the objects you tell it about, and running their entire life cycle of those objects.</p><p>Web applications are more complicated to write than monolithic applications, largely because o
f <em>multithreading</em>. Your code will be servicing many different users simultaneously across many different threads. This tends to complicate the code you write, since some fundamental aspects of object oriented development get called into question: in particular, the use of <em>internal state</em> (values stored inside instance variables), since in a multithreaded environment, that's no longer the safe place it is in traditional development. Shared objects plus internal state plus multiple threads equals an broken, unpredictable application.</p><p>Frameworks such as Tapestry – both the IoC container, and the web framework itself – exist to help.</p><p>When thinking in terms of IoC, <strong>small is beautiful</strong>. What does that mean? It means small classes and small methods are easier to code than large ones. At one extreme, we have servlets circa 1997 (and Visual Basic before that) with methods a thousand lines long, and no distinction between business logic
and view logic. Everything mixed together into an untestable jumble.</p><p>At the other extreme is IoC: small objects, each with a specific purpose, collaborating with other small objects.</p><p>Using unit tests, in collaboration with tools such as <a class="external-link" href="http://easymock.org/" rel="nofollow">EasyMock</a>, you can have a code base that is easy to maintain, easy to extend, and easy to test. And by factoring out a lot of <em>plumbing</em> code, your code base will not only be easier to work with, it will be smaller.</p><h2 id="TapestryIoCOverview-LivingontheFrontier">Living on the Frontier</h2><p>Coding applications the traditional way is like being a homesteader on the American frontier in the 1800's. You're responsible for every aspect of your house: every board, every nail, every stick of furniture is something you personally created. There <em>is</em> a great comfort in total self reliance. Even if your house is small, the windows are a bit drafty or the fl
oorboards creak a little, you know exactly <em>why</em> things are not-quite perfect.</p><p>Flash forward to modern cities or modern suburbia and it's a whole different story. Houses are built to specification from design plans, made from common materials, by many specializing tradespeople. Construction codes dictate how plumbing, wiring and framing should be performed. A home-owner may not even know how to drive a nail, but can still take comfort in draft-free windows, solid floors and working plumbing.</p><p>To extend the metaphor, a house in a town is not alone and self-reliant the way a frontier house is. The town house is situated on a street, in a neighborhood, within a town. The town provides services (utilities, police, fire control, streets and sewers) to houses in a uniform way. Each house just needs to connect up to those services.</p><h2 id="TapestryIoCOverview-TheWorldoftheContainer">The World of the Container</h2><p>So the IoC container is the "town" and in the world o
f the IoC container, everything has a name, a place, and a relationship to everything else in the container. Tapestry calls this world "The Registry".</p><p><span class="confluence-embedded-file-wrapper"><img class="confluence-embedded-image" src="tapestry-ioc-overview.data/ioc-overview.png"></span></p><p>Here we're seeing a few services from the built-in Tapestry IoC module, and a few of the services from the Tapestry web framework module. In fact, there are over 100 services, all interrelated, in the Registry ... and that's before you add your own to the mix. The IoC Registry treats all the services uniformly, regardless of whether they are part of Tapestry, or part of your application, or part of an add-on library.</p><p>Tapestry IoC's job is to make all of these services available to each other, and to the outside world. The outside world could be a standalone application, or it could be an application built on top of the Tapestry web framework.</p><h2 id="TapestryIoCOverview-Se
rviceLifeCycle">Service Life Cycle</h2><p>Tapestry services are <em>lazy</em>, which means they are not fully instantiated until they are absolutely needed. Often, what looks like a service is really a proxy object ... the first time any method of the proxy is invoked, the actual service is instantiated and initialized (Tapestry uses the term <em>realized</em> for this process). Of course, this is all absolutely thread-safe.</p><p>Initially a service is <em>defined</em>, meaning some module has defined the service. Later, the service will be <em>virtual</em>, meaning a proxy has been created. This occurs most often because some other service <em>depends</em> on it, but hasn't gotten around to invoking methods on it. Finally, a service that is ready to use is <em>realized</em>. What's nice is that your code neither knows nor cares about the life cycle of the service, because of the magic of the proxy.</p><p>In fact, when a Tapestry web application starts up, before it services its fi
rst request, only about 20% of the services have been realized; the remainder are defined or virtual.</p><h2 id="TapestryIoCOverview-Classvs.Service">Class vs. Service</h2><p>A Tapestry service is more than just a class. First of all, it is a combination of an <em>interface</em> that defines the operations of the service, and an <em>implementation class</em> that implements the interface.</p><p>Why this extra division? Having a service interface is what lets Tapestry create proxies and perform other operations. It's also a very good practice to code to an interface, rather than a specific implementation. You'll often be surprised at the kinds of things you can accomplish by substituting one implementation for another.</p><p>Tapestry is also very aware that a service will have dependencies on other services. It may also have other needs ... for example, in Tapestry IoC, the container provides services with access to Loggers.</p><p>Tapestry IoC also has support for other configuration
that may be provided to services when they are realized.</p><h2 id="TapestryIoCOverview-DependencyInjection">Dependency Injection</h2><p>Main Article: <a href="injection.html">Injection</a></p><parameter ac:name="style">float:right</parameter><parameter ac:name="title">Related Articles</parameter><parameter ac:name="class">aui-label</parameter><rich-text-body><parameter ac:name="showLabels">false</parameter><parameter ac:name="showSpace">false</parameter><parameter ac:name="title">Related Articles</parameter><parameter ac:name="cql">label = "injection" and space = currentSpace()</parameter></rich-text-body><p>Inversion of Control refers to the fact that the container, here Tapestry IoC's Registry, instantiates your classes. It decides on when the classes get instantiated.</p><p>Dependency Injection is a key part of <em>realization</em>: this is how a service is provided with the other services it needs to operate. For example, a Data Access Object service may be injected with a Co
nnectionPool service.</p><p>In Tapestry, injection occurs through constructors, through parameters to service builder methods, or through direct injection into fields. Tapestry prefers constructor injection, as this emphasizes that dependencies should be stored in <strong>final</strong> variables. This is the best approach towards ensuring thread safety.</p><p>In any case, injection "just happens". Tapestry finds the constructor of your class and analyzes the parameters to determine what to pass in. In some cases, it uses just the parameter type to find a match, in other cases, annotations on the parameters may also be used. It also scans through the fields of your service implementation class to identify which should have injected values written into them.</p><h2 id="TapestryIoCOverview-Whycan'tIjustusenew?">Why can't I just use <code>new</code>?</h2><p>That's a common question. All these concepts seem alien at first. What's wrong with <code>new</code>?</p><p>The problem with new i
s that it rigidly connects one implementation to another implementation. Let's follow a progression that reflects how a lot of projects get written. It will show that in the real world, <code>new</code> is not as simple as it first seems.</p><p>This example is built around some real-world work that involves a Java Messaging Service queue, part of an application performance monitoring subsystem for a large application. Code inside each server collects performance data of various types and sends it, via a shared JMS queue, to a central server for collection and reporting.</p><p>This code is for a metric that periodically counts the number of rows in a key database table. Other implementations of MetricProducer will be responsible for measuring CPU utilization, available disk space, number of requests per second, and so forth.</p><parameter ac:name="">java</parameter><plain-text-body>public class TableMetricProducer implements MetricProducer
{
. . .
@@ -92,9 +78,7 @@
new QueueWriter().sendMetric(metric);
}
}
-</pre>
-</div></div><p>We've omitted some of the details (this code will need a database URL or a connection pool to operate), so as to focus on the one method and it's relationship to the QueueWriter class.</p><p>Obviously, this code has a problem ... we're creating a new QueueWriter for each metric we write into the queue, and the QueueWriter presumably is going to open the JMS queue fresh each time, an expensive operation. Thus:</p><div class="code panel pdl" style="border-width: 1px;"><div class="codeContent panelContent pdl">
-<pre class="brush: java; gutter: false; theme: Default" style="font-size:12px;">public class TableMetricProducer implements MetricProducer
+</plain-text-body><p>We've omitted some of the details (this code will need a database URL or a connection pool to operate), so as to focus on the one method and it's relationship to the QueueWriter class.</p><p>Obviously, this code has a problem ... we're creating a new QueueWriter for each metric we write into the queue, and the QueueWriter presumably is going to open the JMS queue fresh each time, an expensive operation. Thus:</p><parameter ac:name="">java</parameter><plain-text-body>public class TableMetricProducer implements MetricProducer
{
. . .
@@ -105,11 +89,7 @@
int rowCount = . . .;
Metric metric = new Metric("app/clients", System.currentTimeMillis(), rowCount);
queueWriter.sendMetric(metric);
- }</pre>
-</div></div><p>That's better. It's not perfect ... a proper system might know when the application was being shutdown and would shut down the JMS Connection inside the QueueWriter as well.</p><p>Here's a more immediate problem: JMS connections are really meant to be shared, and we'll have lots of little classes collecting different metrics. So we need to make the QueueWriter shareable:</p><div class="code panel pdl" style="border-width: 1px;"><div class="codeContent panelContent pdl">
-<pre class="brush: java; gutter: false; theme: Default" style="font-size:12px;"> private final QueueWriter queueWriter = QueueWriter.getInstance();</pre>
-</div></div><p>... and inside class QueueWriter:</p><div class="code panel pdl" style="border-width: 1px;"><div class="codeContent panelContent pdl">
-<pre class="brush: java; gutter: false; theme: Default" style="font-size:12px;">public class QueueWriter
+ }</plain-text-body><p>That's better. It's not perfect ... a proper system might know when the application was being shutdown and would shut down the JMS Connection inside the QueueWriter as well.</p><p>Here's a more immediate problem: JMS connections are really meant to be shared, and we'll have lots of little classes collecting different metrics. So we need to make the QueueWriter shareable:</p><parameter ac:name="">java</parameter><plain-text-body> private final QueueWriter queueWriter = QueueWriter.getInstance();</plain-text-body><p>... and inside class QueueWriter:</p><parameter ac:name="">java</parameter><plain-text-body>public class QueueWriter
{
private static QueueWriter instance;
@@ -127,9 +107,7 @@
return instance;
}
}
-</pre>
-</div></div><p>Much better! Now all the metric producers running inside all the threads can share a single QueueWriter. Oh wait ...</p><div class="code panel pdl" style="border-width: 1px;"><div class="codeContent panelContent pdl">
-<pre class="brush: java; gutter: false; theme: Default" style="font-size:12px;"> public synchronized static getInstance()
+</plain-text-body><p>Much better! Now all the metric producers running inside all the threads can share a single QueueWriter. Oh wait ...</p><parameter ac:name="">java</parameter><plain-text-body> public synchronized static getInstance()
{
if (instance == null)
{
@@ -137,9 +115,7 @@
}
return instance;
}
-</pre>
-</div></div><p>Is that necessary? Yes. Will the code work without it? Yes – <strong>99.9% of the time</strong>. In fact, this is a very common error in systems that manually code a lot of these construction patterns: forgetting to properly synchronize access. These things often work in development and testing, but fail (with infuriating infrequency) in production, as it takes two or more threads running simultaneously to reveal the coding error.</p><p>Wow, we're a long way from a simple <code>new</code> already, and we're talking about just one service. But let's detour into <em>testing</em>.</p><p>How would you test TableMetricProducer? One way would be to let it run and try to find the message or messages it writes in the queue, but that seems fraught with difficulties. It's more of an integration test, and is certainly something that you'd want to execute at some stage of your development, but not as part of a quick-running unit test suite.</p><p>Instead, let's split QueueW
riter in two: a QueueWriter interface, and a QueueWriterImpl implementation class. This will allow us to run TableMetricProducer against a <em>mock implementation</em> of QueueWriter, rather than the real thing. This is one of the immediate benefits of <em>coding to an interface</em> rather than <em>coding to an implementation</em>.</p><p>We'll need to change TableMetricProducer to take the QueueWriter as a constructor parameter.</p><div class="code panel pdl" style="border-width: 1px;"><div class="codeContent panelContent pdl">
-<pre class="brush: java; gutter: false; theme: Default" style="font-size:12px;">public class TableMetricProducer implements MetricProducer
+</plain-text-body><p>Is that necessary? Yes. Will the code work without it? Yes – <strong>99.9% of the time</strong>. In fact, this is a very common error in systems that manually code a lot of these construction patterns: forgetting to properly synchronize access. These things often work in development and testing, but fail (with infuriating infrequency) in production, as it takes two or more threads running simultaneously to reveal the coding error.</p><p>Wow, we're a long way from a simple <code>new</code> already, and we're talking about just one service. But let's detour into <em>testing</em>.</p><p>How would you test TableMetricProducer? One way would be to let it run and try to find the message or messages it writes in the queue, but that seems fraught with difficulties. It's more of an integration test, and is certainly something that you'd want to execute at some stage of your development, but not as part of a quick-running unit test suite.</p><p>Instead, let's split
QueueWriter in two: a QueueWriter interface, and a QueueWriterImpl implementation class. This will allow us to run TableMetricProducer against a <em>mock implementation</em> of QueueWriter, rather than the real thing. This is one of the immediate benefits of <em>coding to an interface</em> rather than <em>coding to an implementation</em>.</p><p>We'll need to change TableMetricProducer to take the QueueWriter as a constructor parameter.</p><parameter ac:name="">java</parameter><plain-text-body>public class TableMetricProducer implements MetricProducer
{
private final QueueWriter queueWriter;
@@ -170,9 +146,7 @@
queueWriter.sendMetric(metric);
}
}
-</pre>
-</div></div><p>This still isn't ideal, as we still have an explicit linkage between TableMetricProducer and QueueWriterImpl.</p><p>What we're seeing here is that there are multple <em>concerns</em> inside the little bit of code in this example. TableMetricProducer has an unwanted <em>construction concern</em> about which implementation of QueueWriter to instantiate (this shows up as two constructors, rather than just one). QueueWriterImpl has an additional <em>life cycle concern</em>, in terms of managing the singleton.</p><p>These extra concerns, combined with the use of static variables and methods, are a <em>bad design smell</em>. It's not yet very stinky, because this example is so small, but these problems tend to multiply as an application grows larger and more complex, especially as services start to truly collaborate in earnest.</p><p>For comparison, lets see what the Tapestry IoC implementation would look like:</p><div class="code panel pdl" style="border-width: 1px;"><div
class="codeContent panelContent pdl">
-<pre class="brush: java; gutter: false; theme: Default" style="font-size:12px;">public class MonitorModule
+</plain-text-body><p>This still isn't ideal, as we still have an explicit linkage between TableMetricProducer and QueueWriterImpl.</p><p>What we're seeing here is that there are multple <em>concerns</em> inside the little bit of code in this example. TableMetricProducer has an unwanted <em>construction concern</em> about which implementation of QueueWriter to instantiate (this shows up as two constructors, rather than just one). QueueWriterImpl has an additional <em>life cycle concern</em>, in terms of managing the singleton.</p><p>These extra concerns, combined with the use of static variables and methods, are a <em>bad design smell</em>. It's not yet very stinky, because this example is so small, but these problems tend to multiply as an application grows larger and more complex, especially as services start to truly collaborate in earnest.</p><p>For comparison, lets see what the Tapestry IoC implementation would look like:</p><parameter ac:name="">java</parameter><plain-text-body
>public class MonitorModule
{
public static void bind(ServiceBinder binder)
{
@@ -185,8 +159,7 @@
configuration.add(new TableMetricProducer(queueWriter, . . .))
}
}
-</pre>
-</div></div><p>Again, we've omitted a few details related to the database the TableMetricProducer will point at (in fact, Tapestry IoC provides a lot of support for configuration of this type as well, which is yet another concern).</p><p>The MonitorModule class is a Tapestry IoC module: a class that defines and configures services.</p><p>The bind() method is the principle way that services are made known to the Registry: here we're binding a service interface to a service implementation. QueueWriter we've discussed already, and MetricScheduler is a service that is responsible for determining when MetricProducer instances run.</p><p>The contributeMetricScheduler() method allows the module to <em>contribute</em> into the MetricProducer service's <em>configuration</em>. More testability: the MetricProducer isn't tied to a pre-set list of producers, instead it will have a Collection<MetricProducer> injected into its constructor. Thus, when we're coding the MetricProducerImpl class
, we can test it against mock implementations of MetricProducer.</p><p>The QueueWriter service is injected into the contributeMetricScheduler() method. Since there's only one QueueWriter service, Tapestry IoC is able to "find" the correct service based entirely on type. If, eventually, there's more than one QueueWriter service (perhaps pointing at different JMS queues), you would use an annotation on the parameter to help Tapestry connect the parameter to the appropriate service.</p><p>Presumably, there would be a couple of other parameters to the contributeMetricScheduler() method, to inject in a database URL or connection pool (that would, in turn, be passed to TableMetricProducer).</p><p>A new TableMetricProducer instance is created and contributed in. We could contribute as many producers as we like here. Other modules could also define a contributeMetricScheduler() method and contribute their own MetricProducer instances.</p><p>Meanwhile, the QueueWriterImpl class no longer nee
ds the <code>instance</code> variable or getInstance() method, and the TableMetricProducer only needs a single constructor.</p><h2 id="TapestryIoCOverview-AdvantagesofIoC:Summary">Advantages of IoC: Summary</h2><p>It would be ludicrous for us to claim that applications built without an IoC container are doomed to failure. There is overwhelming evidence that applications have been built without containers and have been perfectly successful.</p><p>What we are saying is that IoC techniques and discipline will lead to applications that are:</p><ul><li>More testable – smaller, simpler classes; coding to interfaces allows use of mock implementations</li><li>More robust – smaller, simpler classes; use of final variables; thread safety baked in</li><li>More scalable – thread safety baked in</li><li>Easier to maintain – less code, simpler classes</li><li>Easier to extend – new features are often additions (new services, new contributions) rather than changes to
existing classes</li></ul><p>What we're saying is that an IoC container allows you to work faster and smarter.</p><p>Many of these traits work together; for example, a more testable application is inherently more robust. Having a test suite makes it easier to maintain and extend your code, because its much easier to see if new features break existing ones. Simpler code plus tests also lowers the cost of entry for new developers coming on board, which allows for more developers to work efficiently on the same code base. The clean separation between interface and implementation also allows multiple developers to work on different aspects of the same code base with a lowered risk of interference and conflict.</p><p>By contrast, traditional applications, which we term <em>monolithic</em> applications, are often very difficult to test, because there are fewer classes, and each class has multiple concerns. A lack of tests makes it more difficult to add new features without breaking existi
ng features. Further, the monolithic approach more often leads to implementations being linked to other implementations, yet another hurdle standing in the way of testing.</p><p>Let's end with a metaphor.</p><p>Over a decade ago, when Java first came on the scene, it was the first mainstream language to support garbage collection. This was very controversial: the garbage collector was seen as unnecessary, and a waste of resources. Among C and C++ developers, the attitude was "Why do I need a garbage collector? If I call malloc() I can call free()."</p><p>But now, most developers would never want to go back to a non-garbage collected environment. Having the GC around makes it much easier to code in a way we find natural: many small related objects working together. It turns out that knowing when to call free() is more difficult than it sounds. The Objective-C language tried to solve this with retain counts on objects and that still lead to memory leaks when it was applied to object <
em>graphs</em> rather than object <em>trees</em>.</p><p>Roll the clock forward a decade and the common consensus has shifted considerably. Objective-C 2.0 features true garbage collection and GC libraries are available for C and C++. All scripting languages, including Ruby and Python, feature garbage collection as well. A new language <em>without</em> garbage collection is now considered an anomaly.</p><p>The point is, the life cycle of objects turns out to be far more complicated than it looks at first glance. We've come to accept that our own applications lack the ability to police their objects as they are no longer needed (they literally lack the ability to determine <em>when</em> an object is no longer needed) and the garbage collector, a kind of higher authority, takes over that job very effectively. The end result? Less code and fewer bugs. And a careful study shows that the Java memory allocator and garbage collector (the two are quite intimately tied together) is actually <
strong>more</strong> efficient than malloc() and free().</p><p>So we've come to accept that the <em>death concern</em> is better handled outside of our own code. The use of Inversion of Control is simply the flip side of that: the <em>life cycle and construction concerns</em> are also better handled by an outside authority as well: the IoC container. These concerns govern when a service is <em>realized</em> and how its dependencies and configuration are injected. As with the garbage collector, ceding these chores to the container results in less code and fewer bugs, and lets you concentrate on the things that should matter to you: your business logic, your application – and not a whole bunch of boilerplate plumbing!</p><p> </p><p></p></div>
+</plain-text-body><p>Again, we've omitted a few details related to the database the TableMetricProducer will point at (in fact, Tapestry IoC provides a lot of support for configuration of this type as well, which is yet another concern).</p><p>The MonitorModule class is a Tapestry IoC module: a class that defines and configures services.</p><p>The bind() method is the principle way that services are made known to the Registry: here we're binding a service interface to a service implementation. QueueWriter we've discussed already, and MetricScheduler is a service that is responsible for determining when MetricProducer instances run.</p><p>The contributeMetricScheduler() method allows the module to <em>contribute</em> into the MetricProducer service's <em>configuration</em>. More testability: the MetricProducer isn't tied to a pre-set list of producers, instead it will have a Collection<MetricProducer> injected into its constructor. Thus, when we're coding the MetricProducerImpl
class, we can test it against mock implementations of MetricProducer.</p><p>The QueueWriter service is injected into the contributeMetricScheduler() method. Since there's only one QueueWriter service, Tapestry IoC is able to "find" the correct service based entirely on type. If, eventually, there's more than one QueueWriter service (perhaps pointing at different JMS queues), you would use an annotation on the parameter to help Tapestry connect the parameter to the appropriate service.</p><p>Presumably, there would be a couple of other parameters to the contributeMetricScheduler() method, to inject in a database URL or connection pool (that would, in turn, be passed to TableMetricProducer).</p><p>A new TableMetricProducer instance is created and contributed in. We could contribute as many producers as we like here. Other modules could also define a contributeMetricScheduler() method and contribute their own MetricProducer instances.</p><p>Meanwhile, the QueueWriterImpl class no long
er needs the <code>instance</code> variable or getInstance() method, and the TableMetricProducer only needs a single constructor.</p><h2 id="TapestryIoCOverview-AdvantagesofIoC:Summary">Advantages of IoC: Summary</h2><p>It would be ludicrous for us to claim that applications built without an IoC container are doomed to failure. There is overwhelming evidence that applications have been built without containers and have been perfectly successful.</p><p>What we are saying is that IoC techniques and discipline will lead to applications that are:</p><ul><li>More testable – smaller, simpler classes; coding to interfaces allows use of mock implementations</li><li>More robust – smaller, simpler classes; use of final variables; thread safety baked in</li><li>More scalable – thread safety baked in</li><li>Easier to maintain – less code, simpler classes</li><li>Easier to extend – new features are often additions (new services, new contributions) rather than chang
es to existing classes</li></ul><p>What we're saying is that an IoC container allows you to work faster and smarter.</p><p>Many of these traits work together; for example, a more testable application is inherently more robust. Having a test suite makes it easier to maintain and extend your code, because its much easier to see if new features break existing ones. Simpler code plus tests also lowers the cost of entry for new developers coming on board, which allows for more developers to work efficiently on the same code base. The clean separation between interface and implementation also allows multiple developers to work on different aspects of the same code base with a lowered risk of interference and conflict.</p><p>By contrast, traditional applications, which we term <em>monolithic</em> applications, are often very difficult to test, because there are fewer classes, and each class has multiple concerns. A lack of tests makes it more difficult to add new features without breaking
existing features. Further, the monolithic approach more often leads to implementations being linked to other implementations, yet another hurdle standing in the way of testing.</p><p>Let's end with a metaphor.</p><p>Over a decade ago, when Java first came on the scene, it was the first mainstream language to support garbage collection. This was very controversial: the garbage collector was seen as unnecessary, and a waste of resources. Among C and C++ developers, the attitude was "Why do I need a garbage collector? If I call malloc() I can call free()."</p><p>But now, most developers would never want to go back to a non-garbage collected environment. Having the GC around makes it much easier to code in a way we find natural: many small related objects working together. It turns out that knowing when to call free() is more difficult than it sounds. The Objective-C language tried to solve this with retain counts on objects and that still lead to memory leaks when it was applied to ob
ject <em>graphs</em> rather than object <em>trees</em>.</p><p>Roll the clock forward a decade and the common consensus has shifted considerably. Objective-C 2.0 features true garbage collection and GC libraries are available for C and C++. All scripting languages, including Ruby and Python, feature garbage collection as well. A new language <em>without</em> garbage collection is now considered an anomaly.</p><p>The point is, the life cycle of objects turns out to be far more complicated than it looks at first glance. We've come to accept that our own applications lack the ability to police their objects as they are no longer needed (they literally lack the ability to determine <em>when</em> an object is no longer needed) and the garbage collector, a kind of higher authority, takes over that job very effectively. The end result? Less code and fewer bugs. And a careful study shows that the Java memory allocator and garbage collector (the two are quite intimately tied together) is actu
ally <strong>more</strong> efficient than malloc() and free().</p><p>So we've come to accept that the <em>death concern</em> is better handled outside of our own code. The use of Inversion of Control is simply the flip side of that: the <em>life cycle and construction concerns</em> are also better handled by an outside authority as well: the IoC container. These concerns govern when a service is <em>realized</em> and how its dependencies and configuration are injected. As with the garbage collector, ceding these chores to the container results in less code and fewer bugs, and lets you concentrate on the things that should matter to you: your business logic, your application – and not a whole bunch of boilerplate plumbing!</p><p> </p><p></p></div>
</div>
<div class="clearer"></div>