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Posted to dev@commons.apache.org by de...@apache.org on 2006/01/20 21:57:08 UTC
svn commit: r370898 - in /jakarta/commons/proper/logging/trunk: project.xml
xdocs/guide.xml xdocs/index.xml xdocs/tech.xml
Author: dennisl
Date: Fri Jan 20 12:56:46 2006
New Revision: 370898
URL: http://svn.apache.org/viewcvs?rev=370898&view=rev
Log:
Fix broken and redirected links.
Modified:
jakarta/commons/proper/logging/trunk/project.xml
jakarta/commons/proper/logging/trunk/xdocs/guide.xml
jakarta/commons/proper/logging/trunk/xdocs/index.xml
jakarta/commons/proper/logging/trunk/xdocs/tech.xml
Modified: jakarta/commons/proper/logging/trunk/project.xml
URL: http://svn.apache.org/viewcvs/jakarta/commons/proper/logging/trunk/project.xml?rev=370898&r1=370897&r2=370898&view=diff
==============================================================================
--- jakarta/commons/proper/logging/trunk/project.xml (original)
+++ jakarta/commons/proper/logging/trunk/project.xml Fri Jan 20 12:56:46 2006
@@ -88,13 +88,13 @@
<name>Commons Dev List</name>
<subscribe>commons-dev-subscribe@jakarta.apache.org</subscribe>
<unsubscribe>commons-dev-unsubscribe@jakarta.apache.org</unsubscribe>
- <archive>http://mail-archives.apache.org/eyebrowse/SummarizeList?listName=commons-dev@jakarta.apache.org</archive>
+ <archive>http://mail-archives.apache.org/mod_mbox/jakarta-commons-dev/</archive>
</mailingList>
<mailingList>
<name>Commons User List</name>
<subscribe>commons-user-subscribe@jakarta.apache.org</subscribe>
<unsubscribe>commons-user-unsubscribe@jakarta.apache.org</unsubscribe>
- <archive>http://mail-archives.apache.org/eyebrowse/SummarizeList?listName=commons-user@jakarta.apache.org</archive>
+ <archive>http://mail-archives.apache.org/mod_mbox/jakarta-commons-user/</archive>
</mailingList>
</mailingLists>
Modified: jakarta/commons/proper/logging/trunk/xdocs/guide.xml
URL: http://svn.apache.org/viewcvs/jakarta/commons/proper/logging/trunk/xdocs/guide.xml?rev=370898&r1=370897&r2=370898&view=diff
==============================================================================
--- jakarta/commons/proper/logging/trunk/xdocs/guide.xml (original)
+++ jakarta/commons/proper/logging/trunk/xdocs/guide.xml Fri Jan 20 12:56:46 2006
@@ -94,8 +94,8 @@
</p>
<p>JCL provides thin-wrapper <code>Log</code> implementations for
other logging tools, including
-<a href="http://jakarta.apache.org/log4j/docs/index.html">Log4J</a>,
-<a href="http://jakarta.apache.org/avalon/logkit/index.html">Avalon LogKit</a>,
+<a href="http://logging.apache.org/log4j/docs/index.html">Log4J</a>,
+<a href="http://avalon.apache.org/logkit/index.html">Avalon LogKit</a>,
the Avalon Framework's logging infrastructure,
JDK 1.4, and an implementation of JDK 1.4 logging APIs (JSR-47) for pre-1.4
systems.
@@ -143,16 +143,16 @@
<li>
If the Log4J logging system is available in the application
class path, use the corresponding wrapper class
-(<a href="http://jakarta.apache.org/commons/logging/api/org/apache/commons/logging/impl/Log4JLogger.html">Log4JLogger</a>).
+(<a href="http://jakarta.apache.org/commons/logging/apidocs/org/apache/commons/logging/impl/Log4JLogger.html">Log4JLogger</a>).
</li>
<li>
If the application is executing on a JDK 1.4 system, use
the corresponding wrapper class
-(<a href="http://jakarta.apache.org/commons/logging/api/org/apache/commons/logging/impl/Jdk14Logger.html">Jdk14Logger</a>).
+(<a href="http://jakarta.apache.org/commons/logging/apidocs/org/apache/commons/logging/impl/Jdk14Logger.html">Jdk14Logger</a>).
</li>
<li>
Fall back to the default simple logging wrapper
-(<a href="http://jakarta.apache.org/commons/logging/api/org/apache/commons/logging/impl/SimpleLog.html">SimpleLog</a>).
+(<a href="http://jakarta.apache.org/commons/logging/apidocs/org/apache/commons/logging/impl/SimpleLog.html">SimpleLog</a>).
</li>
</ol>
<p>
@@ -176,7 +176,7 @@
<p>
Log4J is a very commonly used logging implementation (as well as being the JCL primary default),
so a <i>few</i> details are presented herein to get the developer/integrator going.
-Please see the <a href='http://logging.apache.org/log4j'>Log4J Home</a> for more details
+Please see the <a href='http://logging.apache.org/log4j/docs/index.html'>Log4J Home</a> for more details
on Log4J and it's configuration.
</p>
<p>
@@ -314,8 +314,8 @@
The <code>commons-logging.jar</code> file includes the JCL API, the default
<code>LogFactory</code> implemenation and thin-wrapper <code>Log</code>
implementations for
-<a href="http://jakarta.apache.org/log4j/docs/index.html">Log4J</a>,
-<a href="http://jakarta.apache.org/avalon/logkit/index.html">Avalon LogKit</a>,
+<a href="http://logging.apache.org/log4j/docs/index.html">Log4J</a>,
+<a href="http://avalon.apache.org/logkit/index.html">Avalon LogKit</a>,
the Avalon Framework's logging infrastructure,
JDK 1.4, as well as an implementation of JDK 1.4 logging APIs (JSR-47) for
pre-1.4 systems.
@@ -352,7 +352,7 @@
wrapper <code>Log</code> implementations for <code>Log4j</code>,
<code>Avalon</code> and <code>Lumberjack</code>. This jar is intended for
use in specialized containers such as
-<a href='http://jakarta.apache.org/tomcat'>Tomcat</a> that wish to use
+<a href='http://tomcat.apache.org/'>Tomcat</a> that wish to use
JCL internally but also need to make JCL available for use by deployed
applications.
</p>
Modified: jakarta/commons/proper/logging/trunk/xdocs/index.xml
URL: http://svn.apache.org/viewcvs/jakarta/commons/proper/logging/trunk/xdocs/index.xml?rev=370898&r1=370897&r2=370898&view=diff
==============================================================================
--- jakarta/commons/proper/logging/trunk/xdocs/index.xml (original)
+++ jakarta/commons/proper/logging/trunk/xdocs/index.xml Fri Jan 20 12:56:46 2006
@@ -68,7 +68,7 @@
<strong>Note:</strong> <em>this has now been blessed with alpha status.</em>
</p>
<p>
-Please <a href='http://cvs.apache.org/dist/jakarta/commons/logging'>download</a> and test this release. Reports should be directed to the commons-dev
+Please <a href='http://cvs.apache.org/dist/jakarta/commons/logging/'>download</a> and test this release. Reports should be directed to the commons-dev
mailing list.
</p>
<p>
@@ -92,10 +92,8 @@
when downloading from a mirror.
</p>
<p>
- Binary releases are available
- <a href="http://jakarta.apache.org/site/binindex.cgi">here</a>.
- Source releases are available
- <a href="http://jakarta.apache.org/site/sourceindex.cgi">here</a>
+ Binary and source releases are available
+ <a href="http://jakarta.apache.org/site/downloads/downloads_commons-logging.cgi">here</a>.
</p>
<p>
<ul>
@@ -108,9 +106,8 @@
<p>
<ul>
<ul>
-<li><a href="http://jakarta.apache.org/builds/jakarta-commons/release/commons-logging/v1.0.2/">Version 1.0.2</a></li>
-<li><a href="http://jakarta.apache.org/builds/jakarta-commons/release/commons-logging/v1.0.1/">Version 1.0.1</a></li>
-<li><a href="http://jakarta.apache.org/builds/jakarta-commons/release/commons-logging/v1.0/">Version 1.0</a></li>
+<li><a href="http://archive.apache.org/dist/jakarta/commons/logging/binaries/">Binary releases</a></li>
+<li><a href="http://archive.apache.org/dist/jakarta/commons/logging/source/">Source releases</a></li>
</ul>
</ul>
</p>
Modified: jakarta/commons/proper/logging/trunk/xdocs/tech.xml
URL: http://svn.apache.org/viewcvs/jakarta/commons/proper/logging/trunk/xdocs/tech.xml?rev=370898&r1=370897&r2=370898&view=diff
==============================================================================
--- jakarta/commons/proper/logging/trunk/xdocs/tech.xml (original)
+++ jakarta/commons/proper/logging/trunk/xdocs/tech.xml Fri Jan 20 12:56:46 2006
@@ -1,658 +1,658 @@
-<?xml version="1.0"?>
-
-<!--
-
- Copyright 2001-2004 The Apache Software Foundation.
-
- Licensed under the Apache License, Version 2.0 (the "License");
- you may not use this file except in compliance with the License.
- You may obtain a copy of the License at
-
- http://www.apache.org/licenses/LICENSE-2.0
-
- Unless required by applicable law or agreed to in writing, software
- distributed under the License is distributed on an "AS IS" BASIS,
- WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- See the License for the specific language governing permissions and
- limitations under the License.
-
--->
-
-<document>
-
- <properties>
- <title>JCL Technology Guide</title>
- <author email="commons-dev@jakarta.apache.org">Commons Documentation Team</author>
- </properties>
-
- <body>
- <section name='Overview'>
- <subsection name='Contents'>
- <ul>
- <li>
- Overview
- <ul>
- <li>
- Contents
- </li>
- <li>
- <a href='#Introduction'>Introduction</a>
- </li>
- </ul>
- </li>
- <li>
- <a href='#A Short Introduction to Class Loading and Class Loaders'>
- A Short Introduction to Class Loading and Class Loaders
- </a>
- <ul>
- <li>
- <a href='#Preamble'>
- Preamble
- </a>
- </li>
- <li>
- <a href='#Resolution Of Symbolic References'>
- Resolution Of Symbolic References
- </a>
- </li>
- <li>
- <a href='#Loading'>
- Loading
- </a>
- </li>
- <li>
- <a href='#Linking'>
- Linking
- </a>
- </li>
- <li>
- <a href='#Loading Classes'>
- Loading Classes
- </a>
- </li>
- <li>
- <a href='#Bootstrap Classloader'>
- Bootstrap Classloader
- </a>
- </li>
- <li>
- <a href='#Runtime Package'>
- Runtime Package
- </a>
- </li>
- <li>
- <a href='#Loader Used To Resolve A Symbolic Reference'>
- Loader Used To Resolve A Symbolic Reference
- </a>
- </li>
- <li>
- <a href='#Bibliography'>
- Bibliography
- </a>
- </li>
- </ul>
- </li>
- <li>
- <a href='#A Short Guide To Hierarchical Class Loading'>
- A Short Guide To Hierarchical Class Loading
- </a>
- <ul>
- <li>
- <a href='#Delegating Class Loaders'>
- Delegating Class Loaders
- </a>
- </li>
- <li>
- <a href='#Parent-First And Child-First Class Loaders'>
- Parent-First And Child-First Class Loaders
- </a>
- </li>
- <li>
- <a href='#Class ClassLoader'>
- Class ClassLoader
- </a>
- </li>
- <li>
- <a href='#Context ClassLoader'>
- Context ClassLoader
- </a>
- </li>
- <li>
- <a href='#The Context Classloader in Container Applications'>
- The Context Classloader in Container Applications
- </a>
- </li>
- <li>
- <a href='#Issues with Context ClassLoaders'>
- Issues with Context ClassLoaders
- </a>
- </li>
- <li>
- <a href='#Reflection And The Context ClassLoader'>
- Reflection And The Context ClassLoader
- </a>
- </li>
- <li>
- <a href='#More Information'>
- More Information
- </a>
- </li>
- </ul>
- </li>
- <li>
- <a href='#A Short Theory Guide To JCL'>
- A Short Theory Guide To JCL
- </a>
- <ul>
- <li>
- <a href='#Isolation And The Context Class Loader'>
- Isolation And The Context Class Loader
- </a>
- </li>
- <li>
- <a href='#Log And LogFactory'>
- Log And LogFactory
- </a>
- </li>
- <li>
- <a href='#Log Implementations'>
- Log Implementations
- </a>
- </li>
- <li>
- <a href='#Using Reflection To Load Log Implementations'>
- Using Reflection To Load Log Implementations
- </a>
- </li>
- </ul>
- </li>
- </ul>
- </subsection>
- <subsection name='Introduction'>
- <p>
- This guide is aimed at describing the technologies that JCL developers and expert users
- (and users who need to become experts)
- should be familiar with. The aim is to give an understanding whilst being precise but brief.
- Details which are not relevent for JCL have been suppressed.
- References have been included.
- </p>
- <p>
- These topics are a little difficult and it's easy for even experience developers to make
- mistakea. We need you to help us get it right! Please submit corrections, comments, additional references
- and requests for clarification
- by either:
- </p>
- <ul>
- <li>
- posting to the <a href='http://jakarta.apache.org/site/mail.html'>jakarta commons-dev mailing list</a> or
- </li>
- <li>
- creating an issue in <a href='http://issues.apache.org/bugzilla'>Bugzilla</a>.
- </li>
- </ul>
- <p>
- TIA
- </p>
- </subsection>
-
- </section>
- <section name='A Short Introduction to Class Loading and Class Loaders'>
- <subsection name='Preamble'>
- <p>
- This is intended to present a guide to the process by which Java bytecode uses bytecode in other classes
- from the perspective of the language and virtual machine specifications. The focus will be on deciding
- which bytecode will be used (rather than the mechanics of the usage). It focusses on facts and terminology.
- </p>
- <p>
- The process is recursive: it is therefore difficult to pick a starting point.
- Sun's documentation starts from the persective of the startup of a new application.
- This guide starts from the perspective of an executing application.
- </p>
- <p>
- During this discussion, please assume that each time that <em>class</em> is mentioned,
- the comments applied equally well to interfaces.
- </p>
- <p>
- This document is targeted at Java 1.2 and above.
- </p>
- </subsection>
- <subsection name='Resolution Of Symbolic References'>
- <p>
- (<a href='http://java.sun.com/docs/books/jls/second_edition/html/execution.doc.html#44524'>LangSpec 12.3.3</a>)
- The bytecode representation of a class contains symbolic names for other classes referenced.
- </p>
- <p>
- <em>
- In practical development terms: If a class is imported (either explicitly in the list of imports at the top of
- the source file or implicitly through a fully qualified name in the source code) it is referenced symbolically.
- </em>
- </p>
- <p>
- (<a href='http://java.sun.com/docs/books/vmspec/2nd-edition/html/ConstantPool.doc.html#73492'>VMSpec 5.4.3</a>)
- Resolution of a symbolic reference occurs dymanically at runtime and is carried out by
- the Java Vitual Machine. Resolution of a symbolic reference requires loading and linking of the new class.
- </p>
- <p>
- <em>
- Note: references are not statically resolved at compile time.
- </em>
- </p>
- </subsection>
- <subsection name='Loading'>
- <p>
- (<a href='http://java.sun.com/docs/books/vmspec/2nd-edition/html/Concepts.doc.html#19175'>VMSpec 2.17.2</a>)
- Loading is the name given process by which a binary form of a class is obtained
- by the Java Virutal Machine.
- Java classes are always loaded and linked dynamically by the Java Virtual Machine
- (rather than statically by the compiler).
- </p>
- <p>
- <em>
- In practical development terms:
- This means that the developer has no certain knowledge about the actual
- bytecode that will be used to execute any external call (one made outside the class). This is determined only
- at execution time and is effected by the way that the code is deployed.
- </em>
- </p>
- </subsection>
- <subsection name='Linking'>
- <p>
- (<a href='http://java.sun.com/docs/books/vmspec/2nd-edition/html/Concepts.doc.html#22574'>VMSpec 2.17.3</a>)
- Linking is the name used for combining the
- binary form of a class into the Java Virtual Machine. This must happen before the class can be used.
- </p>
- <p>
- (<a href='http://java.sun.com/docs/books/vmspec/2nd-edition/html/Concepts.doc.html#22574'>VMSpec 2.17.3</a>)
- Linking is composed of verification, preparation and resolution (of symbolic references).
- Flexibility is allowed over the timing of resolution. (Within limit) this may happen at any time after
- preparation and before that reference is used.
- </p>
- <p>
- <em>
- In practical development terms: This means that different JVMs may realize that a reference cannot be
- resolved at different times during execution. Consequently, the actual behaviour cannot be precisely predicted
- without intimate knowledge of the JVM (on which the bytecode will be executed).
- This makes it hard to give universal guildance to users.
- </em>
- </p>
- </subsection>
-
- <subsection name='Loading Classes'>
- <p>
- (<a href='http://java.sun.com/docs/books/vmspec/2nd-edition/html/Concepts.doc.html#19175'>VMSpec 2.17.2</a>)
- The loading process is performed by a <code>ClassLoader</code>.
- </p>
- <p>
- (<a href='http://java.sun.com/docs/books/vmspec/2nd-edition/html/ConstantPool.doc.html#72007'>VMSpec 5.3</a>)
- A classloader may create a class either by delegation or by defining it directly.
- The classloader that initiates loading of a class is known as the initiating loader.
- The classloader that defines the class is known as the defining loader.
- </p>
- <p>
- <em>
- In practical terms: understanding and appreciating this distinction is crucial when debugging issues
- concerning classloaders.
- </em>
- </p>
- </subsection>
-
- <subsection name='Bootstrap Classloader'>
- <p>
- (<a href='http://java.sun.com/docs/books/vmspec/2nd-edition/html/ConstantPool.doc.html#72007'>VMSPEC 5.3</a>)
- The bootstrap is the base <code>ClassLoader</code> supplied by the Java Virtual Machine.
- All others are user (also known as application) <code>ClassLoader</code>'s.
- </p>
- <p>
- <em>
- In practical development terms: The System classloader returned by <code>Classloader.getSystemClassLoader()</code>
- will be either the bootstrap classloader or a direct descendent of the bootstrap classloader.
- Only when debugging issues concerning the system classloader should there be any need to consider the detailed
- differences between the bootstrap classloader and the system classloader.
- </em>
- </p>
- </subsection>
- <subsection name='Runtime Package'>
- <p>
- (<a href='http://java.sun.com/docs/books/vmspec/2nd-edition/html/ConstantPool.doc.html#72007'>VMSpec 5.3</a>)
- At runtime, a class (or interface) is determined by it's fully qualified name
- and by the classloader that defines it. This is known as the class's runtime package.
- </p>
- <p>
- (<a href='http://java.sun.com/docs/books/vmspec/2nd-edition/html/ConstantPool.doc.html#75929'>VMSpec 5.4.4</a>)
- Only classes in the same runtime package are mutually accessible.
- </p>
- <p>
- <em>
- In practical development terms: two classes with the same symbolic name can only be used interchangably
- if they are defined by the same classloader. A classic symptom indicative of a classloader issue is that
- two classes with the same fully qualified name are found to be incompatible during a method call.
- This may happen when a member is expecting an interface which is (seemingly) implemented by a class
- but the class is in a different runtime package after being defined by a different classloader. This is a
- fundemental java language security feature.
- </em>
- </p>
- </subsection>
-
- <subsection name='Loader Used To Resolve A Symbolic Reference'>
- <p>
- (<a href='http://java.sun.com/docs/books/vmspec/2nd-edition/html/ConstantPool.doc.html#72007'>VMSpec 5.3</a>)
- The classloader which defines the class (whose reference is being resolved) is the one
- used to initiate loading of the class referred to.
- </p>
- <p>
- <em>
- In practial development terms: This is very important to bear in mind when trying to solve classloader issues.
- A classic misunderstanding is this: suppose class A defined by classloader C has a symbolic reference to
- class B and further that when C initiates loading of B, this is delegated to classloader D which defines B.
- Class B can now only resolve symbols that can be loaded by D, rather than all those which can be loaded by C.
- This is a classic recipe for classloader problems.
- </em>
- </p>
- </subsection>
-
- <subsection name='Bibliography'>
- <ul>
- <li>
- <a href='http://java.sun.com/docs/books/vmspec/'>VMSpec</a> <em>The Java Virtual Machine Specification, Second Edition</em>
- </li>
- <li>
- <a href='http://java.sun.com/docs/books/jls/'>LangSpec</a> <em>The Java Language Specification Second Edition</em>
- </li>
- </ul>
- </subsection>
- </section>
- <section name='A Short Guide To Hierarchical Class Loading'>
- <subsection name='Delegating Class Loaders'>
- <p>
- When asked to load a class, a class loader may either define the class itself or delegate.
- The base <code>ClassLoader</code> class insists that every implementation has a parent class loader.
- This delegation model therefore naturally forms a tree structure rooted in the bootstrap classloader.
- </p>
- <p>
- Containers (i.e. applications such as servlet engines or application servers
- that manage and provide support services for a number of "contained" applications
- that run inside of them) often use complex trees to allow isolation of different applications
- running within the container. This is particularly true of J2EE containers.
- </p>
- </subsection>
-
- <subsection name='Parent-First And Child-First Class Loaders'>
- <p>
- When a classloader is asked to load a class, a question presents itself: should it immediately
- delegate the loading to it's parent (and thus only define those classes not defined by it's parent)
- or should it try to define it first itself (and only delegate to it's parent those classes it does
- not itself define). Classloaders which universally adopt the first approach are termed parent-first
- and the second child-first.
- </p>
- <p>
- <strong>Note:</strong> the term child-first (though commonly used) is misleading.
- A better term (and one which may be encountered on the mailing list) is parent-last.
- This more accurately describes the actual process of classloader performed
- by such a classloader.
- </p>
- <p>
- Parent-first loading has been the standard mechanism in the JDK
- class loader, at least since Java 1.2 introduced hierarchical classloaders.
- The primary reason for this is safety -- parent-first
- makes it impossible for malicious code to trick the JVM into
- replacing a core class (say, <code>java.security.SecurityManager</code>) with a
- class of the same name loaded from a child classloader.
- </p>
- <p>
- Child-first classloading has the advantage of helping to improve isolation
- between containers and the applications inside them. If an application
- uses a library jar that is also used by the container, but the version of
- the jar used by the two is different, child-first classloading allows the
- contained application to load its version of the jar without affecting the
+<?xml version="1.0"?>
+
+<!--
+
+ Copyright 2001-2004 The Apache Software Foundation.
+
+ Licensed under the Apache License, Version 2.0 (the "License");
+ you may not use this file except in compliance with the License.
+ You may obtain a copy of the License at
+
+ http://www.apache.org/licenses/LICENSE-2.0
+
+ Unless required by applicable law or agreed to in writing, software
+ distributed under the License is distributed on an "AS IS" BASIS,
+ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ See the License for the specific language governing permissions and
+ limitations under the License.
+
+-->
+
+<document>
+
+ <properties>
+ <title>JCL Technology Guide</title>
+ <author email="commons-dev@jakarta.apache.org">Commons Documentation Team</author>
+ </properties>
+
+ <body>
+ <section name='Overview'>
+ <subsection name='Contents'>
+ <ul>
+ <li>
+ Overview
+ <ul>
+ <li>
+ Contents
+ </li>
+ <li>
+ <a href='#Introduction'>Introduction</a>
+ </li>
+ </ul>
+ </li>
+ <li>
+ <a href='#A Short Introduction to Class Loading and Class Loaders'>
+ A Short Introduction to Class Loading and Class Loaders
+ </a>
+ <ul>
+ <li>
+ <a href='#Preamble'>
+ Preamble
+ </a>
+ </li>
+ <li>
+ <a href='#Resolution Of Symbolic References'>
+ Resolution Of Symbolic References
+ </a>
+ </li>
+ <li>
+ <a href='#Loading'>
+ Loading
+ </a>
+ </li>
+ <li>
+ <a href='#Linking'>
+ Linking
+ </a>
+ </li>
+ <li>
+ <a href='#Loading Classes'>
+ Loading Classes
+ </a>
+ </li>
+ <li>
+ <a href='#Bootstrap Classloader'>
+ Bootstrap Classloader
+ </a>
+ </li>
+ <li>
+ <a href='#Runtime Package'>
+ Runtime Package
+ </a>
+ </li>
+ <li>
+ <a href='#Loader Used To Resolve A Symbolic Reference'>
+ Loader Used To Resolve A Symbolic Reference
+ </a>
+ </li>
+ <li>
+ <a href='#Bibliography'>
+ Bibliography
+ </a>
+ </li>
+ </ul>
+ </li>
+ <li>
+ <a href='#A Short Guide To Hierarchical Class Loading'>
+ A Short Guide To Hierarchical Class Loading
+ </a>
+ <ul>
+ <li>
+ <a href='#Delegating Class Loaders'>
+ Delegating Class Loaders
+ </a>
+ </li>
+ <li>
+ <a href='#Parent-First And Child-First Class Loaders'>
+ Parent-First And Child-First Class Loaders
+ </a>
+ </li>
+ <li>
+ <a href='#Class ClassLoader'>
+ Class ClassLoader
+ </a>
+ </li>
+ <li>
+ <a href='#Context ClassLoader'>
+ Context ClassLoader
+ </a>
+ </li>
+ <li>
+ <a href='#The Context Classloader in Container Applications'>
+ The Context Classloader in Container Applications
+ </a>
+ </li>
+ <li>
+ <a href='#Issues with Context ClassLoaders'>
+ Issues with Context ClassLoaders
+ </a>
+ </li>
+ <li>
+ <a href='#Reflection And The Context ClassLoader'>
+ Reflection And The Context ClassLoader
+ </a>
+ </li>
+ <li>
+ <a href='#More Information'>
+ More Information
+ </a>
+ </li>
+ </ul>
+ </li>
+ <li>
+ <a href='#A Short Theory Guide To JCL'>
+ A Short Theory Guide To JCL
+ </a>
+ <ul>
+ <li>
+ <a href='#Isolation And The Context Class Loader'>
+ Isolation And The Context Class Loader
+ </a>
+ </li>
+ <li>
+ <a href='#Log And LogFactory'>
+ Log And LogFactory
+ </a>
+ </li>
+ <li>
+ <a href='#Log Implementations'>
+ Log Implementations
+ </a>
+ </li>
+ <li>
+ <a href='#Using Reflection To Load Log Implementations'>
+ Using Reflection To Load Log Implementations
+ </a>
+ </li>
+ </ul>
+ </li>
+ </ul>
+ </subsection>
+ <subsection name='Introduction'>
+ <p>
+ This guide is aimed at describing the technologies that JCL developers and expert users
+ (and users who need to become experts)
+ should be familiar with. The aim is to give an understanding whilst being precise but brief.
+ Details which are not relevent for JCL have been suppressed.
+ References have been included.
+ </p>
+ <p>
+ These topics are a little difficult and it's easy for even experience developers to make
+ mistakea. We need you to help us get it right! Please submit corrections, comments, additional references
+ and requests for clarification
+ by either:
+ </p>
+ <ul>
+ <li>
+ posting to the <a href='http://jakarta.apache.org/site/mail.html'>jakarta commons-dev mailing list</a> or
+ </li>
+ <li>
+ creating an issue in <a href='http://issues.apache.org/bugzilla/'>Bugzilla</a>.
+ </li>
+ </ul>
+ <p>
+ TIA
+ </p>
+ </subsection>
+
+ </section>
+ <section name='A Short Introduction to Class Loading and Class Loaders'>
+ <subsection name='Preamble'>
+ <p>
+ This is intended to present a guide to the process by which Java bytecode uses bytecode in other classes
+ from the perspective of the language and virtual machine specifications. The focus will be on deciding
+ which bytecode will be used (rather than the mechanics of the usage). It focusses on facts and terminology.
+ </p>
+ <p>
+ The process is recursive: it is therefore difficult to pick a starting point.
+ Sun's documentation starts from the persective of the startup of a new application.
+ This guide starts from the perspective of an executing application.
+ </p>
+ <p>
+ During this discussion, please assume that each time that <em>class</em> is mentioned,
+ the comments applied equally well to interfaces.
+ </p>
+ <p>
+ This document is targeted at Java 1.2 and above.
+ </p>
+ </subsection>
+ <subsection name='Resolution Of Symbolic References'>
+ <p>
+ (<a href='http://java.sun.com/docs/books/jls/second_edition/html/execution.doc.html#44524'>LangSpec 12.3.3</a>)
+ The bytecode representation of a class contains symbolic names for other classes referenced.
+ </p>
+ <p>
+ <em>
+ In practical development terms: If a class is imported (either explicitly in the list of imports at the top of
+ the source file or implicitly through a fully qualified name in the source code) it is referenced symbolically.
+ </em>
+ </p>
+ <p>
+ (<a href='http://java.sun.com/docs/books/vmspec/2nd-edition/html/ConstantPool.doc.html#73492'>VMSpec 5.4.3</a>)
+ Resolution of a symbolic reference occurs dymanically at runtime and is carried out by
+ the Java Vitual Machine. Resolution of a symbolic reference requires loading and linking of the new class.
+ </p>
+ <p>
+ <em>
+ Note: references are not statically resolved at compile time.
+ </em>
+ </p>
+ </subsection>
+ <subsection name='Loading'>
+ <p>
+ (<a href='http://java.sun.com/docs/books/vmspec/2nd-edition/html/Concepts.doc.html#19175'>VMSpec 2.17.2</a>)
+ Loading is the name given process by which a binary form of a class is obtained
+ by the Java Virutal Machine.
+ Java classes are always loaded and linked dynamically by the Java Virtual Machine
+ (rather than statically by the compiler).
+ </p>
+ <p>
+ <em>
+ In practical development terms:
+ This means that the developer has no certain knowledge about the actual
+ bytecode that will be used to execute any external call (one made outside the class). This is determined only
+ at execution time and is effected by the way that the code is deployed.
+ </em>
+ </p>
+ </subsection>
+ <subsection name='Linking'>
+ <p>
+ (<a href='http://java.sun.com/docs/books/vmspec/2nd-edition/html/Concepts.doc.html#22574'>VMSpec 2.17.3</a>)
+ Linking is the name used for combining the
+ binary form of a class into the Java Virtual Machine. This must happen before the class can be used.
+ </p>
+ <p>
+ (<a href='http://java.sun.com/docs/books/vmspec/2nd-edition/html/Concepts.doc.html#22574'>VMSpec 2.17.3</a>)
+ Linking is composed of verification, preparation and resolution (of symbolic references).
+ Flexibility is allowed over the timing of resolution. (Within limit) this may happen at any time after
+ preparation and before that reference is used.
+ </p>
+ <p>
+ <em>
+ In practical development terms: This means that different JVMs may realize that a reference cannot be
+ resolved at different times during execution. Consequently, the actual behaviour cannot be precisely predicted
+ without intimate knowledge of the JVM (on which the bytecode will be executed).
+ This makes it hard to give universal guildance to users.
+ </em>
+ </p>
+ </subsection>
+
+ <subsection name='Loading Classes'>
+ <p>
+ (<a href='http://java.sun.com/docs/books/vmspec/2nd-edition/html/Concepts.doc.html#19175'>VMSpec 2.17.2</a>)
+ The loading process is performed by a <code>ClassLoader</code>.
+ </p>
+ <p>
+ (<a href='http://java.sun.com/docs/books/vmspec/2nd-edition/html/ConstantPool.doc.html#72007'>VMSpec 5.3</a>)
+ A classloader may create a class either by delegation or by defining it directly.
+ The classloader that initiates loading of a class is known as the initiating loader.
+ The classloader that defines the class is known as the defining loader.
+ </p>
+ <p>
+ <em>
+ In practical terms: understanding and appreciating this distinction is crucial when debugging issues
+ concerning classloaders.
+ </em>
+ </p>
+ </subsection>
+
+ <subsection name='Bootstrap Classloader'>
+ <p>
+ (<a href='http://java.sun.com/docs/books/vmspec/2nd-edition/html/ConstantPool.doc.html#72007'>VMSPEC 5.3</a>)
+ The bootstrap is the base <code>ClassLoader</code> supplied by the Java Virtual Machine.
+ All others are user (also known as application) <code>ClassLoader</code>'s.
+ </p>
+ <p>
+ <em>
+ In practical development terms: The System classloader returned by <code>Classloader.getSystemClassLoader()</code>
+ will be either the bootstrap classloader or a direct descendent of the bootstrap classloader.
+ Only when debugging issues concerning the system classloader should there be any need to consider the detailed
+ differences between the bootstrap classloader and the system classloader.
+ </em>
+ </p>
+ </subsection>
+ <subsection name='Runtime Package'>
+ <p>
+ (<a href='http://java.sun.com/docs/books/vmspec/2nd-edition/html/ConstantPool.doc.html#72007'>VMSpec 5.3</a>)
+ At runtime, a class (or interface) is determined by it's fully qualified name
+ and by the classloader that defines it. This is known as the class's runtime package.
+ </p>
+ <p>
+ (<a href='http://java.sun.com/docs/books/vmspec/2nd-edition/html/ConstantPool.doc.html#75929'>VMSpec 5.4.4</a>)
+ Only classes in the same runtime package are mutually accessible.
+ </p>
+ <p>
+ <em>
+ In practical development terms: two classes with the same symbolic name can only be used interchangably
+ if they are defined by the same classloader. A classic symptom indicative of a classloader issue is that
+ two classes with the same fully qualified name are found to be incompatible during a method call.
+ This may happen when a member is expecting an interface which is (seemingly) implemented by a class
+ but the class is in a different runtime package after being defined by a different classloader. This is a
+ fundemental java language security feature.
+ </em>
+ </p>
+ </subsection>
+
+ <subsection name='Loader Used To Resolve A Symbolic Reference'>
+ <p>
+ (<a href='http://java.sun.com/docs/books/vmspec/2nd-edition/html/ConstantPool.doc.html#72007'>VMSpec 5.3</a>)
+ The classloader which defines the class (whose reference is being resolved) is the one
+ used to initiate loading of the class referred to.
+ </p>
+ <p>
+ <em>
+ In practial development terms: This is very important to bear in mind when trying to solve classloader issues.
+ A classic misunderstanding is this: suppose class A defined by classloader C has a symbolic reference to
+ class B and further that when C initiates loading of B, this is delegated to classloader D which defines B.
+ Class B can now only resolve symbols that can be loaded by D, rather than all those which can be loaded by C.
+ This is a classic recipe for classloader problems.
+ </em>
+ </p>
+ </subsection>
+
+ <subsection name='Bibliography'>
+ <ul>
+ <li>
+ <a href='http://java.sun.com/docs/books/vmspec/'>VMSpec</a> <em>The Java Virtual Machine Specification, Second Edition</em>
+ </li>
+ <li>
+ <a href='http://java.sun.com/docs/books/jls/'>LangSpec</a> <em>The Java Language Specification Second Edition</em>
+ </li>
+ </ul>
+ </subsection>
+ </section>
+ <section name='A Short Guide To Hierarchical Class Loading'>
+ <subsection name='Delegating Class Loaders'>
+ <p>
+ When asked to load a class, a class loader may either define the class itself or delegate.
+ The base <code>ClassLoader</code> class insists that every implementation has a parent class loader.
+ This delegation model therefore naturally forms a tree structure rooted in the bootstrap classloader.
+ </p>
+ <p>
+ Containers (i.e. applications such as servlet engines or application servers
+ that manage and provide support services for a number of "contained" applications
+ that run inside of them) often use complex trees to allow isolation of different applications
+ running within the container. This is particularly true of J2EE containers.
+ </p>
+ </subsection>
+
+ <subsection name='Parent-First And Child-First Class Loaders'>
+ <p>
+ When a classloader is asked to load a class, a question presents itself: should it immediately
+ delegate the loading to it's parent (and thus only define those classes not defined by it's parent)
+ or should it try to define it first itself (and only delegate to it's parent those classes it does
+ not itself define). Classloaders which universally adopt the first approach are termed parent-first
+ and the second child-first.
+ </p>
+ <p>
+ <strong>Note:</strong> the term child-first (though commonly used) is misleading.
+ A better term (and one which may be encountered on the mailing list) is parent-last.
+ This more accurately describes the actual process of classloader performed
+ by such a classloader.
+ </p>
+ <p>
+ Parent-first loading has been the standard mechanism in the JDK
+ class loader, at least since Java 1.2 introduced hierarchical classloaders.
+ The primary reason for this is safety -- parent-first
+ makes it impossible for malicious code to trick the JVM into
+ replacing a core class (say, <code>java.security.SecurityManager</code>) with a
+ class of the same name loaded from a child classloader.
+ </p>
+ <p>
+ Child-first classloading has the advantage of helping to improve isolation
+ between containers and the applications inside them. If an application
+ uses a library jar that is also used by the container, but the version of
+ the jar used by the two is different, child-first classloading allows the
+ contained application to load its version of the jar without affecting the
container.
- </p>
- <p>
- The ability for a servlet container to offer child-first classloading
- is made available, as an option, by language in the servlet spec (Section
- 9.7.2) that allows a container to offer child-first loading with
- certain restrictions, such as not allowing replacement of java.* or
- javax.* classes, or the container's implementation classes.
- </p>
- <p>
- Though child-first and parent-first are not the only strategies possible,
- they are by far the most common.
- All other strategies are rare.
- However, it is not uncommon to be faced with a mixture of parent-first and child-first
- classloaders within the same hierarchy.
- </p>
- </subsection>
-
- <subsection name='Class ClassLoader'>
- <p>
- The class loader used to define a class is available programmatically by calling
- the <code>getClassLoader</code> method
- on the class in question. This is often known as the class classloader.
- </p>
- </subsection>
-
- <subsection name='Context ClassLoader'>
- <p>
- Java 1.2 introduces a mechanism which allows code to access classloaders
- which are not the class classloader or one of its parents.
- A thread may have a class loader associated with it by it's creator for use
- by code running in the thread when loading resources and classes.
- This classloader is accessed by the <code>getContextClassLoader</code>
- method on <code>Thread</code>. It is therefore often known as the context classloader.
- </p>
- <p>
- Note that the quality and appropriateness of the context classloader depends on the
- care with which the thread's owner manages it.
- </p>
- </subsection>
-
- <subsection name='The Context Classloader in Container Applications'>
- <p>
- The Javadoc for
- <a href="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Thread.html#setContextClassLoader(java.lang.ClassLoader)">
- <code>Thread.setContextClassLoader</code></a> emphasizes the setting of the
- context classloader as an aspect of thread creation. However, in many
- applications the context classloader is not fixed at thread creation but
- rather is changed throughout the life of thread as thread execution moves
- from one context to another. This usage of the context classloader is
- particularly important in container applications.
- </p>
- <p>
- For example, in a hypothetical servlet container, a pool of threads
- is created to handle HTTP requests. When created these threads have their
- context classloader set to a classloader that loads container classes.
- After the thread is assigned to handle a request, container code parses
- the request and then determines which of the deployed web applications
- should handle it. Only when the container is about to call code associated
- with a particular web application (i.e. is about to cross an "application
- boundary") is the context classloader set to the classloader used to load
- the web app's classes. When the web application finishes handling the
- request and the call returns, the context classloader is set back to the
- container classloader.
- </p>
- <p>
- In a properly managed container, changes in the context classloader are
- made when code execution crosses an application boundary. When contained
- application <code>A</code> is handling a request, the context classloader
- should be the one used to load <code>A</code>'s resources. When application
- <code>B</code> is handling a request, the context classloader should be
- <code>B</code>'s.
- </p>
- <p>
- While a contained application is handling a request, it is not
- unusual for it to call system or library code loaded by the container.
- For example, a contained application may wish to call a utility function
- provided by a shared library. This kind of call is considered to be
- within the "application boundary", so the context classloader remains
- the contained application's classloader. If the system or library code
- needs to load classes or other resources only visible to the contained
- application's classloader, it can use the context classloader to access
- these resources.
- </p>
- <p>
- If the context classloader is properly managed, system and library code
- that can be accessed by multiple applications can not only use it to load
- application-specific resources, but also can use it to detect which
- application is making a call and thereby provided services tailored to the
- caller.
- </p>
- </subsection>
-
- <subsection name='Issues with Context ClassLoaders'>
- <p>
- In practice, context classloaders vary in quality and issues sometimes arise
- when using them.
- The owner of the thread is responsible for setting the classloader.
- If the context classloader is not set then it will default to the system
- classloader.
- Any container doing so will cause difficulties for any code using the context classloader.
- </p>
- <p>
- The owner is also at liberty to set the classloader as they wish.
- Containers may set the context classloader so that it is neither a child nor a parent
- of the classloader that defines the class using that loader.
- Again, this will cause difficulties.
- </p>
- <p>
- Introduced in <a href='http://java.sun.com/j2ee/j2ee-1_3-fr-spec.pdf'>Java J2EE 1.3</a>
- is a requirement for vendors to appropriately set the context classloader.
- Section 6.2.4.8 (1.4 text):
- </p>
-<source>
-This specification requires that J2EE containers provide a per thread
-context class loader for the use of system or library classes in
-dynamicly loading classes provided by the application. The EJB
-specification requires that all EJB client containers provide a per
-thread context class loader for dynamicly loading system value classes.
-The per thread context class loader is accessed using the Thread method
-getContextClassLoader.
-
-The classes used by an application will typically be loaded by a
-hierarchy of class loaders. There may be a top level application class
-loader, an extension class loader, and so on, down to a system class
-loader. The top level application class loader delegates to the lower
-class loaders as needed. Classes loaded by lower class loaders, such as
-portable EJB system value classes, need to be able to discover the top
-level application class loader used to dynamicly load application
-classes.
-
-We require that containers provide a per thread context class loader
-that can be used to load top level application classes as described
-above.
-</source>
- <p>
- This specification leaves quite a lot of freedom for vendors.
- (As well as using unconventional terminology and containing the odd typo.)
- It is a difficult passage (to say the least).
- </p>
- </subsection>
-
- <subsection name='Reflection And The Context ClassLoader'>
- <p>
- Reflection cannot bypass restrictions imposed by the java language security model, but, by avoiding symbolic
- references, reflection can be used to load classes which could not otherwise be loaded. Another <code>ClassLoader</code>
- can be used to load a class and then reflection used to create an instance.
- </p>
- <p>
- Recall that the runtime packaging is used to determine accessibility.
- Reflection cannot be used to avoid basic java security.
- Therefore, the runtime packaging becomes an issue when attempting to cast classes
- created by reflection using other class loaders.
- When using this strategy, various modes of failure are possible
- when common class references are defined by the different class loaders.
- </p>
- <p>
- Reflection is often used with the context classloader. In theory, this allows a class defined in
- a parent classloader to load any class that is loadable by the application.
- In practice, this only works well when the context classloader is set carefully.
- </p>
- </subsection>
-
- <subsection name='More Information'>
- <ul>
- <li>
- Articles On Class Loaders And Class Loading
- <ul>
- <li>
- <a
- href='http://www.onjava.com/pub/a/onjava/2001/07/25/ejb.html'>
- Article on J2EE class loading
- </a>
- </li>
- <li>
- <a
- href='http://www.onjava.com/pub/a/onjava/2003/11/12/classloader.html'>
- Article on class loading
- </a>
- </li>
- <li>
- <a
- href='http://www.javaworld.com/javaworld/javaqa/2003-06/01-qa-0606-load.html'>
- Article on context class loaders
- </a>
- </li>
- </ul>
- </li>
- <li>Specific Containers
- <ul>
- <li>
- <a
- href='http://jakarta.apache.org/tomcat/tomcat-4.1-doc/class-loader-howto.html'>
- Tomcat 4.1 ClassLoader Guide
- </a>
- </li>
- <li>
- <a
- href='http://jakarta.apache.org/tomcat/tomcat-5.0-doc/class-loader-howto.html'>
- Tomcat 5.0 ClassLoader Guide
- </a>
- </li>
- <li>
- <a
-href='http://publib.boulder.ibm.com/infocenter/ws60help/index.jsp?topic=/com.ibm.websphere.express.doc/info/exp/ae/trun_classload_web.html'>
- Classloading In WebSphere
- </a>
- </li>
- </ul>
- </li>
- </ul>
- </subsection>
- </section>
-
- <section name='A Short Theory Guide To JCL'>
- <subsection name='Isolation And The Context Class Loader'>
- <p>
- JCL takes the view that different context class loader indicate boundaries between applications
- running in a container environment. Isolation requires that JCL honours these boundaries
- and therefore allows different isolated applications to configure their logging systems
- independently.
- </p>
- </subsection>
- <subsection name='Log And LogFactory'>
- <p>
- Performance dictates that symbolic references to these classes are present in the calling application code
- (reflection would simply be too slow). Therefore, these classes must be loadable by the classloader
- that loads the application code.
- </p>
- </subsection>
- <subsection name='Log Implementations'>
- <p>
- Performance dictates that symbolic references to the logging systems are present in the implementation
- classes (again, reflection would simply be too slow). So, for an implementation to be able to function,
- it is neccessary for the logging system to be loadable by the classloader that defines the implementing class.
- </p>
- </subsection>
-
- <subsection name='Using Reflection To Load Log Implementations'>
- <p>
- However, there is actually no reason why <code>LogFactory</code> requires symbolic references to particular <code>Log</code>
- implementations. Reflection can be used to load these from an appropriate classloader
- without unacceptable performance degradation.
- This is the strategy adopted by JCL.
- </p>
- <p>
- JCL uses the context classloader to use the <code>Log</code> implementation.
- </p>
- </subsection>
- </section>
-</body>
+ </p>
+ <p>
+ The ability for a servlet container to offer child-first classloading
+ is made available, as an option, by language in the servlet spec (Section
+ 9.7.2) that allows a container to offer child-first loading with
+ certain restrictions, such as not allowing replacement of java.* or
+ javax.* classes, or the container's implementation classes.
+ </p>
+ <p>
+ Though child-first and parent-first are not the only strategies possible,
+ they are by far the most common.
+ All other strategies are rare.
+ However, it is not uncommon to be faced with a mixture of parent-first and child-first
+ classloaders within the same hierarchy.
+ </p>
+ </subsection>
+
+ <subsection name='Class ClassLoader'>
+ <p>
+ The class loader used to define a class is available programmatically by calling
+ the <code>getClassLoader</code> method
+ on the class in question. This is often known as the class classloader.
+ </p>
+ </subsection>
+
+ <subsection name='Context ClassLoader'>
+ <p>
+ Java 1.2 introduces a mechanism which allows code to access classloaders
+ which are not the class classloader or one of its parents.
+ A thread may have a class loader associated with it by it's creator for use
+ by code running in the thread when loading resources and classes.
+ This classloader is accessed by the <code>getContextClassLoader</code>
+ method on <code>Thread</code>. It is therefore often known as the context classloader.
+ </p>
+ <p>
+ Note that the quality and appropriateness of the context classloader depends on the
+ care with which the thread's owner manages it.
+ </p>
+ </subsection>
+
+ <subsection name='The Context Classloader in Container Applications'>
+ <p>
+ The Javadoc for
+ <a href="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Thread.html#setContextClassLoader(java.lang.ClassLoader)">
+ <code>Thread.setContextClassLoader</code></a> emphasizes the setting of the
+ context classloader as an aspect of thread creation. However, in many
+ applications the context classloader is not fixed at thread creation but
+ rather is changed throughout the life of thread as thread execution moves
+ from one context to another. This usage of the context classloader is
+ particularly important in container applications.
+ </p>
+ <p>
+ For example, in a hypothetical servlet container, a pool of threads
+ is created to handle HTTP requests. When created these threads have their
+ context classloader set to a classloader that loads container classes.
+ After the thread is assigned to handle a request, container code parses
+ the request and then determines which of the deployed web applications
+ should handle it. Only when the container is about to call code associated
+ with a particular web application (i.e. is about to cross an "application
+ boundary") is the context classloader set to the classloader used to load
+ the web app's classes. When the web application finishes handling the
+ request and the call returns, the context classloader is set back to the
+ container classloader.
+ </p>
+ <p>
+ In a properly managed container, changes in the context classloader are
+ made when code execution crosses an application boundary. When contained
+ application <code>A</code> is handling a request, the context classloader
+ should be the one used to load <code>A</code>'s resources. When application
+ <code>B</code> is handling a request, the context classloader should be
+ <code>B</code>'s.
+ </p>
+ <p>
+ While a contained application is handling a request, it is not
+ unusual for it to call system or library code loaded by the container.
+ For example, a contained application may wish to call a utility function
+ provided by a shared library. This kind of call is considered to be
+ within the "application boundary", so the context classloader remains
+ the contained application's classloader. If the system or library code
+ needs to load classes or other resources only visible to the contained
+ application's classloader, it can use the context classloader to access
+ these resources.
+ </p>
+ <p>
+ If the context classloader is properly managed, system and library code
+ that can be accessed by multiple applications can not only use it to load
+ application-specific resources, but also can use it to detect which
+ application is making a call and thereby provided services tailored to the
+ caller.
+ </p>
+ </subsection>
+
+ <subsection name='Issues with Context ClassLoaders'>
+ <p>
+ In practice, context classloaders vary in quality and issues sometimes arise
+ when using them.
+ The owner of the thread is responsible for setting the classloader.
+ If the context classloader is not set then it will default to the system
+ classloader.
+ Any container doing so will cause difficulties for any code using the context classloader.
+ </p>
+ <p>
+ The owner is also at liberty to set the classloader as they wish.
+ Containers may set the context classloader so that it is neither a child nor a parent
+ of the classloader that defines the class using that loader.
+ Again, this will cause difficulties.
+ </p>
+ <p>
+ Introduced in <a href='http://java.sun.com/j2ee/j2ee-1_3-fr-spec.pdf'>Java J2EE 1.3</a>
+ is a requirement for vendors to appropriately set the context classloader.
+ Section 6.2.4.8 (1.4 text):
+ </p>
+<source>
+This specification requires that J2EE containers provide a per thread
+context class loader for the use of system or library classes in
+dynamicly loading classes provided by the application. The EJB
+specification requires that all EJB client containers provide a per
+thread context class loader for dynamicly loading system value classes.
+The per thread context class loader is accessed using the Thread method
+getContextClassLoader.
+
+The classes used by an application will typically be loaded by a
+hierarchy of class loaders. There may be a top level application class
+loader, an extension class loader, and so on, down to a system class
+loader. The top level application class loader delegates to the lower
+class loaders as needed. Classes loaded by lower class loaders, such as
+portable EJB system value classes, need to be able to discover the top
+level application class loader used to dynamicly load application
+classes.
+
+We require that containers provide a per thread context class loader
+that can be used to load top level application classes as described
+above.
+</source>
+ <p>
+ This specification leaves quite a lot of freedom for vendors.
+ (As well as using unconventional terminology and containing the odd typo.)
+ It is a difficult passage (to say the least).
+ </p>
+ </subsection>
+
+ <subsection name='Reflection And The Context ClassLoader'>
+ <p>
+ Reflection cannot bypass restrictions imposed by the java language security model, but, by avoiding symbolic
+ references, reflection can be used to load classes which could not otherwise be loaded. Another <code>ClassLoader</code>
+ can be used to load a class and then reflection used to create an instance.
+ </p>
+ <p>
+ Recall that the runtime packaging is used to determine accessibility.
+ Reflection cannot be used to avoid basic java security.
+ Therefore, the runtime packaging becomes an issue when attempting to cast classes
+ created by reflection using other class loaders.
+ When using this strategy, various modes of failure are possible
+ when common class references are defined by the different class loaders.
+ </p>
+ <p>
+ Reflection is often used with the context classloader. In theory, this allows a class defined in
+ a parent classloader to load any class that is loadable by the application.
+ In practice, this only works well when the context classloader is set carefully.
+ </p>
+ </subsection>
+
+ <subsection name='More Information'>
+ <ul>
+ <li>
+ Articles On Class Loaders And Class Loading
+ <ul>
+ <li>
+ <a
+ href='http://www.onjava.com/pub/a/onjava/2001/07/25/ejb.html'>
+ Article on J2EE class loading
+ </a>
+ </li>
+ <li>
+ <a
+ href='http://www.onjava.com/pub/a/onjava/2003/11/12/classloader.html'>
+ Article on class loading
+ </a>
+ </li>
+ <li>
+ <a
+ href='http://www.javaworld.com/javaworld/javaqa/2003-06/01-qa-0606-load.html'>
+ Article on context class loaders
+ </a>
+ </li>
+ </ul>
+ </li>
+ <li>Specific Containers
+ <ul>
+ <li>
+ <a
+ href='http://tomcat.apache.org/tomcat-4.1-doc/class-loader-howto.html'>
+ Tomcat 4.1 ClassLoader Guide
+ </a>
+ </li>
+ <li>
+ <a
+ href='http://tomcat.apache.org/tomcat-5.0-doc/class-loader-howto.html'>
+ Tomcat 5.0 ClassLoader Guide
+ </a>
+ </li>
+ <li>
+ <a
+href='http://publib.boulder.ibm.com/infocenter/wasinfo/v6r0/index.jsp?topic=/com.ibm.websphere.express.doc/info/exp/ae/trun_classload_web.html'>
+ Classloading In WebSphere
+ </a>
+ </li>
+ </ul>
+ </li>
+ </ul>
+ </subsection>
+ </section>
+
+ <section name='A Short Theory Guide To JCL'>
+ <subsection name='Isolation And The Context Class Loader'>
+ <p>
+ JCL takes the view that different context class loader indicate boundaries between applications
+ running in a container environment. Isolation requires that JCL honours these boundaries
+ and therefore allows different isolated applications to configure their logging systems
+ independently.
+ </p>
+ </subsection>
+ <subsection name='Log And LogFactory'>
+ <p>
+ Performance dictates that symbolic references to these classes are present in the calling application code
+ (reflection would simply be too slow). Therefore, these classes must be loadable by the classloader
+ that loads the application code.
+ </p>
+ </subsection>
+ <subsection name='Log Implementations'>
+ <p>
+ Performance dictates that symbolic references to the logging systems are present in the implementation
+ classes (again, reflection would simply be too slow). So, for an implementation to be able to function,
+ it is neccessary for the logging system to be loadable by the classloader that defines the implementing class.
+ </p>
+ </subsection>
+
+ <subsection name='Using Reflection To Load Log Implementations'>
+ <p>
+ However, there is actually no reason why <code>LogFactory</code> requires symbolic references to particular <code>Log</code>
+ implementations. Reflection can be used to load these from an appropriate classloader
+ without unacceptable performance degradation.
+ This is the strategy adopted by JCL.
+ </p>
+ <p>
+ JCL uses the context classloader to use the <code>Log</code> implementation.
+ </p>
+ </subsection>
+ </section>
+</body>
</document>
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