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Posted to users@tomcat.apache.org by sr...@gmail.com on 2015/02/18 15:25:46 UTC
Why is Class.forName required to load drivers in Tomcat 8 ?
I am writing a simple Servlet that uses JDBC to talk to MySQL and deploying it in Tomcat. Why do I need to add the line -:
Class.forName("com.mysql.jdbc.Driver");
to load the JDBC driver ? If I don’t do this I get an exception of -:
java.sql.SQLException: No suitable driver was found.
Why is it when I use a JDBC connection in Servlets and deploy it in Tomcat must the class be loaded dynamically ? Why can’t Tomcat do that for me when I have already added it to the build path ?
From Java 6 onwards there is no need for the JDBC Driver to be loaded dynamically. So why when using JRE 8 and Tomcat 8 must this be done ?
Cant it be loaded automatically like for normal apps ? It sure would be a lot more cleaner code.
Regards
Sreyan Chakravarty
Re: Why is Class.forName required to load drivers in Tomcat 8 ?
Posted by sr...@gmail.com.
What is the SYSTEM PATH that you are referring to ? Is it the JAVA_HOME or the PATH system variable ?
What is the CLASSPATH just before Tomcat properly starts up ? What is it after it starts up ? Is the same jar files below -:
bin/bootstrap.jar
bin/tomcat-juli.jar
Correct me if I am wrong that means that whatever dependencies I have in my project is not added to the SYSTEM CLASSPATH but is instead loaded by TOMCAT. So if a module searches only the SYSTEM CLASSPATH it won’t be able to find the dependencies. Am I right ?
Regards
Sreyan Chakravarty
From: Christopher Schultz
Sent: Wednesday, February 18, 2015 8:30 PM
To: Tomcat Users List
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA256
Sreyan,
On 2/18/15 9:25 AM, sreyan32@gmail.com wrote:
> I am writing a simple Servlet that uses JDBC to talk to MySQL and
> deploying it in Tomcat. Why do I need to add the line -:
>
> Class.forName("com.mysql.jdbc.Driver");
>
> to load the JDBC driver ? If I don’t do this I get an exception of
> -:
>
> java.sql.SQLException: No suitable driver was found.
>
> Why is it when I use a JDBC connection in Servlets and deploy it
> in Tomcat must the class be loaded dynamically ? Why can’t Tomcat
> do that for me when I have already added it to the build path ?
>
> From Java 6 onwards there is no need for the JDBC Driver to be
> loaded dynamically. So why when using JRE 8 and Tomcat 8 must this
> be done ?
>
> Cant it be loaded automatically like for normal apps ? It sure
> would be a lot more cleaner code.
My guess is that DriverManager only searches the system CLASSPATH
which only contains the following JAR files when launching Tomcat
using the standard scripts:
bin/bootstrap.jar
bin/tomcat-juli.jar
The bootstrap class then configures a new ClassLoader (including
lib/*.jar) for the Tomcat server and Tomcat properly launches using
/that/.
I think your JDBC driver simply isn't visible to DriverManager's
auto-loading. But when you call Class.forName, the driver registers
itself with DriverManager, which is okay to do if DriverManager didn't
know about your driver.
- -chris
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Re: Why is Class.forName required to load drivers in Tomcat 8 ?
Posted by Christopher Schultz <ch...@christopherschultz.net>.
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA256
Sreyan,
On 2/18/15 10:15 AM, sreyan32@gmail.com wrote:
> What is the SYSTEM PATH that you are referring to ? Is it the
> JAVA_HOME or the PATH system variable ?
Neither of those. It's commonly referred to as the CLASSPATH
environment variable, which should pretty much never be set because
it's horribly confusing.
Instead, Tomcat uses "java -classpath [the classpath]" to launch the
JVM. The classpath used there includes two JAR files:
>
>
> What is the CLASSPATH just before Tomcat properly starts up ? What
> is it after it starts up ? Is the same jar files below -:
>
>
>
>
> Correct me if I am wrong that means that whatever dependencies I
> have in my project is not added to the SYSTEM CLASSPATH but is
> instead loaded by TOMCAT. So if a module searches only the SYSTEM
> CLASSPATH it won’t be able to find the dependencies. Am I right ?
>
>
> Regards Sreyan Chakravarty
>
>
>
>
>
> From: Christopher Schultz Sent: Wednesday, February 18,
> 2015 8:30 PM To: Tomcat Users List
>
>
>
>
>
> Sreyan,
>
> On 2/18/15 9:25 AM, sreyan32@gmail.com wrote:
>> I am writing a simple Servlet that uses JDBC to talk to MySQL and
>> deploying it in Tomcat. Why do I need to add the line -:
>
>> Class.forName("com.mysql.jdbc.Driver");
>
>> to load the JDBC driver ? If I don’t do this I get an exception
>> of -:
>
>> java.sql.SQLException: No suitable driver was found.
>
>> Why is it when I use a JDBC connection in Servlets and deploy it
>> in Tomcat must the class be loaded dynamically ? Why can’t Tomcat
>> do that for me when I have already added it to the build path ?
>
>> From Java 6 onwards there is no need for the JDBC Driver to be
>> loaded dynamically. So why when using JRE 8 and Tomcat 8 must
>> this be done ?
>
>> Cant it be loaded automatically like for normal apps ? It sure
>> would be a lot more cleaner code.
>
> My guess is that DriverManager only searches the system CLASSPATH
> which only contains the following JAR files when launching Tomcat
> using the standard scripts:
>
> bin/bootstrap.jar bin/tomcat-juli.jar
>
> The bootstrap class then configures a new ClassLoader (including
> lib/*.jar) for the Tomcat server and Tomcat properly launches using
> /that/.
>
> I think your JDBC driver simply isn't visible to DriverManager's
> auto-loading. But when you call Class.forName, the driver registers
> itself with DriverManager, which is okay to do if DriverManager
> didn't know about your driver.
>
> -chris
>
> ---------------------------------------------------------------------
>
>
>
To unsubscribe, e-mail: users-unsubscribe@tomcat.apache.org
> For additional commands, e-mail: users-help@tomcat.apache.org
>
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Re: Why is Class.forName required to load drivers in Tomcat 8 ?
Posted by Christopher Schultz <ch...@christopherschultz.net>.
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Hash: SHA256
Sreyan,
On 4/23/15 9:22 AM, Sreyan Chakravarty wrote:
> So what happens when I add something to the build path of a
> web-app?
The "build path" is an Eclipse concept that has no meaning after the
application has been built.
> The dependency is not registered?
What dependency? Where would it be registered? The only think Java
knows about is the current thread's context ClassLoader and what it
can see. And the DriverManager always uses the System classloader to
find drivers automatically. Have a look at the source code for
DriverManager. It should be in src.zip:java/sql/DriverManager.java.
The method you're looking for is "loadInitialDrivers" (at least in
Oracle's JRE.. it's not a public part of the API).
> Also does that mean that whatever dependency I need I will have to
> put it in the /lib folder?
What other dependency?
> I just don't understand why it doesn't work if I add it to the
> Build Path from Eclipse like I do for a normal Java Application.
Because Tomcat does ClassLoading in a more complicated way than with a
standard Java application, where every library is just thrown into the
system classpath.
Tomcat does this to enforce certain security requirements, to provide
insulation against other components, to support embedded usage, and to
support plugability of certain features.
I'm sorry to say that you simply must call Class.forName to load your
driver. If you use either of Tomcat's built-in connection pools (one
based upon commons-dbcp, the other called tomcat-pool), you won't have
to call Class.forName because the pool will do that for you.
You don't have to put the JDBC driver into Tomcat's lib/ directory;
you can put it into your web application's WEB-INF/lib directory
instead, just like all the other libraries that your web application
requires. But be aware that *many* JDBC drivers are poorly-written,
and can cause all kinds of memory leaks when the application is
undeployed or re-loaded.
Hope that helps,
- -chris
> On Thu, Feb 19, 2015 at 2:19 AM, Christopher Schultz <
> chris@christopherschultz.net> wrote:
>
> Sreyan,
>
> (Sorry, I sent way before I was done writing. Full intentional
> message follows.)
>
> On 2/18/15 10:15 AM, sreyan32@gmail.com wrote:
>>>> What is the SYSTEM PATH that you are referring to ? Is it
>>>> the JAVA_HOME or the PATH system variable ?
>
> Neither of those. It's commonly referred to as the CLASSPATH
> environment variable, which should pretty much never be set
> because it's horribly confusing.
>
> Instead, Tomcat uses "java -classpath [the classpath]" to launch
> the JVM. The classpath used there includes two JAR files:
>
>>>> bin/bootstrap.jar bin/tomcat-juli.jar
>
> So the only thing that javax.sql.DriverManager can scan is the
> boot classpath (that includes all the JVM internals) plus the
> system classpath (which includes just those two specific JARs
> above).
>
>>>> What is the CLASSPATH just before Tomcat properly starts up?
>
> The boot class path includes JVM stuff (rt.jar, etc.)
>
> The system class path includes anything the JVM environment was
> instructed to load when launched (either from the CLASSPATH
> environment variable, or from the "-classpath" argument on the
> command-line).
>
> Tomcat's bootstrap class creates a new ClassLoader which includes
> everything in lib/*.jar and then starts the Tomcat container using
> *that* as the effective "class path".
>
>>>> What is it after it starts up?
>
> Well, Tomcat can see everything in the boot and system class paths
> (ClassLoaders) plus everything in lib/*.jar.
>
>>>> Correct me if I am wrong that means that whatever
>>>> dependencies I have in my project is not added to the SYSTEM
>>>> CLASSPATH but is instead loaded by TOMCAT.
>
> Correct.
>
>>>> So if a module searches only the SYSTEM CLASSPATH it won’t be
>>>> able to find the dependencies. Am I right ?
>
> Correct. And DriverManager can't search *down* into ClassLoaders
> it doesn't know about -- including Tomcat's ClassLoader. So,
> DriverManager can't load your JDBC driver using its standard
> detection algorithm.
>
> So, you still need to call Class.forName to register the driver.
>
> -chris
>>
>> ---------------------------------------------------------------------
>>
>>
To unsubscribe, e-mail: users-unsubscribe@tomcat.apache.org
>> For additional commands, e-mail: users-help@tomcat.apache.org
>>
>>
>
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Re: Why is Class.forName required to load drivers in Tomcat 8 ?
Posted by Sreyan Chakravarty <sr...@gmail.com>.
So what happens when I add something to the build path of a web-app ? The
dependency is not registered ? Also does that mean that whatever dependency
I need I will have to put it in the /lib folder ? I just don't understand
why it doesn't work if I add it to the Build Path from Eclipse like I do
for a normal Java Application.
On Thu, Feb 19, 2015 at 2:19 AM, Christopher Schultz <
chris@christopherschultz.net> wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA256
>
> Sreyan,
>
> (Sorry, I sent way before I was done writing. Full intentional message
> follows.)
>
> On 2/18/15 10:15 AM, sreyan32@gmail.com wrote:
> > What is the SYSTEM PATH that you are referring to ? Is it the
> > JAVA_HOME or the PATH system variable ?
>
> Neither of those. It's commonly referred to as the CLASSPATH
> environment variable, which should pretty much never be set because
> it's horribly confusing.
>
> Instead, Tomcat uses "java -classpath [the classpath]" to launch the
> JVM. The classpath used there includes two JAR files:
>
> > bin/bootstrap.jar bin/tomcat-juli.jar
>
> So the only thing that javax.sql.DriverManager can scan is the boot
> classpath (that includes all the JVM internals) plus the system
> classpath (which includes just those two specific JARs above).
>
> > What is the CLASSPATH just before Tomcat properly starts up?
>
> The boot class path includes JVM stuff (rt.jar, etc.)
>
> The system class path includes anything the JVM environment was
> instructed to load when launched (either from the CLASSPATH
> environment variable, or from the "-classpath" argument on the
> command-line).
>
> Tomcat's bootstrap class creates a new ClassLoader which includes
> everything in lib/*.jar and then starts the Tomcat container using
> *that* as the effective "class path".
>
> > What is it after it starts up?
>
> Well, Tomcat can see everything in the boot and system class paths
> (ClassLoaders) plus everything in lib/*.jar.
>
> > Correct me if I am wrong that means that whatever dependencies I
> > have in my project is not added to the SYSTEM CLASSPATH but is
> > instead loaded by TOMCAT.
>
> Correct.
>
> > So if a module searches only the SYSTEM CLASSPATH it won’t be able
> > to find the dependencies. Am I right ?
>
> Correct. And DriverManager can't search *down* into ClassLoaders it
> doesn't know about -- including Tomcat's ClassLoader. So,
> DriverManager can't load your JDBC driver using its standard detection
> algorithm.
>
> So, you still need to call Class.forName to register the driver.
>
> - -chris
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> ---------------------------------------------------------------------
> To unsubscribe, e-mail: users-unsubscribe@tomcat.apache.org
> For additional commands, e-mail: users-help@tomcat.apache.org
>
>
Re: Why is Class.forName required to load drivers in Tomcat 8 ?
Posted by Christopher Schultz <ch...@christopherschultz.net>.
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA256
Sreyan,
(Sorry, I sent way before I was done writing. Full intentional message
follows.)
On 2/18/15 10:15 AM, sreyan32@gmail.com wrote:
> What is the SYSTEM PATH that you are referring to ? Is it the
> JAVA_HOME or the PATH system variable ?
Neither of those. It's commonly referred to as the CLASSPATH
environment variable, which should pretty much never be set because
it's horribly confusing.
Instead, Tomcat uses "java -classpath [the classpath]" to launch the
JVM. The classpath used there includes two JAR files:
> bin/bootstrap.jar bin/tomcat-juli.jar
So the only thing that javax.sql.DriverManager can scan is the boot
classpath (that includes all the JVM internals) plus the system
classpath (which includes just those two specific JARs above).
> What is the CLASSPATH just before Tomcat properly starts up?
The boot class path includes JVM stuff (rt.jar, etc.)
The system class path includes anything the JVM environment was
instructed to load when launched (either from the CLASSPATH
environment variable, or from the "-classpath" argument on the
command-line).
Tomcat's bootstrap class creates a new ClassLoader which includes
everything in lib/*.jar and then starts the Tomcat container using
*that* as the effective "class path".
> What is it after it starts up?
Well, Tomcat can see everything in the boot and system class paths
(ClassLoaders) plus everything in lib/*.jar.
> Correct me if I am wrong that means that whatever dependencies I
> have in my project is not added to the SYSTEM CLASSPATH but is
> instead loaded by TOMCAT.
Correct.
> So if a module searches only the SYSTEM CLASSPATH it won’t be able
> to find the dependencies. Am I right ?
Correct. And DriverManager can't search *down* into ClassLoaders it
doesn't know about -- including Tomcat's ClassLoader. So,
DriverManager can't load your JDBC driver using its standard detection
algorithm.
So, you still need to call Class.forName to register the driver.
- -chris
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Re: Why is Class.forName required to load drivers in Tomcat 8 ?
Posted by Christopher Schultz <ch...@christopherschultz.net>.
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA256
Sreyan,
On 2/18/15 9:25 AM, sreyan32@gmail.com wrote:
> I am writing a simple Servlet that uses JDBC to talk to MySQL and
> deploying it in Tomcat. Why do I need to add the line -:
>
> Class.forName("com.mysql.jdbc.Driver");
>
> to load the JDBC driver ? If I don’t do this I get an exception of
> -:
>
> java.sql.SQLException: No suitable driver was found.
>
> Why is it when I use a JDBC connection in Servlets and deploy it
> in Tomcat must the class be loaded dynamically ? Why can’t Tomcat
> do that for me when I have already added it to the build path ?
>
> From Java 6 onwards there is no need for the JDBC Driver to be
> loaded dynamically. So why when using JRE 8 and Tomcat 8 must this
> be done ?
>
> Cant it be loaded automatically like for normal apps ? It sure
> would be a lot more cleaner code.
My guess is that DriverManager only searches the system CLASSPATH
which only contains the following JAR files when launching Tomcat
using the standard scripts:
bin/bootstrap.jar
bin/tomcat-juli.jar
The bootstrap class then configures a new ClassLoader (including
lib/*.jar) for the Tomcat server and Tomcat properly launches using
/that/.
I think your JDBC driver simply isn't visible to DriverManager's
auto-loading. But when you call Class.forName, the driver registers
itself with DriverManager, which is okay to do if DriverManager didn't
know about your driver.
- -chris
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