Question

I’m trying to practice myself with custom class loaders, and I’ve some questions. Is there a way to indicate the JVM to use my custom class loader globally? For example, I wrote small app running under Tomcat 6. The servlet is managed by the container, where should I set my class loader? In addition, the webapp uses some 3rd party jars, can I control the classes loading of those jars?

Are the answers to the above will be different in case of standalone app?

Thanks!

Answer 1

You can set the system default class loader as a JVM argument:

java -Djava.system.class.loader
    =com.test.YourCustomClassLoader com.test.YourMainClass

As Tomcat starts as a java application, you can set this parameter too, at the %TOMCAT_HOME%\bin\run.bat or $TOMCAT_HOME/bin/run.sh executable.

Edit for completion:
If you set your classloader as de System class loader, it will be used to load Tomcat component classes, the different libraries, and your own classes.
If you want your class loader to be used only for your application classes (including libraries and so), you should configure a Loader element for your context.
The Loader element must be defined inside your Context element, wich can be defined at a context.xml file.
More information:

• Apache Tomcat: Class Loader HOW-TO: defines how the ClassLoaders work in Tomcat.
• Tomcat Configuration Reference: The Context Container: how to define your Context element
• Tomcat Configuration Reference: The Loader Component: How to define your custom Loader element for your own Context.