如何将Java类从一个Web容器加载到另一个Web容器？

Question

I have an application running in a jetty container where a custom class loader, will load classes from /SHARED/< jar files>.. Now I want any of these loaded classes to use another classes (WEB-INF/libs) from applications running in another jetty container at run time. As of now, am getting a class not found exception.

Web Container 1:

Jar location: /ci/Shared/< jars> To load the above jars, we have a customer class loader called SharedJarsClassLoader..

Web Container 2:

App_1/WEB-INF/libs App_2/WEB-INF/libs

My requirement is to supports for the classes in /ci/Shared/< jars> in container 1 to loaded classes from app1 & app2 jars from WEB-INF/libs..

So of now, when the app starts, its shows class not found exception. App is not able to load the classes from App_1/WEB-INF/libs & App_2/WEB-INF/libs.

This is the class loader hierarchy..

sun.misc.Launcher$ExtClassLoader@4921a90                                         (Extensions class loader)
  sun.misc.Launcher$AppClassLoader@3da997a                                       (System class loader)
    startJarLoader@68758d51                                                      (Jetty class loader)
      com.test.jetty_ext.NewWebAppProvider$SharedJarsClassLoader@374d1cbd        (Shared jars class loader)
        WebAppClassLoader=settings@2fee2019                                      (Webapp class loader)
        WebAppClassLoader=taskt@6a57cf0b                                         (2nd Webapp class loader)

Here is the stack trace..

Caused by: java.lang.ClassNotFoundException: com.test.ClassAInContainer3
    at java.net.URLClassLoader$1.run(URLClassLoader.java:366) ~[na:1.7.0_55]
    at java.net.URLClassLoader$1.run(URLClassLoader.java:355) ~[na:1.7.0_55]
    at java.security.AccessController.doPrivileged(Native Method) ~[na:1.7.0_55]
    at java.net.URLClassLoader.findClass(URLClassLoader.java:354) ~[na:1.7.0_55]
    at java.lang.ClassLoader.loadClass(ClassLoader.java:425) ~[na:1.7.0_55]
    at java.lang.ClassLoader.loadClass(ClassLoader.java:358) ~[na:1.7.0_55]
    at org.springframework.util.ClassUtils.forName(ClassUtils.java:258) ~[spring-core.jar:3.2.0.RELEASE]
    at org.springframework.amqp.support.converter.DefaultJavaTypeMapper.getClassIdType(DefaultJavaTypeMapper.java:82) ~[spring-amqp.jar:na]
    ... 16 common frames omitted
2015-08-19 05:38:16.123 UTC,WARN ,App1-web,messaging,com.test.Rabbit,null,SimpleAsyncTaskExecutor-1,Caught an exception while handling a rabbitmq message, Publisher/Consumer should have handled this.
org.springframework.amqp.support.converter.MessageConversionException: failed to resolve class name. Class not found [com.test.ClassAInContainer3]
    at org.springframework.amqp.support.converter.DefaultJavaTypeMapper.getClassIdType(DefaultJavaTypeMapper.java:85) ~[spring-amqp.jar:na]
    at org.springframework.amqp.support.converter.DefaultJavaTypeMapper.toJavaType(DefaultJavaTypeMapper.java:53) ~[spring-amqp.jar:na]
    at org.springframework.amqp.support.converter.JsonMessageConverter.fromMessage(JsonMessageConverter.java:117) ~[spring-amqp.jar:na]

Any inputs on how to resolve this issue?

Answer 1

To be able to do that, your custom class loader (CCL) must be a child of the web app classloader. The reason is that a classloader only ever asks it's parent for missing classes. In your case, it's the other way around. That means:

• app code accesses a custom class via the CCL. This works since the CCL is the parent of your app's class loader.
• The custom class now tries to discover another app class. It asks CCL which doesn't know about is. Then CCL asks parents which also has no idea -> doom.

If you flip this upside down, custom classes will be able to access app code. Now you will have a problem to access the custom classes. In order to solve that, you must pass the CCL reference around and manually load types via the classloader API using the CCL and instantiate the type you need. You can't have Java "automatically" discover this classes.