强制转换为同一对象时发生ClassCastException

Question

What are the possible reasons why Java will throws Cannot Cast Exception when loading class dynamically?

Example:

    String classToLoad = null;
    try {
        classToLoad = extractMainClassManifest(jar);
        LOGGER.info("Class to load: " + classToLoad);
        JarByteClassloader loader = new JarByteClassloader(jar); 
        // e.g. class MyJarEntryObject extends JarEntryObject
        Class c = loader.loadClass(classToLoad); 
        JarEntryObject jarEntry = (JarEntryObject) c.newInstance();
        // other stuff
    } catch (Exception e) {
        e.printStackTrace();
    }

Where this code throws that error, I don't suppose that there is another Class loader here (the case of multiple class loaders).

This code works fine when running with JUnit or just running in a main() method. The cast problem appears when running on a Servlet container (eg mvn jetty:run )

Log:

java.lang.ClassCastException: com.mywebapp.example.MyJarEntryObject cannot be cast to com.mysdk.JarEntryObject
    at org.restlet.routing.Filter.doHandle(Filter.java:150)
    at org.restlet.routing.Filter.handle(Filter.java:197)
    at org.restlet.routing.Router.doHandle(Router.java:422)
    at org.restlet.routing.Router.handle(Router.java:641)
    at org.restlet.routing.Filter.doHandle(Filter.java:150)
    at org.restlet.routing.Filter.handle(Filter.java:197)
    at org.restlet.routing.Filter.doHandle(Filter.java:150)
    at org.restlet.routing.Filter.handle(Filter.java:197)
    at org.restlet.routing.Filter.doHandle(Filter.java:150)
    at org.restlet.routing.Filter.handle(Filter.java:197)
    at org.restlet.routing.Filter.doHandle(Filter.java:150)
    at org.restlet.engine.application.StatusFilter.doHandle(StatusFilter.java:140)
    at org.restlet.routing.Filter.handle(Filter.java:197)
    at org.restlet.routing.Filter.doHandle(Filter.java:150)
    at org.restlet.routing.Filter.handle(Filter.java:197)
    at org.restlet.engine.CompositeHelper.handle(CompositeHelper.java:202)
    at org.restlet.engine.application.ApplicationHelper.handle(ApplicationHelper.java:77)
    at org.restlet.Application.handle(Application.java:385)
    at org.restlet.routing.Filter.doHandle(Filter.java:150)
    at org.restlet.routing.Filter.handle(Filter.java:197)
    at org.restlet.routing.Router.doHandle(Router.java:422)
    at org.restlet.routing.Router.handle(Router.java:641)
    at org.restlet.routing.Filter.doHandle(Filter.java:150)
    at org.restlet.routing.Filter.handle(Filter.java:197)
    at org.restlet.routing.Router.doHandle(Router.java:422)
    at org.restlet.routing.Router.handle(Router.java:641)
    at org.restlet.routing.Filter.doHandle(Filter.java:150)
    at org.restlet.routing.Filter.handle(Filter.java:197)
    at org.restlet.engine.CompositeHelper.handle(CompositeHelper.java:202)
    at org.restlet.Component.handle(Component.java:408)
    at org.restlet.Server.handle(Server.java:507)
    at org.restlet.engine.connector.ServerHelper.handle(ServerHelper.java:63)
    at org.restlet.engine.adapter.HttpServerHelper.handle(HttpServerHelper.java:143)
    at org.restlet.ext.servlet.ServerServlet.service(ServerServlet.java:1117)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:820)
    at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:487)
    at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:362)
    at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
    at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:181)
    at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:726)
    at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:405)
    at org.mortbay.jetty.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:206)
    at org.mortbay.jetty.handler.HandlerCollection.handle(HandlerCollection.java:114)
    at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
    at org.mortbay.jetty.Server.handle(Server.java:324)
    at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:505)
    at org.mortbay.jetty.HttpConnection$RequestHandler.headerComplete(HttpConnection.java:829)
    at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:514)
    at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:211)
    at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:380)
    at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:395)
    at org.mortbay.thread.QueuedThreadPool$PoolThread.run(QueuedThreadPool.java:488)

Answer 1

It looks like your JarByteClassloader is not joining the default class loader's hierarchy. Ensure that the constructor(s) call super(ClassLoader) . Something like this might work:

public class JarByteClassloader extends ClassLoader {
  public JarByteClassloader(Object objectLoadedByDefaultClassLoader) {
    super(objectLoadedByDefaultClassLoader.getClass().getClassLoader());
  }
}

And since you're already passing in a jar that was loaded by the default class loader, you're done!

Actually, I've just remember getSystemClassLoader() , so this code is probably better, and more self-documenting:

public class JarByteClassloader extends ClassLoader {
  public JarByteClassloader() {
    super(ClassLoader.getSystemClassLoader());
  }
}