RMI ClassCastException

Question

Hello, world!
My problem is the following: I'm developping a peer-to-peer application based on RMI. This application includes applets -called peerlets- that can be exposed to and used by other peers. Hence I have two remote interfaces:
public interface Peerlet extends Remote {
    // stuff that throws RemoteException
public interface Peer extends Remote {
    public Peerlet getPeerlet(/*some Peerlet ID here*/) throws RemoteException;
}As you can see Peer is the broker that passes references on local peerlets to remote peers. There are many possible peerlets, such as:
public interface APeerlet extends Peerlet {
    // stuff that throws RemoteException
}Creating a Peer instance, retrieve it from another remote Peer instance via RMI is fine. My problem occurs when I try to get references on remote peerlets. Concretely the situation is the following:
- peerA and peerB are two running Peer instances, they have a reference on each other; peerA hosts a APeerlet.
- if peerB tries the following:
Peerlet p = (APeerlet) peerA.getPeerlet(/*appropriate ID to get the desired APeerlet*/);
//...everything goes fine, peerB can use p normaly. But this is not what I want, since this piece of code needs to have APeerlet defined. If I try to get simply a Peerlet - that would be used latter by another piece of code who knows what to do with a APeerlet - ie:
Peerlet p = (Peerlet) peerA.getPeerlet(/*...*/);
//...I get a
java.lang.ClassCastException: $Proxy3 cannot be cast to ....PeerletI guess it's a class loading issue, but I can't figure out why the cast to Peerlet fails whereas a cast to APeerlet does not. I have found interesting behaviour with this two variations in peerB's code:
import ....APeerlet;
Class<? extends Peerlet> clazz = APeerlet.class;
Peerlet p = clazz.cast(peerA.getPeerlet(/*...*/););
// no import ....APeerlet;
Class<? extends Peerlet> clazz = /*something that returns me APeerlet.class*/;
Peerlet p = clazz.cast(peerA.getPeerlet(/*...*/););The first version is still not what I want since it needs a definition of APeerlet, but it works. The second one throws ClassCastException... I have hidden many details, such as the http server I use for dynamic class loading; it does not receive the same queries when the ClassCastException is thrown. Could it be the source of my trouble?
S
Edited by: sylf on Aug 14, 2010 3:09 PM

Answer 1

I see your point. Only Peer should be in the codebase, since it is the only class bound to the RMIRegistry, Peerlet and APeerlet should NOT be in the codebase but only in the classpath of both peers.
However, if Peerlet is not in the codebase, I get a ClassNotFoundException : Peerlet at startup, since the Peer interface uses Peerlet as a return type of Peer.getPeerlet(...). So I had no choice but to keep Peerlet in the codebase, and now peerB gets a ClassNotFoundException : APeerlet when trying to get a APeerlet from peerA...
java.rmi.ServerException: RemoteException occurred in server thread; nested exception is:
        java.rmi.UnmarshalException: error unmarshalling arguments; nested exception is:
        java.lang.ClassNotFoundException: ....APeerlet
        at sun.rmi.server.UnicastServerRef.dispatch(UnicastServerRef.java:336)
        at sun.rmi.transport.Transport$1.run(Transport.java:159)
        at java.security.AccessController.doPrivileged(Native Method)
        at sun.rmi.transport.Transport.serviceCall(Transport.java:155)
        at sun.rmi.transport.tcp.TCPTransport.handleMessages(TCPTransport.java:535)
        at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(TCPTransport.java:790)
        at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(TCPTransport.java:649)
        at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
        at java.lang.Thread.run(Thread.java:619)
        at sun.rmi.transport.StreamRemoteCall.exceptionReceivedFromServer(StreamRemoteCall.java:273)
        at sun.rmi.transport.StreamRemoteCall.executeCall(StreamRemoteCall.java:251)
        at sun.rmi.server.UnicastRef.invoke(UnicastRef.java:160)
        at java.rmi.server.RemoteObjectInvocationHandler.invokeRemoteMethod(RemoteObjectInvocationHandler.java:194)
        at java.rmi.server.RemoteObjectInvocationHandler.invoke(RemoteObjectInvocationHandler.java:148)
        at $Proxy0.getPeerlet(Unknown Source)
        at ...Peer.getPeerlet(Peer.java:112)
        at //...
Caused by: java.rmi.UnmarshalException: error unmarshalling arguments; nested exception is:
        java.lang.ClassNotFoundException: ...APeerlet
        at sun.rmi.server.UnicastServerRef.dispatch(UnicastServerRef.java:296)
        at sun.rmi.transport.Transport$1.run(Transport.java:159)
        at java.security.AccessController.doPrivileged(Native Method)
        at sun.rmi.transport.Transport.serviceCall(Transport.java:155)
        at sun.rmi.transport.tcp.TCPTransport.handleMessages(TCPTransport.java:535)
        at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(TCPTransport.java:790)
        at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(TCPTransport.java:649)
        at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
        at java.lang.Thread.run(Thread.java:619)
Caused by: java.lang.ClassNotFoundException: ...APeerlet
        at java.net.URLClassLoader$1.run(URLClassLoader.java:200)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:307)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:252)
        at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:320)
        at java.lang.Class.forName0(Native Method)
        at java.lang.Class.forName(Class.java:247)
        at sun.rmi.server.LoaderHandler.loadClass(LoaderHandler.java:434)
        at sun.rmi.server.LoaderHandler.loadClass(LoaderHandler.java:165)
        at java.rmi.server.RMIClassLoader$2.loadClass(RMIClassLoader.java:620)
        at java.rmi.server.RMIClassLoader.loadClass(RMIClassLoader.java:247)
        at sun.rmi.server.MarshalInputStream.resolveClass(MarshalInputStream.java:197)
        at java.io.ObjectInputStream.readNonProxyDesc(ObjectInputStream.java:1575)
        at java.io.ObjectInputStream.readClassDesc(ObjectInputStream.java:1496)
        at java.io.ObjectInputStream.readClass(ObjectInputStream.java:1462)
        at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1312)
        at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:1947)
        at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1871)
        at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1753)
        at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1329)
        at java.io.ObjectInputStream.readObject(ObjectInputStream.java:351)
        at sun.rmi.server.UnicastRef.unmarshalValue(UnicastRef.java:306)
        at sun.rmi.server.UnicastServerRef.dispatch(UnicastServerRef.java:290)
        ... 9 moreIt looks like it's the RMIServer who can't find APeerlet... but why?
I get the same Exception after having changed the return type of Peer.getPeerlet() to Remote so that Peerlet has not to be in the codebase anymore.
S