Re: What causes an object to be removed from the ObjectTable?

Ok, I answered my own question:

The first example is fine, it is the second example that uses m_stub
that has a problem.

public class FooService extends UnicastRemoteObject implements Foo
{
    private Foo m_stub;

    public FooService() throws RemoteException
    {
        m_stub = (Station)UnicastRemoteObject.toStub( this );
        LocateRegistry.getRegistry( 
            getLocalServicePort() ).rebind( 
                Foo.class.getName(), 
                m_stub );
    }

    ....
} 

The constructor to UnicastRemoteObject will export "this". Basically
wrapping it in a Target and adding it (as a WeakReference) to
ObjectTable. Because I'm still in the constructor, toStub has no problem
looking up that target in the ObjectTable again. Unfortunately, I am
binding "m_stub" and not "this". So while "this" AND the regsitry are
maintaining a strong reference to "m_stub", nothing is maintaining a
strong reference to "this". Once the constructor exits, "this" can be
collected and when the reaper runs (however often it runs) it will
remove the Target for FooService from the ObjectTable. Duh.

R.

-----Original Message-----
From: Robert DiFalco 
Sent: Friday, December 16, 2005 2:48 PM
To: [email protected]
Cc: Brian McFeely
Subject: What causes an object to be removed from the ObjectTable?

I'm really confused at this point. It is my understanding that if I have
code like this:

public class FooService extends UnicastRemoteObject implements Foo {
    public FooService() throws RemoteException
    {
        super();

        LocateRegistry.getRegistry( 
            getLocalServicePort() ).rebind( 
                Foo.class.getName(), this );
    }

    ...
}

Then if I have a client of Foo that always looks the class up in the
remote registry that I should NEVER get a NoSuchObjectException. (Maybe
it could get this exception if it cached the stub and the server
restarted, but not otherwise).

However, what I'm seeing is very strange and different.

Eventually, in the sever log I see this:

 Dec 16, 2005 1:24:00 PM sun.rmi.transport.tcp.TCPChannel$Reaper run
 FINER: RMI ConnectionExpiration-[10.69.10.62:9898]: exit  Dec 16, 2005
1:25:31 PM sun.rmi.transport.ObjectTable removeTarget
 FINER: RMI Reaper: remove object [0] <<--- [0] is Foo

Next, when the client would like to get the Foo stub/objid to invoke a
method on the FooServer I see this in the server's log (which seems to
succeed):

 FINER: RMI TCP Connection(99)-10.69.10.62: [10.69.10.62:
sun.rmi.registry.RegistryImpl[0:0:0, 0]: java.rmi.Remote
lookup(java.lang.String)]

Finally, when the client invokes a method on the Foo stub I get:

 FINE: RMI TCP Connection(99)-10.69.10.62: [10.69.10.62] exception: 
 java.rmi.NoSuchObjectException: no such object in table
        at sun.rmi.transport.Transport.serviceCall(Transport.java:112)
        at
sun.rmi.transport.tcp.TCPTransport.handleMessages(TCPTransport.java:460)
        at
sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(TCPTransport.ja
va:701)
        at java.lang.Thread.run(Thread.java:534)

And that is pretty much game over. 

So somehow the WeakRef to the impl in the server's ObjectTable is being
cleaned up, causing the reaper to remove the target from the table all
together. Any idea how this could happen?

All clients and agents have these settings:

        -Dsun.rmi.dgc.server.gcInterval=3600000
        -Dsun.rmi.dgc.client.gcInterval=3600000
        -Dsun.rmi.dgc.checkInterval=1800000

Finally, if the above code were re-written like this (using the stub
directly) could that create the problem I am seeing?


public class FooService extends UnicastRemoteObject implements Foo {
    private Foo m_stub;

    public FooService() throws RemoteException
    {
        m_stub = (Station)UnicastRemoteObject.toStub( this );
        LocateRegistry.getRegistry( 
            getLocalServicePort() ).rebind( 
                Foo.class.getName(), 
                m_stub );
    }
}

Not the registration of m_stub instead of "this".

R.

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