rmi vs cajo

[Mark Wielaard]

Hi,

While playing a bit with Cajo
(http://wiki.java.net/bin/view/Communications/ProxyUsage) I got the
following error:

java.lang.NullPointerException
   at gnu.cajo.invoke.Remote.hashCode (Remote.java:510)
   at java.util.Hashtable.hash (Hashtable.java:822)
   at java.util.Hashtable.put (Hashtable.java:432)
   at gnu.java.rmi.server.UnicastServer.exportObject (UnicastServer.java:66)
   at gnu.java.rmi.server.UnicastServerRef.exportObject 
(UnicastServerRef.java:110)
   at java.rmi.server.UnicastRemoteObject.exportObject 
(UnicastRemoteObject.java:83)
   at java.rmi.server.UnicastRemoteObject.<init> (UnicastRemoteObject.java:69)
   at gnu.cajo.invoke.Remote.<init> (Remote.java:486)
   at gnu.cajo.utils.ItemServer.bind (ItemServer.java:206)
   at ProxyTest.main (ProxyTest.java:38)

It seems we are to eager to export the Remote object immediately (from
the constructor). We want to put it in a Hashtable and call hasCode(),
but for the cajo Remote object the item field used to calculate the
hashCode() hasn't been set yet so that gives a NullPointerException.

Looking at the code I think we might actually want a IdentityHashMap
here that uses the System.identityHashCode() for an object since we are
not really interested in equal objects here.

With the attached patch I get a little further, but then hit the
following error (on the server side):

java.io.IOException: bad protocol header
   at gnu.java.rmi.server.UnicastConnection.acceptConnection 
(UnicastConnection.java:79)
   at gnu.java.rmi.server.UnicastConnectionManager.getServerConnection 
(UnicastConnectionManager.java:265)
   at gnu.java.rmi.server.UnicastConnectionManager.run 
(UnicastConnectionManager.java:371)
   at java.lang.Thread.run (Thread.java:673)

I didn't found time to investigate this further, but I wanted to post
the patch that I used in case someone else had some time to investigate
our rmi implementation a bit more.

Cheers,

Mark

Index: gnu/java/rmi/server/UnicastServer.java
===================================================================
RCS file: /cvsroot/classpath/classpath/gnu/java/rmi/server/UnicastServer.java,v
retrieving revision 1.9
diff -u -r1.9 UnicastServer.java
--- gnu/java/rmi/server/UnicastServer.java      2 Jul 2005 20:32:14 -0000       
1.9
+++ gnu/java/rmi/server/UnicastServer.java      11 Sep 2005 19:37:57 -0000
@@ -51,13 +51,16 @@
 import java.rmi.ServerError;
 import java.rmi.server.ObjID;
 import java.rmi.server.UID;
+import java.util.Collections;
+import java.util.Map;
 import java.util.Hashtable;
+import java.util.IdentityHashMap;
 
 public class UnicastServer
        implements ProtocolConstants {
 
 static private Hashtable objects = new Hashtable();  //mapping OBJID to server 
ref
-static private Hashtable refcache = new Hashtable(); //mapping obj itself to 
server ref
+static private Map refcache = Collections.synchronizedMap(new 
IdentityHashMap()); //mapping obj itself to server ref
 static private DGCImpl dgc;
 
 public static void exportObject(UnicastServerRef obj) {

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