RMI server object getting garbage collected

Hi all,
I have seen a number of posts regarding the ConnectException and found that this can occur in a number of situations.
I am having a problem here.
I am having an RMI server that is always up and running. And the server object gets requests from the client at regular intervals. But, when the server object is not receiving any requests for a long time (ex: 1 day), then I think the remote object itself is getting garbage collected. And so, tough I am able to get the remote reference using the lookup method, I am getting "Connection refused to host: 192.168.0.216; nested exception is:
java.net.ConnectException: Connection refused" when I call a method using this reference.
I believe that this is because the server object getting garbage collected as there are no requests for the server since long. Please correct me if my assumption is wrong.
I want to know, after how much time the server object gets garbage collected if no requests are received. But, my requirement is that the server object should always be available and WHENEVER a client request comes then that should be processed. What should I do to accomplish this task.
If any one have any suggestions, please reply as soon as possible.
Thanks in advance,
srik4u

You might do some research into using an activatable remote object. You run rmid (the rmi activation deamon) and register a subclass of java.rmi.activation.Activatable (instead of the usual UnicastRemoteObject) with it. With an activatable object, the remote reference to the activatable object doesn't need to have a live object behind it. If an activatable object is not running (ie: it hasn't been constructed yet, or it has been garbage collected ...as in your case) a remote reference to the object can still be exported to a client. The first time the client calls a method on the remote object, the activation service on the server sees the object is not active, and activates the object for the client. If the object is running ...it is rmi as usual. But if it gets gc'd again, the next invocation on the remote object will trigger the activation service again.
(The above explanation paraphrases author David Flanagan from Java Enterprise in a Nutshell, O'Reilly)
I have only built one of these, which loosely followed an example from the above mentioned book. It's a whole other ballgame over and above a regular rmi object. But like anything else, if you dig in and get your head wrapped around it, it eventually makes sense. Ok, why lie ...it confused the hell out of me and left me a little queasy. But you know the drill, by the time you build a few of them it will probably seem as easy as mapping the human genome, right? At any rate, it seems like what you might be after ...so have a look at it. Good luck, and wear your lifejacket.

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Hi Pyung,
How about trying this instead:
InitialContext ctx = new InitialContext();
MediatorInterface mediator = (MediatorInterface) ctx.lookup("TestMediator");
This assumes your startup class binds the Mediator to the JNDI name "TestMediator".
- Matt
Pyung Yoon wrote:
Hi all,
I am getting MarshalException when I call java.rmi.Naming.lookup() in a session
bean. Following is the code and exception. Please note that I am using java.rmi
package instead of weblogic.rmi and that both the session bean and RMI server
object (a startup class) is deployed on the same machine.
Thanks for your help in advance.
// a simple, replica-aware session bean method
// some code here
try {
MediatorInterface mediator = (MediatorInterface) java.rmi.Naming.lookup("rmi://localhost:7001/TestMediator);
catch (Exception e) {
// log the exception
The exception:
java.rmi.MarshalException: Error marshaling transport header; nested exception
i
s:
java.io.EOFException
java.io.EOFException
at java.io.DataInputStream.readByte(DataInputStream.java:224)
at sun.rmi.transport.tcp.TCPChannel.createConnection(TCPChannel.java:206
at sun.rmi.transport.tcp.TCPChannel.newConnection(TCPChannel.java:174)
at sun.rmi.server.UnicastRef.newCall(UnicastRef.java:318)
at sun.rmi.registry.RegistryImpl_Stub.lookup(Unknown Source)
at java.rmi.Naming.lookup(Naming.java:84)
at test.mgmtop.CreateNetworkOpHandler.execute(CreateNetworkOpHandler.jav
a:88)
at test.mgmtop.CreateNetworkOpHandler.perform(CreateNetworkOpHandler.jav
a:28)
at test.ejb.MgmtServiceBean.create(MgmtServiceBean.java:57)
at test.ejb.MgmtServiceSession_idi8yo_EOImpl.create(MgmtServiceSession_i
di8yo_EOImpl.java:46)
at test.ejb.MgmtServiceSession_idi8yo_EOImpl_WLSkel.invoke(Unknown Sourc
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at weblogic.rmi.internal.BasicServerRef.invoke(BasicServerRef.java:346)
at weblogic.rmi.cluster.ReplicaAwareServerRef.invoke(ReplicaAwareServerR
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at weblogic.rmi.internal.BasicServerRef$1.run(BasicServerRef.java:300)
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at weblogic.rmi.internal.BasicServerRef.handleRequest(BasicServerRef.jav
a:295)
at weblogic.rmi.internal.BasicExecuteRequest.execute(BasicExecuteRequest
java:30)
at weblogic.kernel.ExecuteThread.execute(ExecuteThread.java:152)
at weblogic.kernel.ExecuteThread.run(ExecuteThread.java:133)

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"Pyung Yoon" <[email protected]> writes:
MediatorInterface mediator = (MediatorInterface) java.rmi.Naming.lookup("rmi://localhost:7001/TestMediator);This implies JRMP which the server does not support. You need to use t3 or iiop.
andy

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Refer to my post on misc. Do not cross-post.
Cheers - Wei
aroshan <@cisco.com> wrote in message news:[email protected]..
Hi,
I'm using Weblogic 5.1, on Windows NT.
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"Pyung Yoon" <[email protected]> writes:
MediatorInterface mediator = (MediatorInterface) java.rmi.Naming.lookup("rmi://localhost:7001/TestMediator);This implies JRMP which the server does not support. You need to use t3 or iiop.
andy

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How do I design and write stateful RMI client/server applications?
I just find the examples on the Web that introduce RMI with some sample RMI client/server programs which are stateless, that means the RMI client and server do not maintain session information.
I would like to design and write stateful client/server applications with RMI, i.e. a new instance of server object is invoked to handle a client Java applet connection session, and maintains state/session information such as security context in the server instance's local variables.
Is that possible? Where can I find sample programs and design guidelines for this topic??

I suggest that you approach this slightly differently.
1. RMI generally gives access to a common server object, with each remote request being handled in a different thread. You can just as well have that common object do a lookup on a (synchronized) table of sessions. In other words, when a new client makes a request, look that client up, and if it is not in the table, then create a new session object and put it in the table. So now you have saved state.
2. A variation on this: Make your common server object create a session object, and return a remote reference to the caller. All subsequent requests go to the individual session object. (The terminology hear is probably misleading; we have done such designs, and call the registered object an "entry", and the individual objects "servers".

At which line , Object is Garbage collected.

1. public class GC {
2. private Object o;
3.   private void doSomethingElse(Object obj) { o = obj; }
4.   public void doSomething() {
5. Object o = new Object();
6. doSomethingElse(o);
7.   o = new Object();
8.   doSomethingElse(null);
9 o = null; 10. }
} When the doSomething method is called, after which line does the Object created in line 5 become available for garbage collection?
A. Line 5
B. Line 6
C. Line 7
D. Line 8
E. Line 9
F. Line 10
I think it should be line 9.but answer is 8....
How is it line 8?
Edited by: user12203354 on Nov 4, 2012 9:38 PM

You've already ask a similar question in your other thread where I ask you to answer some questions.
Question on Garbage collection
You may want to work on one question like this at a time until you understand the difference between an object and a variable that references an object.

Object [5610] : garbage collection is off. Anyone tell me how to turn it on ?

Can anyone tell me how to turn garbage collection on ? I'm getting objct: [5610] garbage collection is off.

Press Cmd-F5 together (that's Cmd + the F5 key, two keys only, not F-5.)

Manage behaviour of an object at garbage collection??

Hello.
I would like to specify that a counter is eventually decremented when an iterator is no longer in use. Is there some way to define the behaviour of an object when JRE realises that no references to it exist and proceeds to kill it?
I don't know very much about what Java does with data that is no longer used, so perhaps the notion is a preposterous one. Please advise.
Grape

I am also writing the collection thoroughly because I'll keep it in a general library
of data structures and patterns that I am building up
over time for reuse in the future.
Sounds cool but wouldn't it be easier to use one of the many container APIs that already exist and have been tested and are known to be reliable? That way you could concentrate on modeling the automata and not worry about measly collections. It doesn't sound very useful even as a learning experience since you must already be familiar with most kinds of containers. If you later find that you need a feature that's not in the standard collections, you can easily extend it or use an additional package like Collections from Jakarta Commons. http://jakarta.apache.org/commons/collections/
Should it be the iterator or the collection that calls the exception?You are the designer, it's your call.
Is there some kind of
ModificationListener that Iterator could implement,
so that when the collection is modified it sends out
something like a ModificationEvent that all Iterators
would get...? And the Iterators could then verify
that an exception should be thrown by checking
numModsAtStart != event.getSource().numMods()
But that would imply that the collection is aware of its iterators and also that iterators have to be both registered and de-registered with the collection. This approach has the same problems as the counting one you suggested before.
This use of the Observer pattern might be useful for other purposes.

Preventing garbage-collection of a RMIRegistered server object

I am developping a client/server RMI application, and keep facing occasional ObjectNotFioundException: no such object in table, when the client tries to invoke method on the server stub retrived from the RMIRegistry.
The Javadoc and online documentation say this means the server object has been GC'ed since it has been registered in the RMIRegistry.
Following the advice found at: http://www.nabble.com/java.rmi.NoSuchObjectException:-no-such-object-in-table-t260095.html,
I register the server in a RMIRegistry I obtain through LocateRegistry.getRegistry(), and not createRegistry().
But I still face these exceptions.
Assuming the issue is really due to the server object being garbage-collected, I tried to keep a static reference to the server but it didn't help (the Main class was probably garbage-collected itself).
As a last hope, I've set up a "keep-alive" thread in the server VM, that keeps a direct reference to the server object (not the stub), and regularly invokes something on it. With this mechanism I never face the exception.
This solution looks obviously clumsy.
Is there a neater way to prevent garbage collection of the server object?
Note that I use dynamic stubs (Java5-style).

Without seeing some code, I'm not sure what you are doing. Also the link you supplied is broken.
You always need to keep a live reference to your implementation class. How you do this is your business. Using a separate thread is over-kill.
I set a reference to the implementation class in the start up class and use a never ending wait() to make sure the start up class thread (with the main()) lives forever.

Nested object reference after garbage collection

I have a question about how GC works.
Assume I have a Class A which has a reference to class B inside it. I create a object of class B and assign to member variable of class A.
Now when obja is no longer needed, I set it null. And when GC runs, the object will be removed from memory. But should I also need to explicitly set objb to null. If i don't set it, will it also get garbage collected when obja is being removed from memory?
public class ClassA {
private ClassB objB = new ClassB();
private static void main(String args[]) {
ClassA obja = new ClassA();
obja = null;
}

801625 wrote:
But should I also need to explicitly set objb to null.No.
If i don't set it, will it also get garbage collected when obja is being removed from memory?If there are no other references outside of classA to the object referenced by objB (in your case an instance of ClassB) then it will be garbage collected.
Note - the only time one needs to set a reference to null is if one wants the object it references to be eligible for GC before the reference goes out of scope. There is a slight complication to this - depending on the JVM implementation, if a reference is defined in a block internal to a method then even though it goes out of scope when execution goes out of the block anything it references may not be eligible for GC until the method exits.

RMI Server exits early when using SSL

Hi,
Maybe someone else has encountered this one.
My RMI server works fine when I use standard sockets. I create an instance and bind it into the rmiregistry. It then waits indefinitely for connections.
When I use SSL, I can create the object instance. I can also bind it into the registry. However, it only waits a few seconds and then exits.
No exception or error is thrown (I have a catch Throwable block on my main method to be sure of this.)
As my code is adapted from the RMI SSL 'Hello World' examples found on this site) I decided to compile and run those examples. I get the same problem.
Any ideas?
This is the relevant part of my code:
public static void main(String[] args)
    HelloImpl obj = null;
    try
        // use -clear option to get standard sockets
        if (args.length > 0 && args[0].equals("-clear"))
            obj = new HelloImpl();
        else // otherwise default to SSL sockets
            RMIClientSocketFactory csf = new RMISSLClientSocketFactory();
            RMIServerSocketFactory ssf = new RMISSLServerSocketFactory();
            obj = new HelloImpl(csf,ssf);
        Naming.rebind("HelloServer", obj);
        System.out.println("HelloServer listening for requests\n";
    catch (RemoteException ex)
        ex.printStackTrace();
    catch (MalformedURLException ex)
        ex.printStackTrace();
    catch (Throwable ex)
        ex.printStackTrace();

Thanks for taking an interest.
I just use:
c:\j2sdk1.4.1_02\bin\rmiregistry
to start the registry.
I think the problem is with my SSL sockets. I'm guessing the reason a UnicastRemoteObject is normally kept alive (not garbage collected) in the registry is because it has a ServerSocket which is waiting for connections.
In this case, I don't think the SSLServerSocket is getting created and so my UnicastRemoteObject gets garbage collected.
However, I'm not sure why exactly.

Avoiding Garbage Collection

Hi,
Does anyone know of a general design pattern that allows an object to remain
in memory without being garbage collected ? I'm not clear whether or not
the singleton pattern fulfills this requirement. I basically want to have a
Global constants class (constants are loaded from a properties file) that
remains in memory so that it can be used by various components. The
constants are loaded from a properties file initialially. So if that class
gets garbage collected, then the next time that class is accessed, it will
have to reload from the props file. This is a performance issue, and I
would like to find a solution around it.
I will probably want to have other services such as a LoggingService,
JNDIService that I want started up, and for it to remain in memory. I know
you can register startup classes with WL, but do those classes remain in
memory ?
I'm been trying to find an answer to these questions. Hopefully someone
will have them.
Thanks.

One way to do it is to bind your constants class into JNDI during
start-up.
-- Rob
Jamie Tsao wrote:
>
Hi,
Does anyone know of a general design pattern that allows an object to remain
in memory without being garbage collected ? I'm not clear whether or not
the singleton pattern fulfills this requirement. I basically want to have a
Global constants class (constants are loaded from a properties file) that
remains in memory so that it can be used by various components. The
constants are loaded from a properties file initialially. So if that class
gets garbage collected, then the next time that class is accessed, it will
have to reload from the props file. This is a performance issue, and I
would like to find a solution around it.
I will probably want to have other services such as a LoggingService,
JNDIService that I want started up, and for it to remain in memory. I know
you can register startup classes with WL, but do those classes remain in
memory ?
I'm been trying to find an answer to these questions. Hopefully someone
will have them.
Thanks.--
Coming Soon: Building J2EE Applications & BEA WebLogic Server
by Michael Girdley, Rob Woollen, and Sandra Emerson
http://learnweblogic.com

RMI server object getting garbage collected

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