Java RMI and ClassNotFoundException

Question

I am just starting to learn how to use RMI, and I have a question. I have the following directory structure:

compute.jar
client
     |
     org\examples\rmi\client
                           |--> ComputePi     // client main
                           |--> Pi            // implements Task
     org\examples\rmi\compute
                           |--> Compute       // interface
                           |--> Task          // interface

server
     |
     org\examples\rmi\engine
                           |--> ComputeEngine // server main, implements Compute
     org\examples\rmi\compute
                           |--> Compute       // interface
                           |--> Task          // interface

Here's the main method in the ComputePi class:

if (System.getSecurityManager() == null) {
  System.setSecurityManager(new SecurityManager());
}
try {
  String name = "Compute";
  // args[0] = 127.0.0.1, args[1] is irrelevant
  Registry registry = LocateRegistry.getRegistry(args[0], 0);
  Compute comp = (Compute) registry.lookup(name);
  Pi task = new Pi(Integer.parseInt(args[1]));
  BigDecimal pi = comp.executeTask(task);
  System.out.println(pi);
}
catch (Exception e) {
  System.err.println("ComputePi exception:");
  e.printStackTrace();
}

Here's the main method in the ComputeEngine class:

if (System.getSecurityManager() == null) {
  System.setSecurityManager(new SecurityManager());
}
try {
  String name = "Compute";
  Compute engine = new ComputeEngine();
  Compute stub = (Compute) UnicastRemoteObject.exportObject(engine, 0);
  Registry registry = LocateRegistry.getRegistry();
  registry.rebind(name, stub);
  System.out.println("ComputeEngine bound.");
}
catch (Exception e) {
  System.err.println("ComputeEngine exception: ");
  e.printStackTrace();
}

Here's the executeTask method, also in the ComputeEngine class:

  public <T> T executeTask(Task<T> task) throws RemoteException {
    if (task == null) {
      throw new IllegalArgumentException("task is null");
    }
    return task.execute();
  }

The RMI registry and server start up just fine. Here are the params for the server:

C:\Users\Public\RMI\server>set CLASSPATH=
C:\Users\Public\RMI\server>start rmiregistry
C:\Users\Public\RMI\server>java -Djava.rmi.server.codebase="file:/C:/Users/Public/RMI/compute.jar" -Djava.rmi.server.hostname=127.0.0.1 -Djava.security.policy=server.policy org.examples.rmi.engine.ComputeEngine

Here are the params for the client:

C:\Users\Public\RMI\client>java -Djava.rmi.server.codebase="file:/C:/Users/Public/RMI/compute.jar" -Djava.security.policy=client.policy org.examples.rmi.client.ComputePi 127.0.0.1 45

However, I get the following exception when I try to run the client:

java.rmi.ServerException: RemoteException occurred in server thread; nested exception is:
        java.rmi.UnmarshalException: error unmarshalling arguments; nested exception is:
        java.lang.ClassNotFoundException: org.examples.rmi.client.Pi
        at sun.rmi.server.UnicastServerRef.dispatch(Unknown Source)
        at sun.rmi.transport.Transport$1.run(Unknown Source)
        at sun.rmi.transport.Transport$1.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at sun.rmi.transport.Transport.serviceCall(Unknown Source)
        at sun.rmi.transport.tcp.TCPTransport.handleMessages(Unknown Source)
        at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(Unknown Source)
        at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
        at java.lang.Thread.run(Unknown Source)
        at sun.rmi.transport.StreamRemoteCall.exceptionReceivedFromServer(Unknown Source)
        at sun.rmi.transport.StreamRemoteCall.executeCall(Unknown Source)
        at sun.rmi.server.UnicastRef.invoke(Unknown Source)
        at java.rmi.server.RemoteObjectInvocationHandler.invokeRemoteMethod(Unknown Source)
        at java.rmi.server.RemoteObjectInvocationHandler.invoke(Unknown Source)
        at $Proxy0.executeTask(Unknown Source)
        at org.examples.rmi.client.ComputePi.main(ComputePi.java:38)
Caused by: java.rmi.UnmarshalException: error unmarshalling arguments; nested exception is:
        java.lang.ClassNotFoundException: org.examples.rmi.client.Pi
        at sun.rmi.server.UnicastServerRef.dispatch(Unknown Source)
        at sun.rmi.transport.Transport$1.run(Unknown Source)
        at sun.rmi.transport.Transport$1.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at sun.rmi.transport.Transport.serviceCall(Unknown Source)
        at sun.rmi.transport.tcp.TCPTransport.handleMessages(Unknown Source)
        at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(Unknown Source)
        at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
        at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.ClassNotFoundException: org.examples.rmi.client.Pi
        at java.net.URLClassLoader$1.run(Unknown Source)
        at java.net.URLClassLoader$1.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at java.lang.Class.forName0(Native Method)
        at java.lang.Class.forName(Unknown Source)
        at sun.rmi.server.LoaderHandler.loadClass(Unknown Source)
        at sun.rmi.server.LoaderHandler.loadClass(Unknown Source)
        at java.rmi.server.RMIClassLoader$2.loadClass(Unknown Source)
        at java.rmi.server.RMIClassLoader.loadClass(Unknown Source)
        at sun.rmi.server.MarshalInputStream.resolveClass(Unknown Source)
        at java.io.ObjectInputStream.readNonProxyDesc(Unknown Source)
        at java.io.ObjectInputStream.readClassDesc(Unknown Source)
        at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
        at java.io.ObjectInputStream.readObject0(Unknown Source)
        at java.io.ObjectInputStream.readObject(Unknown Source)
        at sun.rmi.server.UnicastRef.unmarshalValue(Unknown Source)
        ... 11 more

But if I add the Pi.class file to the server directory:

server
     |
     org\examples\rmi\engine
                           |--> ComputeEngine // server main, implements Compute
     org\examples\rmi\compute
                           |--> Compute       // interface
                           |--> Task          // interface
     org\examples\rmi\client
                           |--> Pi            // same as Pi for client

The program works. My question is, does Pi.class really need to be on the server for my program to work? My understanding is (and please correct me if I'm wrong) that I send an instance of that class to the server, and the server would know what to do with it, i.e. it doesn't care about the implementation. Can someone explain how RMI is working in my case? I really appreciate it. Thanks!

Answer 1

My question is, does Pi.class really need to be on the server for my program to work? My understanding is (and please correct me if I'm wrong) that I send an instance of that class to the server, and the server would know what to do with it, i.e. it doesn't care about the implementation.

You understood correctly. Pi.class doesn't need to be on server when you compile it, but the server does need to download it at runtime! (Pi must be serializable)

The question is: How does a server know where to download the Pi.class when does he need it?

And the answer is: by the value of java.rmi.server.codebase setting provided by the client. The client must set the java.rmi.server.codebase option. You have to say where the Pi.class is. It is a common habit to put a copy of Pi.class in a public directory for deployment. Therefore the complete solution is:

1. The Structure:

   compute.jar
   client\
   |-org\
   |   |-examples\
   |       |-rmi\
   |           |client\
   |               |--> ComputePi     // client main
   |               |--> Pi            // implements Task
   |-deploy\            
   |   |-org\
   |       |-examples\
   |           |-rmi\
   |               |-client\ // directory that will contain the deployment copy of Pi.class
   |--> client.policy  
   
   server\
   |-org\
   |   |-examples\
   |       |-rmi\
   |           |-engine\
   |               |--> ComputeEngine // server main, implements Compute
   |--> server.policy
   

   where compute.jar is a jar file previously created

   cd C:\Users\Public\RMI\
   javac compute\Compute.java compute\Task.java
   jar cvf compute.jar compute\*.class
   

   Did you set correctly the package and import commands in your java files? Because you modified the original structure of the tutorial...

2. Compile the Server:

   C:\Users\Public\RMI\> cd server
   C:\Users\Public\RMI\server> javac -cp ..\compute.jar org\examples\rmi\engine\ComputeEngine.java
   

3. Compile the Client:

   C:\Users\Public\RMI\> cd client
   C:\Users\Public\RMI\client> javac -cp ..\compute.jar org\examples\rmi\client\ComputePi.java org\examples\rmi\client\Pi.java
   

4. Move the Pi.class into the deploy directory

   C:\Users\Public\RMI\> cp client\org\examples\rmi\client\Pi.class client\deploy
   

5. Run the rmi registry. If you are using java 7 set the option -J-Djava.rmi.server.useCodebaseOnly=false, as suggested by muyong

   C:\Users\Public\RMI\> start rmiregistry -J-Djava.rmi.server.useCodebaseOnly=false
   

6. Run the server. If you are using java 7 set the option -J-Djava.rmi.server.useCodebaseOnly=false, as suggested by muyong

   C:\Users\Public\RMI\> cd server
   C:\Users\Public\RMI\server> java -cp .;..\compute.jar 
       -Djava.rmi.server.useCodebaseOnly=false 
       -Djava.rmi.server.codebase=file:/c:/Users/Public/RMI/compute.jar
       -Djava.rmi.server.hostname=127.0.0.1
       -Djava.security.policy=server.policy
       org.examples.rmi.engine.ComputeEngine
   

7. Run the Client. NOTE: watch out the java.rmi.server.codebase setting (REMEMBER the conclusive /)

   C:\Users\Public\RMI\> cd client
   C:\Users\Public\RMI\client> java -cp .;..\compute.jar
       -Djava.rmi.server.codebase=file:/c:/Users/Public/RMI/client/deploy/
       -Djava.security.policy=client.policy
       org.examples.rmi.client.Compute.Pi 127.0.0.1 45
   

Let me know if it works!

P.s. I don't use Windows OS but Linux, I could have made confusion between '\' and '/'

Answer 2

from jdk 1.7 the default value of useCodebaseOnly is true, that mean it wont look for other codebase unless it is in the same directory.

Set this vm argument -Djava.rmi.server.useCodebaseOnly=false for running both server and client and also provide the path to codebase and hostname as well. See below below examples.

Below is my implementation and it's according to my directory structure. For running in windows replace : (colon) with ; (semi-colon).

java -cp classes:classes/compute.jar -Djava.rmi.server.useCodebaseOnly=false -Djava.rmi.server.codebase=url:http://localhost:4800/ -Djava.rmi.server.hostname=localhost -Djava.security.policy=client.policy client.ComputePi localhost 45

Answer 3

I tried this example with two PC in same network. One with Java 1.7.0_40 working as server, another with Java 1.7.0_45 as client. Both PCs are Windows based. I met the same problem raised by denshaotoko.

The solution is:

Server side:

C:\>start rmiregistry -J-Djava.rmi.server.useCodebaseOnly=false

The -J-Djava.rmi.server.useCodebaseOnly option is needed for Java 7 since the default value is true, which means the RMI Registry will not look for other code base except the directory it is started from. Then the next step starting the server will fail. Details see here: http://docs.oracle.com/javase/7/docs/technotes/guides/rmi/enhancements-7.html

C:\>java -cp c:\rmi;c:\rmi\compute.jar -Djava.rmi.server.useCodebaseOnly=false -Djava.rmi.server.codebase=file:/c:/rmi/compute.jar -Djava.rmi.server.hostname=192.168.1.124 -Djava.security.policy=c:\rmi\server.policy engine.ComputeEngine

Again the java.rmi.server.useCodebaseOnly should be set to false. Otherwise the server won't use the codebase provided by the client. Then client side will get the class not found exception. The hostname of 192.168.1.124 is the server's IP address

You should get "ComputeEngine bound"

Client side:

C:\>java -cp c:\rmi;c:\rmi\compute.jar -Djava.rmi.server.codebase=http://54.200.126.244/rmi/ -Djava.security.policy=c:\rmi\client.policy client.ComputePi 192.168.1.124 45 

I trid the file:/ URL but not successful. I think the reason is simple. There're so many security limits that make the server not possible to access a file on the client PC. So I put the Pi.class file on my web server which is at http://54.200.126.244 under rmi directory. My web server use Apache. Any PC can access http://54.200.126.244/rmi/ so the problem is solved cleanly.

Finally, you should be able to start the rmiregistry, the server and the client from any directory using the same commands. Otherwise, some settings may still not be correct even if you can succeed. For example, if you start rmiregistry from the directory contain the "compute" directory (in my case is C:\rmi), the rmiregistry will directly load Compute.class and Task.class from it's starting directory, so the setting of -Djava.rmi.server.codebase=file:/c:/rmi/compute.jar become useless.

Answer 4

You are trying to send a serialized object of a class that is unknown to the server.

When you execute:

  Pi task = new Pi(Integer.parseInt(args[1]));
  BigDecimal pi = comp.executeTask(task);

The server doesn't really know what is Pi. And since the Pi class is a part of your API, it should be loaded on server, too.

When I have an application that needs to execute something remotely, using for example RMI, Spring Remoting or similar, I divide my project in 3 projects: API, Server and Client. The API project will have all interfaces and model classes relevant to the functionality (this project will result in a jar, and is more or less like your computer JAR). The server will import the API JAR, will implement the interfaces and make the service available through an Remote layer (like you did with your server), and the client as you did with your client.

When you work with serialization, the class itself must be known by both sides. What is then transferred is the state of the objects in order to rebuild it on the other side.

Serialization is the mechanism used by RMI to pass objects between JVMs, either as arguments in a method invocation from a client to a server or as return values from a method invocation.

A bit of Serialization on RMI By William Grosso (October 2001). And here a bit more info.