Java RMI和ClassNotFoundException [英] Java RMI and ClassNotFoundException

问题描述

我刚刚开始学习如何使用RMI，但我有一个问题.我具有以下目录结构:

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

这是 ComputePi 类中的main方法:

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();
}

这是 ComputeEngine 类中的main方法:

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();
}

这是executeTask方法，也在 ComputeEngine 类中:

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

RMI注册表和服务器启动正常.这是服务器的参数:

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

以下是客户端的参数:

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

但是，当我尝试运行客户端时，出现以下异常:

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

但是，如果我将 Pi.class 文件添加到服务器目录:

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

该程序正常运行.我的问题是， Pi.class 是否真的需要在服务器上才能运行我的程序?我的理解是(如果我错了，请纠正我)将此类的实例发送到服务器，服务器将知道如何处理该类，即它不在乎实现.有人可以解释RMI在我的情况下如何工作吗?对此，我真的非常感激.谢谢！

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!

推荐答案

您正在尝试发送服务器未知的类的序列化对象.

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

执行时:

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

服务器真的不知道什么是Pi.而且，由于Pi类是您的API的一部分，因此也应该将其加载到服务器上.

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.

当我有一个需要远程执行某些内容的应用程序时，例如，使用RMI，Spring Remoting或类似的方法，我将项目分为3个项目:API，服务器和客户端. API项目将具有与该功能相关的所有接口和模型类(该项目将产生jar，并且或多或少类似于您的计算机JAR).服务器将导入API JAR，将实现接口并使服务通过Remote层可用(就像您对服务器所做的一样)，并通过客户端对客户端进行访问.

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.

使用序列化时，类双方必须都知道类本身.然后传输的是对象的状态，以便在另一侧重建它.

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.

串行化是RMI在对象之间传递对象的机制 JVM，作为从客户端到客户端的方法调用中的参数 服务器或方法调用的返回值.

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.

关于RMI的一些序列化作者:威廉·格罗索(William Grosso)(2001年10月).并且此处多一点信息.

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

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