如何远程关闭Java RMI服务器

Question

我有一个非常简单的Java RMI服务器，看起来像下面这样：

    import java.rmi.*;
    import java.rmi.server.*;

    public class CalculatorImpl extends UnicastRemoteObject implements Calculator {

        private String mServerName;

        public CalculatorImpl(String serverName) throws RemoteException
        {
            super();
            mServerName = serverName;
        }

        public int calculate(int op1, int op2) throws RemoteException
        {
            return op1 + op2;
        }

        public void exit() throws RemoteException
        {
            try{
                Naming.unbind(mServerName);
                System.out.println("CalculatorServer exiting.");
            }
            catch(Exception e){}

            System.exit(1);
        }

        public static void main(String args[]) throws Exception
        {
            System.out.println("Initializing CalculatorServer.");

            String serverObjName = "rmi://localhost/Calculator";

            Calculator calc = new CalculatorImpl(serverObjName);

            Naming.rebind(serverObjName, calc);

            System.out.println("CalculatorServer running.");
        }
}

当我调用exit方法System.exit(1)时，会抛出以下异常：

CalculatorServer exiting.
java.rmi.UnmarshalException: Error unmarshaling return header; nested exception is:
        java.io.EOFException
        at sun.rmi.transport.StreamRemoteCall.executeCall(StreamRemoteCall.java:203)
        at sun.rmi.server.UnicastRef.invoke(UnicastRef.java:126)
        at CalculatorImpl_Stub.exit(Unknown Source)
        at CalculatorClient.<init>(CalculatorClient.java:17)
        at CalculatorClient.main(CalculatorClient.java:29)
Caused by: java.io.EOFException
        at java.io.DataInputStream.readByte(DataInputStream.java:243)
        at sun.rmi.transport.StreamRemoteCall.executeCall(StreamRemoteCall.java:189)
        ... 4 more
[2]+  Exit 1                  java CalculatorImpl

在这个方法中，我做错了什么？

- Clayton

Answer 1

如果有人遇到类似的问题，我自己找到了答案。这是我的exit()方法：

public void exit() throws RemoteException
{
    try{
        // Unregister ourself
        Naming.unbind(mServerName);

        // Unexport; this will also remove us from the RMI runtime
        UnicastRemoteObject.unexportObject(this, true);

        System.out.println("CalculatorServer exiting.");
    }
    catch(Exception e){}
}

- Clayton

---

感谢提供unexportObject示例。 - Philip Durbin

Answer 2

实际上，仅取消注册并立即调用System.exit()并不能进行干净的关闭。它基本上在通知客户端消息已完成之前就中断了连接。可行的方法是启动一个小线程来关闭系统，如下所示：

public void quit() throws RemoteException {
  System.out.println("quit");
  Registry registry = LocateRegistry.getRegistry();
  try {
    registry.unbind(_SERVICENAME);
    UnicastRemoteObject.unexportObject(this, false);
  } catch (NotBoundException e) {
    throw new RemoteException("Could not unregister service, quiting anyway", e);
  }

  new Thread() {
    @Override
    public void run() {
      System.out.print("Shutting down...");
      try {
        sleep(2000);
      } catch (InterruptedException e) {
        // I don't care
      }
      System.out.println("done");
      System.exit(0);
    }

  }.start();
}

线程需要能够在将来发生某些事情的同时仍然从退出方法返回。

- Paul de Vrieze

---

整个线程部分是个好的想法，但并非必需。你只需要将UnicastRemoteObject.unexportObject(this, false) 改为 UnicastRemoteObject.unexportObject(this, true) 就可以了。我刚刚尝试了同样的问题。 - Anonymous

我曾经遇到过同样的问题，设置unexportObject的第二个参数并没有帮助，但在一个线程中运行System.exit确实有所帮助。 - simon