在Java中使用Socket與ServerSocket建立客戶機和伺服器時，若採用ObjectInputStream與ObjectOutputStream建立通訊，則需要注意兩個流的順序。否則會發生兩方互相等待導致死鎖。

下面通過一個例子來證明：

伺服器:

package com.gary;

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.net.ServerSocket;
import java.net.Socket;

public class Server implements Runnable {
	private ServerSocket serverSocket;
	private int port;
	private boolean goon;
	private ObjectInputStream ois;
	private ObjectOutputStream oos;
	
	public Server() {
		goon = false;
	}
	

	public Server(int port) {
		this();
		this.port = port;
	}

	public void setPort(int port) {
		this.port = port;
	}
	
	public void stopServer() {
		this.goon = false;
		try {
			this.serverSocket.close();
		} catch (IOException e) {
			e.printStackTrace();
		}
	}
	
	public void startServer() throws Exception {
		if (this.port == 0) {
			// TODO 拋異常
			return;
		}
		this.serverSocket = new ServerSocket(port);
		this.goon = true;
		new Thread(this, "SERVER").start();
	}
	
	
	@Override
	public void run() {
		while(goon) {
			try {
				Socket client = serverSocket.accept();
				ois = new ObjectInputStream(client.getInputStream());
				oos = new ObjectOutputStream(client.getOutputStream());
				oos.writeObject(new String("去死"));
				String str = (String) ois.readObject();
				System.out.println("伺服器收到客戶端[" + client.getInetAddress().getHostAddress() + "]訊息：" + str);
			} catch (IOException e) {
				goon = false;
				e.printStackTrace();
			} catch (ClassNotFoundException e) {
				e.printStackTrace();
			}
		}
		stopServer();
	}
	
}
 

客戶機：

package com.gary;

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.net.Socket;

public class Client {
	private String serverIp;
	private int serverPort;
	
	public Client(String serverIp, int serverPort) {
		this.serverIp = serverIp;
		this.serverPort = serverPort;
	}
	
	private void closeSocket(ObjectInputStream ois, ObjectOutputStream oos, Socket socket) {
		try {
			if (ois != null) {
				ois.close();
			}
		} catch (IOException e) {
			e.printStackTrace();
		} finally {
			ois = null;
		}
		try {
			if (oos != null) {
				oos.close();
			}
		} catch (IOException e) {
			e.printStackTrace();
		} finally {
			oos = null;
		}
		try {
			if (socket != null && !socket.isClosed()) {
				socket.close();
			}
		} catch (IOException e) {
			e.printStackTrace();
		} finally {
			socket = null;
		}
	}	

	public void start() throws Exception {
		Socket socket = new Socket(serverIp, serverPort);
                ObjectInputStream ois = new ObjectInputStream(socket.getInputStream());
		ObjectOutputStream oos = new ObjectOutputStream(socket.getOutputStream());
		oos.writeObject(new String("好的"));
		
		Object result = ois.readObject();
		System.out.println("client收到：" + result);
		closeSocket(ois, oos, socket);
	}
}
 

測試：

package com.gary;

public class TestServer {

	public static void main(String[] args) {
		Server rpcServer = new Server();
		rpcServer.setPort(54189);
		
		try {
			rpcServer.startServer();
		} catch (Exception e) {
			e.printStackTrace();
		}
	}

}

package com.gary;

public class TestClient {
	
	public static void main(String[] args) {
		Client client = new Client("172.21.126.74", 54189);
		try {
			client.start();
		} catch (Exception e) {
			e.printStackTrace();
		}
		
	}
	
}
 

結果

分析原因：

仔細看ObjectInputStream的API

第一段的意思是，建立一個從指定的InputStream讀取的ObjectInputStream，序列化的流的頭是從這個Strem中讀取並驗證的。此構造方法會一直阻塞直到相應的ObjectOutputStream已經寫入並重新整理頭。

所以上述程式碼執行後會都阻塞，如果將建立ObjectInputStream的順序修改成其他的順序，便可正常通訊。