高并发编程：8、多线程死锁分析

一、概述

• 甲有两个儿子A和B，乙有两个儿子C和D。
• 甲有一把锁钥匙在A儿子身上，乙有一把锁钥匙在C儿子身上。
• A认识D，需要找D帮忙拿一下东西，但是D没有钥匙，钥匙在大哥C身上，等待大哥C办完事才能拿到钥匙。
• C认识B，需要找B帮忙拿一下东西，但B没有钥匙，钥匙在大哥A身上，等待大哥A办完事才能拿到钥匙。
• 这样就产生了大家都在等待对方大哥办完事情，但是这事情永远不能完成，这就叫死锁。

二、代码

package com.cfl.thread;

/**
 * 多线程死锁分析，案列介绍
 * @author chenfenli
 *
 */
public class Thread9 {
    public static void main(String[] args) throws Exception {
	final DeadLock deadLock = new DeadLock();
	final OtherServer otherServer = new OtherServer(deadLock);
	deadLock.setOtherServer(otherServer);
	
	new Thread(new Runnable() {
	    public void run() {
		while (true) {
		    deadLock.d1();
		}
	    }
	},"甲").start();
	new Thread(new Runnable() {
	    public void run() {
		while (true) {
		    otherServer.s2();
		}
	    }
	},"乙").start();
    }
    static class DeadLock {	
	private static Object LOCK =  new Object();	
	private OtherServer otherServer;
	public void d1() {
	    System.out.println("A ================== start");
	    synchronized(LOCK) {
		System.out.println("A ================== synchronized");
		otherServer.s1();
		System.out.println("A ================== end");
		}
	    }
	    public void d2() {
		System.out.println("B ================== start");
		synchronized(LOCK) {
		System.out.println("B ================== synchronized");
	    }
        }
	public OtherServer getOtherServer() {
	    return otherServer;
	}
	public void setOtherServer(OtherServer otherServer) {
	    this.otherServer = otherServer;
	}
    }
    static class OtherServer {	
	private final Object LOCK =  new Object();
	private DeadLock deadLock;	
	public OtherServer(DeadLock deadLock) {
	    this.deadLock = deadLock;
	}	
	public void s1() {
	    System.out.println("C ================== start");
	    synchronized(LOCK) {
		System.out.println("C ================== synchronized");
	    }
	}
	public void s2() {
	    System.out.println("D ================== start");
	    synchronized (LOCK) {
	        System.out.println("D ================== synchronized");
		deadLock.d2();
		System.out.println("D ================== end");
	    }
        }
    }
}

.....
D ================== end
A ================== end
A ================== start
A ================== wait...
D ================== start
C ================== start
D ================== end
A ================== end
A ================== start
A ================== wait...
D ================== start
C ================== wait...
B ================== start

• 可以看到随后死锁了，A一直在等待，C也一直在等待。