java死锁

java 死锁产生的四个必要条件：

• 1、互斥使用，即当资源被一个线程使用(占有)时，别的线程不能使用
• 2、不可抢占，资源请求者不能强制从资源占有者手中夺取资源，资源只能由资源占有者主动释放。
• 3、请求和保持，即当资源请求者在请求其他的资源的同时保持对原有资源的占有。
• 4、循环等待，即存在一个等待队列：P1占有P2的资源，P2占有P3的资源，P3占有P1的资源。这样就形成了一个等待环路。

package com.thread;

import java.util.Date;
import java.util.concurrent.TimeUnit;

/**
 * 
 * @author zy
 *
 */
public class LockTest {
    public static void main(String[] args) {
        String o1 = "o1";
        String o2 = "o2";
        String o3 = "o3";

　　   　　new Thread(new LockA(o1, o2), "t1").start();
　　　   　new Thread(new LockA(o2, o1), "t2").start();
　　　   　//new Thread(new LockA(o2, o3), "t2").start();
　　　   　// new Thread(new LockA(o3, o1), "t3").start();

    }
}

class LockA implements Runnable {
    private String o1;
    private String o2;

    public LockA(String o1, String o2) {
        this.o1 = o1;
        this.o2 = o2;
    }

    public void run() {
        try {
            String name = Thread.currentThread().getName();
            System.out.println(new Date().toString() + " " + name + "   " + " acquiring lock on " + o1);
            synchronized (o1) {
                System.out.println(new Date().toString() + " " + name + "   " + " lock on " + o1);
                TimeUnit.MILLISECONDS.sleep(300);
                System.out.println(new Date().toString() + " " + name + "   " + " acquiring lock on " + o2);

                synchronized (o2) {
                    System.out.println(new Date().toString() + " " + name + "   " + " lock on " + o2);
                    TimeUnit.MILLISECONDS.sleep(300);
                }
                System.out.println(new Date().toString() + " " + name + " released lock on " + o2);
            }
            System.out.println(new Date().toString() + " " + name + " released lock on " + o1);
            System.out.println(new Date().toString() + " " + name + " finished execution.");
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

运行结果为：

Fri May 05 09:53:55 CST 2017 t2 acquiring lock on o2
Fri May 05 09:53:55 CST 2017 t2 lock on o2
Fri May 05 09:53:55 CST 2017 t1 acquiring lock on o1
Fri May 05 09:53:55 CST 2017 t1 lock on o1
Fri May 05 09:53:55 CST 2017 t1 acquiring lock on o2
Fri May 05 09:53:55 CST 2017 t2 acquiring lock on o1

这是两个线程死锁的时候，也可以让多个线程相互之间关联形成死锁，将注释去掉，再把第二个线程注释掉，就有三个资源相互关联的线程，运行结果:

Fri May 05 10:06:07 CST 2017 t3 acquiring lock on o3
Fri May 05 10:06:07 CST 2017 t3 lock on o3
Fri May 05 10:06:07 CST 2017 t1 acquiring lock on o1
Fri May 05 10:06:07 CST 2017 t1 lock on o1
Fri May 05 10:06:07 CST 2017 t2 acquiring lock on o2
Fri May 05 10:06:07 CST 2017 t2 lock on o2
Fri May 05 10:06:07 CST 2017 t3 acquiring lock on o1
Fri May 05 10:06:07 CST 2017 t2 acquiring lock on o3
Fri May 05 10:06:07 CST 2017 t1 acquiring lock on o2