死锁

在实际编程中，死锁虽然不常见，但是如果遇到死锁便是致命的。接下来了解一下《操作系统》对于死锁产生条件的描述。

死锁产生的条件：

1. 互斥条件：所谓互斥就是进程在某一时间内独占资源。

2. 请求与保持条件：一个进程因请求资源而阻塞时，对已获得的资源保持不放。

3. 不剥夺条件:进程已获得资源，在末使用完之前，不能强行剥夺。

4. 循环等待条件:若干进程之间形成一种头尾相接的循环等待资源关系。

死锁的例子

public class Test { 
        public static void main(String[] args) { 
                DeadlockRisk dead = new DeadlockRisk(); 
                MyThread t1 = new MyThread(dead, 1, 2); 
                MyThread t2 = new MyThread(dead, 3, 4); 
                MyThread t3 = new MyThread(dead, 5, 6); 
                MyThread t4 = new MyThread(dead, 7, 8); 

                t1.start(); 
                t2.start(); 
                t3.start(); 
                t4.start(); 
        } 

} 

class MyThread extends Thread { 
        private DeadlockRisk dead; 
        private int a, b; 


        MyThread(DeadlockRisk dead, int a, int b) { 
                this.dead = dead; 
                this.a = a; 
                this.b = b; 
        } 

        @Override 
        public void run() { 
                dead.read(); 
                dead.write(a, b); 
        } 
} 

class DeadlockRisk { 
        private static class Resource { 
                public int value; 
        } 

        private Resource resourceA = new Resource(); 
        private Resource resourceB = new Resource(); 

        public int read() { 
                synchronized (resourceA) { 
                        System.out.println("read():" + Thread.currentThread().getName() + "获取了resourceA的锁！"); 
                        synchronized (resourceB) { 
                                System.out.println("read():" + Thread.currentThread().getName() + "获取了resourceB的锁！"); 
                                return resourceB.value + resourceA.value; 
                        } 
                } 
        } 

        public void write(int a, int b) { 
                synchronized (resourceB) { 
                        System.out.println("write():" + Thread.currentThread().getName() + "获取了resourceA的锁！"); 
                        synchronized (resourceA) { 
                                System.out.println("write():" + Thread.currentThread().getName() + "获取了resourceB的锁！"); 
                                resourceA.value = a; 
                                resourceB.value = b; 
                        } 
                } 
        } 
}
／／运行结果
read():Thread-0获取了resourceA的锁！
read():Thread-0获取了resourceB的锁！
write():Thread-0获取了resourceA的锁！
read():Thread-3获取了resourceA的锁！

获取线程dump

1. jps获得当前Java虚拟机进程的pid
   ➜ ~ jps
   657
   2564 Jps
   2525 AppMain
   2526 Launcher

2. jstack打印堆栈
   Found one Java-level deadlock:
   =============================
   "Thread-3":
   waiting to lock monitor 0x00007fd5ca80c368 (object 0x00000007d56d1550, a deadlock.DeadlockRisk$Resource),
   which is held by "Thread-0"
   "Thread-0":
   waiting to lock monitor 0x00007fd5ca80ed58 (object 0x00000007d56d1540, a deadlock.DeadlockRisk$Resource),
   which is held by "Thread-3"

    Java stack information for the threads listed above:
    ===================================================
    "Thread-3":
            at deadlock.DeadlockRisk.read(Test.java:52)
            - waiting to lock <0x00000007d56d1550> (a deadlock.DeadlockRisk$Resource)
            - locked <0x00000007d56d1540> (a deadlock.DeadlockRisk$Resource)
            at deadlock.MyThread.run(Test.java:35)
    "Thread-0":
            at deadlock.DeadlockRisk.write(Test.java:62)
            - waiting to lock <0x00000007d56d1540> (a deadlock.DeadlockRisk$Resource)
            - locked <0x00000007d56d1550> (a deadlock.DeadlockRisk$Resource)
            at deadlock.MyThread.run(Test.java:36)
   
    Found 1 deadlock.
   

更多关于线程dump可以参考：http://my.oschina.net/u/161458/blog/266313

如何避免死锁

1. 避免一个线程同时获取多个锁。

2. 避免一个线程在锁内同时占用多个资源，尽量保证每个锁只占用一个资源。

3. 尝试使用定时锁，使用lock.tryLock（timeout）来替代使用内部锁机制。

4. 对于数据库锁，加锁和解锁必须在一个数据库连接里，否则会出现解锁失败的情况。

活锁

活锁：马路中间有条小桥，只能容纳一辆车经过，桥两头开来两辆车A和B，A比较礼貌，示意B先过，B也比较礼貌，示意A先过，结果两人一直谦让谁也过不去。