Java多线程:线程死锁

发生死锁的原因一般是两个对象的锁相互等待造成的。
下面用一个实例来构造这种情况：

package basic.e_deadlock;import org.apache.log4j.Logger;public class TestDeadLock {public static void main(String[] args) {DeadlockRisk dead = new DeadlockRisk();MyThread t1 = new MyThread(dead, 1, 2, "线程1");MyThread t2 = new MyThread(dead, 3, 4, "线程2");MyThread t3 = new MyThread(dead, 5, 6, "线程3");MyThread t4 = new MyThread(dead, 7, 8, "线程4");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, String threadName) {this.dead = dead;this.a = a;this.b = b;this.setName(threadName);}@Overridepublic void run() {dead.read();dead.write(a, b);}}class DeadlockRisk {private static Logger logger = Logger.getLogger(DeadlockRisk.class);private static class Resource {public int value;}private Resource resourceA = new Resource();private Resource resourceB = new Resource();public void read() {logger.debug("===========read  begin===========");synchronized (resourceA) {logger.debug("read():" + Thread.currentThread().getName() + "获取了resourceA 的锁！");synchronized (resourceB) {logger.debug("read():" + Thread.currentThread().getName() + "获取了resourceB 的锁！");}}logger.debug("===========read  end=============");}public void write(int a, int b) {logger.debug("===========write begin===========");synchronized (resourceB) {logger.debug("write():" + Thread.currentThread().getName() + "获取了resourceB 的锁！");synchronized (resourceA) {logger.debug("write():" + Thread.currentThread().getName() + "获取了resourceA 的锁！");resourceA.value = a;resourceB.value = b;}}logger.debug("===========write end=============");}}

执行结果：

0    [线程1] DEBUG basic.e_deadlock.DeadlockRisk - ===========read  begin===========0    [线程2] DEBUG basic.e_deadlock.DeadlockRisk - ===========read  begin===========0    [线程4] DEBUG basic.e_deadlock.DeadlockRisk - ===========read  begin===========0    [线程3] DEBUG basic.e_deadlock.DeadlockRisk - ===========read  begin===========0    [线程1] DEBUG basic.e_deadlock.DeadlockRisk - read():线程1获取了resourceA 的锁！0    [线程1] DEBUG basic.e_deadlock.DeadlockRisk - read():线程1获取了resourceB 的锁！0    [线程1] DEBUG basic.e_deadlock.DeadlockRisk - ===========read  end=============0    [线程2] DEBUG basic.e_deadlock.DeadlockRisk - read():线程2获取了resourceA 的锁！0    [线程1] DEBUG basic.e_deadlock.DeadlockRisk - ===========write being===========1    [线程2] DEBUG basic.e_deadlock.DeadlockRisk - read():线程2获取了resourceB 的锁！1    [线程2] DEBUG basic.e_deadlock.DeadlockRisk - ===========read  end=============1    [线程2] DEBUG basic.e_deadlock.DeadlockRisk - ===========write being===========1    [线程1] DEBUG basic.e_deadlock.DeadlockRisk - write():线程1获取了resourceB 的锁！1    [线程4] DEBUG basic.e_deadlock.DeadlockRisk - read():线程4获取了resourceA 的锁！

注意：此时线程1在等待resourceB的资源，线程2在等待resourceA的资源。两个线程在相互等待，出现死锁。