Java多線程:線程死結

發生死結的原因一般是兩個對象的鎖相互等待造成的。
下面用一個執行個體來構造這種情況：

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, "線程1");MyThread t2 = new MyThread(dead, 3, 4, "線程2");MyThread t3 = new MyThread(dead, 5, 6, "線程3");MyThread t4 = new MyThread(dead, 7, 8, "線程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() + "擷取了resourceA 的鎖！");synchronized (resourceB) {logger.debug("read():" + Thread.currentThread().getName() + "擷取了resourceB 的鎖！");}}logger.debug("===========read  end=============");}public void write(int a, int b) {logger.debug("===========write begin===========");synchronized (resourceB) {logger.debug("write():" + Thread.currentThread().getName() + "擷取了resourceB 的鎖！");synchronized (resourceA) {logger.debug("write():" + Thread.currentThread().getName() + "擷取了resourceA 的鎖！");resourceA.value = a;resourceB.value = b;}}logger.debug("===========write end=============");}}

執行結果：

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

注意：此時線程1在等待resourceB的資源，線程2在等待resourceA的資源。兩個線程在相互等待，出現死結。