死锁例子

Java产生死锁的一个简单例子

思路是创建两个字符串a和b，再创建两个线程A和B，让每个线程都用synchronized锁住字符串（A先锁a，再去锁b；B先锁b，再锁a），如果A锁住a，B锁住b，A就没办法锁住b，B也没办法锁住a，这时就陷入了死锁。直接贴代码：
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public class DeadLock {
public static String obj1 = “obj1”;
public static String obj2 = “obj2”;
public static void main(String[] args){
Thread a = new Thread(new Lock1());
Thread b = new Thread(new Lock2());
a.start();
b.start();
}
}
class Lock1 implements Runnable{
@Override
public void run(){
try{
System.out.println(“Lock1 running”);
while(true){
synchronized(DeadLock.obj1){
System.out.println(“Lock1 lock obj1”);
Thread.sleep(3000);//获取obj1后先等一会儿，让Lock2有足够的时间锁住obj2
synchronized(DeadLock.obj2){
System.out.println(“Lock1 lock obj2”);
}
}
}
}catch(Exception e){
e.printStackTrace();
}
}
}
class Lock2 implements Runnable{
@Override
public void run(){
try{
System.out.println(“Lock2 running”);
while(true){
synchronized(DeadLock.obj2){
System.out.println(“Lock2 lock obj2”);
Thread.sleep(3000);
synchronized(DeadLock.obj1){
System.out.println(“Lock2 lock obj1”);
}
}
}
}catch(Exception e){
e.printStackTrace();
}
}
}

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运行的结果如图所示：

可以看到，Lock1获取obj1，Lock2获取obj2，但是它们都没有办法再获取另外一个obj，因为它们都在等待对方先释放锁，这时就是死锁。

如果我们只运行Lock1呢？修改一下main函数，把线程b注释掉。
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public class DeadLock {
public static String obj1 = “obj1”;
public static String obj2 = “obj2”;
public static void main(String[] args){
Thread a = new Thread(new Lock1());
//Thread b = new Thread(new Lock2());
a.start();
//b.start();
}
}
class Lock1 implements Runnable{
@Override
public void run(){
try{
System.out.println(“Lock1 running”);
while(true){
synchronized(DeadLock.obj1){
System.out.println(“Lock1 lock obj1”);
Thread.sleep(3000);
synchronized(DeadLock.obj2){
System.out.println(“Lock1 lock obj2”);
}
}
}
}catch(Exception e){
e.printStackTrace();
}
}
}
class Lock2 implements Runnable{
@Override
public void run(){
try{
System.out.println(“Lock2 running”);
while(true){
synchronized(DeadLock.obj2){
System.out.println(“Lock2 lock obj2”);
Thread.sleep(3000);
synchronized(DeadLock.obj1){
System.out.println(“Lock2 lock obj1”);
}
}
}
}catch(Exception e){
e.printStackTrace();
}
}
}

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运行结果为：

由于没有其它线程和Lock1争夺obj1和obj2，Lock1可以不断地循环获取并释放它们，这时没有死锁。