无涯教程-Java - 处理线程死锁函数死锁描述了一种情况，其中两个或多个线程永远被阻塞，互相等待，当多个线程需要相

死锁描述了一种情况，其中两个或多个线程永远被阻塞，互相等待，当多个线程需要相同的锁但以不同的顺序获得它们时，就会发生死锁。 Java多线程程序可能会遇到死锁情况，因为 synchronized 关键字会导致正在执行的线程在等待与指定对象关联的锁时被阻塞。这是一个示例。

public class TestThread {
   public static Object Lock1 = new Object();
   public static Object Lock2 = new Object();
public static void main(String args[]) {
ThreadDemo1 T1 = new ThreadDemo1();
ThreadDemo2 T2 = new ThreadDemo2();
T1.start();
T2.start();
}
private static class ThreadDemo1 extends Thread {
public void run() {
synchronized (Lock1) {
System.out.println("Thread 1: Holding lock 1...");
        </span><span class="kwd">try</span><span class="pln"> </span><span class="pun">{</span><span class="pln"> </span><span class="typ">Thread</span><span class="pun">.</span><span class="pln">sleep</span><span class="pun">(</span><span class="lit">10</span><span class="pun">);</span><span class="pln"> </span><span class="pun">}</span><span class="pln">
        </span><span class="kwd">catch</span><span class="pln"> </span><span class="pun">(</span><span class="typ">InterruptedException</span><span class="pln"> e</span><span class="pun">)</span><span class="pln"> </span><span class="pun">{}</span><span class="pln">
        </span><span class="typ">System</span><span class="pun">.</span><span class="kwd">out</span><span class="pun">.</span><span class="pln">println</span><span class="pun">(</span><span class="str">"Thread 1: Waiting for lock 2..."</span><span class="pun">);</span><span class="pln">
        
        </span><span class="kwd">synchronized</span><span class="pln"> </span><span class="pun">(</span><span class="typ">Lock2</span><span class="pun">)</span><span class="pln"> </span><span class="pun">{</span><span class="pln">
           </span><span class="typ">System</span><span class="pun">.</span><span class="kwd">out</span><span class="pun">.</span><span class="pln">println</span><span class="pun">(</span><span class="str">"Thread 1: Holding lock 1 &amp; 2..."</span><span class="pun">);</span><span class="pln">
        </span><span class="pun">}</span><span class="pln">
     </span><span class="pun">}</span><span class="pln">
  </span><span class="pun">}</span><span class="pln">

}
private static class ThreadDemo2 extends Thread {
public void run() {
synchronized (Lock2) {
System.out.println("Thread 2: Holding lock 2...");
        </span><span class="kwd">try</span><span class="pln"> </span><span class="pun">{</span><span class="pln"> </span><span class="typ">Thread</span><span class="pun">.</span><span class="pln">sleep</span><span class="pun">(</span><span class="lit">10</span><span class="pun">);</span><span class="pln"> </span><span class="pun">}</span><span class="pln">
        </span><span class="kwd">catch</span><span class="pln"> </span><span class="pun">(</span><span class="typ">InterruptedException</span><span class="pln"> e</span><span class="pun">)</span><span class="pln"> </span><span class="pun">{}</span><span class="pln">
        </span><span class="typ">System</span><span class="pun">.</span><span class="kwd">out</span><span class="pun">.</span><span class="pln">println</span><span class="pun">(</span><span class="str">"Thread 2: Waiting for lock 1..."</span><span class="pun">);</span><span class="pln">
        
        </span><span class="kwd">synchronized</span><span class="pln"> </span><span class="pun">(</span><span class="typ">Lock1</span><span class="pun">)</span><span class="pln"> </span><span class="pun">{</span><span class="pln">
           </span><span class="typ">System</span><span class="pun">.</span><span class="kwd">out</span><span class="pun">.</span><span class="pln">println</span><span class="pun">(</span><span class="str">"Thread 2: Holding lock 1 &amp; 2..."</span><span class="pun">);</span><span class="pln">
        </span><span class="pun">}</span><span class="pln">
     </span><span class="pun">}</span><span class="pln">
  </span><span class="pun">}</span><span class="pln">

}
}

编译并执行上述程序时，发现死锁情况，以下是该程序产生的输出:

Thread 1: Holding lock 1...
Thread 2: Holding lock 2...
Thread 1: Waiting for lock 2...
Thread 2: Waiting for lock 1...

上面的程序将永远挂起，因为两个线程都无法继续进行并等待彼此释放锁，因此可以通过按CTRL + C退出程序。

死锁解决示例

让无涯教程更改同一程序的锁定和运行顺序，以查看两个线程是否仍然彼此等待-

public class TestThread {
   public static Object Lock1 = new Object();
   public static Object Lock2 = new Object();
public static void main(String args[]) {
ThreadDemo1 T1 = new ThreadDemo1();
ThreadDemo2 T2 = new ThreadDemo2();
T1.start();
T2.start();
}
private static class ThreadDemo1 extends Thread {
public void run() {
synchronized (Lock1) {
System.out.println("Thread 1: Holding lock 1...");
        </span><span class="kwd">try</span><span class="pln"> </span><span class="pun">{</span><span class="pln">
           </span><span class="typ">Thread</span><span class="pun">.</span><span class="pln">sleep</span><span class="pun">(</span><span class="lit">10</span><span class="pun">);</span><span class="pln">
        </span><span class="pun">}</span><span class="pln"> </span><span class="kwd">catch</span><span class="pln"> </span><span class="pun">(</span><span class="typ">InterruptedException</span><span class="pln"> e</span><span class="pun">)</span><span class="pln"> </span><span class="pun">{}</span><span class="pln">
        </span><span class="typ">System</span><span class="pun">.</span><span class="kwd">out</span><span class="pun">.</span><span class="pln">println</span><span class="pun">(</span><span class="str">"Thread 1: Waiting for lock 2..."</span><span class="pun">);</span><span class="pln">
        
        </span><span class="kwd">synchronized</span><span class="pln"> </span><span class="pun">(</span><span class="typ">Lock2</span><span class="pun">)</span><span class="pln"> </span><span class="pun">{</span><span class="pln">
           </span><span class="typ">System</span><span class="pun">.</span><span class="kwd">out</span><span class="pun">.</span><span class="pln">println</span><span class="pun">(</span><span class="str">"Thread 1: Holding lock 1 &amp; 2..."</span><span class="pun">);</span><span class="pln">
        </span><span class="pun">}</span><span class="pln">
     </span><span class="pun">}</span><span class="pln">
  </span><span class="pun">}</span><span class="pln">

}
private static class ThreadDemo2 extends Thread {
public void run() {
synchronized (Lock1) {
System.out.println("Thread 2: Holding lock 1...");
        </span><span class="kwd">try</span><span class="pln"> </span><span class="pun">{</span><span class="pln">
           </span><span class="typ">Thread</span><span class="pun">.</span><span class="pln">sleep</span><span class="pun">(</span><span class="lit">10</span><span class="pun">);</span><span class="pln">
        </span><span class="pun">}</span><span class="pln"> </span><span class="kwd">catch</span><span class="pln"> </span><span class="pun">(</span><span class="typ">InterruptedException</span><span class="pln"> e</span><span class="pun">)</span><span class="pln"> </span><span class="pun">{}</span><span class="pln">
        </span><span class="typ">System</span><span class="pun">.</span><span class="kwd">out</span><span class="pun">.</span><span class="pln">println</span><span class="pun">(</span><span class="str">"Thread 2: Waiting for lock 2..."</span><span class="pun">);</span><span class="pln">
        
        </span><span class="kwd">synchronized</span><span class="pln"> </span><span class="pun">(</span><span class="typ">Lock2</span><span class="pun">)</span><span class="pln"> </span><span class="pun">{</span><span class="pln">
           </span><span class="typ">System</span><span class="pun">.</span><span class="kwd">out</span><span class="pun">.</span><span class="pln">println</span><span class="pun">(</span><span class="str">"Thread 2: Holding lock 1 &amp; 2..."</span><span class="pun">);</span><span class="pln">
        </span><span class="pun">}</span><span class="pln">
     </span><span class="pun">}</span><span class="pln">
  </span><span class="pun">}</span><span class="pln">

}
}

因此，只需更改锁的顺序即可防止程序陷入死锁情况，并完成以下输出:

Thread 1: Holding lock 1...
Thread 1: Waiting for lock 2...
Thread 1: Holding lock 1 & 2...
Thread 2: Holding lock 1...
Thread 2: Waiting for lock 2...
Thread 2: Holding lock 1 & 2...

无涯教程-Java - 处理线程死锁函数

死锁解决示例

参考链接