我们如何避免在Java中发生死锁？

在Java中，死锁是一种编程情况，其中两个或多个线程被永久阻塞。死锁条件至少会发生在两个线程和两个或多个资源之间。

如何避免死锁

• 避免嵌套锁：死锁主要发生在给多个线程提供锁时。如果我们已经给一个线程提供了锁，就避免给其他线程提供锁。
• 避免不必要的锁：我们只需要给那些必要的成员提供锁。不必要地提供锁可能导致死锁。
• 使用Thread.join()：当一个线程在等待另一个线程完成时，会出现死锁条件。如果出现这种情况，我们可以使用Thread.join()，并设置最长执行时间。

示例

public class DeadlockTest {
   public static void main(String[] args) throws InterruptedException {
      Object obj1 = new Object();
      Object obj2 = new Object();
      Object obj3 = new Object();
      Thread t1 = new Thread(new SyncThread(obj1, obj2), "t1");
      Thread t2 = new Thread(new SyncThread(obj2, obj3), "t2");
      t1.start();
      Thread.sleep(2000);
      t2.start();
      Thread.sleep(2000);
   }
}
class SyncThread implements Runnable {
   private Object obj1;
   private Object obj2;
   public SyncThread(Object o1, Object o2){
      this.obj1=o1;
      this.obj2=o2;
   }
   @Override
   public void run() {
      String name = Thread.currentThread().getName();
      System.out.println(name + " acquiring lock on " + obj1);
      <strong>synchronized (obj1)</strong> {
         System.out.println(name + " acquired lock on " + obj1);
         work();
      }
      System.out.println(name + " released lock on " + obj1);
      System.out.println(name + " acquiring lock on " + obj2);
      <strong>synchronized (obj2)</strong> {
         System.out.println(name + " acquired lock on " + obj2);
         work();
      }
      System.out.println(name + " released lock on " + obj2);
      System.out.println(name + " finished execution.");
   }
   private void work() {
      try {
         Thread.sleep(5000);
      } catch (InterruptedException ie) {
         ie.printStackTrace();
      }
   }
}

输出

t1 acquiring lock on java.lang.Object@917d8d4
t1 acquired lock on java.lang.Object@917d8d4
t2 acquiring lock on java.lang.Object@5c4b42fb
t2 acquired lock on java.lang.Object@5c4b42fb
t1 released lock on java.lang.Object@917d8d4
t1 acquiring lock on java.lang.Object@5c4b42fb
t1 acquired lock on java.lang.Object@5c4b42fb
t2 released lock on java.lang.Object@5c4b42fb
t2 acquiring lock on java.lang.Object@528cb702
t2 acquired lock on java.lang.Object@528cb702
t1 released lock on java.lang.Object@5c4b42fb
t2 released lock on java.lang.Object@528cb702
t1 finished execution.
t2 finished execution.

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