Java中的wait，notify和notifyAll

￡神魔★判官ぃ 2022-11-05 00:56 114阅读 0赞

# 资源锁 #

在Java中，如果一个资源被上了锁，则其他线程必须要等待该资源被释放后才能使用。如下：

String student = "test";
    Runnable runnable = () -> { 
        synchronized (student) { 
            System.out.println(Thread.currentThread().getName() + "start");
            try { 
                Thread.sleep(1000);
            } catch (Exception e) { 
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName() + "end");
        }
    };
    Thread thread1 = new Thread(runnable);
    Thread thread2 = new Thread(runnable);
    Thread thread3 = new Thread(runnable);
    thread1.start();
    thread2.start();
    thread3.start();

该代码的输出结果为：

Thread-0start
    Thread-0end
    Thread-1start
    Thread-1end
    Thread-2start
    Thread-2end

因为线程0把student对象给占用了，所以其他线程在占用该对象之前，必须等待该对象的释放。

# wait的作用 #

wait可以阻塞自己，然后暂时释放自己对资源的占用，等待其他对象调用notify来唤醒他，然后再次对资源进行占用。例如

输出结果为：

Thread-0start
    Thread-1start
    Thread-2start
    
    // 阻塞在这个地方

0线程在输出start后，调用了wait方法暂时释放了student的资源锁。所以线程1就可以进入sychronized代码。但是由于没有线程调用notify，所以代码最终阻塞了。

# notify #

如果调用了notify，则会唤醒一个wait中的资源。网上有说是随机，但是经测试，应该是唤醒最先被阻塞的资源。

String student = "test";
      Runnable runnable = () -> { 
          synchronized (student) { 
              System.out.println(Thread.currentThread().getName() + "start");
              try { 
                  student.wait();  // 暂时释放资源
              } catch (InterruptedException e) { 
                  e.printStackTrace();
              }
              System.out.println(Thread.currentThread().getName() + "end");
          }
      };
    
      Runnable notifyR = () -> { 
          synchronized (student) { 
              student.notify(); // 唤醒一个结果
          }
      };
      Thread thread1 = new Thread(runnable);
      Thread.sleep(1000);
      Thread thread2 = new Thread(runnable);
      Thread.sleep(1000);
      Thread thread3 = new Thread(runnable);
      thread1.start();
      thread2.start();
      thread3.start();
    
      Thread notifyT = new Thread(notifyR);
      Thread.sleep(1000);
      notifyT.start();

输出结果为：

Thread-0start
    Thread-1start
    Thread-2start
    Thread-0end
    //在这里阻塞了

因为只调用了一次notify，所以只唤醒了一个wait，1和2线程还在等待notify来唤醒他们

# notifyAll #

唤醒所有wait资源。优先唤醒最后阻塞的线程。比如，阻塞顺序为 A-B-C，则唤醒时的顺序为 C-B-A。且C线程释放资源后，B线程才会被唤醒。

String student = "test";
      Runnable runnable = () -> { 
          synchronized (student) { 
              System.out.println(Thread.currentThread().getName() + "start");
              try { 
                  student.wait();  // 暂时释放资源
                  Thread.sleep(1000);
              } catch (InterruptedException e) { 
                  e.printStackTrace();
              }
              System.out.println(Thread.currentThread().getName() + "end");
          }
      };
    
      Runnable notifyR = () -> { 
          synchronized (student) { 
              student.notifyAll(); // 唤醒所有wait
          }
      };
      Thread thread1 = new Thread(runnable);
      Thread.sleep(1000);
      Thread thread2 = new Thread(runnable);
      Thread.sleep(1000);
      Thread thread3 = new Thread(runnable);
      thread1.start();
      thread2.start();
      thread3.start();
    
      Thread notifyT = new Thread(notifyR);
      Thread.sleep(1000);
      notifyT.start();

输出结果为：

Thread-0start
    Thread-1start
    Thread-2start
    Thread-2end
    Thread-1end
    Thread-0end
    // 到这里程序结束，没有阻塞

因为这次调用的notifyAll，所有的wait都会被唤醒（不是同时唤醒，而是先等前面的线程把资源锁释放后，才会被唤醒）。所以不会有wait被无限阻塞。