Java 线程同步（wait、notify、notifyAll）

一、方法介绍

1、void wait()

使得线程进入等待状态，直到它被其他线程通过notify()或者notifyAll唤醒。该方法只能在同步方法（void synchronized methodName( args... ){ ... }）或者 “同步块内部“ （synchronized(object){ ... }）被调用。如果当前线程不是锁的持有者，该方法会抛出一个IllegalMonitorStateException异常。

即如果锁住的是object，那么你只能调用object的wait()方法。

2、void notify()

随机选择一个在该对象上调用wait方法的线程，解除其阻塞状态。该方法只能在同步方法（void synchronized methodName( args... ){ ... }）或者 “同步块内部“ （synchronized(object){ ... }）被调用。如果当前线程不是锁的持有者，该方法会抛出一个IllegalMonitorStateException异常。

3、void notifyAll()

解除所有那些在该对象上调用wait方法的线程的阻塞状态。该方法只能在同步方法（void synchronized methodName( args... ){ ... }）或者 “同步块内部“ （synchronized(object){ ... }）被调用。如果当前线程不是锁的持有者，该方法会抛出一个IllegalMonitorStateException异常。

二、范例讲解（ 注意测试类A只能声明为外部类，下面代码在 class A 和 main方法 之间 省略了MainClass { ... } ）

1、“同步块”测试

（1）加synchronized(){}

      class A{}
      public static void main(String[] args) {
     	A object = new A();
      	System.out.println("Main Thread Id = " + Thread.currentThread().getId());
     	synchronized (object) {
     		try {
      			System.out.println("This Thread Id = " + Thread.currentThread().getId());
      			object.wait();
      		} catch (InterruptedException e) {
      			</span>e.printStackTrace();
      		}
      	}
      }
  
 

输出

（2）不加synchronized(){}

      class A{}
      public static void main(String[] args) {
     	A object = new A();
      	System.out.println("Main Thread Id = " + Thread.currentThread().getId());
     	//synchronized (object) {
     		try {
      			</span>System.out.println("This Thread Id = " + Thread.currentThread().getId());
      			</span>object.wait();
      		} catch (InterruptedException e) {
      			</span>e.printStackTrace();
      		}
     	//}
      }
  
 

2、同步方法测试

（1）synchronized方法前缀

      class A
      {
     	public synchronized void test() throws InterruptedException {
     		//synchronized (this) {
      			System.out.println("Object Thread Id = " + Thread.currentThread().getId());
      			wait();
     		//}
      	};
      }
      public static void main(String[] args) {
     	A object = new A();
      	System.out.println("Main Thread Id = " + Thread.currentThread().getId());
     	//synchronized (object) {
     		try {
      			System.out.println("This Thread Id = " + Thread.currentThread().getId());
     			//object.wait();
      			object.test();
      		} catch (InterruptedException e) {
      			e.printStackTrace();
      		}
     	//}
      }
  
 

输出

（2）不synchronized方法前缀

      class A
      {
     	public void test() throws InterruptedException {
     		//synchronized (this) {
      			System.out.println("Object Thread Id = " + Thread.currentThread().getId());
      			wait();
     		//}
      	};
      }
      public static void main(String[] args) {
     	A object = new A();
      	System.out.println("Main Thread Id = " + Thread.currentThread().getId());
     	//synchronized (object) {
     		try {
      			System.out.println("This Thread Id = " + Thread.currentThread().getId());
     			//object.wait();
      			object.test();
      		} catch (InterruptedException e) {
      			e.printStackTrace();
      		}
     	//}
      }
  
 

输出

3、使用其它锁对象测试

      class A{}
      public static void main(String[] args) {
     	A object = new A();
     	Object object2 = new Object();
      	System.out.println("Main Thread Id = " + Thread.currentThread().getId());
     	synchronized (object) {
     	//synchronized (object2) {
     		try {
      			System.out.println("This Thread Id = " + Thread.currentThread().getId());
      			object.wait();
     			//object.test();
      		} catch (InterruptedException e) {
      			e.printStackTrace();
      		}
      	}
      }
  
 

输出：

4、wait、notify进行线程同步

（1）一（wait）对一（notify）模式

      class A{}
      public static void main(String[] args) {
     	final A object = new A();
      	System.out.println("main thread id = " + Thread.currentThread().getId());
     	new Thread(new Runnable() {
     		@Override
     		public void run() {
     			synchronized (object) {
     				try {
      					System.out.println("before wait thread id = " + Thread.currentThread().getId());
      					object.wait();
      					System.out.println("after wait thread id = " + Thread.currentThread().getId());
      				} catch (InterruptedException e) {
      					e.printStackTrace();
      				}
      			}
      		}
      	}).start();
     	new Thread(new Runnable() {
     		@Override
     		public void run() {
     			try {
      				Thread.sleep(1000);
      			} catch (InterruptedException e) {
      				e.printStackTrace();
      			}
     			synchronized (object) {
      				System.out.println("before notify thread id = " + Thread.currentThread().getId());
      				object.notify();
      				System.out.println("after notify thread id = " + Thread.currentThread().getId());
      			}
      		}
      	}).start();
      }
  
 

输出

（2）多（wait）对一（notify）模式

      class A{}
      public static void main(String[] args) {
     	final A object = new A();
      	System.out.println("main thread id = " + Thread.currentThread().getId());
     	new Thread(new Runnable() {
     		@Override
     		public void run() {
     			synchronized (object) {
     				try {
      					System.out.println("before wait1 thread id = " + Thread.currentThread().getId());
      					object.wait();
      					System.out.println("after wait1 thread id = " + Thread.currentThread().getId());
      				} catch (InterruptedException e) {
      					e.printStackTrace();
      				}
      			}
      		}
      	}).start();
     	new Thread(new Runnable() {
     			@Override
     			public void run() {
     				synchronized (object) {
     					try {
      						System.out.println("before wait2 thread id = " + Thread.currentThread().getId());
      						object.wait();
      						System.out.println("after wait2 thread id = " + Thread.currentThread().getId());
      					} catch (InterruptedException e) {
      						e.printStackTrace();
      					}
      				}
      			}
      		}).start();
     	new Thread(new Runnable() {
     		@Override
     		public void run() {
     			try {
      				Thread.sleep(1000);
      			} catch (InterruptedException e) {
      				e.printStackTrace();
      			}
     			synchronized (object) {
      				System.out.println("before notify thread id = " + Thread.currentThread().getId());
      				object.notify();
      				System.out.println("after notify thread id = " + Thread.currentThread().getId());
      			}
      		}
      	}).start();
      }
  
 

5、notifyAll

      class A{}
      public static void main(String[] args) {
     	final A object = new A();
      	System.out.println("main thread id = " + Thread.currentThread().getId());
     	new Thread(new Runnable() {
     		@Override
     		public void run() {
     			synchronized (object) {
     				try {
      					System.out.println("before wait1 thread id = " + Thread.currentThread().getId());
      					object.wait();
      					System.out.println("after wait1 thread id = " + Thread.currentThread().getId());
      				} catch (InterruptedException e) {
      					e.printStackTrace();
      				}
      			}
      		}
      	}).start();
     	new Thread(new Runnable() {
     			@Override
     			public void run() {
     				synchronized (object) {
     					try {
      						System.out.println("before wait2 thread id = " + Thread.currentThread().getId());
      						object.wait();
      						System.out.println("after wait2 thread id = " + Thread.currentThread().getId());
      					} catch (InterruptedException e) {
      						e.printStackTrace();
      					}
      				}
      			}
      		}).start();
     	new Thread(new Runnable() {
     		@Override
     		public void run() {
     			try {
      				Thread.sleep(1000);
      			} catch (InterruptedException e) {
      				e.printStackTrace();
      			}
     			synchronized (object) {
      				System.out.println("before notify thread id = " + Thread.currentThread().getId());
      				object.notifyAll();
      				System.out.println("after notify thread id = " + Thread.currentThread().getId());
      			}
      		}
      	}).start();
      }
  
 

三、总结：

（1）由1和2可以看出：object.wait()方法只能在同步方法（void synchronized methodName( args... ){ ... }）或者 “同步块内部“ （synchronized(object){ ... }）被调用，否则会抛出一个IllegalMonitorStateException异常。

（2）由3可以看出：object.wait()方法只能在锁住自己的情况下被调用，否则会抛出一个IllegalMonitorStateException异常。

（3）由4可得：一个notify只能唤醒一个wait的线程

（4）由5可得：一个notifyAll可以唤醒多个wait的线程

四、注意点：（网上很多博客都讲的不清不楚的，龙神我估计他们就没有花时间真正的去测试过）

wait、notify和notifyAll方法是Object类的final native方法。所以这些方法不能被子类重写，Object类是所有类的超类，因此在程序中有以下三种形式调用wait等方法：

1、wait()

2、this.wait()

3、super.wait()

这三个方法是等价的。

文章来源: blog.csdn.net，作者：福州-司马懿，版权归原作者所有，如需转载，请联系作者。

原文链接：blog.csdn.net/chy555chy/article/details/51864773