java线程间同步----wait、notify、synchronized

一、wait、notify

wait、notify 是java 根级父类Obeject 中定义得两个方法，其相关作用如下：
object.wait()：执行该语句，会让获取了该object对象锁得线程进入WAIT状态，并释放该object对象锁；
object.notify()：执行该语句，会唤醒此object对象监视器上等待的单个线程，但是并不会释放锁，而是会继续执行剩余语句，直到执行完synchronized块，才会释放object对象锁；
object.notifAll()：同上，但是会唤醒此对象监视器上等待的所有线程；

二、代码实践

WaitThread

public class WaitThread extends Thread {
	private final Object lock;

    public WaitThread(Object lock) {
        super();
        this.lock = lock;
    }
    @Override
    public void run() {
    	System.out.println("this is wait");
        try {
            synchronized (lock) {
                long start = System.currentTimeMillis();
                System.out.println("开始 wait time= " + start);
                lock.wait();
                long end = System.currentTimeMillis();
                System.out.println("结束 wait time= " + end);
                System.out.print("wait time = " + (end - start));
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

}

NotifyThread

public class NotifyThread extends Thread  {
	
	private final Object lock;

    public NotifyThread(Object lock) {
        super();
        this.lock = lock;
    }
    @Override
    public void run() {
    	System.out.println("this is notify");
        synchronized (lock) {
  
            try {
                Thread.sleep(3000);
            } catch (Exception e) {
                e.printStackTrace();
            }
            // 业务逻辑完成了...
            System.out.println("开始 notify time= " + System.currentTimeMillis());
            lock.notify();
            try {
				Thread.sleep(10000);
			} catch (InterruptedException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
            System.out.println("结束 notify time= " + System.currentTimeMillis());
        }
    }

}

main

public class Main {
	
	public static void main(String[] args) {
		
		Object lock = new Object();
        WaitThread t1 = new WaitThread(lock);
        t1.start();
        NotifyThread t2 = new NotifyThread(lock);
        t2.start();		
	}
}

执行结果

this is wait
this is notify
开始 wait time= 1713457641122
开始 notify time= 1713457644123
结束 notify time= 1713457654137
结束 wait time= 1713457654137
wait time = 13015

代码逻辑简介

1、主线程创建了两个线程WaitThread t1 和 NotifyThread t2 并运行着两个线程；
2、线程WaitThread t1先获取lock 同步锁，然后 NotifyThread t2只能等待；
3、线程WaitThread t1获取锁之后执行相关代码，直到object.wait()，WaitThread t1进入等待状态，并释放锁；
4、锁一释放，NotifyThread t2 就会获取该对象得同步锁，并执行后续代码；
5、NotifyThread t2线程执行到object.wait()，该语句可以唤醒因object.wait()而处于WAIT状态得线程WaitThread t1，但是此时锁并没有释放，所以WaitThread t1会继续等待锁；直到NotifyThread t2的synchronized模块执行完成，才会释放锁；此时WaitThread t1会获取锁并继续执行，直到线程执行完毕；
相关流程如下图：