Java多线程（一）——Thread之join()方法

最新推荐文章于 2025-06-14 12:03:32 发布

liberg

最新推荐文章于 2025-06-14 12:03:32 发布

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分类专栏： Java功底

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本文介绍如何使用Java的Thread类及其join方法来确保线程按指定顺序执行。通过示例代码展示了如何让T2线程在T1执行完后启动，T3在线程T2执行完后启动，并提供了一种更简洁的实现方式。

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线程Thread的join()方法能够将调用线程挂起（会释放锁并且不消耗CPU时间，区别于sleep）直到被调用join方法的线程执行结束。
查看JDK源码发现Thread的join方法内部是通过Object.wait实现的线程同步的。
主线程开启了T1、T2、T3三个线程，如何保证T2在T1执行完后执行，T3在T2执行完后执行？

public static void main(String[] args) throws Exception {
		System.out.println(Thread.currentThread() + " test begin...");
		final Thread t1 = new Thread(new Runnable() {
			@Override
			public void run() {
				work(3000);//模拟耗时任务
				System.out.println(Thread.currentThread()+ " exit");
			}
		});
		t1.start();
		
		final Thread t2 = new Thread(new Runnable() {
			@Override
			public void run() {
				try {
					t1.join();//保证T2在T1执行完之后执行
				} catch (InterruptedException e1) {
					e1.printStackTrace();
				}
				work(3000);
				System.out.println(Thread.currentThread()+ " exit");
			}
		});
		t2.start();
		
		Thread t3 = new Thread(new Runnable() {
			@Override
			public void run() {
				try {
					t2.join();//等待T2执行完毕
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
				work(3000);
				System.out.println(Thread.currentThread()+ " exit");
			}
		});
		t3.start();
		t3.join();//主线程等待T3结束
		System.out.println("Bye");
	}
	
	public static void work(long milliSeconds) {
		try {
			System.out.println(Thread.currentThread() + " doing work!");
			Thread.sleep(milliSeconds);
		} catch (InterruptedException e) {
			System.out.println(Thread.currentThread() + " work exception...");
			e.printStackTrace();
		}
	}

输出如下:

Thread[main,5,main] test begin...
Thread[Thread-0,5,main] doing work...
Thread[Thread-0,5,main] exit
Thread[Thread-1,5,main] doing work...
Thread[Thread-1,5,main] exit
Thread[Thread-2,5,main] doing work...
Thread[Thread-2,5,main] exit
Bye

稍微封装了下，如下：

//WorkThread.java
public class WorkThread extends Thread {
	long mWorkLoad;
	Thread mRunAfterThread;
	
	public WorkThread(long workLoad, Thread preThread) {
		mWorkLoad = workLoad;
		mRunAfterThread = preThread;
	}
	
	@Override
	public void run() {
		try {
			if(mRunAfterThread != null) {
				mRunAfterThread.join();//等待前一个线程执行结束
			}
			System.out.println(Thread.currentThread() + " doing work...");
			Thread.sleep(mWorkLoad);
		} catch (InterruptedException e) {
			System.out.println(Thread.currentThread() + " work exception!");
			e.printStackTrace();
		}
		System.out.println(Thread.currentThread() + " exit");
	}
	
}

//测试代码
public static void main(String[] args) throws Exception {
		System.out.println(Thread.currentThread() + " test begin...");
		WorkThread t1 = new WorkThread(3000, null);
		WorkThread t2 = new WorkThread(3000, t1);//在t1之后执行
		WorkThread t3 = new WorkThread(3000, t2);//在t2之后执行
		t1.start();
		t2.start();
		t3.start();
		t3.join();//主线程等待t3结束
		System.out.println("Bye");
	}