Java 多线程中wait、notify、nptifyAll、yield、join、sleep的使用方法及区别

在Java多线程编程中，会涉及到wait、notify、nptifyAll、yield、join、sleep线程间交互的方法，这些比较晦涩难懂，现在全面介绍下这些方法的使用方式。

1. 总览

这些方法可大致分为两类，一类是继承自Object的方法，全部为native实现，一类是Thread的方法，也都是依托native实现的。

1) 继承自Object

• notify()
• notifyAll()
• wait()
• wait(long timeout)
• wait(long timeout, int nanos)

2) Thread的方法

• static yield()
• static sleep(long millis)
• static sleep(long millis, int nanos)
• join(long millis)
• join(long millis, int nanos)
• join()

2. 线程状态转换

2.使用说明

1) wait/wait(time) 与 notify/notifyAll

2) sleep

3) yield

• 代码样例

import java.time.LocalTime;

public class YieldTest {

    public static void main(String[] args) {
        Thread thread = new Thread(new MyThread());
        thread.start();

        Thread thread2 = new Thread(new MyThread());
        thread2.start();
    }

    public static class MyThread implements Runnable{

        @Override
        public void run() {
            for (int i = 0; i < 10; i++) {
                System.out.println(Thread.currentThread().getName() + " " + i + " " + LocalTime.now());
                Thread.yield();
                System.out.println(Thread.currentThread().getName() + " " + i + " yield " + LocalTime.now());
            }
        }
    }
}

4) join/join(time)

如下代码表示主线程调用了thread的join方法，形成阻塞，thread睡了2秒后，主线程继续执行。

• 代码样例

import java.time.LocalTime;

public class JoinTest {

    public static void main(String[] args) throws InterruptedException {
        Thread thread = new Thread(new MyThread());
        thread.start();

        System.out.println("Main join wait " + LocalTime.now());
        thread.join();
        System.out.println("Main end " + LocalTime.now());
    }

    public static class MyThread implements Runnable{

        @Override
        public void run() {
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

• 输出

Main join wait 00:35:33.265
Main end 00:35:35.199

• 源码解读

public final synchronized void join(long millis)
throws InterruptedException {
    long base = System.currentTimeMillis();
    long now = 0;

    if (millis < 0) {
        throw new IllegalArgumentException("timeout value is negative");
    }

    if (millis == 0) {
        //无限等待至该线程可用死亡(!isAlive())
        //isAlive与wait都是native方法
        while (isAlive()) {
            wait(0); // 复用Object的wait方法
        }
    } else {
        while (isAlive()) {
            long delay = millis - now;
            if (delay <= 0) {
                break;
            }
            wait(delay);
            now = System.currentTimeMillis() - base;
        }
    }
}

参考

1. Java线程源码解析之yield和sleep