重学Java之线程的 join 方法

线程的 join 方法 API 说明，

Waits for this thread to die.
An invocation of this method behaves in exactly the same way as the invocation
join(0)
Throws:
InterruptedException – if any thread has interrupted the current thread. The interrupted status of the current thread is cleared when this exception is thrown.
public final void join() throws InterruptedException {
	join(0);
}

翻译过来就是：
等待这个线程死亡。
此方法的调用行为与 join(0) 调用完全相同
抛出 InterruptedException 异常 – 如果任何线程中断了当前线程。 抛出此异常时清除当前线程的中断状态。

嗯，没看明白。

查了资料，结合 API 说明，意思是：
假设有 A 、 B 两个线程，要求 B 先执行完毕，然后再执行 A 。
简言之，B 优先执行，A 在 B 执行完再执行。

举个栗子，

例1

public class JoinThreadTest {

    public static void main(String[] args) throws InterruptedException {
        
        Thread aThread = new Thread("aThread"){
            @Override
            public void run() {
                super.run();
                try {
                    Thread.sleep(2000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName() + " finish");
            }
        };
        aThread.start();
        System.out.println(Thread.currentThread().getName() + " finish");
    }
}

创建一个 Thread ，添加延时，
运行结果，

main finish
aThread finish

如果我想让 aThread 优先执行，就用 join 方法，示例，

public class JoinThreadTest {

    public static void main(String[] args) throws InterruptedException {
        
        Thread aThread = new Thread("aThread"){
            @Override
            public void run() {
                super.run();
                try {
                    Thread.sleep(2000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName() + " finish");
            }
        };
        aThread.start();
        aThread.join(); //添加这行
        System.out.println(Thread.currentThread().getName() + " finish");
    }
}

运行结果，符合预期

aThread finish
main finish

例2

新建 3 个线程试试，都不用 join 方法，

public class JoinThreadTest {

    public static void main(String[] args) throws InterruptedException {
        Thread thread1 = new Thread(){
            @Override
            public void run() {
                super.run();
                for (int i = 0 ; i < 6; i++){
                    System.out.println("thread1 running , i = " + i);
                }
                System.out.println("thread1 finish");
            }
        };

        Thread thread2 = new Thread(){
            @Override
            public void run() {
                super.run();
                for (int i = 0 ; i < 6; i++){
                    System.out.println("thread2 running , i = " + i);
                }
                System.out.println("thread2 finish");
            }
        };

        Thread thread3 = new Thread(){
            @Override
            public void run() {
                super.run();

                for (int i = 0 ; i < 6; i++){
                    System.out.println("thread3 running , i = " + i);
                }
                System.out.println("thread3 finish");
            }
        };

       thread1.start();
       thread2.start();
       thread3.start();
}

运行结果，

thread3 running , i = 0
thread3 running , i = 1
thread3 running , i = 2
thread3 running , i = 3
thread3 running , i = 4
thread3 running , i = 5
thread3 finish
thread1 running , i = 0
thread1 running , i = 1
thread1 running , i = 2
thread1 running , i = 3
thread1 running , i = 4
thread1 running , i = 5
thread1 finish
thread2 running , i = 0
thread2 running , i = 1
thread2 running , i = 2
thread2 running , i = 3
thread2 running , i = 4
thread2 running , i = 5
thread2 finish

可以看到，3 个线程的运行顺序是不固定的，多运行几次，结果是不一样的。

使用 join 方法，

thread2 中调用 thread1.join() ，确保 thread1 执行完后再执行 thread2 ，

thread3 中调用 thread2.join() ， 确保 thread2 执行完后再执行 thread3 ，

public class JoinThreadTest {

    public static void main(String[] args) throws InterruptedException {
        Thread thread1 = new Thread(){
            @Override
            public void run() {
                super.run();
                for (int i = 0 ; i < 6; i++){
                    System.out.println("thread1 running , i = " + i);
                }
                System.out.println("thread1 finish");
            }
        };

        Thread thread2 = new Thread(){
            @Override
            public void run() {
                super.run();
                try {
                    thread1.join();// thread1 优先执行，thread2 等待 thread1 执行完再执行
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                for (int i = 0 ; i < 6; i++){
                    System.out.println("thread2 running , i = " + i);
                }
                System.out.println("thread2 finish");
            }
        };

        Thread thread3 = new Thread(){
            @Override
            public void run() {
                super.run();
                try {
                    thread2.join();// thread2 优先执行，thread3 等待 thread2 执行完再执行
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }

                for (int i = 0 ; i < 6; i++){
                    System.out.println("thread3 running , i = " + i);
                }
                System.out.println("thread3 finish");
            }
        };

        thread1.start();
        thread2.start();
        thread3.start();
    }
}

多试几次，运行结果都是

thread1 running , i = 0
thread1 running , i = 1
thread1 running , i = 2
thread1 running , i = 3
thread1 running , i = 4
thread1 running , i = 5
thread1 finish
thread2 running , i = 0
thread2 running , i = 1
thread2 running , i = 2
thread2 running , i = 3
thread2 running , i = 4
thread2 running , i = 5
thread2 finish
thread3 running , i = 0
thread3 running , i = 1
thread3 running , i = 2
thread3 running , i = 3
thread3 running , i = 4
thread3 running , i = 5
thread3 finish

说明 join 方法生效了。