join方法

测试案例

------------------------------------------------------------测试类------------------------------------------------------------

package com.qf.test;

/**
 * @author qf
 * @create 2018-09-20 15:32
 */
public class Run {
    static int count = 0;
    public static void main(String[] args) {
        Thread thread = new Thread(new Runnable() {
            @Override
            public void run() {
                for (int i = 0; i < 10; i++) {
                    count++;
                }
            }
        });
        thread.start();
        System.out.println("count = " + count);
    }
}

-----------------------------------------------------------打印结果-----------------------------------------------------------

count = 0

结果是0，并没有被修改成10，说明main线程在thread线程之前执行完成了

如何获取修改后的count值？使用join方法

------------------------------------------------------------测试类------------------------------------------------------------

package com.qf.test;

/**
 * @author qf
 * @create 2018-09-20 15:32
 */
public class Run {
    static int count = 0;
    public static void main(String[] args) throws InterruptedException {
        Thread thread = new Thread(new Runnable() {
            @Override
            public void run() {
                for (int i = 0; i < 10; i++) {
                    count++;
                }
            }
        });
        thread.start();
        thread.join();
        System.out.println("count = " + count);
    }
}

-----------------------------------------------------------打印结果-----------------------------------------------------------

count = 10

原因分析

join源码

    
    public final void join() throws InterruptedException {
        join(0);
    }
==============================================================================
    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) {
            while (isAlive()) {
                wait(0);
            }
        } else {
            while (isAlive()) {// thread对象调用的isAlive方法，所以只要thread还存活，就会循环
                long delay = millis - now;
                if (delay <= 0) {
                    break;
                }
                wait(delay);// wait方法，使当前线程等待，当前线程是main线程
                now = System.currentTimeMillis() - base;
            }
        }
    }

join方法使得当前线程进入阻塞状态，进入排队队列的作用，且必须等待调用join的线程对象执行完run方法才能执行