多线程交替输出问题3种常用解法（多线程经典面试题）

题目：让两个多线程交替输出
举例：“ABCDEF”和“123456”，
输出结果为固定“A1B2C3D4E5F6”

1.利用synchronized+wait/notify

package com.mooc.house.user.controller;


public class sync_wait_notify {

    public static void main(String[] args) {
        final Object o = new Object();

        char[] aI = "ABCDEF".toCharArray();
        char[] aC = "123456".toCharArray();


        new Thread(()->{
            synchronized (o) {
                for(char c : aI) {
                    System.out.print(c);
                    try {
                        o.notify();
                        o.wait(); //让出锁
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                o.notify(); //必须，否则无法停止程序
            }

        }, "t1").start();

        new Thread(()->{
            synchronized (o) {
                for(char c : aC) {
                    System.out.print(c);
                    try {
                        o.notify();
                        o.wait();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                o.notify();
            }
        }, "t2").start();
    }
}

2.利用condition和ReentrantLock联合使用

package com.mooc.house.user.controller;

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;


public class T01_00_Question {

    private char[] a1 = {'1','2','3','4','5','6'};
    private char[] a2 = {'A','B','C','D','E','F'};
    private ReentrantLock lock = new ReentrantLock();
    private Condition c1 = lock.newCondition();
    private Condition c2 = lock.newCondition();

    public void m1(){
        int count = 0;
        lock.lock();
        //这里模拟前面四个数
        while(count <= 5){
            System.out.print(a2[count++]);
            try {
                c2.signal();
                c1.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        c2.signal();
        lock.unlock();
    }

    public void m2(){
        int count = 0;
        lock.lock();
        while(count <= 5){
            System.out.print(a1[count++]);
            try {
                c1.signal();
                c2.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        c1.signal();
        lock.unlock();
    }
    public static void main(String[] args) {
        //要求用线程顺序打印A1B2C3....Z26
        T01_00_Question t = new T01_00_Question();
        new Thread(()->{
            t.m1();
        }).start();
        new Thread(()->{
            t.m2();
        }).start();
    }
}

3.利用LockSupport

package com.mooc.house.user.controller;

import java.util.concurrent.locks.LockSupport;


public class Test1 {

    static Thread t1 = null,t2=null;

    public static void main(String[] args) {
        char[] a1= "ABCDEF".toCharArray();
        char[] c1= "123456".toCharArray();

        t1 = new Thread(() -> {
            for(char c : a1) {
                System.out.print(c);
                LockSupport.unpark(t2); //叫醒T2
                LockSupport.park(); //T1阻塞
            }
        }, "t1");

        t2 = new Thread(() -> {
            for(char c : c1) {
                LockSupport.park(); //t2阻塞
                System.out.print(c);
                LockSupport.unpark(t1); //叫醒t1
            }
        }, "t2");

        t1.start();
        t2.start();
    }
}