1.循环打印
public class OutWord {
private String word = "输出A";
public synchronized void outA() throws InterruptedException {
if (word.equals("B输出")){
this.wait();
}
//输出word
System.out.println("word = " + word);
word="B输出";
this.notify();
}
public synchronized void outB() throws InterruptedException {
if (word.equals("输出A")){
this.wait();
}
//输出word
System.out.println("word = " + word);
word="输出A";
this.notify();
}
}
public class Test {
public static void main(String[] args) {
OutWord outWord = new OutWord();
new Thread(()->{
for (int i = 0; i < 10; i++) {
try {
outWord.outA();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
},"线程一").start();
new Thread(()->{
for (int i = 0; i < 10; i++) {
try {
outWord.outB();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
},"线程二").start();
}
}
输出结果
word = 输出A
word = B输出
word = 输出A
word = B输出
word = 输出A
.........
2.虚假唤醒
2.1代码演示
加入我们增加两个线程 新增的线程分别是 打印A 和 打印B
public class Test {
public static void main(String[] args) {
OutWord outWord = new OutWord();
new Thread(()->{
for (int i = 0; i < 5; i++) {
try {
outWord.outA();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
},"线程一").start();
new Thread(()->{
for (int i = 0; i < 5; i++) {
try {
outWord.outB();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
},"线程二").start();
new Thread(()->{
for (int i = 0; i < 5; i++) {
try {
outWord.outA();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
},"线程三").start();
new Thread(()->{
for (int i = 0; i < 5; i++) {
try {
outWord.outB();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
},"线程四").start();
}
}
public class OutWord {
private String word = "输出A";
public synchronized void outA() throws InterruptedException {
if (word.equals("B输出")){
this.wait();
}
//输出word
System.out.println("word = " + word);
word="B输出";
this.notify();
}
public synchronized void outB() throws InterruptedException {
if (word.equals("输出A")){
this.wait();
}
//输出word
System.out.println("word = " + word);
word="输出A";
this.notify();
}
}
输出结果
word = 输出A
word = B输出
word = 输出A
word = B输出
word = 输出A
word = B输出
word = 输出A
word = B输出
word = 输出A
word = B输出
word = 输出A
word = B输出
word = B输出
word = B输出
word = B输出
word = B输出
2.2 什么是虚假唤醒
可以看到打印顺序已经乱了,出现了虚假唤醒 : 现在有四条线程执行打印任务 假设其中两条打印B的线程先后抢到锁,但是都不满足if条件,进入wait()等待,并释放了锁,此时打印A的线程拿到了锁,并且打印了A,随机唤醒了打印B的一条线程,线程打印B后又唤醒了另外一条打印B的线程,此时就会出现打印乱序的情况了 原因:notify随机唤醒一条线程,被唤醒的线程将会继续执行wait后的代码
2.2.3 解决办法
将 if条件换成 while 线程被唤醒后依然要进行条件判断 如果不满足,继续wait 释放锁,等待被唤醒
public class OutWord {
private String word = "输出A";
public synchronized void outA() throws InterruptedException {
// System.out.println(Thread.currentThread().getName()+"拿到了锁");
while (word.equals("B输出")){
// System.out.println(Thread.currentThread().getName()+"等待被唤醒");
this.wait();
// System.out.println(Thread.currentThread().getName()+"被唤醒,继续运行");
}
//输出word
System.out.println("word = " + word);
word="B输出";
//System.out.println(Thread.currentThread().getName()+"随机唤醒一条线程");
this.notify();
// System.out.println(Thread.currentThread().getName()+"执行完毕");
}
public synchronized void outB() throws InterruptedException {
// System.out.println(Thread.currentThread().getName()+"拿到了锁");
while (word.equals("输出A")){
// System.out.println(Thread.currentThread().getName()+"等待被唤醒");
this.wait();
// System.out.println(Thread.currentThread().getName()+"被唤醒,继续运行");
}
//输出word
System.out.println("word = " + word);
word="输出A";
// System.out.println(Thread.currentThread().getName()+"随机唤醒一条线程");
this.notify();
// System.out.println(Thread.currentThread().getName()+"执行完毕");
}
}
输出结果
word = 输出A
word = B输出
word = 输出A
word = B输出
word = 输出A
word = B输出
word = 输出A
word = B输出
word = 输出A
word = B输出
word = 输出A
word = B输出
word = 输出A
word = B输出
word = 输出A
word = B输出
word = 输出A
word = B输出
word = 输出A
word = B输出
打印顺序已经恢复正常,那是不是就没有其它问题了呢? 我们多运行几次 发现输出台居然卡住了 是死锁了吗
3.解决虚假唤醒问题后出现的程序卡死问题
首先我们分析 锁的都是同一个对象 wait会释放锁,被唤醒后也需要重新竞争锁对象
死锁的四个条件 除了互斥 其他三个都不满足 所以不可能死锁
而代码中,只使用了 while循环 是while的原因吗?
while(条件){
this.wait()
}
下一贴详细说明问题出现的原因 欢迎留言说下你的答案