先来看看下面的程序
A、C两个线程做number的增1，B、D 两个线程做number的减1
class MpCust {
private int number = 0;
public synchronized void increment() throws InterruptedException {
if (number != 0){
this.wait();
}
number++;
System.out.println(Thread.currentThread().getName()+"\n"+number);
this.notifyAll();
}
public synchronized void decrement() throws InterruptedException {
if (number==0){
this.wait();
}
number--;
System.out.println(Thread.currentThread().getName()+"\n"+number);
this.notifyAll();
}
public static void main(String[] args) {
MpCust mpCust = new MpCust();
new Thread(new Runnable() {
@Override
public void run() {
for (int i = 0; i < 10; i++) {
try {
Thread.sleep(300);
mpCust.increment();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
},"A").start();
new Thread(new Runnable() {
@Override
public void run() {
for (int i = 0; i < 10; i++) {
try {
Thread.sleep(400);
mpCust.decrement();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
},"B").start();
new Thread(new Runnable() {
@Override
public void run() {
for (int i = 0; i < 10; i++) {
try {
Thread.sleep(300);
mpCust.increment();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
},"C").start();
new Thread(new Runnable() {
@Override
public void run() {
for (int i = 0; i < 10; i++) {
try {
Thread.sleep(400);
mpCust.decrement();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
},"D").start();
}
}
运行结果中有值打印为2，这是不正常的！
image.png
因为increment()方法执行到这里，number 的值不为0的话就会执行Object.wait()方法，当前线程会进入等待。
if (number != 0){
this.wait();
}
因为这段代码是在多线程下执行的，线程执行到 this.wait();等待，苏醒后继续执行。这时候如果别的线程已经将number的值改为1，那么就不会再去判断一次number 是否等于0，就直接往下执行number++，导致number的值为2。这种想象我们称之为虚假唤醒
所以需要一种机制让线程苏醒之后再次判断number的值是否等于0。这时候java中的while循环正好排上用场。
使用while循环代替if条件
while(number != 0){
this.wait();
}
class MpCust {
private int number = 0;
public synchronized void increment() throws InterruptedException {
while (number != 0){
this.wait();
}
number++;
System.out.println(Thread.currentThread().getName()+"\n"+number);
this.notifyAll();
}
public synchronized void decrement() throws InterruptedException {
while (number==0){
this.wait();
}
number--;
System.out.println(Thread.currentThread().getName()+"\n"+number);
this.notifyAll();
}
public static void main(String[] args) {
MpCust mpCust = new MpCust();
new Thread(new Runnable() {
@Override
public void run() {
for (int i = 0; i < 10; i++) {
try {
Thread.sleep(300);
mpCust.increment();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
},"A").start();
new Thread(new Runnable() {
@Override
public void run() {
for (int i = 0; i < 10; i++) {
try {
Thread.sleep(400);
mpCust.decrement();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
},"B").start();
new Thread(new Runnable() {
@Override
public void run() {
for (int i = 0; i < 10; i++) {
try {
Thread.sleep(300);
mpCust.increment();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
},"C").start();
new Thread(new Runnable() {
@Override
public void run() {
for (int i = 0; i < 10; i++) {
try {
Thread.sleep(400);
mpCust.decrement();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
},"D").start();
}
}
完美~~
image.png
