虚假唤醒
例子
wait（）是object类自带的方法，在jdk有介绍，有可能出现中断、虚假唤醒
也就是在下面的例子中
if(number != 0){ this.wait(); }当线程成功进入if语句块中，发生了中断，cpu跑去调度别的进程了，再次调度这个线程的时候，应该需要再经历一次if的判断，但是并没有这样做。
于是下面的程序运行结果：
/**
* @author zkw
* @Description TODO
*
*/
public class ThreadWaitNotify {
public static void main(String[] args) {
Resource resource = new Resource();
//increment
new Thread(()->{
for (int i = 0; i < 10; i++) {
try {
resource.increment();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
},"A-1").start();
new Thread(()->{
for (int i = 0; i < 10; i++) {
try {
resource.increment();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
},"A-2").start();
//decrement
new Thread(()->{
for (int i = 0; i < 10; i++) {
try {
resource.decrement();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
},"B-1").start();
new Thread(()->{
try {
resource.decrement();
} catch (InterruptedException e) {
e.printStackTrace();
}
},"B-2").start();
}
}
class Resource{
private int number = 0;
//number++
public synchronized void increment() throws InterruptedException {
if(number != 0){
this.wait();
}
number++;
System.out.println(Thread.currentThread().getName()+":"+number);
this.notifyAll();
}
//number--
public synchronized void decrement() throws InterruptedException {
if(number == 0){
this.wait();
}
number--;
System.out.println(Thread.currentThread().getName()+":"+number);
this.notifyAll();
}
}解决办法
按照jdk文档的指示，应该将if换成while
while(number == 0){
this.wait();
}修改后，正确的运行结果如下：
精准唤醒
JUC那套 lock、await、signal和 Object那套 synchronized、wait、notify的区别：
condition这一套能实现精确唤醒某一个线程，而不是像notifyAll那样唤醒全部线程。
下面这个例子中，一共创建了三个Condition，可以根据业务对这三个Condition依次唤醒。来达到精准唤醒的功能
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
/**
* @author zkw
* @Description 线程顺序访问
* 多线程之间的顺序调用 A-》B-》C
* 三个线程启动，要求如下：
* AA打印5次，BB打印10次，CC打印15次
* 接着
* AA打印5次，BB打印10次，CC打印15次
* ....来10轮
*
* 1 高内聚低耦合的前提下，线程 操作 资源类
* 2 判断/干活/通知
* 3 多线程交互中，必须要防止多线程的虚假唤醒，也即（判断只用while不用if）
* 4 标志位
*
*/
public class ThreadOrderAccess {
public static void main(String[] args) {
ShareResource shareResource = new ShareResource();
new Thread(()->{
try {
for (int i = 0; i < 10; i++) {
shareResource.printf5();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
},"Thread1").start();
new Thread(()->{
try {
for (int i = 0; i < 10; i++) {
shareResource.printf10();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
},"Thread2").start();
new Thread(()->{
try {
for (int i = 0; i < 10; i++) {
shareResource.printf15();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
},"Thread3").start();
}
}
class ShareResource{
private int number = 1; //1:A 2:B 3C
private Lock lock = new ReentrantLock();
private Condition condition1 = lock.newCondition();
private Condition condition2 = lock.newCondition();
private Condition condition3 = lock.newCondition();
public void printf5() throws InterruptedException {
lock.lock();
try {
while (number!=1){
condition1.await();
}
for (int i = 0; i < 5; i++) {
System.out.println(Thread.currentThread().getName()+"--AA");
}
number = 2;
condition2.signal();
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
public void printf10() throws InterruptedException {
lock.lock();
try {
while (number!=2){
condition2.await();
}
for (int i = 0; i < 10; i++) {
System.out.println(Thread.currentThread().getName()+"--BB");
}
number = 3;
condition3.signal();
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
public void printf15() throws InterruptedException {
lock.lock();
try {
while (number!=3){
condition3.await();
}
for (int i = 0; i < 15; i++) {
System.out.println(Thread.currentThread().getName()+"--CC");
}
number = 1;
condition1.signal();
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
}
