CountDownLatch 简介和例子
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2022-07-12 10:35:31
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在一些应用场合中,某段程序需要等待某个条件达到要求后才能执行,或者等待一定长的时间后此行,从jdk1.5开始就可以使用CountDownLatch实现,
CountDownLatch类是一个同步倒数计数器,构造时传入int参数,该参数就是计数器的初始值,每调用一次countDown()方法,计数器减1,计数器大于0 时,await()方法会阻塞后面程序执行,直到计数器为0,await(long timeout, TimeUnit unit),是等待一定时间,然后执行,不管计数器是否到0了。
下面举一个等车的例子:
10个同学上车,车等待同学上车,如果有等待时间限制,到时间就开走,不管学生上没上完。如果没有等待时间,学生上完了再开: public class CountDownLatchTest {
public static int numberOfPeople = 10;//等车的学生数
public static boolean isGone = false;//车开的标志
public static int carWaitTime = 3;//车等的时间
public static void main(String[] args) throws InterruptedException {
CountDownLatch waitStudentsGetOn = new CountDownLatch(numberOfPeople);
new Thread(new GetOn(waitStudentsGetOn)).start();
waitStudentGetOn(waitStudentsGetOn);//等所有的学生上车
driveHome();//开车走
}
private static void waitStudentGetOn(CountDownLatch waitStudentsGetOn) throws InterruptedException {
System.out.println("赶紧的,抓紧时间上车..");
waitStudentsGetOn.await(carWaitTime, TimeUnit.SECONDS);//等5秒,还没上车,就开走。。
}
private static void driveHome() throws InterruptedException {
System.out.println("开车,鞋儿破 帽儿破 身上的袈裟破 你笑我 他笑我 一把扇儿破");
isGone = true;
}
}
class GetOn implements Runnable{
private CountDownLatch waitStudentsGetOn;
GetOn(CountDownLatch waitStudentsGetOn){
this.waitStudentsGetOn = waitStudentsGetOn;
}
public void run() {
for (int i = 0; i < CountDownLatchTest.numberOfPeople; i++) {
try {
if(CountDownLatchTest.isGone){
System.out.println("妈的,还差:"+waitStudentsGetOn.getCount()+" 个没娃上车呢.怎么车走了");
break;
}
boolean goonSuccess = new Student(i+1).getOn();//顺序上车
if(goonSuccess)waitStudentsGetOn.countDown();
} catch (InterruptedException e) {}
if(waitStudentsGetOn.getCount()!=0l){
System.out.println("还差:"+(waitStudentsGetOn.getCount())+" 个没上车");
}else{
System.out.println("都上车了");
}
}
}
class Student{
private int myNum;//学生编号
public Student(int num){
this.myNum = num;
}
//上车
public boolean getOn() throws InterruptedException{
Thread.currentThread().sleep(new Random().nextInt(2)*1000);//上车使用的时间,随机
if(CountDownLatchTest.isGone){
return false;//不能上了,上车失败
}
System.out.print("编号为:"+myNum+"的同学上车了..");
return true;
}
}
}
CountDownLatch类是一个同步倒数计数器,构造时传入int参数,该参数就是计数器的初始值,每调用一次countDown()方法,计数器减1,计数器大于0 时,await()方法会阻塞后面程序执行,直到计数器为0,await(long timeout, TimeUnit unit),是等待一定时间,然后执行,不管计数器是否到0了。
下面举一个等车的例子:
10个同学上车,车等待同学上车,如果有等待时间限制,到时间就开走,不管学生上没上完。如果没有等待时间,学生上完了再开: public class CountDownLatchTest {
public static int numberOfPeople = 10;//等车的学生数
public static boolean isGone = false;//车开的标志
public static int carWaitTime = 3;//车等的时间
public static void main(String[] args) throws InterruptedException {
CountDownLatch waitStudentsGetOn = new CountDownLatch(numberOfPeople);
new Thread(new GetOn(waitStudentsGetOn)).start();
waitStudentGetOn(waitStudentsGetOn);//等所有的学生上车
driveHome();//开车走
}
private static void waitStudentGetOn(CountDownLatch waitStudentsGetOn) throws InterruptedException {
System.out.println("赶紧的,抓紧时间上车..");
waitStudentsGetOn.await(carWaitTime, TimeUnit.SECONDS);//等5秒,还没上车,就开走。。
}
private static void driveHome() throws InterruptedException {
System.out.println("开车,鞋儿破 帽儿破 身上的袈裟破 你笑我 他笑我 一把扇儿破");
isGone = true;
}
}
class GetOn implements Runnable{
private CountDownLatch waitStudentsGetOn;
GetOn(CountDownLatch waitStudentsGetOn){
this.waitStudentsGetOn = waitStudentsGetOn;
}
public void run() {
for (int i = 0; i < CountDownLatchTest.numberOfPeople; i++) {
try {
if(CountDownLatchTest.isGone){
System.out.println("妈的,还差:"+waitStudentsGetOn.getCount()+" 个没娃上车呢.怎么车走了");
break;
}
boolean goonSuccess = new Student(i+1).getOn();//顺序上车
if(goonSuccess)waitStudentsGetOn.countDown();
} catch (InterruptedException e) {}
if(waitStudentsGetOn.getCount()!=0l){
System.out.println("还差:"+(waitStudentsGetOn.getCount())+" 个没上车");
}else{
System.out.println("都上车了");
}
}
}
class Student{
private int myNum;//学生编号
public Student(int num){
this.myNum = num;
}
//上车
public boolean getOn() throws InterruptedException{
Thread.currentThread().sleep(new Random().nextInt(2)*1000);//上车使用的时间,随机
if(CountDownLatchTest.isGone){
return false;//不能上了,上车失败
}
System.out.print("编号为:"+myNum+"的同学上车了..");
return true;
}
}
}
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