java并发之同步辅助类CountDownLatch
CountDownLatch
含义：
CountDownLatch可以理解为一个计数器在初始化时设置初始值，当一个线程需要等待某些操作先完成时，需要调用await()方法。这个方法让线程进入休眠状态直到等待的所有线程都执行完成。每调用一次countDown()方法内部计数器减1,直到计数器为0时唤醒。这个可以理解为特殊的CyclicBarrier。线程同步点比较特殊，为内部计数器值为0时开始。
方法：
核心方法两个：countDown()和await()
countDown():使CountDownLatch维护的内部计数器减1,每个被等待的线程完成的时候调用
await():线程在执行到CountDownLatch的时候会将此线程置于休眠
例子
开会的例子：会议室里等与会人员到齐了会议才能开始。
import java.util.concurrent.CountDownLatch;
public class VideoConference implements Runnable {
private final CountDownLatch controller;
public VideoConference(int number) {
controller = new CountDownLatch(number);
}
public void arrive(String name) {
System.out.printf("%s has arrived.\n", name);
controller.countDown();// 调用countDown()方法，使内部计数器减1
System.out.printf("VideoConference: Waiting for %d participants.\n", controller.getCount());
}
@Override
public void run() {
System.out.printf("VideoConference: Initialization: %d participants.\n", controller.getCount());
try {
controller.await();// 等待，直到CoutDownLatch计数器为0
System.out.printf("VideoConference: All the participants have come\n");
System.out.printf("VideoConference: Let's start...\n");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
参加会议人员类
import java.util.concurrent.Semaphore;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class PrintQueue {
//信号量
private Semaphore semaphore;
//是否空闲打印机
private boolean freePrinters[];
private Lock lockPrinters;
public PrintQueue(){
//初始化三个信号
semaphore=new Semaphore(3);
//三台空闲打印机
freePrinters=new boolean[3];
for (int i=0; i<3; i++){
freePrinters[i]=true;
}
lockPrinters=new ReentrantLock();
}
public void printJob (Object document){
try {
//获取信号量
semaphore.acquire();
int assignedPrinter=getPrinter();
Long duration=(long)(Math.random()*10);
System.out.printf("%s: PrintQueue: Printing a Job in Printer %d during %d seconds\n",Thread.currentThread().getName(),assignedPrinter,duration);
TimeUnit.SECONDS.sleep(duration);
freePrinters[assignedPrinter]=true;
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
// Free the semaphore
semaphore.release();
}
}
private int getPrinter() {
int ret=-1;
try {
lockPrinters.lock();
for (int i=0; i<freePrinters.length; i++) {
if (freePrinters[i]){
ret=i;
freePrinters[i]=false;
break;
}
}
} catch (Exception e) {
e.printStackTrace();
} finally {
lockPrinters.unlock();
}
return ret;
}
}
测试类：
public class CountDownLatchMain {
public static void main(String[] args) {
VideoConference conference = new VideoConference(10);
Thread threadConference = new Thread(conference);
threadConference.start();// 开启await()方法，在内部计数器为0之前线程处于等待状态
for (int i = 0; i < 10; i++) {
Participant p = new Participant(conference, "Participant " + i);
Thread t = new Thread(p);
t.start();
}
}
}
海量视频获取 springboot springcloud
