java執行緒之Latch設計模式見解
阿新 • • 發佈：2018-11-28
CountDownLatch :（個人理解）使用閥門值，直到閥門值為０之前，一直阻塞執行緒。實則使用物件鎖，不釋放物件鎖，一直佔用資源，（這裡是一個缺點）。閥門值為０時，呼叫釋放物件鎖的方法，釋放資源。應用的場景，我覺得是需要一些執行緒先完成的前提下，再使用其他執行緒。也就是我就是要一些重要的執行緒（也不是特指重要）完成任務之後，接著再執行其他執行緒。
Table of Contents
本文引用的demo均為轉載，其他內容是自己原創。感謝這些demo的提供者。為了部落....Ｏ（∩＿∩）Ｏ哈哈～
demo1-重寫CountDownLatch
這個是我在《java高併發詳解》中找到的demo (注：有一些自己的調整)：
(readme: 重寫了CountDownLatch類以及一些方法。建立了一個自定義異常TimeoutException。應用了一些鎖的知識點。（synchronized會一直等待執行緒釋放物件鎖而造成阻塞，wait方法會釋放物件鎖，本程式如果不加該方法的後果就是會一直被阻塞，造成超時。）
四個程式設計師約定在某個時間到某地聚會，每人都會採用交通工具，最後對按時到達的程式設計師，輸出按時到達。
package com.mzs.entity; import java.util.concurrent.TimeUnit; public abstract class Latch { protected int limit; // 閥門值 protected boolean isLate; // 是否遲到 public Latch(int limit, boolean isLate) { this.limit = limit; this.isLate = isLate; } /** * 模擬等待 * @param unit 時間單位 * @param time 預定的到達時間 * @throws InterruptedException 被打斷時丟擲該異常 * @throws TimeoutException 自定義的時間超時異常，當時間超過預定的到達時間時，丟擲該異常 */ public abstract void await(TimeUnit unit, long time) throws InterruptedException, TimeoutException; /** * 閥門值減一 */ public abstract void countDown(); /** * 統計未到達的人數 * @return 未到達的人數 */ public abstract int getUnarrived(); }
package com.mzs.entity; import java.util.concurrent.TimeUnit; public class CountDownLatch extends Latch { public CountDownLatch(int limit, boolean isLate) { super(limit, isLate); } @Override public void await(TimeUnit unit, long time) throws InterruptedException, TimeoutException { if (time < 0) throw new IllegalArgumentException("argument is invalid"); // 表示預定的到達時間 long remainingTime = unit.toNanos(time); long endTime = System.nanoTime() + remainingTime; synchronized (this) { while (limit > 0) { // 剩餘時間小於０，則未按時到達，標記遲到，並丟擲自定義超時異常 if (TimeUnit.NANOSECONDS.toMillis(remainingTime) < 0) { this.isLate = true; throw new TimeoutException("time is over"); } // 等待的過程中，被打斷時，時間進行新的處理。 this.wait(TimeUnit.NANOSECONDS.toMillis(remainingTime)); // 計算剩餘的時間 remainingTime = endTime - System.nanoTime(); } } } @Override public void countDown() { synchronized (this) { // 對閥門值的檢查，如果小於０，丟擲該異常 if (limit < 0) throw new IllegalStateException("the number of limit is illegal"); // 閥門值自減，表示已到達 limit--; // 通知阻塞執行緒 this.notifyAll(); } } @Override public int getUnarrived() { return limit; } }
package com.mzs.entity;
public class TimeoutException extends Exception {
/**
*
*/
private static final long serialVersionUID = -6499958945796073069L;
private String message;
public TimeoutException(String message) {
super(message);
}
}
package com.mzs.entity;
import java.util.concurrent.ThreadLocalRandom;
import java.util.concurrent.TimeUnit;
import java.util.logging.Logger;
public class ProgrammerTravel extends Thread {
private final Latch latch; // 閥門值
private final String programmer; // 程式設計師
private final String transportation; // 交通工具
private Logger logger = Logger.getLogger(getClass().getName());
public ProgrammerTravel(Latch latch, String programmer, String transportation) {
this.latch = latch;
this.programmer = programmer;
this.transportation = transportation;
}
@Override
public void run() {
logger.info(programmer + " starts to take the transportation [ " + transportation + " ]");
try {
// 模擬程式設計師到達目的地花費時間的隨機性
TimeUnit.SECONDS.sleep(ThreadLocalRandom.current().nextInt(10));
} catch (InterruptedException e) {
e.printStackTrace();
}
latch.countDown();
if (!latch.isLate)
logger.info(programmer + " arrived on time!!!");
if (latch.getUnarrived() == 0)
logger.info("all the programmers arrived");
}
public static void main(String[] args) {
// 設定閥門值為４，並標記未遲到
Latch latch = new CountDownLatch(4, false);
new ProgrammerTravel(latch, "Tom", "bike").start();
new ProgrammerTravel(latch, "Selina", "bike").start();
new ProgrammerTravel(latch, "King", "Car").start();
new ProgrammerTravel(latch, "Khan", "Bus").start();
try {
// 設定預定時間為５秒
latch.await(TimeUnit.SECONDS, 5);
} catch (InterruptedException e) {
e.printStackTrace();
} catch (TimeoutException e) {
System.err.println(e);
}
}
}
demo2-子執行緒任務完成之後執行主執行緒
重寫runnable run()，使用java.util的CountDownLatch，實現多個子執行緒任務完成之後，執行主執行緒的任務。
package com.mzs.demo1;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.ThreadLocalRandom;
import java.util.logging.Logger;
public class CountDownLatchTest1 {
private static Logger logger = Logger.getLogger("com.mzs.demo1.CountDownLatchTest1");
public static void main(String[] args) {
ExecutorService service = Executors.newFixedThreadPool(3);
CountDownLatch latch = new CountDownLatch(3);
for (int i = 0; i < 3; i++) {
Runnable runnable = new Runnable() {
@Override
public void run() {
logger.info("child thread [" + Thread.currentThread().getName() + "] starts to execute");
try {
Thread.sleep(ThreadLocalRandom.current().nextInt(20));
} catch (InterruptedException e) {
e.printStackTrace();
}
logger.info("child thread [" + Thread.currentThread().getName() + "] finished");
latch.countDown();
}
};
service.execute(runnable);
}
logger.info("main thread [" + Thread.currentThread().getName() + "] is waiting");
try {
latch.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
logger.info("main thread [" + Thread.currentThread().getName() + "] starts to execute");
}
}
demo3-多個CountDownLatch的應用
package com.mzs.demo1;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.ThreadLocalRandom;
import java.util.logging.Logger;
public class CountDownLatchTest2 {
private static Logger logger = Logger.getLogger("com.mzs.demo1.CountDownLatchTest2");
public static void main(String[] args) {
ExecutorService service = Executors.newCachedThreadPool();
CountDownLatch latch = new CountDownLatch(1);
CountDownLatch latch1 = new CountDownLatch(4);
for (int i = 0; i < 4; i++) {
Runnable runnable = new Runnable() {
@Override
public void run() {
logger.info("運動員" + Thread.currentThread().getName() + "等待裁判命令");
try {
latch.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
logger.info("運動員" + Thread.currentThread().getName() + "接收裁判命令");
try {
Thread.sleep(ThreadLocalRandom.current().nextInt(20));
} catch (InterruptedException e) {
e.printStackTrace();
}
logger.info("運動員" + Thread.currentThread().getName() + "到達終點");
latch1.countDown();
}
};
service.execute(runnable);
}
latch.countDown();
try {
latch1.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
logger.info("裁判" + Thread.currentThread().getName() + "評判結果");
}
}