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本文介绍两种BlockingDeque在多线程任务处理时正确结束的方法

一般最开始简单的多线程处理任务过程

• 把总任务放入BlockingDeque
• 创建多个线程，每个线程内逻辑时，判断BlockingDeque任务是否处理完，处理完退出，还有任务就BlockingDeque.take()取任务处理
• 主线程join等待多线程处理完，收尾处理完成任务。

最开始版本代码，10个任务，3个线程来处理

package org.example;import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.BlockingDeque;
import java.util.concurrent.LinkedBlockingDeque;public class Main3 {public static void main(String[] args){System.out.println("start");BlockingDeque<Integer> task = new LinkedBlockingDeque<>();for (int i = 0; i < 10; i++) {task.add(i);}List<Thread> workers = new ArrayList<>();for (int i = 0; i < 3; i++) {Thread worker = new Thread(()->{while (true) {Integer data = null;try {if (task.size()==0) {System.out.println(Thread.currentThread().getName() +" quit");break;}
//                        Thread.sleep(100); // 默认任务耗时data = task.take();} catch (InterruptedException e) {throw new RuntimeException(e);}System.out.println(Thread.currentThread().getName() +" do "+ data);}});workers.add(worker);worker.start();}for (Thread worker: workers) {try {worker.join();} catch (InterruptedException e) {throw new RuntimeException(e);}}System.out.println("job done");}
}

运行之后，感觉非常好，完美实现逻辑

但是当把上面的任务数加到200，线程数加到30，上面线程sleep的注释打开，再次运行，就会发现主进程最后会被一直卡着不结束，说明多线程没有正确判断任务结束，线程不安全

上面的子线程内的size()等于0到下面的BlockingDeque.take()取任务这段之间的代码，这段不是线程安全的

让线程正确判断任务结束，而且要线程安全的三种方法，推荐第二种，兼顾效率和兼容正确性

• 判断任务结束这段代码加synchronized约束起来，实现线程安全（太慢）
• 给总任务task内，加入和线程相同数量的停止标志marker
• 使用BlockingDeque.poll(超时时间) + 异常数据检查（需要检查异常数据）

使用synchronized约束

package org.example;import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.BlockingDeque;
import java.util.concurrent.LinkedBlockingDeque;public class Main {public static void main(String[] args) {System.out.println("start");BlockingDeque<Integer> task = new LinkedBlockingDeque<>();for (int i = 0; i < 20; i++) {task.add(i);}List<Thread> workers = new ArrayList<>();for (int i = 0; i < 3; i++) {Thread worker = new Thread(()->{while (true) {Integer data = null;synchronized (task) {if (task.size() ==0) {System.out.println(Thread.currentThread().getName() +" quit");break;}try {data = task.take();} catch (InterruptedException e) {throw new RuntimeException(e);}}try {Thread.sleep(300);} catch (InterruptedException e) {throw new RuntimeException(e);}System.out.println(Thread.currentThread().getName() +" do "+ data);}});workers.add(worker);worker.start();}for (Thread worker: workers) {try {worker.join();} catch (InterruptedException e) {throw new RuntimeException(e);}}System.out.println("job done");}
}

总任务添加stop marker停止标志

package org.example;import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.BlockingDeque;
import java.util.concurrent.LinkedBlockingDeque;public class Main2 {public static void main(String[] args){System.out.println("start");BlockingDeque<Integer> task = new LinkedBlockingDeque<>();for (int i = 0; i < 20; i++) {task.add(i);}List<Thread> workers = new ArrayList<>();for (int i = 0; i < 3; i++) task.add(99);for (int i = 0; i < 3; i++) {Thread worker = new Thread(()->{while (true) {Integer data = null;try {data = task.take();if (data == 99) {System.out.println(Thread.currentThread().getName() +" quit");break;}} catch (InterruptedException e) {throw new RuntimeException(e);}try {Thread.sleep(300);} catch (InterruptedException e) {throw new RuntimeException(e);}System.out.println(Thread.currentThread().getName() +" do "+ data);}});workers.add(worker);worker.start();}for (Thread worker: workers) {try {worker.join();} catch (InterruptedException e) {throw new RuntimeException(e);}}System.out.println("job done");}
}

使用BlockingDeque.poll(超时时间)，避免了take的永久性等待问题，但是会取到null值，要加判断处理

package org.example;import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.BlockingDeque;
import java.util.concurrent.LinkedBlockingDeque;
import java.util.concurrent.TimeUnit;public class Main4 {public static void main(String[] args){System.out.println("start");BlockingDeque<Integer> task = new LinkedBlockingDeque<>();for (int i = 0; i < 200; i++) {task.add(i);}List<Thread> workers = new ArrayList<>();for (int i = 0; i < 30; i++) {Thread worker = new Thread(()->{while (true) {Integer data = null;try {if (task.size()==0) {System.out.println(Thread.currentThread().getName() +" quit");break;}Thread.sleep(100); // 默认任务耗时data = task.poll(1000, TimeUnit.MILLISECONDS);if (data == null) {System.out.println(Thread.currentThread().getName() +" get null");continue;}} catch (InterruptedException e) {throw new RuntimeException(e);}System.out.println(Thread.currentThread().getName() +" do "+ data);}});workers.add(worker);worker.start();}for (Thread worker: workers) {try {worker.join();} catch (InterruptedException e) {throw new RuntimeException(e);}}System.out.println("job done");}
}