DiscardOldestPolicy策略

DiscardOldestPolicy策略是当任务添加到线程池中被拒绝时，线程池会放弃等待队列中最旧的未处理任务，将被拒绝的任务添加到等待队列中。
线程执行代码如下：

public class TheRunnable implements Runnable {
 
    public String getUsername() {
        return username;
    }
 
    public void setUsername(String username) {
        this.username = username;
    }
 
    private String username;
 
    public TheRunnable(String username){
        super();
        this.username=username;
    }
 
 
 
    @Override
    public void run() {
        try {
            SimpleDateFormat simpleDateFormat = new SimpleDateFormat("HH:mm:ss");
            System.out.println("线程名："+username+"  开始时间："+simpleDateFormat.format(new Date()));
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

运行类代码如下：

@Slf4j
public class DiscardOldestPolicyRun {
    public static void main(String[] args) {
        ArrayBlockingQueue arrayBlockingQueue = new ArrayBlockingQueue(2);
        ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(2, 3, 10, TimeUnit.SECONDS, arrayBlockingQueue, new ThreadPoolExecutor.DiscardOldestPolicy());
        for (int i = 0; i < 5 ; i++) {
            TheRunnable runnable = new TheRunnable("Runnable " + i );
            threadPoolExecutor.execute(runnable);
        }
        try {
            Thread.sleep(500);
            Iterator iterator = arrayBlockingQueue.iterator();
            while (iterator.hasNext()){
                Object object = iterator.next();
                log.info("线程名:{}",((TheRunnable)object).getUsername());
            }
            threadPoolExecutor.execute(new TheRunnable("Runnable 6"));
            threadPoolExecutor.execute(new TheRunnable("Runnable 7"));
            iterator = arrayBlockingQueue.iterator();
            while(iterator.hasNext()){
                Object object = iterator.next();
                log.info("线程名：{}",((TheRunnable)object).getUsername());
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

运行结果如下：

线程名：Runnable 4 开始时间：16:47:54
线程名：Runnable 1 开始时间：16:47:54
线程名：Runnable 0 开始时间：16:47:54
16:47:55.509 [main] INFO com.ozx.concurrentprogram.executor.controller.DiscardOldestPolicyRun - 线程名:Runnable 2
16:47:55.516 [main] INFO com.ozx.concurrentprogram.executor.controller.DiscardOldestPolicyRun - 线程名:Runnable 3
16:47:55.517 [main] INFO com.ozx.concurrentprogram.executor.controller.DiscardOldestPolicyRun - 线程名：Runnable 6
16:47:55.517 [main] INFO com.ozx.concurrentprogram.executor.controller.DiscardOldestPolicyRun - 线程名：Runnable 7
线程名：Runnable 7 开始时间：16:47:56
线程名：Runnable 6 开始时间：16:47:56
从运行结果看出早期放入队列中两个任务被取消了，执行了后来添加的两个任务。

DiscardPolicy策略

DiscardPolicy策略是当任务添加到线程池中被拒绝时，线程池将丢弃被拒绝的任务。

运行类代码如下：

@Slf4j
public class DiscardPolicyRun {
    public static void main(String[] args) throws InterruptedException {
        Runnable runnable = new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(5000);
                    log.info(Thread.currentThread().getName()+"执行结束");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        };
 
        ArrayBlockingQueue arrayBlockingQueue = new ArrayBlockingQueue(2);
        ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(2, 3, 10, TimeUnit.SECONDS, arrayBlockingQueue, new ThreadPoolExecutor.DiscardPolicy());
        for (int i = 0; i < 8 ; i++) {
            threadPoolExecutor.execute(runnable);
        }
        Thread.sleep(8000);
        log.info("线程池中最大线程数：{},队列数：{}",threadPoolExecutor.getPoolSize(),arrayBlockingQueue.size());
    }
}

运行结果如下：
17:40:14.301 [pool-1-thread-1] INFO com.ozx.concurrentprogram.executor.controller.DiscardPolicyRun - pool-1-thread-1执行结束
17:40:14.301 [pool-1-thread-2] INFO com.ozx.concurrentprogram.executor.controller.DiscardPolicyRun - pool-1-thread-2执行结束
17:40:14.301 [pool-1-thread-3] INFO com.ozx.concurrentprogram.executor.controller.DiscardPolicyRun - pool-1-thread-3执行结束
17:40:17.278 [main] INFO com.ozx.concurrentprogram.executor.controller.DiscardPolicyRun - 线程池中最大线程数：3,队列数：0
17:40:19.326 [pool-1-thread-1] INFO com.ozx.concurrentprogram.executor.controller.DiscardPolicyRun - pool-1-thread-1执行结束
17:40:19.326 [pool-1-thread-2] INFO com.ozx.concurrentprogram.executor.controller.DiscardPolicyRun - pool-1-thread-2执行结束
从运行结果看出多余的任务直接被取消执行。