背景

線程池的技術(shù)在項(xiàng)目中使用廣泛，線程池提供了四種拒絕策略，大家是否了解這四種拒絕的策略呢？本文將詳細(xì)的講解ThreadPoolExecutor的四種拒絕策略，以及相關(guān)的注意事項(xiàng)。

線程池基本原理

線程池的原理如下圖：

說明：

• 當(dāng)前運(yùn)行的線程少于corePoolSize，則創(chuàng)建新線程來執(zhí)行任務(wù)。
• 運(yùn)行的線程等于或多于corePoolSize，則將任務(wù)添加到隊(duì)列中。
• 當(dāng)任務(wù)隊(duì)列已滿，則在非corePool中創(chuàng)建新的線程來處理任務(wù)。
• 創(chuàng)建新線程將使當(dāng)前運(yùn)行的線程超出maximumPoolSize，任務(wù)將被拒絕，并調(diào)用RejectedExecutionHandler.rejectedExecution()方法。

線程池拒絕策略

線程池為我們提供了四種拒絕策略分別是：CallerRunsPolicy,AbortPolicy,DiscardPolicy,DiscardOldestPolicy

AbortPolicy

ThreadPoolExecutor中默認(rèn)的拒絕策略就是AbortPolicy直接拋出異常,具體實(shí)現(xiàn)如下

public static class AbortPolicy implements RejectedExecutionHandler {
    public AbortPolicy() { }
    public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
        throw new RejectedExecutionException("Task " + r.toString() +
                                             " rejected from " +
                                             e.toString());
    }
}

說明：這種策略非常簡單粗暴，直接拋出RejectedExecutionException異常，也不會(huì)執(zhí)行后續(xù)的任務(wù)。

示例說明：

public class ThreadPoolTest
{
    public static void main(String[] args)
    {
        ThreadPoolExecutor threadPoolExecutor  = new ThreadPoolExecutor(
                2,
                5,
                10,
                TimeUnit.MICROSECONDS,
                new LinkedBlockingDeque<>(1),
                new ThreadPoolExecutor.AbortPolicy());
        
        //異步執(zhí)行
        for(int i=0; i<10;i++)
        {
          System.out.println("添加第"+i+"個(gè)任務(wù)");
          threadPoolExecutor.execute(new TestThread("線程"+i));
        }        
    }
}

public class TestThread implements Runnable
{
    private String name;
    public TestThread(String name){
        this.name=name;
    }
    
    @Override
    public void run()
    {
        try
        {
            Thread.sleep(1000);
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
        System.out.println("thread name:"+Thread.currentThread().getName()+",執(zhí)行:"+name);
    }
}

執(zhí)行結(jié)果：

Exception in thread "main" java.util.concurrent.RejectedExecutionException: Task com.skywares.fw.juc.thread.TestThread@55f96302 rejected from java.util.concurrent.ThreadPoolExecutor@3d4eac69[Running, pool size = 5, active threads = 5, queued tasks = 1, completed tasks = 0]
    at java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:2047)
    at java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:823)
    at java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:1369)
    at com.skywares.fw.juc.thread.ThreadPoolTest.main(ThreadPoolTest.java:26)
thread name:pool-1-thread-5,執(zhí)行:線程5
thread name:pool-1-thread-2,執(zhí)行:線程1
thread name:pool-1-thread-4,執(zhí)行:線程4
thread name:pool-1-thread-3,執(zhí)行:線程3
thread name:pool-1-thread-1,執(zhí)行:線程0
thread name:pool-1-thread-5,執(zhí)行:線程2

從執(zhí)行結(jié)果我們得知，采用AbortPolicy策略當(dāng)任務(wù)執(zhí)行到第七個(gè)任務(wù)時(shí)會(huì)直接報(bào)錯(cuò)，導(dǎo)致后續(xù)的業(yè)務(wù)邏輯不會(huì)執(zhí)行。

CallerRunsPolicy

CallerRunsPolicy在任務(wù)被拒絕添加后，會(huì)用調(diào)用execute函數(shù)的上層線程去執(zhí)行被拒絕的任務(wù)。

相關(guān)示例

public class ThreadPoolTest
{
    public static void main(String[] args)
    {
        ThreadPoolExecutor threadPoolExecutor  = new ThreadPoolExecutor(
                2,
                5,
                10,
                TimeUnit.MICROSECONDS,
                new LinkedBlockingDeque<>(1),
                new ThreadPoolExecutor.CallerRunsPolicy());
        
        //異步執(zhí)行
        for(int i=0; i<10;i++)
        {
          System.out.println("添加第"+i+"個(gè)任務(wù)");
          threadPoolExecutor.execute(new TestThread("線程"+i));
        }
    }
}

執(zhí)行結(jié)果：

添加第0個(gè)任務(wù)
添加第1個(gè)任務(wù)
添加第2個(gè)任務(wù)
添加第3個(gè)任務(wù)
添加第4個(gè)任務(wù)
添加第5個(gè)任務(wù)
添加第6個(gè)任務(wù)
thread name:main,執(zhí)行:線程6
thread name:pool-1-thread-3,執(zhí)行:線程3
thread name:pool-1-thread-1,執(zhí)行:線程0
thread name:pool-1-thread-4,執(zhí)行:線程4
thread name:pool-1-thread-2,執(zhí)行:線程1
thread name:pool-1-thread-5,執(zhí)行:線程5
添加第7個(gè)任務(wù)
添加第8個(gè)任務(wù)
thread name:main,執(zhí)行:線程8
thread name:pool-1-thread-1,執(zhí)行:線程7
thread name:pool-1-thread-3,執(zhí)行:線程2
添加第9個(gè)任務(wù)
thread name:pool-1-thread-1,執(zhí)行:線程9

從執(zhí)行的結(jié)果我們可以得知，當(dāng)執(zhí)行到第7個(gè)任務(wù)時(shí)，由于線程池拒絕策略，此任務(wù)由主線程來執(zhí)行，當(dāng)線程池有空閑時(shí)，才繼續(xù)執(zhí)行其他的任務(wù)。所以此策略可能會(huì)阻塞主線程。

DiscardPolicy

這種拒絕策略比較簡單，線程池拒絕的任務(wù)直接拋棄，不會(huì)拋異常也不會(huì)執(zhí)行

示例

修改上述的代碼，將拒絕策略修改為DiscardPolicy

 ThreadPoolExecutor threadPoolExecutor  = new ThreadPoolExecutor(
                2,
                5,
                10,
                TimeUnit.MICROSECONDS,
                new LinkedBlockingDeque<>(1),
                new ThreadPoolExecutor.CallerRunsPolicy());

執(zhí)行結(jié)果

invoke dealStock success
goodsId:手機(jī)
thread name:pool-1-thread-1,執(zhí)行:線程0
thread name:pool-1-thread-4,執(zhí)行:線程4
thread name:pool-1-thread-5,執(zhí)行:線程5
thread name:pool-1-thread-3,執(zhí)行:線程3
thread name:pool-1-thread-2,執(zhí)行:線程1
thread name:pool-1-thread-1,執(zhí)行:線程2

從執(zhí)行的結(jié)果來看只執(zhí)行了6個(gè)任務(wù)，其他的任務(wù)都被拋棄了。

DiscardOldestPolicy

DiscardOldestPolicy 當(dāng)任務(wù)拒絕添加時(shí)，會(huì)拋棄任務(wù)隊(duì)列中最先加入隊(duì)列的任務(wù)，再把新任務(wù)添加進(jìn)去。

示例說明

 ThreadPoolExecutor threadPoolExecutor  = new ThreadPoolExecutor(
                1,
                2,
                10,
                TimeUnit.MICROSECONDS,
                new LinkedBlockingDeque<>(2),
                new ThreadPoolExecutor.CallerRunsPolicy());

執(zhí)行結(jié)果：

添加第0個(gè)任務(wù)
添加第1個(gè)任務(wù)
添加第2個(gè)任務(wù)
添加第3個(gè)任務(wù)
添加第4個(gè)任務(wù)
添加第5個(gè)任務(wù)
invoke dealStock success
goodsId:手機(jī)
thread name:pool-1-thread-2,執(zhí)行:線程3
thread name:pool-1-thread-1,執(zhí)行:線程0
thread name:pool-1-thread-1,執(zhí)行:線程2
thread name:pool-1-thread-2,執(zhí)行:線程1

自定義拒絕策略

當(dāng)線程池提供的拒絕策略無法滿足要求時(shí)，我們可以采用自定義的拒絕策略，只需要實(shí)現(xiàn)RejectedExecutionHandler接口即可

public class CustRejectedExecutionHandler implements RejectedExecutionHandler
{
    @Override
    public void rejectedExecution(Runnable r, ThreadPoolExecutor executor)
    {
        new Thread(r,"線程:"+new Random().nextInt(10)).start();
    }
}

  ThreadPoolExecutor threadPoolExecutor  = new ThreadPoolExecutor(
                1,
                2,
                10,
                TimeUnit.MICROSECONDS,
                new LinkedBlockingDeque<>(2),
                new CustRejectedExecutionHandler());

執(zhí)行結(jié)果：

thread name:客戶線程:6,執(zhí)行:線程5
thread name:pool-1-thread-1,執(zhí)行:線程0
thread name:客戶線程:8,執(zhí)行:線程4
thread name:pool-1-thread-2,執(zhí)行:線程3
thread name:pool-1-thread-1,執(zhí)行:線程1
thread name:pool-1-thread-2,執(zhí)行:線程2

從執(zhí)行的結(jié)果來看，被拒絕的任務(wù)都在客戶的新線程中執(zhí)行。

小結(jié)

• AbortPolicy：直接拋出異常，后續(xù)的任務(wù)不會(huì)執(zhí)行
• CallerRunsPolicy：子任務(wù)執(zhí)行的時(shí)間過長，可能會(huì)阻塞主線程。
• DiscardPolicy：不拋異常，任務(wù)直接丟棄
• DiscardOldestPolicy；丟棄最先加入隊(duì)列的任務(wù)

總結(jié)

本文對(duì)于線程的池的幾種策略進(jìn)行詳細(xì)的講解，在實(shí)際的生產(chǎn)中需要集合相關(guān)的場(chǎng)景來選擇合適的拒絕策略，如有疑問，請(qǐng)隨時(shí)反饋。

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