線程池之ThreadPoolExecutor使用

ThreadPoolExecutor提供了四個構造方法：

 

 

ThreadPoolExecutor構造方法.png

 

咱們以最後一個構造方法（參數最多的那個），對其參數進行解釋：

public ThreadPoolExecutor(int corePoolSize, // 1
                              int maximumPoolSize,  // 2
                              long keepAliveTime,  // 3
                              TimeUnit unit,  // 4
                              BlockingQueue<Runnable> workQueue, // 5
                              ThreadFactory threadFactory,  // 6
                              RejectedExecutionHandler handler ) { //7
        if (corePoolSize < 0 ||
            maximumPoolSize <= 0 ||
            maximumPoolSize < corePoolSize ||
            keepAliveTime < 0)
            throw new IllegalArgumentException();
        if (workQueue == null || threadFactory == null || handler == null)
            throw new NullPointerException();
        this.corePoolSize = corePoolSize;
        this.maximumPoolSize = maximumPoolSize;
        this.workQueue = workQueue;
        this.keepAliveTime = unit.toNanos(keepAliveTime);
        this.threadFactory = threadFactory;
        this.handler = handler;
    }

序號	名稱	類型	含義
1	corePoolSize	int	核心線程池大小
2	maximumPoolSize	int	最大線程池大小
3	keepAliveTime	long	線程最大空閒時間
4	unit	TimeUnit	時間單位
5	workQueue	BlockingQueue<Runnable>	線程等待隊列
6	threadFactory	ThreadFactory	線程建立工廠
7	handler	RejectedExecutionHandler	拒絕策略

若是對這些參數做用有疑惑的請看 ThreadPoolExecutor概述。
知道了各個參數的做用後，咱們開始構造符合咱們期待的線程池。首先看JDK給咱們預約義的幾種線程池：

1、預約義線程池

1. FixedThreadPool

public static ExecutorService newFixedThreadPool(int nThreads) {
        return new ThreadPoolExecutor(nThreads, nThreads,
                                      0L, TimeUnit.MILLISECONDS,
                                      new LinkedBlockingQueue<Runnable>());
    }

• corePoolSize與maximumPoolSize相等，即其線程全爲核心線程，是一個固定大小的線程池，是其優點；
• keepAliveTime = 0 該參數默認對核心線程無效，而FixedThreadPool所有爲核心線程；
• workQueue 爲LinkedBlockingQueue（無界阻塞隊列），隊列最大值爲Integer.MAX_VALUE。若是任務提交速度持續大餘任務處理速度，會形成隊列大量阻塞。由於隊列很大，頗有可能在拒絕策略前，內存溢出。是其劣勢；
• FixedThreadPool的任務執行是無序的；

適用場景：可用於Web服務瞬時削峯，但需注意長時間持續高峯狀況形成的隊列阻塞。

1. CachedThreadPool

public static ExecutorService newCachedThreadPool() {
        return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                                      60L, TimeUnit.SECONDS,
                                      new SynchronousQueue<Runnable>());
    }

• corePoolSize = 0，maximumPoolSize = Integer.MAX_VALUE，即線程數量幾乎無限制；
• keepAliveTime = 60s，線程空閒60s後自動結束。
• workQueue 爲 SynchronousQueue 同步隊列，這個隊列相似於一個接力棒，入隊出隊必須同時傳遞，由於CachedThreadPool線程建立無限制，不會有隊列等待，因此使用SynchronousQueue；

適用場景：快速處理大量耗時較短的任務，如Netty的NIO接受請求時，可以使用CachedThreadPool。

1. SingleThreadExecutor

public static ExecutorService newSingleThreadExecutor() {
        return new FinalizableDelegatedExecutorService
            (new ThreadPoolExecutor(1, 1,
                                    0L, TimeUnit.MILLISECONDS,
                                    new LinkedBlockingQueue<Runnable>()));
    }

咋一瞅，不就是newFixedThreadPool(1)嗎？定眼一看，這裏多了一層FinalizableDelegatedExecutorService包裝，這一層有什麼用呢，寫個dome來解釋一下：

public static void main(String[] args) {
        ExecutorService fixedExecutorService = Executors.newFixedThreadPool(1);
        ThreadPoolExecutor threadPoolExecutor = (ThreadPoolExecutor) fixedExecutorService;
        System.out.println(threadPoolExecutor.getMaximumPoolSize());
        threadPoolExecutor.setCorePoolSize(8);
        
        ExecutorService singleExecutorService = Executors.newSingleThreadExecutor();
//      運行時異常 java.lang.ClassCastException
//      ThreadPoolExecutor threadPoolExecutor2 = (ThreadPoolExecutor) singleExecutorService;
    }

對比能夠看出，FixedThreadPool能夠向下轉型爲ThreadPoolExecutor，並對其線程池進行配置，而SingleThreadExecutor被包裝後，沒法成功向下轉型。所以，SingleThreadExecutor被定之後，沒法修改，作到了真正的Single。

1. ScheduledThreadPool

public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
        return new ScheduledThreadPoolExecutor(corePoolSize);
    }

newScheduledThreadPool調用的是ScheduledThreadPoolExecutor的構造方法，而ScheduledThreadPoolExecutor繼承了ThreadPoolExecutor，構造是仍是調用了其父類的構造方法。

public ScheduledThreadPoolExecutor(int corePoolSize) {
        super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,
              new DelayedWorkQueue());
    }

對於ScheduledThreadPool本文不作描述，其特性請關注後續篇章。

2、自定義線程池

如下是自定義線程池，使用了有界隊列，自定義ThreadFactory和拒絕策略的demo：

public class ThreadTest {

    public static void main(String[] args) throws InterruptedException, IOException {
        int corePoolSize = 2;
        int maximumPoolSize = 4;
        long keepAliveTime = 10;
        TimeUnit unit = TimeUnit.SECONDS;
        BlockingQueue<Runnable> workQueue = new ArrayBlockingQueue<>(2);
        ThreadFactory threadFactory = new NameTreadFactory();
        RejectedExecutionHandler handler = new MyIgnorePolicy();
        ThreadPoolExecutor executor = new ThreadPoolExecutor(corePoolSize, maximumPoolSize, keepAliveTime, unit,
                workQueue, threadFactory, handler);
        executor.prestartAllCoreThreads(); // 預啓動全部核心線程
        
        for (int i = 1; i <= 10; i++) {
            MyTask task = new MyTask(String.valueOf(i));
            executor.execute(task);
        }

        System.in.read(); //阻塞主線程
    }

    static class NameTreadFactory implements ThreadFactory {

        private final AtomicInteger mThreadNum = new AtomicInteger(1);

        @Override
        public Thread newThread(Runnable r) {
            Thread t = new Thread(r, "my-thread-" + mThreadNum.getAndIncrement());
            System.out.println(t.getName() + " has been created");
            return t;
        }
    }

    public static class MyIgnorePolicy implements RejectedExecutionHandler {

        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
            doLog(r, e);
        }

        private void doLog(Runnable r, ThreadPoolExecutor e) {
            // 可作日誌記錄等
            System.err.println( r.toString() + " rejected");
//          System.out.println("completedTaskCount: " + e.getCompletedTaskCount());
        }
    }

    static class MyTask implements Runnable {
        private String name;

        public MyTask(String name) {
            this.name = name;
        }

        @Override
        public void run() {
            try {
                System.out.println(this.toString() + " is running!");
                Thread.sleep(3000); //讓任務執行慢點
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

        public String getName() {
            return name;
        }

        @Override
        public String toString() {
            return "MyTask [name=" + name + "]";
        }
    }
}

輸出結果以下：

 

 

image.png

 

其中線程線程1-4先佔滿了核心線程和最大線程數量，而後四、5線程進入等待隊列，7-10線程被直接忽略拒絕執行，等1-4線程中有線程執行完後通知四、5線程繼續執行。

總結，經過自定義線程池，咱們能夠更好的讓線程池爲咱們所用，更加適應個人實際場景。