線程池之ThreadPoolExecutor使用

轉載自：https://www.jianshu.com/p/f030aa5d7a28

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。

2. 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。

3. 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。

4. 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。

做者：徐志毅 連接：https://www.jianshu.com/p/f030aa5d7a28 來源：簡書 簡書著做權歸做者全部，任何形式的轉載都請聯繫做者得到受權並註明出處。