Java ThreadPoolExecutor的參數深入理解
一、使用Executors創建線程池
之前創建線程的時候都是用的Executors的newFixedThreadPool(),newSingleThreadExecutor(),newCachedThreadPool()這三個方法。當然Executors也是用不同的參數去new ThreadPoolExecutor
1. newFixedThreadPool()
創建線程數固定大小的線程池。 由于使用了LinkedBlockingQueue所以maximumPoolSize 沒用，當corePoolSize滿了之后就加入到LinkedBlockingQueue隊列中。每當某個線程執行完成之后就從LinkedBlockingQueue隊列中取一個。所以這個是創建固定大小的線程池。
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public static ExecutorService newFixedThreadPool(int nThreads) { return new ThreadPoolExecutor(nThreads, nThreads, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>()); }public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue) { this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, Executors.defaultThreadFactory(), defaultHandler); } |
2.newSingleThreadPool()
創建線程數為1的線程池，由于使用了LinkedBlockingQueue所以maximumPoolSize 沒用，corePoolSize為1表示線程數大小為1,滿了就放入隊列中，執行完了就從隊列取一個。
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public static ExecutorService newSingleThreadExecutor() { return new FinalizableDelegatedExecutorService (new ThreadPoolExecutor(1, 1, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>())); } |
3.newCachedThreadPool()
創建可緩沖的線程池。沒有大小限制。由于corePoolSize為0所以任務會放入SynchronousQueue隊列中，SynchronousQueue只能存放大小為1，所以會立刻新起線程，由于maxumumPoolSize為Integer.MAX_VALUE所以可以認為大小為2147483647。受內存大小限制。
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public static ExecutorService newCachedThreadPool() { return new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60L, TimeUnit.SECONDS, new SynchronousQueue<Runnable>()); }public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue) { this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, Executors.defaultThreadFactory(), defaultHandler); } |
二、使用ThreadPoolExecutor創建線程池
ThreadPoolExecutor的構造函數
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public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory, RejectedExecutionHandler handler) { 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; } |
參數：
1、corePoolSize核心線程數大小，當線程數<corePoolSize ，會創建線程執行runnable
2、maximumPoolSize 最大線程數， 當線程數 >= corePoolSize的時候，會把runnable放入workQueue中
3、keepAliveTime 保持存活時間，當線程數大于corePoolSize的空閑線程能保持的最大時間。
4、unit 時間單位
5、workQueue 保存任務的阻塞隊列
6、threadFactory 創建線程的工廠
7、handler 拒絕策略
任務執行順序：
1、當線程數小于corePoolSize時，創建線程執行任務。
2、當線程數大于等于corePoolSize并且workQueue沒有滿時，放入workQueue中
3、線程數大于等于corePoolSize并且當workQueue滿時，新任務新建線程運行，線程總數要小于maximumPoolSize
4、當線程總數等于maximumPoolSize并且workQueue滿了的時候執行handler的rejectedExecution。也就是拒絕策略。
ThreadPoolExecutor默認有四個拒絕策略：
1、ThreadPoolExecutor.AbortPolicy() 直接拋出異常RejectedExecutionException
2、ThreadPoolExecutor.CallerRunsPolicy() 直接調用run方法并且阻塞執行
3、ThreadPoolExecutor.DiscardPolicy() 直接丟棄后來的任務
4、ThreadPoolExecutor.DiscardOldestPolicy() 丟棄在隊列中隊首的任務
當然可以自己繼承RejectedExecutionHandler來寫拒絕策略.
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int corePoolSize = 1; int maximumPoolSize = 2; int keepAliveTime = 10;// BlockingQueue<Runnable> workQueue = new LinkedBlockingQueue<Runnable>(); BlockingQueue<Runnable> workQueue = new ArrayBlockingQueue<Runnable>(5); ThreadFactory threadFactory = Executors.defaultThreadFactory(); //線程池和隊列滿了之后的處理方式 //1.跑出異常 RejectedExecutionHandler handler = new ThreadPoolExecutor.AbortPolicy(); RejectedExecutionHandler handler2 = new ThreadPoolExecutor.CallerRunsPolicy(); RejectedExecutionHandler handler3 = new ThreadPoolExecutor.DiscardPolicy(); RejectedExecutionHandler handler4 = new ThreadPoolExecutor.DiscardOldestPolicy(); ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(corePoolSize, maximumPoolSize, keepAliveTime, TimeUnit.SECONDS, workQueue, threadFactory, handler2); for (int j = 1; j < 15; j++) { threadPoolExecutor.execute(new Runnable() { public void run() { try { System.out.println(Thread.currentThread().getName()); TimeUnit.SECONDS.sleep(1); } catch (InterruptedException e) { e.printStackTrace(); } } }); } System.out.println(threadPoolExecutor); } |
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原文鏈接：https://my.oschina.net/u/2250599/blog/498787
