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Java ThreadPoolExecutor的參數深入理解
#一、使用Executors建立執行緒池
先前建立執行緒的時候都是使用的Executors的newFixedThreadPool(),newSinglereadExecutor(),newCachedTh#Pool(ThreadExecutor(),newCachedTh#Pool(Thread 。當然Executors也是用不同的參數去new ThreadPoolExecutor
1. newFixedThreadPool()
# 建立執行緒數固定大小的執行緒池。 由於使用了LinkedBlockingQueue所以maximumPoolSize 沒用,當corePoolSize滿了之後就加入到LinkedBlockingQueue佇列中。每當某個執行緒執行完成之後就從LinkedBlockingQueue佇列中取一個。所以這個是創建固定大小的線程池。
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()
建立執行緒數為1的執行緒池,由於使用了LinkedBlockingQueue所以maximumPoolSize 沒用,corePoolSize為1表示執行緒數大小為1表示執行緒數大小為1表示執行緒數1,滿了就放入佇列中,執行完了就從佇列取一個。public static ExecutorService newSingleThreadExecutor() { return new FinalizableDelegatedExecutorService (new ThreadPoolExecutor(1, 1, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>())); }
3.newCachedThreadPool()
建立可緩衝的執行緒池。沒有大小限制。由於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,由於maxumumPoolSize為Integer.MAX_VALUE所以可以認為大小為2147483647。受記憶體大小限制。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建立執行緒池
ThreadPoolExecutor的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核心執行緒數大小,當執行緒數
1、執行緒數小於corePoolSize時,建立執行緒執行任務。 2、當執行緒數大於等於corePoolSize且workQueue沒有滿時,放入workQueue中
## 3、執行緒數大於等於corePoolSize且當workQueue執行滿時,新執行緒執行執行數總數要小於maximumPoolSize 4、當總線程數等於maximumPoolSize且workQueue滿了的時候執行handler的rejectedExecution。也就是拒絕策略。ThreadPoolExecutor預設有四個拒絕策略:
1、ThreadPoolExecutor.AbortPolicy() 直接#拋出異常RejectedExecution#ception 2、ThreadPoolExecutor.CallerRunsPolicy() 直接呼叫run方法並且阻斷執行
3、ThreadPoolExecutor.DiscardPolicy 3、ThreadPoolExecutor.DiscardPolicy 3、ThreadPoolExecutor.DiscardPolicy) Policy() 丟棄在佇列中隊首的任務當然可以自己繼承
RejectedExecutionHandler來寫拒絕策略.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(); //线程池和队列满了之后的处理方式 //1.跑出异常 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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