Java ThreadPoolExecutor的參數(shù)深入理解
Java ThreadPoolExecutor的參數(shù)深入理解
一、使用Executors創(chuàng)建線程池
之前創(chuàng)建線程的時(shí)候都是用的Executors的newFixedThreadPool(),newSingleThreadExecutor(),newCachedThreadPool()這三個(gè)方法。當(dāng)然Executors也是用不同的參數(shù)去new ThreadPoolExecutor
1. newFixedThreadPool()
創(chuàng)建線程數(shù)固定大小的線程池。 由于使用了LinkedBlockingQueue所以maximumPoolSize 沒(méi)用,當(dāng)corePoolSize滿了之后就加入到LinkedBlockingQueue隊(duì)列中。每當(dāng)某個(gè)線程執(zhí)行完成之后就從LinkedBlockingQueue隊(duì)列中取一個(gè)。所以這個(gè)是創(chuàng)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()
創(chuàng)建線程數(shù)為1的線程池,由于使用了LinkedBlockingQueue所以maximumPoolSize 沒(méi)用,corePoolSize為1表示線程數(shù)大小為1,滿了就放入隊(duì)列中,執(zhí)行完了就從隊(duì)列取一個(gè)。
public static ExecutorService newSingleThreadExecutor() { return new FinalizableDelegatedExecutorService (new ThreadPoolExecutor(1, 1, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>())); }
3.newCachedThreadPool()
創(chuàng)建可緩沖的線程池。沒(méi)有大小限制。由于corePoolSize為0所以任務(wù)會(huì)放入SynchronousQueue隊(duì)列中,SynchronousQueue只能存放大小為1,所以會(huì)立刻新起線程,由于maxumumPoolSize為Integer.MAX_VALUE所以可以認(rèn)為大小為2147483647。受內(nèi)存大小限制。
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創(chuàng)建線程池
ThreadPoolExecutor的構(gòu)造函數(shù)
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(shù):
1、corePoolSize核心線程數(shù)大小,當(dāng)線程數(shù)<corePoolSize ,會(huì)創(chuàng)建線程執(zhí)行runnable
2、maximumPoolSize 最大線程數(shù), 當(dāng)線程數(shù) >= corePoolSize的時(shí)候,會(huì)把runnable放入workQueue中
3、keepAliveTime 保持存活時(shí)間,當(dāng)線程數(shù)大于corePoolSize的空閑線程能保持的最大時(shí)間。
4、unit 時(shí)間單位
5、workQueue 保存任務(wù)的阻塞隊(duì)列
6、threadFactory 創(chuàng)建線程的工廠
7、handler 拒絕策略
任務(wù)執(zhí)行順序:
1、當(dāng)線程數(shù)小于corePoolSize時(shí),創(chuàng)建線程執(zhí)行任務(wù)。
2、當(dāng)線程數(shù)大于等于corePoolSize并且workQueue沒(méi)有滿時(shí),放入workQueue中
3、線程數(shù)大于等于corePoolSize并且當(dāng)workQueue滿時(shí),新任務(wù)新建線程運(yùn)行,線程總數(shù)要小于maximumPoolSize
4、當(dāng)線程總數(shù)等于maximumPoolSize并且workQueue滿了的時(shí)候執(zhí)行handler的rejectedExecution。也就是拒絕策略。
ThreadPoolExecutor默認(rèn)有四個(gè)拒絕策略:
1、ThreadPoolExecutor.AbortPolicy() 直接拋出異常RejectedExecutionException
2、ThreadPoolExecutor.CallerRunsPolicy() 直接調(diào)用run方法并且阻塞執(zhí)行
3、ThreadPoolExecutor.DiscardPolicy() 直接丟棄后來(lái)的任務(wù)
4、ThreadPoolExecutor.DiscardOldestPolicy() 丟棄在隊(duì)列中隊(duì)首的任務(wù)
當(dāng)然可以自己繼承RejectedExecutionHandler來(lái)寫(xiě)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(); //線程池和隊(duì)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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