之前對(duì)線程池中屬性：keepAliveTime比較模糊，而且看過之后過一段時(shí)間就會(huì)忘掉，于是就在此記錄一下。

當(dāng)線程數(shù)大于核心時(shí)，此為終止前多余的空閑線程等待新任務(wù)的最長(zhǎng)時(shí)間。

說的讓人感覺比較模糊，總結(jié)一下大概意思為：比如說線程池中最大的線程數(shù)為50，而其中只有40個(gè)線程任務(wù)在跑，相當(dāng)于有10個(gè)空閑線程，這10個(gè)空閑線程不能讓他一直在開著，因?yàn)榫€程的存在也會(huì)特別好資源的，所有就需要設(shè)置一個(gè)這個(gè)空閑線程的存活時(shí)間，這么解釋應(yīng)該就很清楚了。

這樣以后忘記了就過來看看就OK了。

補(bǔ)充：線程池的狀態(tài)及KeepAliveTime參數(shù)

// runState is stored in the high-order bits private static final int RUNNING = -1 << COUNT_BITS; private static final int SHUTDOWN = 0 << COUNT_BITS; private static final int STOP = 1 << COUNT_BITS; private static final int TIDYING = 2 << COUNT_BITS; private static final int TERMINATED = 3 << COUNT_BITS;循環(huán)getTask方法

/** * Performs blocking or timed wait for a task, depending on * current configuration settings, or returns null if this worker * must exit because of any of: * 1. There are more than maximumPoolSize workers (due to * a call to setMaximumPoolSize). * 2. The pool is stopped. * 3. The pool is shutdown and the queue is empty. * 4. This worker timed out waiting for a task, and timed-out * workers are subject to termination (that is, * {@code allowCoreThreadTimeOut || workerCount > corePoolSize}) * both before and after the timed wait. * * @return task, or null if the worker must exit, in which case * workerCount is decremented */ private Runnable getTask() { boolean timedOut = false; // Did the last poll() time out? retry: for (;;) { int c = ctl.get(); int rs = runStateOf(c); // Check if queue empty only if necessary. if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) { decrementWorkerCount(); return null; } boolean timed; // Are workers subject to culling? for (;;) { int wc = workerCountOf(c); timed = allowCoreThreadTimeOut || wc > corePoolSize; //默認(rèn)allowCoreThreadTimeOut為false，除非程序指定 //（1）當(dāng)沒有超過核心線程時(shí)，默認(rèn)allowCoreThreadTimeOut為false時(shí) //timed值為false，始終break掉，不會(huì)銷毀線程 //（2）當(dāng)超過核心線程數(shù)，默認(rèn)allowCoreThreadTimeOut為false時(shí) //timed值為true，如果超過最大值，則銷毀；如果timeout過，則銷毀 // 如果allowCoreThreadTimeOut為true，則timed始終為true if (wc <= maximumPoolSize && ! (timedOut && timed)) break; if (compareAndDecrementWorkerCount(c)) return null; c = ctl.get(); // Re-read ctl if (runStateOf(c) != rs) continue retry; // else CAS failed due to workerCount change; retry inner loop } try { Runnable r = timed ? workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) : workQueue.take(); if (r != null) return r; timedOut = true; } catch (InterruptedException retry) { timedOut = false; } } }線程池狀態(tài)大于SHUTDOWN值的兩種情況1、調(diào)用shutdown方法

當(dāng)線程池調(diào)用了shutdown方法，線程池的狀態(tài)會(huì)首先被設(shè)置為SHUTDOWN，然后遍歷線程池中所有線程，調(diào)用一次interrupt方法，如果在休眠中的線程將會(huì)激活，激活后的線程以及調(diào)用shutdown方法本身的線程都會(huì)嘗試去調(diào)用tryTerminate方法，該方法將判定如果線程池中所有記錄的線程數(shù)為0，則將線程狀態(tài)改為TERMINATED，這個(gè)值為3，將大于SHUTDOWN狀態(tài)值。

當(dāng)線程調(diào)用了shutdownNow方法后，首先將線程的狀態(tài)修改為STOP，這個(gè)狀態(tài)是大于SHUTDOWN值的，接下來它也會(huì)通過中斷激活線程，只是它來的更暴力一些，連加鎖和一些基本判斷都沒有，直接中斷；在調(diào)用tryTerminate之前會(huì)先清空阻塞隊(duì)列中所有的元素，這些元素被組裝為一個(gè)List列表作為shutdownNow方法的返回值。換句話說，沒有執(zhí)行的任務(wù)在shutdownNow執(zhí)行后的返回值中可以得到。在程序某些必要的情況下，可以通過線程池的isTerminating，isTerminated，isStopped，isShutdown來對(duì)線程做一些狀態(tài)判定。

KeepAliveTime參數(shù)

workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS)

當(dāng)阻塞隊(duì)列中沒有任務(wù)時(shí)，等待時(shí)間達(dá)到keepAliveTime毫秒值時(shí)就會(huì)被自動(dòng)喚醒，而不會(huì)永遠(yuǎn)地沉睡下去。

keepAliveTime，如果是通過newCachedThreadPool的話，默認(rèn)是1分鐘超時(shí)，如果遇到前面所提到的瞬間沖擊，那么線程池?cái)?shù)量將瞬間快速膨脹，而且這些瞬間膨脹的線程的生命周期最少在1分鐘以上。

如果設(shè)置了該參數(shù)，那么當(dāng)timeout的時(shí)候，就return null，就會(huì)跳出循環(huán)，回收線程。

if (wc <= maximumPoolSize && ! (timedOut && timed)) break; if (compareAndDecrementWorkerCount(c)) return null;

allowCoreThreadTimeout : 默認(rèn)情況下核心線程不會(huì)退出，可通過將該參數(shù)設(shè)置為true，讓核心線程也退出。

默認(rèn)的Executors工廠，只有newCachedThreadPool，timeout為60秒，出現(xiàn)timeout情況下，而且線程數(shù)超過了核心線程數(shù)，會(huì)銷毀銷毀線程。保持在corePoolSize數(shù)（如果是cached的，corePoolSize為0）。

/** * Timeout in nanoseconds for idle threads waiting for work. * Threads use this timeout when there are more than corePoolSize * present or if allowCoreThreadTimeOut. Otherwise they wait * forever for new work. */ private volatile long keepAliveTime; /** * If false (default), core threads stay alive even when idle. * If true, core threads use keepAliveTime to time out waiting * for work. */ private volatile boolean allowCoreThreadTimeOut;

線程池最小是corePoolSize，最大是maximumPoolSize，除非設(shè)置了allowCoreThreadTimeOut和超時(shí)時(shí)間，這種情況線程數(shù)可能減少到0，最大可能是Integer.MAX_VALUE。

Core pool size is the minimum number of workers to keep alive(and not allow to time out etc) unless allowCoreThreadTimeOut is set, in which case the minimum is zero.

/** * Creates a thread pool that creates new threads as needed, but * will reuse previously constructed threads when they are * available. These pools will typically improve the performance * of programs that execute many short-lived asynchronous tasks. * Calls to <tt>execute</tt> will reuse previously constructed * threads if available. If no existing thread is available, a new * thread will be created and added to the pool. Threads that have * not been used for sixty seconds are terminated and removed from * the cache. Thus, a pool that remains idle for long enough will * not consume any resources. Note that pools with similar * properties but different details (for example, timeout parameters) * may be created using {@link ThreadPoolExecutor} constructors. * * @return the newly created thread pool */ public static ExecutorService newCachedThreadPool() { return new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60L, TimeUnit.SECONDS, new SynchronousQueue<Runnable>()); } /** * Creates a thread pool that creates new threads as needed, but * will reuse previously constructed threads when they are * available, and uses the provided * ThreadFactory to create new threads when needed. * @param threadFactory the factory to use when creating new threads * @return the newly created thread pool * @throws NullPointerException if threadFactory is null */ public static ExecutorService newCachedThreadPool(ThreadFactory threadFactory) { return new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60L, TimeUnit.SECONDS, new SynchronousQueue<Runnable>(), threadFactory); }

超時(shí)timeout設(shè)置為0的話，表示不等待

public E poll(long timeout, TimeUnit unit) throws InterruptedException { return pollFirst(timeout, unit); }具體如下

public E pollFirst(long timeout, TimeUnit unit) throws InterruptedException { long nanos = unit.toNanos(timeout); final ReentrantLock lock = this.lock; lock.lockInterruptibly(); try { E x; while ( (x = unlinkFirst()) == null) { if (nanos <= 0) return null; nanos = notEmpty.awaitNanos(nanos); } return x; } finally { lock.unlock(); } }

以上為個(gè)人經(jīng)驗(yàn)，希望能給大家一個(gè)參考，也希望大家多多支持好吧啦網(wǎng)。如有錯(cuò)誤或未考慮完全的地方，望不吝賜教。