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什么是線程池？

很簡單，簡單看名字就知道是裝有線程的池子，我們可以把要執行的多線程交給線程池來處理，和連接池的概念一樣，通過維護一定數量的線程池來達到多個線程的復用。

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線程池的好處

我們知道不用線程池的話，每個線程都要通過new Thread(xxRunnable).start()的方式來創建并運行一個線程，線程少的話這不會是問題，而真實環境可能會開啟多個線程讓系統和程序達到最佳效率，當線程數達到一定數量就會耗盡系統的CPU和內存資源，也會造成GC頻繁收集和停頓，因為每次創建和銷毀一個線程都是要消耗系統資源的，如果為每個任務都創建線程這無疑是一個很大的性能瓶頸。所以，線程池中的線程復用極大節省了系統資源，當線程一段時間不再有任務處理時它也會自動銷毀，而不會長駐內存。

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線程池核心類

在java.util.concurrent包中我們能找到線程池的定義，其中ThreadPoolExecutor是我們線程池核心類，首先看看線程池類的主要參數有哪些。

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;}

• corePoolSize：線程池的核心大小，也可以理解為最小的線程池大小。

• maximumPoolSize：最大線程池大小。

• keepAliveTime：空余線程存活時間，指的是超過corePoolSize的空余線程達到多長時間才進行銷毀。

• unit：銷毀時間單位。

• workQueue：存儲等待執行線程的工作隊列。

• threadFactory：創建線程的工廠，一般用默認即可。

• handler：拒絕策略，當工作隊列、線程池全已滿時如何拒絕新任務，默認拋出異常。

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線程池工作流程

1、如果線程池中的線程小于corePoolSize時就會創建新線程直接執行任務。

2、如果線程池中的線程大于corePoolSize時就會暫時把任務存儲到工作隊列workQueue中等待執行。

3、如果工作隊列workQueue也滿時：當線程數小于最大線程池數maximumPoolSize時就會創建新線程來處理，而線程數大于等于最大線程池數maximumPoolSize時就會執行拒絕策略。

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線程池分類

Executors是jdk里面提供的創建線程池的工廠類，它默認提供了4種常用的線程池應用，而不必我們去重復構造。

• newFixedThreadPool

  固定線程池，核心線程數和最大線程數固定相等，而空閑存活時間為0毫秒，說明此參數也無意義，工作隊列為最大為Integer.MAX_VALUE大小的阻塞隊列。當執行任務時，如果線程都很忙，就會丟到工作隊列等有空閑線程時再執行，隊列滿就執行默認的拒絕策略。

/*** Creates a thread pool that reuses a fixed number of threads* operating off a shared unbounded queue. At any point, at most* {@code nThreads} threads will be active processing tasks.* If additional tasks are submitted when all threads are active,* they will wait in the queue until a thread is available.* If any thread terminates due to a failure during execution* prior to shutdown, a new one will take its place if needed to* execute subsequent tasks. The threads in the pool will exist* until it is explicitly {@link ExecutorService#shutdown shutdown}.** @param nThreads the number of threads in the pool* @return the newly created thread pool* @throws IllegalArgumentException if {@code nThreads <= 0}*/public static ExecutorService newFixedThreadPool(int nThreads) {return new ThreadPoolExecutor(nThreads, nThreads,0L, TimeUnit.MILLISECONDS,new LinkedBlockingQueue<Runnable>());}

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• newCachedThreadPool

? ? ? ? 帶緩沖線程池，從構造看核心線程數為0，最大線程數為Integer最大值大小，超過0個的空閑線程在60秒后銷毀，SynchronousQueue這是一個直接提交的隊列，意味著每個新任務都會有線程來執行，如果線程池有可用線程則執行任務，沒有的話就創建一個來執行，線程池中的線程數不確定，一般建議執行速度較快較小的線程，不然這個最大線程池邊界過大容易造成內存溢出。

/*** 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 {@code execute} 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>());}

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• newSingleThreadExecutor

????????單線程線程池，核心線程數和最大線程數均為1，空閑線程存活0毫秒同樣無意思，意味著每次只執行一個線程，多余的先存儲到工作隊列，一個一個執行，保證了線程的順序執行。

/*** Creates an Executor that uses a single worker thread operating* off an unbounded queue. (Note however that if this single* thread terminates due to a failure during execution prior to* shutdown, a new one will take its place if needed to execute* subsequent tasks.) Tasks are guaranteed to execute* sequentially, and no more than one task will be active at any* given time. Unlike the otherwise equivalent* {@code newFixedThreadPool(1)} the returned executor is* guaranteed not to be reconfigurable to use additional threads.** @return the newly created single-threaded Executor*/public static ExecutorService newSingleThreadExecutor() {return new FinalizableDelegatedExecutorService(new ThreadPoolExecutor(1, 1,0L, TimeUnit.MILLISECONDS,new LinkedBlockingQueue<Runnable>()));}

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• newScheduledThreadPool

  調度線程池，即按一定的周期執行任務，即定時任務，對ThreadPoolExecutor進行了包裝而已。

/*** Creates a thread pool that can schedule commands to run after a* given delay, or to execute periodically.* @param corePoolSize the number of threads to keep in the pool,* even if they are idle* @return a newly created scheduled thread pool* @throws IllegalArgumentException if {@code corePoolSize < 0}*/public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {return new ScheduledThreadPoolExecutor(corePoolSize);}

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拒絕策略

• AbortPolicy

??????簡單粗暴，直接拋出拒絕異常，這也是默認的拒絕策略。

/*** A handler for rejected tasks that throws a* {@code RejectedExecutionException}.*/public static class AbortPolicy implements RejectedExecutionHandler {/*** Creates an {@code AbortPolicy}.*/public AbortPolicy() { }/*** Always throws RejectedExecutionException.** @param r the runnable task requested to be executed* @param e the executor attempting to execute this task* @throws RejectedExecutionException always*/public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {throw new RejectedExecutionException("Task " + r.toString() +" rejected from " +e.toString());}} /*** The default rejected execution handler*/private static final RejectedExecutionHandler defaultHandler =new AbortPolicy();

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• CallerRunsPolicy? ? ?

???????如果線程池未關閉，則會在調用者線程中直接執行新任務，這會導致主線程提交線程性能變慢。

/*** A handler for rejected tasks that runs the rejected task* directly in the calling thread of the {@code execute} method,* unless the executor has been shut down, in which case the task* is discarded.*/public static class CallerRunsPolicy implements RejectedExecutionHandler {/*** Creates a {@code CallerRunsPolicy}.*/public CallerRunsPolicy() { }/*** Executes task r in the caller's thread, unless the executor* has been shut down, in which case the task is discarded.** @param r the runnable task requested to be executed* @param e the executor attempting to execute this task*/public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {if (!e.isShutdown()) {r.run();}}}

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• DiscardPolicy

????? ?從方法看沒做任務操作，即表示不處理新任務，即丟棄。

/*** A handler for rejected tasks that silently discards the* rejected task.*/public static class DiscardPolicy implements RejectedExecutionHandler {/*** Creates a {@code DiscardPolicy}.*/public DiscardPolicy() { }/*** Does nothing, which has the effect of discarding task r.** @param r the runnable task requested to be executed* @param e the executor attempting to execute this task*/public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {}}

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• DiscardOldestPolicy

????? ?拋棄最老的任務，就是從隊列取出最老的任務然后放入新的任務進行執行。? ? ? ??

/*** A handler for rejected tasks that discards the oldest unhandled* request and then retries {@code execute}, unless the executor* is shut down, in which case the task is discarded.*/public static class DiscardOldestPolicy implements RejectedExecutionHandler {/*** Creates a {@code DiscardOldestPolicy} for the given executor.*/public DiscardOldestPolicy() { }/*** Obtains and ignores the next task that the executor* would otherwise execute, if one is immediately available,* and then retries execution of task r, unless the executor* is shut down, in which case task r is instead discarded.** @param r the runnable task requested to be executed* @param e the executor attempting to execute this task*/public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {if (!e.isShutdown()) {e.getQueue().poll();e.execute(r);}}}

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如何提交線程

如可以先隨便定義一個固定大小的線程池

ExecutorService es = Executors.newFixedThreadPool(3);

提交一個線程

es.submit(xxRunnble); es.execute(xxRunnble);

submit和execute分別有什么區別呢？

execute沒有返回值，如果不需要知道線程的結果就使用execute方法，性能會好很多。

submit返回一個Future對象，如果想知道線程結果就使用submit提交，而且它能在主線程中通過Future的get方法捕獲線程中的異常。

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如何關閉線程池

es.shutdown();?

不再接受新的任務，之前提交的任務等執行結束再關閉線程池。

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es.shutdownNow();

不再接受新的任務，試圖停止池中的任務再關閉線程池，返回所有未處理的線程list列表。

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