java高级应用:线程池全面解析
什么是线程池?
很简单,简单看名字就知道是装有线程的池子,我们可以把要执行的多线程交给线程池来处理,和连接池的概念一样,通过维护一定数量的线程池来达到多个线程的复用。
线程池的好处
我们知道不用线程池的话,每个线程都要通过new Thread(xxRunnable).start()的方式来创建并运行一个线程,线程少的话这不会是问题,而真实环境可能会开启多个线程让系统和程序达到最佳效率,当线程数达到一定数量就会耗尽系统的CPU和内存资源,也会造成GC频繁收集和停顿,因为每次创建和销毁一个线程都是要消耗系统资源的,如果为每个任务都创建线程这无疑是一个很大的性能瓶颈。所以,线程池中的线程复用极大节省了系统资源,当线程一段时间不再有任务处理时它也会自动销毁,而不会长驻内存。
线程池核心类
在java.util.concurrent包中我们能找到线程池的定义,其中ThreadPoolExecutor是我们线程池核心类,首先看看线程池类的主要参数有哪些。
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:线程池的核心大小,也可以理解为最小的线程池大小。
maximumPoolSize:最大线程池大小。
keepAliveTime:空余线程存活时间,指的是超过corePoolSize的空余线程达到多长时间才进行销毁。
unit:销毁时间单位。
workQueue:存储等待执行线程的工作队列。
threadFactory:创建线程的工厂,一般用默认即可。
handler:拒绝策略,当工作队列、线程池全已满时如何拒绝新任务,默认抛出异常。
线程池工作流程
1、如果线程池中的线程小于corePoolSize时就会创建新线程直接执行任务。
2、如果线程池中的线程大于corePoolSize时就会暂时把任务存储到工作队列workQueue中等待执行。
3、如果工作队列workQueue也满时:当线程数小于最大线程池数maximumPoolSize时就会创建新线程来处理,而线程数大于等于最大线程池数maximumPoolSize时就会执行拒绝策略。
线程池分类
Executors是jdk里面提供的创建线程池的工厂类,它默认提供了4种常用的线程池应用,而不必我们去重复构造。
newFixedThreadPool
固定线程池,核心线程数和最大线程数固定相等,而空闲存活时间为0毫秒,说明此参数也无意义,工作队列为最大为Integer.MAX_VALUE大小的阻塞队列。当执行任务时,如果线程都很忙,就会丢到工作队列等有空闲线程时再执行,队列满就执行默认的拒绝策略。
/*** 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>());}
newCachedThreadPool
带缓冲线程池,从构造看核心线程数为0,最大线程数为Integer最大值大小,超过0个的空闲线程在60秒后销毁,SynchronousQueue这是一个直接提交的队列,意味着每个新任务都会有线程来执行,如果线程池有可用线程则执行任务,没有的话就创建一个来执行,线程池中的线程数不确定,一般建议执行速度较快较小的线程,不然这个最大线程池边界过大容易造成内存溢出。
/*** 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>());}
newSingleThreadExecutor
单线程线程池,核心线程数和最大线程数均为1,空闲线程存活0毫秒同样无意思,意味着每次只执行一个线程,多余的先存储到工作队列,一个一个执行,保证了线程的顺序执行。
/*** 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>()));}
newScheduledThreadPool
调度线程池,即按一定的周期执行任务,即定时任务,对ThreadPoolExecutor进行了包装而已。
/*** 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);}
拒绝策略
AbortPolicy
简单粗暴,直接抛出拒绝异常,这也是默认的拒绝策略。
/*** 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();
CallerRunsPolicy
如果线程池未关闭,则会在调用者线程中直接执行新任务,这会导致主线程提交线程性能变慢。
/*** 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();}}}
DiscardPolicy
从方法看没做任务操作,即表示不处理新任务,即丢弃。
/*** 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) {}}
DiscardOldestPolicy
抛弃最老的任务,就是从队列取出最老的任务然后放入新的任务进行执行。
/*** 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);}}}
如何提交线程
如可以先随便定义一个固定大小的线程池
ExecutorService es = Executors.newFixedThreadPool(3);
提交一个线程
es.submit(xxRunnble);
es.execute(xxRunnble);
submit和execute分别有什么区别呢?
execute没有返回值,如果不需要知道线程的结果就使用execute方法,性能会好很多。
submit返回一个Future对象,如果想知道线程结果就使用submit提交,而且它能在主线程中通过Future的get方法捕获线程中的异常。
如何关闭线程池
es.shutdown();
不再接受新的任务,之前提交的任务等执行结束再关闭线程池。
es.shutdownNow();
不再接受新的任务,试图停止池中的任务再关闭线程池,返回所有未处理的线程list列表。
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