总体来说,线程池有如下的优势:
(1)降低资源消耗。通过重复利用已创建的线程降低线程创建和销毁造成的消耗。
(2)提高响应速度。当任务到达时,任务可以不需要等到线程创建就能立即执行。
(3)提高线程的可管理性。线程是稀缺资源,如果无限制的创建,不仅会消耗系统资源,还会降低系统的稳定性,使用线程池可以进行统一的分配,调优和监控。
线程池的真正实现类是 ThreadPoolExecutor
,其构造方法有如下4种:
public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue) { this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, Executors.defaultThreadFactory(), defaultHandler); } public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory) { this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, threadFactory, defaultHandler); } public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, RejectedExecutionHandler handler) { this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, Executors.defaultThreadFactory(), handler); } 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; }
可以看到,其需要如下几个参数:
allowCoreThreadTimeout
设置为 true 时,核心线程也会超时回收。allowCoreThreadTimeout
设置为 true 时,核心线程也会超时回收。keepAliveTime
参数的时间单位。常用的有:TimeUnit.MILLISECONDS
(毫秒)、TimeUnit.SECONDS
(秒)、TimeUnit.MINUTES
(分)。execute()
方法提交的 Runnable
对象将存储在该参数中。其采用阻塞队列实现。线程池的使用流程如下:
// 创建线程池 ThreadPoolExecutor threadPool = new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE, TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory); // 向线程池提交任务 threadPool.execute(new Runnable() { @Override public void run() { ... // 线程执行的任务 } }); // 关闭线程池 threadPool.shutdown(); // 设置线程池的状态为SHUTDOWN,然后中断所有没有正在执行任务的线程 threadPool.shutdownNow(); // 设置线程池的状态为 STOP,然后尝试停止所有的正在执行或暂停任务的线程,并返回等待执行任务的列表
下面来描述一下线程池工作的原理,同时对上面的参数有一个更深的了解。其工作原理流程图如下:
通过上图,相信大家已经对所有参数有个了解了。下面再对任务队列、线程工厂和拒绝策略做更多的说明。
任务队列是基于阻塞队列实现的,即采用生产者消费者模式,在 Java 中需要实现 BlockingQueue 接口。但 Java 已经为我们提供了 7 种阻塞队列的实现:
Integer.MAX_VALUE
。Comparable
接口也可以提供 Comparator
来对队列中的元素进行比较。跟时间没有任何关系,仅仅是按照优先级取任务。PriorityBlockingQueue
,是二叉堆实现的无界优先级阻塞队列。要求元素都实现 Delayed
接口,通过执行时延从队列中提取任务,时间没到任务取不出来。take()
方法的时候就会发生阻塞,直到有一个生产者线程生产了一个元素,消费者线程就可以拿到这个元素并返回;生产者线程调用 put()
方法的时候也会发生阻塞,直到有一个消费者线程消费了一个元素,生产者才会返回。FIFO
(先进先出),也可以像栈一样 FILO
(先进后出)。ConcurrentLinkedQueue
、LinkedBlockingQueue
和 SynchronousQueue
的结合体,但是把它用在 ThreadPoolExecutor
中,和 LinkedBlockingQueue
行为一致,但是是无界的阻塞队列。注意有界队列和无界队列的区别:如果使用有界队列,当队列饱和时并超过最大线程数时就会执行拒绝策略;而如果使用无界队列,因为任务队列永远都可以添加任务,所以设置
maximumPoolSize
没有任何意义。
线程工厂指定创建线程的方式,需要实现 ThreadFactory
接口,并实现 newThread(Runnable r)
方法。该参数可以不用指定,Executors
框架已经为我们实现了一个默认的线程工厂:
/** * The default thread factory. */ private static class DefaultThreadFactory implements ThreadFactory { private static final AtomicInteger poolNumber = new AtomicInteger(1); private final ThreadGroup group; private final AtomicInteger threadNumber = new AtomicInteger(1); private final String namePrefix; DefaultThreadFactory() { SecurityManager s = System.getSecurityManager(); group = (s != null) ? s.getThreadGroup() : Thread.currentThread().getThreadGroup(); namePrefix = "pool-" + poolNumber.getAndIncrement() + "-thread-"; } public Thread newThread(Runnable r) { Thread t = new Thread(group, r, namePrefix + threadNumber.getAndIncrement(), 0); if (t.isDaemon()) t.setDaemon(false); if (t.getPriority() != Thread.NORM_PRIORITY) t.setPriority(Thread.NORM_PRIORITY); return t; } }
当线程池的线程数达到最大线程数时,需要执行拒绝策略。拒绝策略需要实现 RejectedExecutionHandler
接口,并实现 rejectedExecution(Runnable r, ThreadPoolExecutor executor)
方法。不过 Executors
框架已经为我们实现了 4 种拒绝策略:
RejectedExecutionException
异常。嫌上面使用线程池的方法太麻烦?其实Executors已经为我们封装好了 4 种常见的功能线程池,如下:
FixedThreadPool
)ScheduledThreadPool
)CachedThreadPool
)SingleThreadExecutor
)创建方法的源码:
public static ExecutorService newFixedThreadPool(int nThreads) { return new ThreadPoolExecutor(nThreads, nThreads, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>()); } public static ExecutorService newFixedThreadPool(int nThreads, ThreadFactory threadFactory) { return new ThreadPoolExecutor(nThreads, nThreads, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>(), threadFactory); }
使用示例:
// 1. 创建定长线程池对象 & 设置线程池线程数量固定为3 ExecutorService fixedThreadPool = Executors.newFixedThreadPool(3); // 2. 创建好Runnable类线程对象 & 需执行的任务 Runnable task =new Runnable(){ public void run() { System.out.println("执行任务啦"); } }; // 3. 向线程池提交任务 fixedThreadPool.execute(task);
创建方法的源码:
private static final long DEFAULT_KEEPALIVE_MILLIS = 10L; public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) { return new ScheduledThreadPoolExecutor(corePoolSize); } public ScheduledThreadPoolExecutor(int corePoolSize) { super(corePoolSize, Integer.MAX_VALUE, DEFAULT_KEEPALIVE_MILLIS, MILLISECONDS, new DelayedWorkQueue()); } public static ScheduledExecutorService newScheduledThreadPool( int corePoolSize, ThreadFactory threadFactory) { return new ScheduledThreadPoolExecutor(corePoolSize, threadFactory); } public ScheduledThreadPoolExecutor(int corePoolSize, ThreadFactory threadFactory) { super(corePoolSize, Integer.MAX_VALUE, DEFAULT_KEEPALIVE_MILLIS, MILLISECONDS, new DelayedWorkQueue(), threadFactory); }
使用示例:
// 1. 创建 定时线程池对象 & 设置线程池线程数量固定为5 ScheduledExecutorService scheduledThreadPool = Executors.newScheduledThreadPool(5); // 2. 创建好Runnable类线程对象 & 需执行的任务 Runnable task =new Runnable(){ public void run() { System.out.println("执行任务啦"); } }; // 3. 向线程池提交任务 scheduledThreadPool.schedule(task, 1, TimeUnit.SECONDS); // 延迟1s后执行任务 scheduledThreadPool.scheduleAtFixedRate(task,10,1000,TimeUnit.MILLISECONDS);// 延迟10ms后、每隔1000ms执行任务
创建方法的源码:
public static ExecutorService newCachedThreadPool() { return new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60L, TimeUnit.SECONDS, new SynchronousQueue<Runnable>()); } public static ExecutorService newCachedThreadPool(ThreadFactory threadFactory) { return new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60L, TimeUnit.SECONDS, new SynchronousQueue<Runnable>(), threadFactory); }
使用示例:
// 1. 创建可缓存线程池对象 ExecutorService cachedThreadPool = Executors.newCachedThreadPool(); // 2. 创建好Runnable类线程对象 & 需执行的任务 Runnable task =new Runnable(){ public void run() { System.out.println("执行任务啦"); } }; // 3. 向线程池提交任务 cachedThreadPool.execute(task);
创建方法的源码:
public static ExecutorService newSingleThreadExecutor() { return new FinalizableDelegatedExecutorService (new ThreadPoolExecutor(1, 1, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>())); } public static ExecutorService newSingleThreadExecutor(ThreadFactory threadFactory) { return new FinalizableDelegatedExecutorService (new ThreadPoolExecutor(1, 1, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>(), threadFactory)); }
使用示例:
// 1. 创建单线程化线程池 ExecutorService singleThreadExecutor = Executors.newSingleThreadExecutor(); // 2. 创建好Runnable类线程对象 & 需执行的任务 Runnable task =new Runnable(){ public void run() { System.out.println("执行任务啦"); } }; // 3. 向线程池提交任务 singleThreadExecutor.execute(task);
Executors 的 4 个功能线程池虽然方便,但现在已经不建议使用了,而是建议直接通过使用 ThreadPoolExecutor
的方式,这样的处理方式让写的同学更加明确线程池的运行规则,规避资源耗尽的风险。
其实 Executors
的 4 个功能线程有如下弊端:
FixedThreadPool
和 SingleThreadExecutor
:主要问题是堆积的请求处理队列均采用 LinkedBlockingQueue
,可能会耗费非常大的内存,甚至 OOM。CachedThreadPool
和 ScheduledThreadPool
:主要问题是线程数最大数是 Integer.MAX_VALUE
,可能会创建数量非常多的线程,甚至 OOM。来源:blog.csdn.net/u013541140/article/details/95225769