简单介绍Java中常用的几种线程池

Bertha 。 2022-10-05 10:46 170阅读 0赞

Java中所有线程的创建都来自于同一个父类：java.util.concurrent.Executors extends Object。只要对Executors的常用API熟悉了，基本上剩下的只是熟练程度的问题了

# 1、 newFixedThreadPool #

public static ExecutorService newFixedThreadPool(int nThreads)

源码：

/**
         * 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>());
        }

初始固定线程池大小。  
创建一个可重用固定线程数的线程池，以共享的无界队列方式来运行这些线程，在需要时使用提供的 ThreadFactory 创建新线程。在任意点，在大多数 nThreads线程会处于处理任务的活动状态。如果在所有线程处于活动状态时提交附加任务，则在有可用线程之前，附加任务将在队列中等待。如果在关闭前的执行期间由于失败而导致任何线程终止，那么一个新线程将代替它执行后续的任务（如果需要）。在某个线程被显式地关闭之前，池中的线程将一直存在。

#         2、 defaultThreadFactory #

public static ThreadFactory defaultThreadFactory()

源码

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;
            }
        }

返回用于创建新线程的默认线程工厂。此工厂创建同一个线程组(ThreadGroup)中Executor 使用的所有新线程。如果有 SecurityManager，则它使用 System.getSecurityManager()返回的组，其他情况则使用调用 defaultThreadFactory 方法的组。每个新线程都作为非守护程序而创建，并且具有设置线程优先级为 Thread.NORM\_PRIORITY 与线程组中允许的最大优先级的较小者。新线程具有可通过 pool-N-thread-M 的 Thread.getName() 来访问的名称，其中 N 是此工厂的序列号，M 是此工厂所创建线程的序列号。

# 3、newCachedThreadPool #

public static ExecutorService newCachedThreadPool()

源码：

/**
         * 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>());
        }
    
        /**
         * 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);
        }

创建一个可根据需要创建新线程的线程池，但是在以前构造的线程可用时将重用它们。对于执行很多短期异步任务的程序而言，这些线程池通常可提高程序性能。调用 execute 将重用以前构造的线程（如果线程可用）。如果现有线程没有可用的，则创建一个新线程并添加到池中。终止并从缓存中移除那些已有 60 秒钟未被使用的线程。因此，长时间保持空闲的线程池不会使用任何资源。注意，可以使用 ThreadPoolExecutor 构造方法创建具有类似属性但细节不同（例如超时参数）的线程池。

# 4、newScheduledThreadPool #

public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize)

源码：

/**
         * Creates a new {@code ScheduledThreadPoolExecutor} with the
         * given core pool size.
         *
         * @param corePoolSize the number of threads to keep in the pool, even
         *        if they are idle, unless {@code allowCoreThreadTimeOut} is set
         * @throws IllegalArgumentException if {@code corePoolSize < 0}
         */
        public ScheduledThreadPoolExecutor(int corePoolSize) {
            super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,
                  new DelayedWorkQueue());
        }

创建一个线程池，它可安排在给定延迟后运行命令或者定期地执行。

# 5、`newSingleThreadExecutor` #

`public static ExecutorService newSingleThreadExecutor()`

/**
         * 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>()));
        }

这是一个比较特殊的线程池，源码里面已经固化了只有一个可执行线程完成任务。这样可以保证所有任务串行执行。避免了资源的抢占死锁问题。

总结下，所有的线程池都可适用于不同的业务场景。根据对应的业务场景挑选不同的线程池，事半功倍。