多线程(七)JDK原生线程池

如同数据库连接一样，线程的创建、切换和销毁同样会耗费大量的系统资源。为了复用创建好的线程，减少频繁创建线程的次数，提高线程利用率可以引用线程池技术。使用线程池的优势有如下几点：
        1、保持一定数量的线程，减少了线程频繁创建和销毁资源消耗。
        2、使用线程的时候直接由线程池中取出线程，省去了创建线程的时间，侧面提高了系统的响应时间。
        3、需要使用线程的时候直接从线程池中取出，避免了人为的不合理创建线程，减少了潜在的风险。
Doug Lea在实现JUC中提供了原生的线程池，并提供了各种线程管理策略来适应于不同的使用场景。使用的时候可通过Executors获取各种线程池实例。

这里提供了六对12个方法来创建ExecutorService，其中每种类型的ExecutorService可以适用于不同的应用场景，对线程的管理策略也各不相同。下面就看一下各个方法的注释：

/**
     * Creates a thread pool that creates new threads as needed, but will reuse previously constructed threads when they are available.
     * 创建一个线程池，需要的时候会创建新的线程，如果有可用的线程则会复用以前已经创建好的线程。
     * These pools will typically improve the performance of programs that execute many short-lived asynchronous tasks.
     * 这些线程池通常情况下可以提升哪些短期异步任务的性能
     * Calls to {@code execute} will reuse previously constructed threads if available.
     * 如果以创建的线程状态可用的话，调用execute可以复用他们
     * If no existing thread is available, a new thread will be created and added to the pool.
     * 如果不存在可用状态的线程，那么将会创建一个新线程同时会把该线程添加到线程池中
     * Threads that have not been used for sixty seconds are terminated and removed from the cache.
     *那些超过60s没用的线程将会被销毁同时从缓存中移除
     * Thus, a pool that remains idle for long enough will not consume any resources.
     *因此长时间空闲的线程池不会消耗任何资源
     * Note that pools with similar properties but different details (for example, timeout parameters) may be created using {@link ThreadPoolExecutor} constructors.
     *可以使用ThreadPoolExecutor创建性质相似但实现细节不同的线程池
     * @return the newly created thread pool
     */

    public static ExecutorService newCachedThreadPool();

    //可以使用自定义的ThreadFactory 类创建线程，其它和无参方法一致
    public static ExecutorService newCachedThreadPool(ThreadFactory threadFactory);

/**
     * Creates a thread pool that reuses a fixed number of threads operating off a shared unbounded queue.
     *创建一个可重用、固定数量线程的线程池
     * At any point, at most {@code nThreads} threads will be active processing tasks.
     *任何时间最多只有 nThreads 个线程被激活来执行任务
     * If additional tasks are submitted when all threads are active, they will wait in the queue until a thread is available.
     * 当无可用空闲线程的时候，如果有新任务被提交，这些新任务将会一直等待直至有可用线程来执行。
     * 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.
     *如果任何线程正常关闭之前在执行过程中因失败而提前终止，那么如果有未被执行的后续任务，则会创建新的线程来继续执行。
     * The threads in the pool will exist  until it is explicitly {@link ExecutorService#shutdown shutdown}.
     * 线程池中的所有线程在明确掉用shutdown之后将会退出
     *
     * @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);
    //可自定义ThreadFactory
    public static ExecutorService newFixedThreadPool(int nThreads, ThreadFactory threadFactory);

/**
     * Creates a thread pool that can schedule commands to run after a given delay, or to execute periodically.
     *  创建一个线程池，该线程在延迟指定时间之后可以周期性的执行线程体
     * @param corePoolSize the number of threads to keep in the pool,
     * even if they are idle
     * @return a newly created scheduled thread pool 注意返回值类型是ScheduledExecutorService，不要使用ExecutorService来接收，否则找不到schedule执行方法
     * @throws IllegalArgumentException if {@code corePoolSize < 0}
     */

    public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize);
    //可自定义ThreadFactory
    public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize, ThreadFactory threadFactory);

/**
     * Creates an Executor that uses a single worker thread operating off an unbounded queue.
     *创建一个Executor，使用一个线程来工作，该线程存储在LinkedBlockingQueue中
     * (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.)
     *注，如果任何线程正常关闭之前在执行过程中因失败而提前终止，那么如果有未被执行的后续任务，则会创建新的线程来继续执行。
     * Tasks are guaranteed to execute sequentially, and no more than one task will be active at any given time.
     *  任务是按顺序执行的，任何时间都只有一个线程来执行任务
     * 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();
    //可自定义ThreadFactory
    public static ExecutorService newSingleThreadExecutor(ThreadFactory threadFactory);

/**
     * Creates a single-threaded executor that can schedule commands
     * to run after a given delay, or to execute periodically.
     * (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 newScheduledThreadPool(1)} the returned executor is
     * guaranteed not to be reconfigurable to use additional threads.
     * @return the newly created scheduled executor
     */

    public static ScheduledExecutorService newSingleThreadScheduledExecutor();

    public static ScheduledExecutorService newSingleThreadScheduledExecutor(ThreadFactory threadFactory)

使用示例：

package thread.blogs.threadpool;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;

/**
 * Created by PerkinsZhu on 2017/8/31 13:59.
 */
public class PoolTest {

    public static void main(String[] args) {
//        testCachedThreadPool();
//        testSingleThreadExecutor();
//        testFixedThreadPool();
        testScheduledThreadPool();
    }
    private static ThreadFactory myFactory = new MyThreadFactory();

    private static void testSingleThreadExecutor() {
        //一个一个的依次执行
        doHandle(Executors.newSingleThreadExecutor(myFactory));
    }

    private static void testFixedThreadPool() {
        //两个两个的一起执行
        doHandle(Executors.newFixedThreadPool(2,myFactory));
    }

    private static void testCachedThreadPool() {
        //10个一起一次性执行完
        doHandle(Executors.newCachedThreadPool(myFactory));
    }

    private static void testScheduledThreadPool() {
        //定时周期执行
        Executors.newScheduledThreadPool(1,myFactory).scheduleAtFixedRate(runnable, 500, 2000, TimeUnit.MILLISECONDS);
    }

    private static Runnable runnable = () -> {
        sleep(1000);
        System.out.println(Thread.currentThread().getName() + "    work!!!");
    };

    private static void doHandle(ExecutorService executorService) {
        for (int i = 0; i < 10; i++) {
            executorService.execute(runnable);
        }
        executorService.shutdown();
    }

    private static void sleep(int time) {
        try {
            Thread.sleep(time);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

class MyThreadFactory implements ThreadFactory {//自定义ThreadFactory
    private static final AtomicInteger poolNumber = new AtomicInteger(1);
    private final ThreadGroup group;
    private final AtomicInteger threadNumber = new AtomicInteger(1);
    private final String namePrefix;

    MyThreadFactory() {
        SecurityManager s = System.getSecurityManager();
        group = (s != null) ? s.getThreadGroup() : Thread.currentThread().getThreadGroup();
        namePrefix = "☆☆☆--" + poolNumber.getAndIncrement() + "-****-";
    }

    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;
    }

}

在使用的时候必须明白各种线程池是否适用于自己的情景，选取合适的线程池进行使用。

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原文链接：多线程(七)JDK原生线程池转载请注明出处！

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