java线程池的使用学习

1. 线程池的创建

线程池的创建使用ThreadPoolExecutor类，有利于编码时更好的明确线程池运行规则。

     //构造函数
         /**
     * Creates a new {@code ThreadPoolExecutor} with the given initial
     * parameters.
     *
     * @param corePoolSize the number of threads to keep in the pool, even
     *        if they are idle, unless {@code allowCoreThreadTimeOut} is set
     * @param maximumPoolSize the maximum number of threads to allow in the
     *        pool
     * @param keepAliveTime when the number of threads is greater than
     *        the core, this is the maximum time that excess idle threads
     *        will wait for new tasks before terminating.
     * @param unit the time unit for the {@code keepAliveTime} argument
     * @param workQueue the queue to use for holding tasks before they are
     *        executed.  This queue will hold only the {@code Runnable}
     *        tasks submitted by the {@code execute} method.
     * @param threadFactory the factory to use when the executor
     *        creates a new thread
     * @param handler the handler to use when execution is blocked
     *        because the thread bounds and queue capacities are reached
     * @throws IllegalArgumentException if one of the following holds:

     *         {@code corePoolSize < 0}

     *         {@code keepAliveTime < 0}

     *         {@code maximumPoolSize <= 0}

     *         {@code maximumPoolSize < corePoolSize}
     * @throws NullPointerException if {@code workQueue}
     *         or {@code threadFactory} or {@code handler} is null
     */
    public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue 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;
    }

参数含义

(1) 核心线程数corePoolSize： 保持在池中的线程数

(2) 最大线程数maximumPoolSize

(3) 保活时间keepAliveTime： 线程数大于corePoolSize，闲置线程最大空闲时间

(4) 时间单位unit

(5) 阻塞队列workQueue
java.util.concurrent.BlockingQueue主要实现类有：

• ArrayBlockingQueue： 数组结构有界阻塞队列，FIFO排序。其构造函数必须设置队列长度。
• LinkedBlockingQueue：链表结构有界阻塞队列，FIFO排序。队列默认最大长度为Integer.MAX_VALUE,故可能会堆积大量请求，导致OOM。
  
• PriorityBlockingQueue：支持优先级排序的无界阻塞队列。默认自然顺序排列，可以通过比较器comparator指定排序规则。
• DelayQueue：支持延时获取元素的无界阻塞队列。队列使用PriorityQueue实现。

(6) 线程创建接口threadFactory

• 默认使用Executors.defaultThreadFactory()。
• 可以自定义ThreadFactory实现或使用第三方实现，方便指定有意义的线程名称。

import com.google.common.util.concurrent.ThreadFactoryBuilder; 
    ...

    ThreadFactory namedThreadFactory = new ThreadFactoryBuilder().setNameFormat("my-pool-%d").build();

public class MyThreadFactory implements ThreadFactory {

    private final AtomicInteger threadNumber = new AtomicInteger(1);
    private final String namePrefix;

    MyThreadFactory(String namePrefix) {
        this.namePrefix = namePrefix+"-";
    }
    @Override
    public Thread newThread(Runnable r) {
        Thread t = new Thread( r,namePrefix + threadNumber.getAndIncrement());
        if (t.isDaemon()) {
            t.setDaemon(true);
        }
        if (t.getPriority() != Thread.NORM_PRIORITY) {
            t.setPriority(Thread.NORM_PRIORITY);
        }
        return t;
    }
}

(7) 饱和策略handler

• ThreadPoolExecutor.AbortPolicy()：终止策略(默认) ， 抛出java.util.concurrent.RejectedExecutionException异常。
  
• ThreadPoolExecutor.CallerRunsPolicy(): 重试添加当前的任务，他会自动重复调用execute()方法。
• ThreadPoolExecutor.DiscardOldestPolicy(): 抛弃下一个即将被执行的任务，然后尝试重新提交新的任务。最好不和优先级队列一起使用，因为它会抛弃优先级最高的任务。
• ThreadPoolExecutor.DiscardPolicy(): 抛弃策略， 抛弃当前任务。

2. 线程池的运行规则

execute添加任务到线程池:
一个任务通过execute(Runnable)方法被添加到线程池。任务是一个 Runnable类型的对象，任务的执行方法就是 Runnable类型对象的run()方法。

线程池运行规则:
当一个任务通过execute(Runnable)方法添加到线程池时：

• 如果此时线程池中的数量小于corePoolSize，即使线程池中的线程都处于空闲状态，也要创建新的线程来处理被添加的任务。
• 如果此时线程池中的数量等于 corePoolSize，但是缓冲队列 workQueue未满，那么任务被放入缓冲队列。
• 如果此时线程池中的数量大于corePoolSize，缓冲队列workQueue满，并且线程池中的数量小于maximumPoolSize，建新的线程来处理被添加的任务。

• 如果此时线程池中的数量大于corePoolSize，缓冲队列workQueue满，并且线程池中的数量等于maximumPoolSize，那么通过 handler所指定的策略来处理此任务。

• 也就是：处理任务的优先级为：

  核心线程corePoolSize - > 任务队列workQueue - > 最大线程maximumPoolSize
  如果三者都满了，使用handler策略处理该任务。

• 当线程池中的线程数量大于 corePoolSize时，如果某线程空闲时间超过keepAliveTime，线程将被终止。这样，线程池可以动态的调整池中的线程数。

    // execute方法源码实现(jdk1.8)
    public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
        /*
         * Proceed in 3 steps:
         *
         * 1. If fewer than corePoolSize threads are running, try to
         * start a new thread with the given command as its first
         * task.  The call to addWorker atomically checks runState and
         * workerCount, and so prevents false alarms that would add
         * threads when it shouldn't, by returning false.
         *
         * 2. If a task can be successfully queued, then we still need
         * to double-check whether we should have added a thread
         * (because existing ones died since last checking) or that
         * the pool shut down since entry into this method. So we
         * recheck state and if necessary roll back the enqueuing if
         * stopped, or start a new thread if there are none.
         *
         * 3. If we cannot queue task, then we try to add a new
         * thread.  If it fails, we know we are shut down or saturated
         * and so reject the task.
         */
        int c = ctl.get();
        if (workerCountOf(c) < corePoolSize) {
            if (addWorker(command, true))
                return;
            c = ctl.get();
        }
        if (isRunning(c) && workQueue.offer(command)) {
            int recheck = ctl.get();
            if (! isRunning(recheck) && remove(command))
                reject(command);
            else if (workerCountOf(recheck) == 0)
                addWorker(null, false);
        }
        else if (!addWorker(command, false))
            reject(command);
    }

3. 线程池的关闭

通过调用线程池的shutdown或shutdownNow方法来关闭线程池。

• shutdown：将线程池的状态设置成SHUTDOWN状态，然后interrupt空闲线程。
• shutdownNow：线程池的状态设置成STOP，然后尝试interrupt所有线程，包括正在运行的。

关于线程池状态，源码中的注释比较清晰：

再看一下源代码：

    // 在关闭中，之前提交的任务会被执行（包含正在执行的，在阻塞队列中的），但新任务会被拒绝。
    public void shutdown() {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            checkShutdownAccess();
            // 状态设置为shutdown
            advanceRunState(SHUTDOWN);
            // interrupt空闲线程
            interruptIdleWorkers();
            onShutdown(); // hook for ScheduledThreadPoolExecutor
        } finally {
            mainLock.unlock();
        }
        // 尝试终止线程池
        tryTerminate();
    }

其中，interruptIdleWorkers()方法往下调用了interruptIdleWorkers(), 这里w.tryLock()比较关键。
中断之前需要先tryLock()获取worker锁，正在运行的worker tryLock()失败(runWorker()方法会先对worker上锁)，故正在运行的worker不能中断。

    // 尝试停止所有正在执行的任务，停止对等待任务的处理，并返回正在等待被执行的任务列表
    public List shutdownNow() {
        List tasks;
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            checkShutdownAccess();
            // 状态设置为STOP
            advanceRunState(STOP);
            // 停止所有线程  interruptWorkers逻辑简单些，循环对所有worker调用interruptIfStarted().(interrupt所有线程)
            interruptWorkers();
            tasks = drainQueue();
        } finally {
            mainLock.unlock();
        }
        tryTerminate();
        return tasks;
    }

4. 线程池的使用场合

（1）单个任务处理的时间比较短；
（2）需要处理的任务数量大；

5. 线程池大小的设置

可根据计算任务类型估算线程池设置大小：

cpu密集型：可采用Runtime.avaliableProcesses()+1个线程；
IO密集型：由于阻塞操作多，可使用更多的线程，如2倍cpu核数。

6 实现举例

场景: ftp服务器收到文件后，触发相关搬移/处理操作。

public class FtpEventHandler extends DefaultFtplet {

    @Override
    public FtpletResult onUploadEnd(FtpSession session, FtpRequest request)
            throws FtpException, IOException {
        // 获取文件名
        String fileName = request.getArgument();
        Integer index = fileName.lastIndexOf("/");
        String realFileName = fileName.substring(index + 1);
        index = realFileName.lastIndexOf("\\");
        realFileName = realFileName.substring(index + 1);

        // **处理文件**
        ThreadPoolExecutor threadPool = new ThreadPoolExecutor(10, 50, 10,
                TimeUnit.SECONDS, new ArrayBlockingQueue(3));
        threadPool.execute(new fileSenderThread(realFileName));

        return FtpletResult.DEFAULT;
    }
}

Spring也提供了ThreadPoolTaskExecutor

     
    
        
        
        
        
        
        
        
    

    //java代码中注入bean
    @Autowired  
    @Qualifier("gkTaskExecutor")  
    private ThreadPoolTaskExecutor gkTaskExecutor;  

end.

转载于:https://www.cnblogs.com/eaglediao/p/7742570.html