java并发：线程池之ThreadPoolExecutor

一、线程池的属性

二、详解ThreadPoolExecutor

　　上文提到可以通过显式的ThreadPoolExecutor构造函数来构造特定形式的线程池，ThreadPoolExecutor是java.util.concurrent包以内部线程池的形式对外提供线程池管理、线程调度等服务，此处我们来了解一下ThreadPoolExecutor

（1）一般使用方式

ExecutorService exec = new ThreadPoolExecutor(8,
                8, 
                0L,
                TimeUnit.MILLISECONDS,
                new LinkedBlockingQueue<Runnable>(100),
                new ThreadPoolExecutor.CallerRunsPolicy());

（2）构造函数

/**
     * Creates a new {@code ThreadPoolExecutor} with the given initial
     * parameters, the default thread factory and the default rejected
     * execution handler.
     *
     * <p>It may be more convenient to use one of the {@link Executors}
     * factory methods instead of this general purpose constructor.
     *
     * @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.
     * @throws IllegalArgumentException if one of the following holds:<br>
     *         {@code corePoolSize < 0}<br>
     *         {@code keepAliveTime < 0}<br>
     *         {@code maximumPoolSize <= 0}<br>
     *         {@code maximumPoolSize < corePoolSize}
     * @throws NullPointerException if {@code workQueue} is null
     */
    public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue<Runnable> workQueue) {
        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
             Executors.defaultThreadFactory(), defaultHandler);
    }

    /**
     * Creates a new {@code ThreadPoolExecutor} with the given initial
     * parameters and {@linkplain ThreadPoolExecutor.AbortPolicy
     * default rejected execution handler}.
     *
     * @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
     * @throws IllegalArgumentException if one of the following holds:<br>
     *         {@code corePoolSize < 0}<br>
     *         {@code keepAliveTime < 0}<br>
     *         {@code maximumPoolSize <= 0}<br>
     *         {@code maximumPoolSize < corePoolSize}
     * @throws NullPointerException if {@code workQueue}
     *         or {@code threadFactory} is null
     */
    public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue<Runnable> workQueue,
                              ThreadFactory threadFactory) {
        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
             threadFactory, defaultHandler);
    }

    /**
     * Creates a new {@code ThreadPoolExecutor} with the given initial
     * parameters and
     * {@linkplain Executors#defaultThreadFactory default thread factory}.
     *
     * @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 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:<br>
     *         {@code corePoolSize < 0}<br>
     *         {@code keepAliveTime < 0}<br>
     *         {@code maximumPoolSize <= 0}<br>
     *         {@code maximumPoolSize < corePoolSize}
     * @throws NullPointerException if {@code workQueue}
     *         or {@code handler} is null
     */
    public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue<Runnable> workQueue,
                              RejectedExecutionHandler handler) {
        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
             Executors.defaultThreadFactory(), handler);
    }

    /**
     * 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:<br>
     *         {@code corePoolSize < 0}<br>
     *         {@code keepAliveTime < 0}<br>
     *         {@code maximumPoolSize <= 0}<br>
     *         {@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<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;
    }

（3）构造函数参数说明

进一步解说：

A、当提交新任务时，若线程池大小小于corePoolSize，将创建一个新的线程来执行任务，即使此时线程池中存在空闲线程；

B、当提交新任务时，若线程池达到corePoolSize大小，新提交的任务将被放入workQueue中，等待线程池调度执行；

C、当提交新任务时，若workQueue已满，且maximumPoolSize>corePoolSize，将创建新的线程来执行任务；

D、当提交新任务时，若任务总数超过maximumPoolSize，新提交的任务将由RejectedExecutionHandler来处理；

E、当线程池中的线程数超过corePoolSize时，若线程的空闲时间达到keepAliveTime，则关闭空闲线程

（4）阻塞队列的选择

 （5）简述SynchronousQueue

Note：

同步队列 SynchronousQueue最多只有一个元素

三、定制ThreadPoolExecutor

（1）修改参数的方式

（2）自定义方式（封装各种参数）

示例（摘自网络）：

import java.util.concurrent.ArrayBlockingQueue;  
import java.util.concurrent.ExecutorService;  
import java.util.concurrent.RejectedExecutionHandler;  
import java.util.concurrent.ThreadFactory;  
import java.util.concurrent.ThreadPoolExecutor;  
import java.util.concurrent.TimeUnit;  
import java.util.concurrent.atomic.AtomicInteger;  
  
  
public class CustomThreadPoolExecutor {  
  
      
    private ThreadPoolExecutor pool = null;  
      
      
    /** 
     * 线程池初始化方法 
     *  
     * corePoolSize 核心线程池大小----10 
     * maximumPoolSize 最大线程池大小----30 
     * keepAliveTime 线程池中超过corePoolSize数目的空闲线程最大存活时间----30+单位TimeUnit 
     * TimeUnit keepAliveTime时间单位----TimeUnit.MINUTES 
     * workQueue 阻塞队列----new ArrayBlockingQueue<Runnable>(10)====10容量的阻塞队列 
     * threadFactory 新建线程工厂----new CustomThreadFactory()====定制的线程工厂 
     * rejectedExecutionHandler 当提交任务数超过maxmumPoolSize+workQueue之和时, 
     *                          即当提交第41个任务时(前面线程都没有执行完,此测试方法中用sleep(100)), 
     *                          任务会交给RejectedExecutionHandler来处理 
     */  
    public void init() {  
        pool = new ThreadPoolExecutor(  
                10,  
                30,  
                30,  
                TimeUnit.MINUTES,  
                new ArrayBlockingQueue<Runnable>(10),  
                new CustomThreadFactory(),new CustomRejectedExecutionHandler());  
    }  
  
      
    public void destory() {  
        if(pool != null) {  
            pool.shutdownNow();  
        }  
    }  
      
      
    public ExecutorService getCustomThreadPoolExecutor() {  
        return this.pool;  
    }  
      
    private class CustomThreadFactory implements ThreadFactory {  
  
        private AtomicInteger count = new AtomicInteger(0);  
          
        @Override  
        public Thread newThread(Runnable r) {  
            Thread t = new Thread(r);  
            String threadName = CustomThreadPoolExecutor.class.getSimpleName() + count.addAndGet(1);  
            System.out.println(threadName);  
            t.setName(threadName);  
            return t;  
        }  
    }
      
    private class CustomRejectedExecutionHandler implements RejectedExecutionHandler {  
  
        @Override  
        public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {  
            // 记录异常  
            // 报警处理等  
            System.out.println("error.............");  
        }  
    }
      
    // 测试构造的线程池  
    public static void main(String[] args) {  
        CustomThreadPoolExecutor exec = new CustomThreadPoolExecutor();  
        // 1.初始化  
        exec.init();  
          
        ExecutorService pool = exec.getCustomThreadPoolExecutor();  
        for(int i=1; i<100; i++) {  
            System.out.println("提交第" + i + "个任务!");  
            pool.execute(new Runnable() {  
                @Override  
                public void run() {  
                    try {  
                        Thread.sleep(300);  
                    } catch (InterruptedException e) {  
                        e.printStackTrace();  
                    }  
                    System.out.println("running=====");  
                }  
            });  
        }    
          
        // 2.销毁----此处不能销毁,因为任务没有提交执行完,如果销毁线程池,任务也就无法执行了  
        // exec.destory();  
          
        try {  
            Thread.sleep(10000);  
        } catch (InterruptedException e) {  
            e.printStackTrace();  
        }  
    }  
}

解读：

上述代码通过init()方法对ThreadPoolExecutor构造函数进行了一些自定义设置，getCustomThreadPoolExecutor()方法返回init()方法配置的ThreadPoolExecutor对象实例（即线程池引用）

四、扩展ThreadPoolExecutor

五、源码视角

从源码视角分析Executors、ThreadPoolExecutor、ExecuteService、Executor之间的关系

（1）Executors

　　从Java5开始新增了Executors类，它有几个静态工厂方法用来创建线程池，这些静态工厂方法返回一个ExecutorService类型的值，此值即为线程池的引用。

（2）Executor

Executor是一个接口，里面只有一个execute方法

public interface Executor {
    void execute(Runnable command);
}

（3）ExecuteService

ExecuteService也是一个接口，此接口继承了Executor接口，其定义如下：

public interface ExecutorService extends Executor {...}

（4）ThreadPoolExecutor

继承AbstractExecutorService，AbstractExecutorService实现ExecutorService接口

public class ThreadPoolExecutor extends AbstractExecutorService {...}

public abstract class AbstractExecutorService implements ExecutorService {...}

AbstractExecutorService已经实现了ExecuteService中定义的几个submit方法，代码如下：

    /**
     * @throws RejectedExecutionException {@inheritDoc}
     * @throws NullPointerException       {@inheritDoc}
     */
    public Future<?> submit(Runnable task) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<Void> ftask = newTaskFor(task, null);
        execute(ftask);
        return ftask;
    }

    /**
     * @throws RejectedExecutionException {@inheritDoc}
     * @throws NullPointerException       {@inheritDoc}
     */
    public <T> Future<T> submit(Runnable task, T result) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<T> ftask = newTaskFor(task, result);
        execute(ftask);
        return ftask;
    }

    /**
     * @throws RejectedExecutionException {@inheritDoc}
     * @throws NullPointerException       {@inheritDoc}
     */
    public <T> Future<T> submit(Callable<T> task) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<T> ftask = newTaskFor(task);
        execute(ftask);
        return ftask;
    }

解读：

这几个submit方法最终是调用了execute方法，该方法依赖于子类的具体实现。

ThreadPoolExecutor实现了Executor中定义的execute方法，代码如下：

    /**
     * Executes the given task sometime in the future.  The task
     * may execute in a new thread or in an existing pooled thread.
     *
     * If the task cannot be submitted for execution, either because this
     * executor has been shutdown or because its capacity has been reached,
     * the task is handled by the current {@link RejectedExecutionHandler}.
     *
     * @param command the task to execute
     * @throws RejectedExecutionException at discretion of
     *         {@code RejectedExecutionHandler}, if the task
     *         cannot be accepted for execution
     * @throws NullPointerException if {@code command} is null
     */
    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);
    }

解读：

此方法首先获取线程池相关信息，如：线程池中处于active的线程数（workerCountOf）以及线程池的状态（isRunning）；然后根据情况新增线程或者将任务放入队列或者通过饱和策略来处理。

Note：

此处成员变量 ctl是一个 Integer的原子变量，用来记录线程池状态和线程池中线程个数，类似于 ReentrantReadWriteLock 使用一个变量来保存两种信息。

相关代码如下：

    private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
    private static final int COUNT_BITS = Integer.SIZE - 3;
    private static final int COUNT_MASK = (1 << COUNT_BITS) - 1;

    // runState is stored in the high-order bits
    private static final int RUNNING    = -1 << COUNT_BITS;
    private static final int SHUTDOWN   =  0 << COUNT_BITS;
    private static final int STOP       =  1 << COUNT_BITS;
    private static final int TIDYING    =  2 << COUNT_BITS;
    private static final int TERMINATED =  3 << COUNT_BITS;

    // Packing and unpacking ctl
    private static int runStateOf(int c)     { return c & ~COUNT_MASK; }
    private static int workerCountOf(int c)  { return c & COUNT_MASK; }
    private static int ctlOf(int rs, int wc) { return rs | wc; }

    /*
     * Bit field accessors that don't require unpacking ctl.
     * These depend on the bit layout and on workerCount being never negative.
     */

    private static boolean runStateLessThan(int c, int s) {
        return c < s;
    }

    private static boolean runStateAtLeast(int c, int s) {
        return c >= s;
    }

    private static boolean isRunning(int c) {
        return c < SHUTDOWN;
    }

    /**
     * Attempts to CAS-increment the workerCount field of ctl.
     */
    private boolean compareAndIncrementWorkerCount(int expect) {
        return ctl.compareAndSet(expect, expect + 1);
    }

    /**
     * Attempts to CAS-decrement the workerCount field of ctl.
     */
    private boolean compareAndDecrementWorkerCount(int expect) {
        return ctl.compareAndSet(expect, expect - 1);
    }

    /**
     * Decrements the workerCount field of ctl. This is called only on
     * abrupt termination of a thread (see processWorkerExit). Other
     * decrements are performed within getTask.
     */
    private void decrementWorkerCount() {
        ctl.addAndGet(-1);
    }