Java教程

线程池的深入理解

本文主要是介绍线程池的深入理解,对大家解决编程问题具有一定的参考价值,需要的程序猿们随着小编来一起学习吧!

  简单研究下线程池的执行原理。以及execute 和 submit 方法的区别。

1. execute 方法接收的是一个Runnable 参数,返回值是void 类型,也就是不接收结果, 方法签名如下:

 java.util.concurrent.Executor#execute

void execute(Runnable command);

2. submit 方法三个重载的方法,如下:

 对应的方法如下:‘

(1) java.util.concurrent.ExecutorService#submit(java.lang.Runnable)

Future<?> submit(Runnable task);

(2)  java.util.concurrent.ExecutorService#submit(java.lang.Runnable, T)

<T> Future<T> submit(Runnable task, T result);

(3) java.util.concurrent.ExecutorService#submit(java.util.concurrent.Callable<T>)

<T> Future<T> submit(Callable<T> task);

1.java.util.concurrent.ThreadPoolExecutor#execute理解

测试代码

        ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(5, 5,
                0L, TimeUnit.MILLISECONDS,
                new LinkedBlockingQueue<Runnable>(3));
        // execute 方法无返回值,相当于执行不带返回结果的任务
        threadPoolExecutor.execute(() -> {
            log.info("--" + Thread.currentThread().getName() + "--A");
            try {
                Thread.sleep(2 * 1000);
            } catch (InterruptedException e) {
            }
            log.info("--" + Thread.currentThread().getName() + "--B");
        });

查看源代码如下:

(1) 接口是Executor 的方法

    void execute(Runnable command);

(2) 实现类java.util.concurrent.ThreadPoolExecutor#execute

    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);
    }

源码跟踪:

(1) addWorker(command, true) 增加worker

    private boolean addWorker(Runnable firstTask, boolean core) {
        retry:
        for (;;) {
            int c = ctl.get();
            int rs = runStateOf(c);

            // Check if queue empty only if necessary.
            if (rs >= SHUTDOWN &&
                ! (rs == SHUTDOWN &&
                   firstTask == null &&
                   ! workQueue.isEmpty()))
                return false;

            for (;;) {
                int wc = workerCountOf(c);
                if (wc >= CAPACITY ||
                    wc >= (core ? corePoolSize : maximumPoolSize))
                    return false;
                if (compareAndIncrementWorkerCount(c))
                    break retry;
                c = ctl.get();  // Re-read ctl
                if (runStateOf(c) != rs)
                    continue retry;
                // else CAS failed due to workerCount change; retry inner loop
            }
        }

        boolean workerStarted = false;
        boolean workerAdded = false;
        Worker w = null;
        try {
            w = new Worker(firstTask);
            final Thread t = w.thread;
            if (t != null) {
                final ReentrantLock mainLock = this.mainLock;
                mainLock.lock();
                try {
                    // Recheck while holding lock.
                    // Back out on ThreadFactory failure or if
                    // shut down before lock acquired.
                    int rs = runStateOf(ctl.get());

                    if (rs < SHUTDOWN ||
                        (rs == SHUTDOWN && firstTask == null)) {
                        if (t.isAlive()) // precheck that t is startable
                            throw new IllegalThreadStateException();
                        workers.add(w);
                        int s = workers.size();
                        if (s > largestPoolSize)
                            largestPoolSize = s;
                        workerAdded = true;
                    }
                } finally {
                    mainLock.unlock();
                }
                if (workerAdded) {
                    t.start();
                    workerStarted = true;
                }
            }
        } finally {
            if (! workerStarted)
                addWorkerFailed(w);
        }
        return workerStarted;
    }

worker 是一个内部类java.util.concurrent.ThreadPoolExecutor.Worker,其包含的重要属性和方法如下:

    private final class Worker
        extends AbstractQueuedSynchronizer
        implements Runnable
    {
        /**
         * This class will never be serialized, but we provide a
         * serialVersionUID to suppress a javac warning.
         */
        private static final long serialVersionUID = 6138294804551838833L;

        /** Thread this worker is running in.  Null if factory fails. */
        final Thread thread;
        /** Initial task to run.  Possibly null. */
        Runnable firstTask;
        /** Per-thread task counter */
        volatile long completedTasks;

        Worker(Runnable firstTask) {
            setState(-1); // inhibit interrupts until runWorker
            this.firstTask = firstTask;
            this.thread = getThreadFactory().newThread(this);
        }

        public void run() {
            runWorker(this);
        }

    final void runWorker(Worker w) {
        Thread wt = Thread.currentThread();
        Runnable task = w.firstTask;
        w.firstTask = null;
        w.unlock(); // allow interrupts
        boolean completedAbruptly = true;
        try {
            while (task != null || (task = getTask()) != null) {
                w.lock();
                // If pool is stopping, ensure thread is interrupted;
                // if not, ensure thread is not interrupted.  This
                // requires a recheck in second case to deal with
                // shutdownNow race while clearing interrupt
                if ((runStateAtLeast(ctl.get(), STOP) ||
                     (Thread.interrupted() &&
                      runStateAtLeast(ctl.get(), STOP))) &&
                    !wt.isInterrupted())
                    wt.interrupt();
                try {
                    beforeExecute(wt, task);
                    Throwable thrown = null;
                    try {
                        task.run();
                    } catch (RuntimeException x) {
                        thrown = x; throw x;
                    } catch (Error x) {
                        thrown = x; throw x;
                    } catch (Throwable x) {
                        thrown = x; throw new Error(x);
                    } finally {
                        afterExecute(task, thrown);
                    }
                } finally {
                    task = null;
                    w.completedTasks++;
                    w.unlock();
                }
            }
            completedAbruptly = false;
        } finally {
            processWorkerExit(w, completedAbruptly);
        }
    }

...    

1》new Worker 的时候会创建线程,线程传入的Runnable 对象是Worker 自身,同时Worker 

2》t.start(); 启动的时候会调用java.util.concurrent.ThreadPoolExecutor.Worker#run 方法

3》然后调用到java.util.concurrent.ThreadPoolExecutor#runWorker, 方法内部调用task.run(); 相当于同步的调用 传给线程池的Runnable 任务的run 方法。

4》当不是第一个任务的时候走的是java.util.concurrent.ThreadPoolExecutor#getTask, 也就是从任务队列中阻塞拿去任务

    private Runnable getTask() {
        boolean timedOut = false; // Did the last poll() time out?

        for (;;) {
            int c = ctl.get();
            int rs = runStateOf(c);

            // Check if queue empty only if necessary.
            if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
                decrementWorkerCount();
                return null;
            }

            int wc = workerCountOf(c);

            // Are workers subject to culling?
            boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;

            if ((wc > maximumPoolSize || (timed && timedOut))
                && (wc > 1 || workQueue.isEmpty())) {
                if (compareAndDecrementWorkerCount(c))
                    return null;
                continue;
            }

            try {
                Runnable r = timed ?
                    workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                    workQueue.take();
                if (r != null)
                    return r;
                timedOut = true;
            } catch (InterruptedException retry) {
                timedOut = false;
            }
        }
    }

 2.  submit理解

1. java.util.concurrent.AbstractExecutorService#submit(java.lang.Runnable) 理解

查看源码如下:java.util.concurrent.AbstractExecutorService#submit(java.lang.Runnable)

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

可以理解大体的思路是对,传给线程池的Runnable 进行了下包装,java.util.concurrent.AbstractExecutorService#newTaskFor(java.lang.Runnable, T) 如下:

    protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
        return new FutureTask<T>(runnable, value);
    }

(1) java.util.concurrent.RunnableFuture 继承了接口Runnable, Future<V>

public interface RunnableFuture<V> extends Runnable, Future<V> {

    void run();
}

java.util.concurrent.Future 接口如下:

public interface Future<V> {

    /**
     * Attempts to cancel execution of this task.  This attempt will
     * fail if the task has already completed, has already been cancelled,
     * or could not be cancelled for some other reason. If successful,
     * and this task has not started when {@code cancel} is called,
     * this task should never run.  If the task has already started,
     * then the {@code mayInterruptIfRunning} parameter determines
     * whether the thread executing this task should be interrupted in
     * an attempt to stop the task.
     *
     * <p>After this method returns, subsequent calls to {@link #isDone} will
     * always return {@code true}.  Subsequent calls to {@link #isCancelled}
     * will always return {@code true} if this method returned {@code true}.
     *
     * @param mayInterruptIfRunning {@code true} if the thread executing this
     * task should be interrupted; otherwise, in-progress tasks are allowed
     * to complete
     * @return {@code false} if the task could not be cancelled,
     * typically because it has already completed normally;
     * {@code true} otherwise
     */
    boolean cancel(boolean mayInterruptIfRunning);

    /**
     * Returns {@code true} if this task was cancelled before it completed
     * normally.
     *
     * @return {@code true} if this task was cancelled before it completed
     */
    boolean isCancelled();

    /**
     * Returns {@code true} if this task completed.
     *
     * Completion may be due to normal termination, an exception, or
     * cancellation -- in all of these cases, this method will return
     * {@code true}.
     *
     * @return {@code true} if this task completed
     */
    boolean isDone();

    /**
     * Waits if necessary for the computation to complete, and then
     * retrieves its result.
     *
     * @return the computed result
     * @throws CancellationException if the computation was cancelled
     * @throws ExecutionException if the computation threw an
     * exception
     * @throws InterruptedException if the current thread was interrupted
     * while waiting
     */
    V get() throws InterruptedException, ExecutionException;

    /**
     * Waits if necessary for at most the given time for the computation
     * to complete, and then retrieves its result, if available.
     *
     * @param timeout the maximum time to wait
     * @param unit the time unit of the timeout argument
     * @return the computed result
     * @throws CancellationException if the computation was cancelled
     * @throws ExecutionException if the computation threw an
     * exception
     * @throws InterruptedException if the current thread was interrupted
     * while waiting
     * @throws TimeoutException if the wait timed out
     */
    V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException;
}

java.util.concurrent.FutureTask 的重要属性和方法如下:

public class FutureTask<V> implements RunnableFuture<V> {

    private volatile int state;
    private static final int NEW          = 0;
    private static final int COMPLETING   = 1;
    private static final int NORMAL       = 2;
    private static final int EXCEPTIONAL  = 3;
    private static final int CANCELLED    = 4;
    private static final int INTERRUPTING = 5;
    private static final int INTERRUPTED  = 6;


    /** The underlying callable; nulled out after running */
    private Callable<V> callable;
    /** The result to return or exception to throw from get() */
    private Object outcome; // non-volatile, protected by state reads/writes
    /** The thread running the callable; CASed during run() */
    private volatile Thread runner;
    /** Treiber stack of waiting threads */
    private volatile WaitNode waiters;

    public FutureTask(Callable<V> callable) {
        if (callable == null)
            throw new NullPointerException();
        this.callable = callable;
        this.state = NEW;       // ensure visibility of callable
    }

    public FutureTask(Runnable runnable, V result) {
        this.callable = Executors.callable(runnable, result);
        this.state = NEW;       // ensure visibility of callable
    }

    public boolean isCancelled() {
        return state >= CANCELLED;
    }

    public boolean isDone() {
        return state != NEW;
    }

    public boolean cancel(boolean mayInterruptIfRunning) {
        if (!(state == NEW &&
              UNSAFE.compareAndSwapInt(this, stateOffset, NEW,
                  mayInterruptIfRunning ? INTERRUPTING : CANCELLED)))
            return false;
        try {    // in case call to interrupt throws exception
            if (mayInterruptIfRunning) {
                try {
                    Thread t = runner;
                    if (t != null)
                        t.interrupt();
                } finally { // final state
                    UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED);
                }
            }
        } finally {
            finishCompletion();
        }
        return true;
    }

    public V get() throws InterruptedException, ExecutionException {
        int s = state;
        if (s <= COMPLETING)
            s = awaitDone(false, 0L);
        return report(s);
    }

    public V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException {
        if (unit == null)
            throw new NullPointerException();
        int s = state;
        if (s <= COMPLETING &&
            (s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
            throw new TimeoutException();
        return report(s);
    }

    protected void done() { }

    protected void set(V v) {
        if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
            outcome = v;
            UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
            finishCompletion();
        }
    }

...

1》java.util.concurrent.Executors#callable(java.lang.Runnable, T) 采用适配器模式,将Runnable 适配为Callable 

    public static <T> Callable<T> callable(Runnable task, T result) {
        if (task == null)
            throw new NullPointerException();
        return new RunnableAdapter<T>(task, result);
    }

java.util.concurrent.Executors.RunnableAdapter 如下:

    static final class RunnableAdapter<T> implements Callable<T> {
        final Runnable task;
        final T result;
        RunnableAdapter(Runnable task, T result) {
            this.task = task;
            this.result = result;
        }
        public T call() {
            task.run();
            return result;
        }
    }

  可以看到是返回固定的结果, 也就是接受Runnable 的时候,返回的结果是固定的结果

2》然后同上面execute 方法一样,执行execute(ftask); 方法,这时候线程池跑的是FutureTask 对象

3》return ftask; 返回Future 对象,使得可以外部拿该对象可以获取执行结果。

4》线程跑任务会调用java.util.concurrent.FutureTask#run, 方法如下:

    public void run() {
        if (state != NEW ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                         null, Thread.currentThread()))
            return;
        try {
            Callable<V> c = callable;
            if (c != null && state == NEW) {
                V result;
                boolean ran;
                try {
                    result = c.call();
                    ran = true;
                } catch (Throwable ex) {
                    result = null;
                    ran = false;
                    setException(ex);
                }
                if (ran)
                    set(result);
            }
        } finally {
            // runner must be non-null until state is settled to
            // prevent concurrent calls to run()
            runner = null;
            // state must be re-read after nulling runner to prevent
            // leaked interrupts
            int s = state;
            if (s >= INTERRUPTING)
                handlePossibleCancellationInterrupt(s);
        }
    }

2. java.util.concurrent.AbstractExecutorService#submit(java.lang.Runnable, T) 理解

    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;
    }

  根据上面的理解,是返回固定的结果result, 只不过java.util.concurrent.AbstractExecutorService#submit(java.lang.Runnable) 的返回结果是null。

3.  java.util.concurrent.AbstractExecutorService#submit(java.util.concurrent.Callable<T>) 理解

(1) 测试代码

        Future<String> submit1 = threadPoolExecutor.submit(new Callable<String>() {
            @Override
            public String call() throws Exception {
                log.info("--" + Thread.currentThread().getName() + "--E");
                try {
                    Thread.sleep(2 * 1000);
                } catch (InterruptedException e) {
                }
                log.info("--" + Thread.currentThread().getName() + "--F");
                return "123456";
            }
        });

(2) 源码:java.util.concurrent.AbstractExecutorService#submit(java.util.concurrent.Callable<T>)

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

1》java.util.concurrent.AbstractExecutorService#newTaskFor(java.util.concurrent.Callable<T>) 创建 FutureTask

    protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
        return new FutureTask<T>(callable);
    }

接着调用:java.util.concurrent.FutureTask#FutureTask(java.util.concurrent.Callable<V>)

    public FutureTask(Callable<V> callable) {
        if (callable == null)
            throw new NullPointerException();
        this.callable = callable;
        this.state = NEW;       // ensure visibility of callable
    }

2》然后调用java.util.concurrent.ThreadPoolExecutor#execute 跑任务,只不过Runnable 对象是java.util.concurrent.FutureTask, 内部的run 方法是调用成员属性Callable 的call 方法,方法执行完之后记录其返回结果, 所以FutureTask 可以拿到返回的结果。

补充:Future 两个重要的方法

(1) java.util.concurrent.FutureTask#isDone 判断是否执行完成

    public boolean isDone() {
        return state != NEW;
    }

(2) 获取结果:

    public V get() throws InterruptedException, ExecutionException {
        int s = state;
        if (s <= COMPLETING)
            s = awaitDone(false, 0L);
        return report(s);
    }

java.util.concurrent.FutureTask#awaitDone 阻塞获取结果:

    private int awaitDone(boolean timed, long nanos)
        throws InterruptedException {
        final long deadline = timed ? System.nanoTime() + nanos : 0L;
        WaitNode q = null;
        boolean queued = false;
        for (;;) {
            if (Thread.interrupted()) {
                removeWaiter(q);
                throw new InterruptedException();
            }

            int s = state;
            if (s > COMPLETING) {
                if (q != null)
                    q.thread = null;
                return s;
            }
            else if (s == COMPLETING) // cannot time out yet
                Thread.yield();
            else if (q == null)
                q = new WaitNode();
            else if (!queued)
                queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                     q.next = waiters, q);
            else if (timed) {
                nanos = deadline - System.nanoTime();
                if (nanos <= 0L) {
                    removeWaiter(q);
                    return state;
                }
                LockSupport.parkNanos(this, nanos);
            }
            else
                LockSupport.park(this);
        }
    }

可以看到这里使用了 java.util.concurrent.locks.LockSupport#park(java.lang.Object) 进行阻塞线程,下面使用的是UNSAFE 类的相关API。

补充:关于LockSupport 的使用

   参考:https://www.jianshu.com/p/8e6b1942ae39

测试代码:

package org.example;

import java.util.concurrent.locks.LockSupport;

public class PlainTest2 {

    public static void main(String[] args) {
        Thread t = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("111222");
                LockSupport.park();
                System.out.println("333444");
                //LockSupport.park(obj);
            }
        });
        t.start();

        try {
            Thread.sleep(10000);
        } catch (InterruptedException e) {
        }

        System.out.println("555555");
        LockSupport.unpark(t);
        System.out.println("555555");
    }
}

查看LockSupport 源码:

    public static void park() {
        UNSAFE.park(false, 0L);
    }

    public static void unpark(Thread thread) {
        if (thread != null)
            UNSAFE.unpark(thread);
    }

strace linux环境下面监测如下:

1》park

clock_gettime(CLOCK_MONOTONIC, {tv_sec=273, tv_nsec=622979062}) = 0
futex(0xf6797ab8, FUTEX_WAIT_PRIVATE, 1, NULL) = 0
futex(0xf6797a9c, FUTEX_WAIT_PRIVATE, 2, NULL) = 0
futex(0xf6797a9c, FUTEX_WAKE_PRIVATE, 1) = 0
clock_gettime(CLOCK_MONOTONIC, {tv_sec=373, tv_nsec=621209093}) = 0
clock_gettime(CLOCK_MONOTONIC, {tv_sec=373, tv_nsec=621237803}) = 0
futex(0xf6797344, FUTEX_WAIT_PRIVATE, 1, {tv_sec=0, tv_nsec=971290}) = 0
futex(0xf6797328, FUTEX_WAIT_PRIVATE, 2, NULL) = 0
futex(0xf6797328, FUTEX_WAKE_PRIVATE, 1) = 0

2》unpark

write(1, "555555", 6)                   = 6
write(1, "\n", 1)                       = 1
clock_gettime(CLOCK_MONOTONIC, {tv_sec=373, tv_nsec=620209800}) = 0
futex(0xf6797ab8, FUTEX_WAKE_OP_PRIVATE, 1, 1, 0xf6797ab4, FUTEX_OP_SET<<28|0<<12|FUTEX_OP_CMP_GT<<24|0x1) = 1
futex(0xf6797a9c, FUTEX_WAKE_PRIVATE, 1) = 1
write(1, "555555", 6)                   = 6

  可以看到到内核调用的也是futex 指令,和synchronized 一样。

 

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