AsyncTask RejectedExecutionException 小结

在使用Asynctask时，相信有些朋友会遇到以下RejectedExecutionException：

Java.util.concurrent.RejectedExecutionException: Task android.os.AsyncTask$3@e3a9753 rejected from java.util.concurrent.ThreadPoolExecutor@63fe890[Running, pool size = 9, active threads = 9, queued tasks = 128, completed tasks = 0]
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

到底是什么原理导致以上RuntimeException呢？ 让我们一起研读源码来一探究竟。

首先，可以使用以下代码来制造上面的Exception：

package com.breakmedia.interview.asyncTask;

import android.os.AsyncTask;
import android.util.Log;

public class AsyncPool {

    private static int TASK_NUMBER = 138;
    private static final String TAG = "jeff";

    public void doTask() {
        for (int i = 0; i <= TASK_NUMBER; i++) {
            String task = "task@ " + i;
            Log.d(TAG, "put " + task);
            MyAsyncTask myAsyncTask = new MyAsyncTask(task);
            myAsyncTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, 0);
        }
    }

    static class MyAsyncTask extends AsyncTask<Integer, Integer, Integer> {
        private static int SLEEP_TIME = 2000;
        private String name;

        public MyAsyncTask(String name) {
            this.name = name;
        }

        protected Integer doInBackground(Integer... arg0) {
            Log.d(TAG, "start .." + name
                    + " thread id: " + Thread.currentThread().getId()
                    + " thread name: " + Thread.currentThread().getName());
            try {
                Thread.sleep(SLEEP_TIME);
            } catch (Exception e) {
                Log.d(TAG, "", e);
            }
            return 0;
        }
    }
}

需要解释的是，我用的CPU 是四核，所以对应的 MAXIMUM_POOL_SIZE 为9， 

private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;

而在AsyncTask 内部有一个static 的变量 ThreadPoolExecutor， 其 workQuene 为容量为128的 LinkedBlockingQueue：

private static final BlockingQueue<Runnable> sPoolWorkQueue =
            new LinkedBlockingQueue<Runnable>(128);

    /**
     * An {@link Executor} that can be used to execute tasks in parallel.
     */
    public static final Executor THREAD_POOL_EXECUTOR;

    static {
        ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(
                CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE_SECONDS, TimeUnit.SECONDS,
                sPoolWorkQueue, sThreadFactory);
        threadPoolExecutor.allowCoreThreadTimeOut(true);
        THREAD_POOL_EXECUTOR = threadPoolExecutor;
    }

综上所述，如果按照  myAsyncTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, 0);

调用，如果Task_NUMBER 设置为138，应该会出现RejectException， 结果是不是和预想一样的呢？ 运行程序，如果得到一样的RuntimeException

java.util.concurrent.RejectedExecutionException: Task android.os.AsyncTask$3@63fe890 rejected from java.util.concurrent.ThreadPoolExecutor@1721589[Running, pool size = 9, active threads = 9, queued tasks = 128, completed tasks = 0]
                                                     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
                                                     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
                                                     at android.app.ActivityThread.-wrap11(ActivityThread.java)
                                                     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
                                                     at android.os.Handler.dispatchMessage(Handler.java:102)
                                                     at android.os.Looper.loop(Looper.java:148)
                                                     at android.app.ActivityThread.main(ActivityThread.java:5417)
                                                     at java.lang.reflect.Method.invoke(Native Method)
                                                     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
                                                     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
                                                  Caused by: java.util.concurrent.RejectedExecutionException: Task android.os.AsyncTask$3@63fe890 rejected from java.util.concurrent.ThreadPoolExecutor@1721589[Running, pool size = 9, active threads = 9, queued tasks = 128, completed tasks = 0]
                                                     at java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:2014)
                                                     at java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:794)
                                                     at java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:1340)
                                                     at android.os.AsyncTask.executeOnExecutor(AsyncTask.java:607)
                                                     at com.breakmedia.interview.asyncTask.AsyncPool.doTask(AsyncPool.java:16)
                                                     at com.ryg.chapter_11.MainActivity.onCreate(MainActivity.java:91)
                                                     at android.app.Activity.performCreate(Activity.java:6237)
                                                     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
                                                     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
                                                     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
                                                     at android.app.ActivityThread.-wrap11(ActivityThread.java) 
                                                     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
                                                     at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                     at android.os.Looper.loop(Looper.java:148) 
                                                     at android.app.ActivityThread.main(ActivityThread.java:5417) 
                                                     at java.lang.reflect.Method.invoke(Native Method) 
                                                     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
                                                     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 
04-25 13:55:12.788 763-833/system_process E/Surface: getSlotFromBufferLocked: unknown buffer: 0xde3024d0

同理，如果我将

 private static int TASK_NUMBER = 136;

是不是程序正常运行呢？个人测试下来，没有碰到crash。

04-25 14:06:21.013 11266-11281/com.ryg.chapter_11 D/jeff: start ..task@ 121 thread id: 277 thread name: AsyncTask #1
04-25 14:06:22.959 11266-11285/com.ryg.chapter_11 D/jeff: start ..task@ 122 thread id: 281 thread name: AsyncTask #5
04-25 14:06:22.960 11266-11289/com.ryg.chapter_11 D/jeff: start ..task@ 124 thread id: 285 thread name: AsyncTask #9
04-25 14:06:22.961 11266-11283/com.ryg.chapter_11 D/jeff: start ..task@ 123 thread id: 279 thread name: AsyncTask #3
04-25 14:06:22.970 11266-11288/com.ryg.chapter_11 D/jeff: start ..task@ 125 thread id: 284 thread name: AsyncTask #8
04-25 14:06:22.970 11266-11286/com.ryg.chapter_11 D/jeff: start ..task@ 126 thread id: 282 thread name: AsyncTask #6
04-25 14:06:22.996 11266-11284/com.ryg.chapter_11 D/jeff: start ..task@ 127 thread id: 280 thread name: AsyncTask #4
04-25 14:06:22.996 11266-11282/com.ryg.chapter_11 D/jeff: start ..task@ 128 thread id: 278 thread name: AsyncTask #2
04-25 14:06:22.996 11266-11287/com.ryg.chapter_11 D/jeff: start ..task@ 129 thread id: 283 thread name: AsyncTask #7
04-25 14:06:23.053 11266-11281/com.ryg.chapter_11 D/jeff: start ..task@ 130 thread id: 277 thread name: AsyncTask #1
04-25 14:06:25.001 11266-11285/com.ryg.chapter_11 D/jeff: start ..task@ 131 thread id: 281 thread name: AsyncTask #5
04-25 14:06:25.001 11266-11283/com.ryg.chapter_11 D/jeff: start ..task@ 132 thread id: 279 thread name: AsyncTask #3

让我们看看AsyncTask为什么到达阈值时，这个RuntimeException的调用过程：

ThreadPoolExecutor 的构造函数如下：

public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue<Runnable> workQueue,
                              ThreadFactory threadFactory) {
        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
             threadFactory, defaultHandler);
    }

其中defaultHandler的实现如下：

private static final RejectedExecutionHandler defaultHandler = new AbortPolicy();

其中AbortPolicy的源码如下：

/**
     * A handler for rejected tasks that throws a
     * {@code RejectedExecutionException}.
     */
    public static class AbortPolicy implements RejectedExecutionHandler {
        /**
         * Creates an {@code AbortPolicy}.
         */
        public AbortPolicy() { }

        /**
         * Always throws RejectedExecutionException.
         *
         * @param r the runnable task requested to be executed
         * @param e the executor attempting to execute this task
         * @throws RejectedExecutionException always
         */
        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
            throw new RejectedExecutionException("Task " + r.toString() +
                                                 " rejected from " +
                                                 e.toString());
        }
    }

所以大家现在应该知道 RejectedExecutionException 是如何产生的吧。

最后需要强调一下，其实google很早就意识到这个问题，所以默认的方式是

myAsyncTask.execute(0);

而不是

myAsyncTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, 0);

使用的默认方式，不会出现RejectedExecutionException，即使 TASK_NUMBER = 500， 不信的同志可以自己试验一下。究其原因，可以看以下代码：

 @MainThread
    public final AsyncTask<Params, Progress, Result> execute(Params... params) {
        return executeOnExecutor(sDefaultExecutor, params);
    }

private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;

private static class SerialExecutor implements Executor {
        final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();
        Runnable mActive;

        public synchronized void execute(final Runnable r) {
            mTasks.offer(new Runnable() {
                public void run() {
                    try {
                        r.run();
                    } finally {
                        scheduleNext();
                    }
                }
            });
            if (mActive == null) {
                scheduleNext();
            }
        }

        protected synchronized void scheduleNext() {
            if ((mActive = mTasks.poll()) != null) {
                THREAD_POOL_EXECUTOR.execute(mActive);
            }
        }
    }

从以上代码可以看出，因为 execute 和  scheduleNext 都加了锁，线程应该是串行执行，而不是并行执行，THREAD_POOL_EXECUTOR.execute(mActive) 每次执行一个，当然不会到达峰值(137)。但是由于是串行执行，又会带来一个新的问题，有人用AsyncTask加载图片的时候特别慢，具体细节，请参考如下 博客：

Android实战技巧：深入解析AsyncTask