在使用Asynctask时,相信有些朋友会遇到以下RejectedExecutionException:
Java.util.concurrent.RejectedExecutionException: Task [email protected] rejected from [email protected][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,我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 = "[email protected] " + 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 [email protected] rejected from [email protected][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$[email protected] rejected from [email protected][Running, pool size = 9, active threads = 9, queued tasks = 128, completed tasks = 0] at java.util.concurrent.ThreadPoolExecutor$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 [email protected] 121 thread id: 277 thread name: AsyncTask #1 04-25 14:06:22.959 11266-11285/com.ryg.chapter_11 D/jeff: start [email protected] 122 thread id: 281 thread name: AsyncTask #5 04-25 14:06:22.960 11266-11289/com.ryg.chapter_11 D/jeff: start [email protected] 124 thread id: 285 thread name: AsyncTask #9 04-25 14:06:22.961 11266-11283/com.ryg.chapter_11 D/jeff: start [email protected] 123 thread id: 279 thread name: AsyncTask #3 04-25 14:06:22.970 11266-11288/com.ryg.chapter_11 D/jeff: start [email protected] 125 thread id: 284 thread name: AsyncTask #8 04-25 14:06:22.970 11266-11286/com.ryg.chapter_11 D/jeff: start [email protected] 126 thread id: 282 thread name: AsyncTask #6 04-25 14:06:22.996 11266-11284/com.ryg.chapter_11 D/jeff: start [email protected] 127 thread id: 280 thread name: AsyncTask #4 04-25 14:06:22.996 11266-11282/com.ryg.chapter_11 D/jeff: start [email protected] 128 thread id: 278 thread name: AsyncTask #2 04-25 14:06:22.996 11266-11287/com.ryg.chapter_11 D/jeff: start [email protected] 129 thread id: 283 thread name: AsyncTask #7 04-25 14:06:23.053 11266-11281/com.ryg.chapter_11 D/jeff: start [email protected] 130 thread id: 277 thread name: AsyncTask #1 04-25 14:06:25.001 11266-11285/com.ryg.chapter_11 D/jeff: start [email protected] 131 thread id: 281 thread name: AsyncTask #5 04-25 14:06:25.001 11266-11283/com.ryg.chapter_11 D/jeff: start [email protected] 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: