AsyncTask源码分析（Android7.0）
ggtx2404
8年前
<h3><strong>1.Android 3.0版本之前的AsyncTask</strong></h3> <p>下面是Android 2.3.7版本的AsyncTask的部分源码。</p> <pre> <code class="language-java">public abstract class AsyncTask<Params, Progress, Result> { private static final String LOG_TAG = "AsyncTask"; private static final int CORE_POOL_SIZE = 5; private static final int MAXIMUM_POOL_SIZE = 128; private static final int KEEP_ALIVE = 1; private static final BlockingQueue<Runnable> sWorkQueue = new LinkedBlockingQueue<Runnable>(10); private static final ThreadFactory sThreadFactory = new ThreadFactory() { private final AtomicInteger mCount = new AtomicInteger(1); public Thread newThread(Runnable r) { return new Thread(r, "AsyncTask #" + mCount.getAndIncrement()); } }; private static final ThreadPoolExecutor sExecutor = new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE, TimeUnit.SECONDS, sWorkQueue, sThreadFactory); ... }</code></pre> <p>在这里又看到了ThreadPoolExecutor，它的核心线程数是5个，线程池允许创建的最大线程数为128，非核心线程空闲等待新任务的最长时间为10秒。采用的阻塞队列是LinkedBlockingQueue，它的容量为10。3.0版本之前的AsyncTask有一个缺点就是，线程池最大的线程数为128，加上阻塞队列的10个任务，所以AsyncTask最多能同时容纳138个任务，当提交第139任务时就会执行饱和策略，默认抛出RejectedExecutionException异常。</p> <h3><strong>2.Android 7.0版本的AsyncTask</strong></h3> <p>在这里采用Android 7.0版本的AsyncTask作为例子，首先来看AsyncTask的构造函数：</p> <pre> <code class="language-java">public AsyncTask() { mWorker = new WorkerRunnable<Params, Result>() {//1 public Result call() throws Exception { mTaskInvoked.set(true); Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); Result result = doInBackground(mParams); Binder.flushPendingCommands(); return postResult(result); } }; mFuture = new FutureTask<Result>(mWorker) {//2 @Override protected void done() { try { postResultIfNotInvoked(get()); } catch (InterruptedException e) { android.util.Log.w(LOG_TAG, e); } catch (ExecutionException e) { throw new RuntimeException("An error occurred while executing doInBackground()", e.getCause()); } catch (CancellationException e) { postResultIfNotInvoked(null); } } }; }</code></pre> <p>从注释1处看这个WorkerRunnable实现了Callable<Result>接口，并实现了它的call方法，在call方法中调用了doInBackground(mParams)来处理任务并得到结果，并最终调用postResult将结果投递出去。注释2处的FutureTask是一个可管理的异步任务，它实现了Runnable和Futrue这两个接口。因此它可以包装Runnable和Callable<V>,并提供给Executor执行。也可以调用线程直接执行（FutureTask.run()）。在这里WorkerRunnable作为参数传递给了FutureTask。这两个变量会暂时保存在内存中，稍后会用到它们。</p> <p>当要执行AsyncTask时，需要调用它的execute方法，代码如下所示。</p> <pre> <code class="language-java">public final AsyncTask<Params, Progress, Result> execute(Params... params) { return executeOnExecutor(sDefaultExecutor, params); }</code></pre> <p>execute方法又调用了executeOnExecutor方法：</p> <pre> <code class="language-java">public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec, Params... params) { if (mStatus != Status.PENDING) { switch (mStatus) { case RUNNING: throw new IllegalStateException("Cannot execute task:" + " the task is already running."); case FINISHED: throw new IllegalStateException("Cannot execute task:" + " the task has already been executed " + "(a task can be executed only once)"); } } mStatus = Status.RUNNING; onPreExecute(); mWorker.mParams = params;//1 exec.execute(mFuture); return this; }</code></pre> <p>这里会首先调用 onPreExecute方法，在注释1处将AsyncTask的参数传给WorkerRunnable，从前面我们知道WorkerRunnable会作为参数传递给了FutureTask，因此，参数被封装到FutureTask中。接下来会调用exec的execute方法，并将mFuture也就是前面讲到的FutureTask传进去。这里exec是传进来的参数sDefaultExecutor，它是一个串行的线程池，它的代码如下所示。</p> <pre> <code class="language-java">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() {//1 public void run() { try { r.run();//2 } finally { scheduleNext(); } } }); if (mActive == null) { scheduleNext(); } } protected synchronized void scheduleNext() { if ((mActive = mTasks.poll()) != null) { THREAD_POOL_EXECUTOR.execute(mActive); } } }</code></pre> <p>从注释1处可以看出，当调用SerialExecutor 的execute方法时，会将FutureTask加入到mTasks中。当任务执行完或者当前没有活动的任务时都会执行scheduleNext方法，它会从mTasks取出FutureTask任务并交由THREAD_POOL_EXECUTOR处理。关于THREAD_POOL_EXECUTOR，后面会介绍。从这里看出SerialExecutor是串行执行的。在注释2处可以看到执行了FutureTask的run方法，它最终会调用WorkerRunnable的call方法。</p> <p>前面我们提到call方法postResult方法将结果投递出去，postResult方法代码如下所示。</p> <pre> <code class="language-java">private Result postResult(Result result) { @SuppressWarnings("unchecked") Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT, new AsyncTaskResult<Result>(this, result)); message.sendToTarget(); return result; }</code></pre> <p>在postResult方法中会创建Message，并将结果赋值给这个Message，通过getHandler方法得到Handler，并通过这个Handler发送消息，getHandler方法如下所示。</p> <pre> <code class="language-java">private static Handler getHandler() { synchronized (AsyncTask.class) { if (sHandler == null) { sHandler = new InternalHandler(); } return sHandler; } }</code></pre> <p>在getHandler方法中创建了InternalHandler，InternalHandler的定义如下所示。</p> <pre> <code class="language-java">private static class InternalHandler extends Handler { public InternalHandler() { super(Looper.getMainLooper()); } @SuppressWarnings({"unchecked", "RawUseOfParameterizedType"}) @Override public void handleMessage(Message msg) { AsyncTaskResult<?> result = (AsyncTaskResult<?>) msg.obj; switch (msg.what) { case MESSAGE_POST_RESULT: // There is only one result result.mTask.finish(result.mData[0]); break; case MESSAGE_POST_PROGRESS: result.mTask.onProgressUpdate(result.mData); break; } } }</code></pre> <p>在接收到MESSAGE_POST_RESULT消息后会调用AsyncTask的finish方法：</p> <pre> <code class="language-java">private void finish(Result result) { if (isCancelled()) { onCancelled(result); } else { onPostExecute(result); } mStatus = Status.FINISHED; }</code></pre> <p>如果AsyncTask任务被取消了则执行onCancelled方法，否则就调用onPostExecute方法。而正是通过onPostExecute方法我们才能够得到异步任务执行后的结果。</p> <p>接着回头来看SerialExecutor ，线程池SerialExecutor主要用来处理排队，将任务串行处理。 SerialExecutor中调用scheduleNext方法时，将任务交给THREAD_POOL_EXECUTOR。THREAD_POOL_EXECUTOR同样是一个线程池，用来执行任务。</p> <pre> <code class="language-java">private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors(); private static final int CORE_POOL_SIZE = Math.max(2, Math.min(CPU_COUNT - 1, 4)); private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1; private static final int KEEP_ALIVE_SECONDS = 30; private static final BlockingQueue<Runnable> sPoolWorkQueue = new LinkedBlockingQueue<Runnable>(128); 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; }</code></pre> <p>THREAD_POOL_EXECUTOR指的就是threadPoolExecutor，他的核心线程和线程池允许创建的最大线程数都是由CPU的核数来计算出来的。它采用的阻塞队列仍旧是LinkedBlockingQueue，容量为128。</p> <p>到此， Android 7.0版本的AsyncTask的源码就分析完了，在AsyncTask中用到了线程池，线程池中运行线程并且又用到了阻塞队列，因此，本章前面介绍的知识在这一节中做了很好的铺垫。Android 3.0及以上版本用SerialExecutor作为默认的线程，它将任务串行的处理保证一个时间段只有一个任务执行，而3.0之前版本是并行处理的。关于3.0之前版本的缺点在3.0之后版本也不会出现，因为线程是一个接一个执行的，不会出现超过任务数而执行饱和策略。如果想要在3.0及以上版本使用并行的线程处理可以使用如下的代码：</p> <pre> <code class="language-java">asyncTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR，"");</code></pre> <p>其中asyncTask是我们自定义的AsyncTask，当然也可以传入Java提供的线程池，比如传入CachedThreadPool。</p> <pre> <code class="language-java">asyncTask.executeOnExecutor(Executors.newCachedThreadPool(),"");</code></pre> <p>也可以传入自定义的线程池：</p> <pre> <code class="language-java">Executor exec =new ThreadPoolExecutor(0, Integer.MAX_VALUE, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>()); asyncTask.executeOnExecutor(exec,"");</code></pre> <p> </p> <p>来自：http://www.jianshu.com/p/ed7d9fb724cf</p> <p> </p>