Android多线程(二)AsyncTask源码分析
最后更新于：2022-04-01 14:46:05
转载请注明出处：[](http://blog.csdn.net/guolin_blog/article/details/42238633)[http://blog.csdn.net/itachi85/article/details/45041923](http://blog.csdn.net/itachi85/article/details/45041923)
AsyncTask的基本用法这里就不在赘述了，是个安卓开发者就会。
**1.android 3.0以前的 AsyncTask**
~~~
private static final int CORE_POOL_SIZE = 5;
private static final int MAXIMUM_POOL_SIZE = 128;
private static final it KEEP_ALIVE = 10;
……
private static final ThreadPoolExecutor sExecutor = new ThreadPoolExecutor(CORE_POOL_SIZE,
MAXIMUM_POOL_SIZE, KEEP_ALIVE, TimeUnit.SECONDS, sWorkQueue, sThreadFactory);
~~~
在这里我们又看到了ThreadPoolExecutor，它的原理我已经在上一篇介绍过了[http://blog.csdn.net/itachi85/article/details/44874511](http://blog.csdn.net/itachi85/article/details/44874511)。
在这里同一时刻能够运行的线程数为5个，线程池总大小为128，当线程数大于核心时，终止前多余的空闲线程等待新任务的最长时间为10秒。在3.0之前的AsyncTask可以同时有5个任务在执行，而3.0之后的AsyncTask同时只能有1个任务在执行。
**2.让我们来看看android 4.3版本的 AsyncTask**
AsyncTask构造函数：
~~~
/**
* Creates a new asynchronous task. This constructor must be invoked on the UI thread.
*/
public AsyncTask() {
mWorker = new WorkerRunnable<Params, Result>() {
public Result call() throws Exception {
mTaskInvoked.set(true);
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
//noinspection unchecked
return postResult(doInBackground(mParams));
}
};
mFuture = new FutureTask<Result>(mWorker) {
@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 occured while executing doInBackground()",
e.getCause());
} catch (CancellationException e) {
postResultIfNotInvoked(null);
}
}
};
}
~~~
这段代码初始化了两个变量，mWorker和mFuture，并在初始化mFuture的时候将mWorker作为参数传入。mWorker是一个Callable对象，mFuture是一个FutureTask对象，这两个变量会暂时保存在内存中，稍后才会用到它们。
我们要运用AsyncTask时，大多时候会调用execute()方法，来看看execute()的源码：
~~~
public final AsyncTask<Params, Progress, Result> execute(Params... params) {
return executeOnExecutor(sDefaultExecutor, params);}
~~~
返回了executeOnExecutor并传进去sDefaultExecutor（默认的线程池）。先看看executeOnExecutor的源码：
~~~
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;
exec.execute(mFuture);
return this;
}
~~~
传入的线程池exec调用了execute方法并将上文提到的mFuture传了进去。
这个传进来的线程池sDefaultExecutor就是默认的线程池SerialExecutor也就是调用了SerialExecutor的execute()方法：
~~~
public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
~~~
SerialExecutor的源码：
~~~
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);
}
}
}
~~~
调用SerialExecutor的execute方法这里可以看到传进来一个Runnable，这个Runnable就是上文提到的mFuture(FutureTask)，第九行执行了FutureTask的run方法：
~~~
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);
}
}
~~~
在run方法中执行了c.call,这里的c就是我们上文提到的mWorker（WorkerRunnable）。执行WorkerRunnable的call方法：
~~~
mWorker = new WorkerRunnable<Params, Result>() {
public Result call() throws Exception {
mTaskInvoked.set(true);
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
//noinspection unchecked
return postResult(doInBackground(mParams));
}
~~~
最后一行postResult()方法源码：
~~~
private Result postResult(Result result) {
@SuppressWarnings("unchecked")
Message message = sHandler.obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(this, result));
message.sendToTarget();
return result;
}
~~~
我们发现就是发送了一个消息，上面的代码发送的消息由这里接受：
~~~
private static class InternalHandler extends Handler {
@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;
}
}
}
~~~
消息是MESSAGE_POST_RESULT所以会执行 result.mTask.finish(result.mData[0]) ，finish源码：
~~~
private void finish(Result result) {
if (isCancelled()) {
onCancelled(result);
} else {
onPostExecute(result);
}
mStatus = Status.FINISHED;
}
~~~
当被取消时会执行 onCancelled(result);否则就会调用 onPostExecute(result);这样我们就可以在onPostExecute方发中得到我们需要的结果result来进行下一步的处理了。
**3.AsyncTask中的线程池 **
AsyncTask中一共定义了两个线程池一个是此前我们已经介绍了线程池SerialExecutor，这个是目前我们调用AsyncTask.execute()方法默认使用的线程池，这个在前一篇文章中已经讲到过了，另一个是3.0版本之前的默认线程池THREAD_POOL_EXECUTOR。现在我们来回顾一下SerialExecutor的源码：
~~~
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();
}
}
~~~
这个默认的线程池同一时间只能处理一个任务，一个任务完成以后才可以执行下一个任务，相当于Executors.newSingleThreadPool()。上面的arrayDeque是一个装载Runnable的队列，如果我们一次性启动了很多个任务，在第一次运行execute()方法的时候会调用ArrayDeque的offer()方法将传入的Runnable对象添加到队列的尾部， 然后判断mActive对象是不是等于null，第一次运行等于null，于是调用scheduleNext()方法。另外在finally中也调用了scheduleNext()方法，这样保证每次当一个任务执行完毕后，下一个任务才会执行。我们来看看scheduleNext()方法的源码：
~~~
protected synchronized void scheduleNext() {
if ((mActive = mTasks.poll()) != null) {
THREAD_POOL_EXECUTOR.execute(mActive);
}
}
~~~
首先从runnable队列的头部取值，如果不为空就赋值给mActive对象，然后调用THREAD_POOL_EXECUTOR去执行取出的Runnable对象。THREAD_POOL_EXECUTOR源码：
~~~
private static final int CORE_POOL_SIZE = 5;
private static final int MAXIMUM_POOL_SIZE = 128;
private static final int KEEP_ALIVE = 1;
.
public static final Executor THREAD_POOL_EXECUTOR
= new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,
TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);
~~~
这是3.0版本之前的线程池，同一时刻能够运行的线程数为5个，workQueue总大小为128。当我们启动10个任务，只有5个任务能够优先执行，其余的任务放在workQueue中，当workQueue大于128时就会调用RejectedExecutionHandler来做拒绝处理。当然在3.0之前是并没有SerialExecutor这个类的。如果不希望用默认线程池我们也可以使用这个3.0版本之前的线程池
~~~
AsyncTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, null);
~~~
同时3.0版本也提供了executeOnExecutor这个方法可以传入AsyncTask定义的线程池也可以传入Executor定义的4种线程池，不知道这四种线程池的可以看[http://blog.csdn.net/itachi85/article/details/44874511](http://blog.csdn.net/itachi85/article/details/44874511)
传入CachedThreadPool:
~~~
LikeListTask mLikeListTask=new LikeListTask();
executeOnExecutor(Executors.newCachedThreadPool(), null);
~~~
当然我们也可以传入自定义的线程池：
~~~
Executor exec =new ThreadPoolExecutor(0, Integer.MAX_VALUE,
0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>());
new LikeListTask().executeOnExecutor(exec, null);
~~~
我们看到这里定义的是一个类似于CachedThreadPool的一个线程池