关于AsyncTask的用法可以参看前面一篇博客《AsyncTask实现断点续传》，本文只解析AsyncTask的源代码。
AsyncTask.execute方法：
public final AsyncTask<Params, Progress, Result> execute(Params... params) {
return executeOnExecutor(sDefaultExecutor, params);
}execute方法里面直接调用了executeOnexecute方法。
AsyncTask.executeOnexecute方法：
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;
}3-13行是检测AsyncTask的状态，如果状态不为PENDING，则会抛异常，这也是为什么一个AsyncTask只能被执行一次的原因。14行将状态改为RUNNING，表示该任务正在运行。然后调用AsyncTask的onPreExecute()方法。
由下面代码可以看出，AsyncTask有三种状态：PENDING（未运行）、RUNNING（正在运行）、FINISHED（已运行完毕）。
public enum Status {
/**
* Indicates that the task has not been executed yet.
*/
PENDING,
/**
* Indicates that the task is running.
*/
RUNNING,
/**
* Indicates that {@link AsyncTask#onPostExecute} has finished.
*/
FINISHED,
}FutureTask代码：
public class FutureTask<V> implements RunnableFuture<V> {
......
//构造方法传入一个Callable对象
public FutureTask(Callable<V> callable) {
if (callable == null)
throw new NullPointerException();
this.callable = callable;
this.state = NEW; // ensure visibility of callable
}
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();//这里调用了callable.call()方法
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);
}
}
......
}AsyncTask构造方法：
public abstract class AsyncTask<Params, Progress, Result> {
......
/**
* 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));
}
};
//创建FutureTask对象的时候传入了mWorker作为Callable
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);
}
}
};
}
......
}由FutureTask源码我们可以看出，run()方法里面调用了c.call()，而AsyncTask
中创建FutureTask的时候传入了mWorker，所以FutureTask.run()方法里面c.call()调用的是mWorker对象的
call()方法，而AsyncTask里mWorker重写了call方法，即上面8-14行，所以c.call()会执行到
mWorker.call()方法来。call方法里面11行将线程的优先级设置为后台线程，这样当多个线程并发后很多无关紧要的线程分配的CPU时间将
会减少，有利于主线程的处理。
接下来11行执行了doInBackground(mParams)方法，通常我们会重写该方法来实现业务逻辑操作。然后执行postResult
方法，并且将结果返回给FutureTask（因为是FutureTask.run方法调用的此call方法，所以需要返回结果到
FutureTask.run方法）。这里我们先看看postResult：
private Result postResult(Result result) {
@SuppressWarnings("unchecked")
Message message = sHandler.obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(this, result));
message.sendToTarget();
return result;
}这里的sHandler是InternalHandler对象。
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;
}
}
}由第9行代码可知最终会执行AsyncTask的finish方法，代码如下：
private void finish(Result result) {
if (isCancelled()) {
onCancelled(result);
} else {
onPostExecute(result);
}
mStatus = Status.FINISHED;
}finish的作用是如果task被取消了就执行onCancelled方法，如果没有被取消而是正常执行完毕，则执行onPostExecute
方法（这也是为什么task被调用了cancel方法，不会执行onPostExecute的原因）。最后将task的状态标记为FINISHED。
上面说到mWorker.call会将执行结果返回给FutureTask.run()方法并且继续往下执行，我们再次看看FutureTask.run方法（20-30行）：
boolean ran;
try {
result = c.call();
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);
}
if (ran)
set(result);由上面代码可以看到，执行完c.call后，会执行set(result)方法。
protected void set(V v) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = v;
UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
finishCompletion();
}
}最终会执行finishCompletion()方法。
private void finishCompletion() {
// assert state > COMPLETING;
for (WaitNode q; (q = waiters) != null;) {
if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
for (;;) {
Thread t = q.thread;
if (t != null) {
q.thread = null;
LockSupport.unpark(t);
}
WaitNode next = q.next;
if (next == null)
break;
q.next = null; // unlink to help gc
q = next;
}
break;
}
}
done();
callable = null; // to reduce footprint
}看到21行代码，会执行FutureTask的done()方法，而这个方法在AsyncTask构造函数中初始化FutureTask对象的时候被重写了。
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);
}
}
};这里主要是验证postResult是否被调用了，如果没有被调用着调用postResult函数，因为前面mWorker.call方法里面调用过了，所以这里不错操作。
顺便提一下，在AsyncTask的doInBackground方法中如果需要更新UI的话，则调用AsyncTask的publishProgress方法即可：
protected final void publishProgress(Progress... values) {
if (!isCancelled()) {
sHandler.obtainMessage(MESSAGE_POST_PROGRESS,
new AsyncTaskResult<Progress>(this, values)).sendToTarget();
}
}publishProgress方法最终也会通过sHandler来调用AsyncTask的onProgressUpdate方法，一般我们如果需要获取进度的话都需要重写AsyncTask的onProgressUpdate。
好了，AsyncTask的源码也分析完了。再次总结一下Asynctask使用的注意事项：
- 异步任务的实例必须在UI线程中创建。
- execute(Params… params)方法必须在UI线程中调用。
- 不要手动调用onPreExecute()，doInBackground(Params… params)，onProgressUpdate(Progress… values)，onPostExecute(Result result)这几个方法。
- 不能在doInBackground(Params… params)中更改UI组件的信息。
- 一个任务实例只能执行一次，如果执行第二次将会抛出异常。
