首先 我们知道AsyncTask 是封装了线程池和Handler ，那就写一个简单的例子开始说起，
class MyAsyncTask extends AsyncTask<String, Void, String>{
String str;
public MyAsyncTask(String str){
this.str = str;
}
public MyAsyncTask(){
}
@Override
protected String doInBackground(String... strings) {
Log.d(TAG, "doInBackground: " + strings[0]);
return strings[0];
}
@Override
protected void onPostExecute(String s) {
super.onPostExecute(s);
Log.d(TAG, "onPostExecute: " + s);
}
}使用方式 ：
new MyAsyncTask().execute("sjh");
这样就使用了AsyncTask ，从 execute（）方法说起，点击该方法，看到：
@MainThread
public final AsyncTask<Params, Progress, Result> execute(Params... params) {
return executeOnExecutor(sDefaultExecutor, params);
}点击exectueOnExecutor方法，可以到
@MainThread
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;
}先判断线程状态，在类中寻找一下相应的状态定义，发现：
/**
* Indicates the current status of the task. Each status will be set only once
* during the lifetime of a task.
*/
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,
}
这是一个内部枚举类定义任务的状态，并且每个状态只会赋值一次，我们知道一个AsyncTask 创建之后只能用一次，在用就会报错，原因可以结合以上分析得到原因。
再来看看这个 sDefaultExecutor 参数，这是一个串行的线程池，
private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
/**
* An {@link Executor} that executes tasks one at a time in serial
* order. This serialization is global to a particular process.
*/
public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
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);
}
}
}
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;
}从以上代码知道这是一个Executor, 在 executeOnExecutor方法中，把params赋值给workers，并执行 exec.execute(mFuture); 在看看mFuture 是什么？
executeOnExecutor 中 mFuture 实现了Runnable接口，并且在AsyncTask的构造方法里 对mFuture作了初始化工作：
/**
* 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);
Result result = null;
try {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
//noinspection unchecked
result = doInBackground(mParams);
Binder.flushPendingCommands();
} catch (Throwable tr) {
mCancelled.set(true);
throw tr;
} finally {
postResult(result);
}
return result;
}
};
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 occurred while executing doInBackground()",
e.getCause());
} catch (CancellationException e) {
postResultIfNotInvoked(null);
}
}
};
}从代码中知道 mFuture 的初始化需要mWorker 对象，并且也对mWorker 作了初始化，另外 也知道mWorker是一个实现了Callable接口的内部类，在其run（）方法中 执行了
result = doInBackground(mParams);由于doInBackground方法是抽象方法，子类必须实现，所以我们在使用的时候就一定要实现此方法并在这里做耗时任务，之后并把结果调用了post方法，
postResult(result);看看这个postResult（）及相关方法源代码：
private Result postResult(Result result) {
@SuppressWarnings("unchecked")
Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(this, result));
message.sendToTarget();
return result;
}
private static Handler getHandler() {
synchronized (AsyncTask.class) {
if (sHandler == null) {
sHandler = new InternalHandler();
}
return sHandler;
}
}
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;
}
}
}
// result.mTask.finish(result.mData[0]);
private void finish(Result result) {
if (isCancelled()) {
onCancelled(result);
} else {
onPostExecute(result);
}
mStatus = Status.FINISHED;
}
显然一目了然，知道这是用handler把结果发送出去。知道这个Handler 是一个内部类 InteralHandler ，然后看到 其构造方法，知道这个Handler用的是主线程的looper，所以这个handler 是在主线程处理消息，再看看熟悉的HandleMessage（Msg），一切都是那么熟悉和清晰。(针对这里不明白为什么要用MainLooper的情况可以参考 Handler机制 )
再次回顾整个流程：
从Exector.execte(Runnable)方法说起，Exector 是里面一个串行 SerialExecutor 对象包含的 THREAD_POOL_EXECUTOR 执行线程池，然后Runable的来源就是在构建AsyncTask的时候初始化的mWorker 和mFuture 对象，mWorker 代表了子线程要执行的任务doInbackground 和 把结果向外抛，利用InternalHandler 对象进行线程切换。
使用：
针对前面的例子，可以
new MyAsyncTask().execute("sjh")
也可以 new MyAsyncTask(“sjh”).execute(); 看到这两种方式，你要明白其不同含义，第一种写法是传入了 params，后面一种写法通过构造函数去传递值，相当于params 为空，没有进行传值，这时候在doInbackground用params 的时候就要注意 params是否有值，我这样写的话用第二种方式会产生异常的，即数组越界。