通过名字就可以知道，AsyncTask主要用于处理android中的异步任务。但是通过源码，我们可以看到它的实现其实还是依赖于Handler的异步消息处理机制。现在我们先来学习它的使用方式，然后再研究源码。
一．AsyncTask的基本用法：
AsyncTask是一个抽象类，在之类继承它时，必须指定三个泛型参数，这三个参数的用途如下：
1. 在执行AsyncTask时需要传入的参数，可用于在后台任务中使用。
2. 后台任何执行时，如果需要在界面上显示当前的进度，则使用这里指定的泛型作为进度单位。3. 当任务执行完毕后，如果需要对结果进行返回，则使用这里指定的泛型作为返回值类型。
自定义的AsyncTask之类如下示例：
public class DownAsyncTask extends AsyncTask<String, Integer, byte[]>
这里我们把AsyncTask的第一个泛型参数指定为String，表示在执行AsyncTask的时候需要传入String参数给后台任务。第二个泛型参数指定为Integer，表示使用整型数据来作为进度显示单位。第三个泛型参数指定为byte[]，则表示使用一个byte数组来反馈执行结果。
当然，目前我们自定义的DownAsyncTask还是一个空任务，并不能进行任何实际的操作，我们还需要去重写AsyncTask中的几个方法才能完成对任务的定制。经常需要去重写的方法有以下四个：
onPreExecute()
这个方法会在后台任务开始执行之间调用，用于进行一些界面上的初始化操作，比如显示一个进度条对话框等。
doInBackground(Params...)
onProgressUpdate(Progress...)
onPostExecute(Result)
public class DownAsyncTask extends AsyncTask<String, Integer, byte[]>{
private String TAG = "DownAsyncTask";
private ProgressDialog mProgress;
private Context mContext;
private ImageView imageButton;
public DownAsyncTask(Context mContext,ImageView imageButton){
this.mContext = mContext;
this.imageButton = imageButton;
}
@Override
protected byte[] doInBackground(String... params) {
// TODO Auto-generated method stub
android.util.Log.v(TAG,"DownAsyncTask--------doInBackground");
// 通过Apache的HttpClient来访问请求网络中的一张图片
HttpClient httpClient = new DefaultHttpClient();
HttpGet httpGet = new HttpGet(params[0]);
byte[] image = new byte[]{};
try
{
HttpResponse httpResponse = httpClient.execute(httpGet);
HttpEntity httpEntity = httpResponse.getEntity();
if(httpEntity != null && httpResponse.getStatusLine().getStatusCode() == HttpStatus.SC_OK)
{
image = EntityUtils.toByteArray(httpEntity);
}
//..根据下载的情况，使用publishProgress()通知onProgressUpdate()更新进度条
}
catch (Exception e)
{
e.printStackTrace();
}
finally
{
httpClient.getConnectionManager().shutdown();
}
return image;
}
@Override
protected void onPreExecute() {
// TODO Auto-generated method stub
android.util.Log.v(TAG,"DownAsyncTask--------onPreExecute");
//设置并显示进度条
mProgress = new ProgressDialog(mContext);
mProgress.setTitle("xiazai");
mProgress.setMessage("download is running");
mProgress.setProgressStyle(ProgressDialog.STYLE_SPINNER);
mProgress.show();
super.onPreExecute();
}
@Override
protected void onPostExecute(byte[] result) {
// TODO Auto-generated method stub
//异步下载结束后，更换控件图片，关闭进度条
android.util.Log.v(TAG,"DownAsyncTask--------onPostExecute:"+result.length+" path:"+path);
Bitmap mBitmap = BitmapFactory.decodeByteArray(result, 0, result.length);
imageButton.setImageBitmap(mBitmap);
mProgress.dismiss();
super.onPostExecute(result);
}
@Override
protected void onProgressUpdate(Integer... values) {
// TODO Auto-generated method stub
android.util.Log.v(TAG,"DownAsyncTask--------onProgressUpdate");
super.onProgressUpdate(values);
}
@Override
protected void onCancelled(byte[] result) {
// TODO Auto-generated method stub
android.util.Log.v(TAG,"DownAsyncTask--------onCancelled");
super.onCancelled(result);
}
@Override
protected void onCancelled() {
// TODO Auto-generated method stub
android.util.Log.v(TAG,"DownAsyncTask--------onCancelled");
super.onCancelled();
}
}
public class MainActivity extends Activity {
private Button downButton;
private ImageView mPictrue;
private String uriPath = "http://ww2.sinaimg.cn/mw690/69c7e018jw1e6hd0vm3pej20fa0a674c.jpg";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
downButton = (Button) findViewById(R.id.button);
mPictrue = (ImageView) findViewById(R.id.image);
downButton.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
DownAsyncTask dat = new DownAsyncTask(MainActivity.this,mPictrue);
dat.execute(uriPath);
}
});
}
}
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);
}
}
};
}
public final AsyncTask<Params, Progress, Result> execute(Params... params) {
return executeOnExecutor(sDefaultExecutor, params);
}
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;
}
public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
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);
}
}
}
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();
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);
}
}
private Result postResult(Result result) {
@SuppressWarnings("unchecked")
Message message = sHandler.obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(this, result));
message.sendToTarget();
return result;
}
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;
}
}
}
private void finish(Result result) {
if (isCancelled()) {
onCancelled(result);
} else {
onPostExecute(result);
}
mStatus = Status.FINISHED;
}
protected final void publishProgress(Progress... values) {
if (!isCancelled()) {
sHandler.obtainMessage(MESSAGE_POST_PROGRESS,
new AsyncTaskResult<Progress>(this, values)).sendToTarget();
}
}
原文：http://blog.csdn.net/sjz4860402/article/details/51182275