深入AsyncTask原理

AsyncTask的本质是一个线程池，所有提交的异步任务都会在这个线程池（想想C3P0）中的工作线程内执行，当工作线程需要跟UI线程交互时，工作线程会通过向在UI线程创建的Handler传递消息的方式，调用相关的回调函数，从而实现UI界面的更新。

还是以昨天的例子为例~来说明AsyncTask的实现原理

package cn.caiwb.aysnc;


import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;

import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.ProgressBar;
import android.widget.Toast;

public class AsyncTaskActivity extends Activity {

private ImageView mImageView;
private Button mButton;
private ProgressBar mProgressBar;

@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);

mImageView= (ImageView) findViewById(R.id.imageView);
mButton = (Button) findViewById(R.id.button);
mProgressBar = (ProgressBar) findViewById(R.id.progressBar);
mButton.setOnClickListener(new OnClickListener() {

@Override
public void onClick(View v) {
GetIteyeLogoTask task = new GetIteyeLogoTask();
task.execute("http://www.iteye.com/images/logo.gif");
}
});
}

class GetIteyeLogoTask extends AsyncTask {//继承AsyncTask

@Override
protected Bitmap doInBackground(String... params) {//处理后台执行的任务，在后台线程执行
publishProgress(0);//将会调用onProgressUpdate(Integer... progress)方法
HttpClient hc = new DefaultHttpClient();
publishProgress(30);
HttpGet hg = new HttpGet(params[0]);//获取csdn的logo
final Bitmap bm;
try {
HttpResponse hr = hc.execute(hg);
bm = BitmapFactory.decodeStream(hr.getEntity().getContent());
} catch (Exception e) {

return null;
}
publishProgress(100);
//mImageView.setImageBitmap(result); 不能在后台线程操作ui
return bm;
}

protected void onProgressUpdate(Integer... progress) {//在调用publishProgress之后被调用，在ui线程执行
mProgressBar.setProgress(progress[0]);//更新进度条的进度
}

protected void onPostExecute(Bitmap result) {//后台任务执行完之后被调用，在ui线程执行
if(result != null) {
Toast.makeText(AsyncTaskActivity.this, "成功获取图片", Toast.LENGTH_LONG).show();
mImageView.setImageBitmap(result);
}else {
Toast.makeText(AsyncTaskActivity.this, "获取图片失败", Toast.LENGTH_LONG).show();
}
}

protected void onPreExecute () {//在 doInBackground(Params...)之前被调用，在ui线程执行
mImageView.setImageBitmap(null);
mProgressBar.setProgress(0);//进度条复位
}

protected void onCancelled () {//在ui线程执行
mProgressBar.setProgress(0);//进度条复位
}

}


}

package cn.caiwb.aysnc; import org.apache.http.HttpResponse; import org.apache.http.client.HttpClient; import org.apache.http.client.methods.HttpGet; import org.apache.http.impl.client.DefaultHttpClient; import android.app.Activity; import android.graphics.Bitmap; import android.graphics.BitmapFactory; import android.os.AsyncTask; import android.os.Bundle; import android.view.View; import android.view.View.OnClickListener; import android.widget.Button; import android.widget.ImageView; import android.widget.ProgressBar; import android.widget.Toast; public class AsyncTaskActivity extends Activity { private ImageView mImageView; private Button mButton; private ProgressBar mProgressBar; @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); mImageView= (ImageView) findViewById(R.id.imageView); mButton = (Button) findViewById(R.id.button); mProgressBar = (ProgressBar) findViewById(R.id.progressBar); mButton.setOnClickListener(new OnClickListener() { @Override public void onClick(View v) { GetIteyeLogoTask task = new GetIteyeLogoTask(); task.execute("http://www.iteye.com/images/logo.gif"); } }); } class GetIteyeLogoTask extends AsyncTask {//继承AsyncTask @Override protected Bitmap doInBackground(String... params) {//处理后台执行的任务，在后台线程执行 publishProgress(0);//将会调用onProgressUpdate(Integer... progress)方法 HttpClient hc = new DefaultHttpClient(); publishProgress(30); HttpGet hg = new HttpGet(params[0]);//获取csdn的logo final Bitmap bm; try { HttpResponse hr = hc.execute(hg); bm = BitmapFactory.decodeStream(hr.getEntity().getContent()); } catch (Exception e) { return null; } publishProgress(100); //mImageView.setImageBitmap(result); 不能在后台线程操作ui return bm; } protected void onProgressUpdate(Integer... progress) {//在调用publishProgress之后被调用，在ui线程执行 mProgressBar.setProgress(progress[0]);//更新进度条的进度 } protected void onPostExecute(Bitmap result) {//后台任务执行完之后被调用，在ui线程执行 if(result != null) { Toast.makeText(AsyncTaskActivity.this, "成功获取图片", Toast.LENGTH_LONG).show(); mImageView.setImageBitmap(result); }else { Toast.makeText(AsyncTaskActivity.this, "获取图片失败", Toast.LENGTH_LONG).show(); } } protected void onPreExecute () {//在 doInBackground(Params...)之前被调用，在ui线程执行 mImageView.setImageBitmap(null); mProgressBar.setProgress(0);//进度条复位 } protected void onCancelled () {//在ui线程执行 mProgressBar.setProgress(0);//进度条复位 } } }

分析：

在分析实现流程之前，我们先了解一下AsyncTask有哪些成员变量。

private static final int CORE_POOL_SIZE =5;//5个核心工作线程
private static final int MAXIMUM_POOL_SIZE = 128;//最多128个工作线程
private static final int KEEP_ALIVE = 1;//空闲线程的超时时间为1秒

private static final BlockingQueue sWorkQueue =
new LinkedBlockingQueue(10);//等待队列

private static final ThreadPoolExecutor***ecutor = new ThreadPoolExecutor(CORE_POOL_SIZE,
MAXIMUM_POOL_SIZE, KEEP_ALIVE, TimeUnit.SECONDS, sWorkQueue,sThreadFactory);//线程池是静态变量，所有的异步任务都会放到这个线程池的工作线程内执行。

private static final int CORE_POOL_SIZE =5;//5个核心工作线程 private static final int MAXIMUM_POOL_SIZE = 128;//最多128个工作线程 private static final int KEEP_ALIVE = 1;//空闲线程的超时时间为1秒 private static final BlockingQueue sWorkQueue = new LinkedBlockingQueue(10);//等待队列 private static final ThreadPoolExecutor***ecutor = new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE, TimeUnit.SECONDS, sWorkQueue,sThreadFactory);//线程池是静态变量，所有的异步任务都会放到这个线程池的工作线程内执行。


回到例子中，点击按钮之后会新建一个GetIteyeLogoTask对象：

GetIteyeLogoTask task = new GetIteyeLogoTask();

GetIteyeLogoTask task = new GetIteyeLogoTask();

此时会调用父类AsyncTask的构造函数：

AsyncTask.java

public AsyncTask() {
mWorker = new WorkerRunnable() {
public Result call() throws Exception {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
return doInBackground(mParams);
}
};

mFuture = new FutureTask(mWorker) {
@Override
protected void done() {
Message message;
Result result = null;

try {
result = 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) {
message = sHandler.obtainMessage(MESSAGE_POST_CANCEL,
new AsyncTaskResult(AsyncTask.this, (Result[]) null));
message.sendToTarget();//取消任务，发送MESSAGE_POST_CANCEL消息
return;
} catch (Throwable t) {
throw new RuntimeException("An error occured while executing "
+ "doInBackground()", t);
}

message = sHandler.obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult(AsyncTask.this, result));//完成任务，发送MESSAGE_POST_RESULT消息并传递result对象
message.sendToTarget();
}
};
}

public AsyncTask() { mWorker = new WorkerRunnable() { public Result call() throws Exception { Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); return doInBackground(mParams); } }; mFuture = new FutureTask(mWorker) { @Override protected void done() { Message message; Result result = null; try { result = 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) { message = sHandler.obtainMessage(MESSAGE_POST_CANCEL, new AsyncTaskResult(AsyncTask.this, (Result[]) null)); message.sendToTarget();//取消任务，发送MESSAGE_POST_CANCEL消息 return; } catch (Throwable t) { throw new RuntimeException("An error occured while executing " + "doInBackground()", t); } message = sHandler.obtainMessage(MESSAGE_POST_RESULT, new AsyncTaskResult(AsyncTask.this, result));//完成任务，发送MESSAGE_POST_RESULT消息并传递result对象 message.sendToTarget(); } }; }


WorkerRunnable类实现了callable接口的call()方法，该函数会调用我们在AsyncTask子类中实现的doInBackground(mParams)方法，由此可见，WorkerRunnable封装了我们要执行的异步任务。FutureTask中的protected void done() {}方法实现了异步任务状态改变后的操作。当异步任务被取消，会向UI线程传递MESSAGE_POST_CANCEL消息，当任务成功执行，会向UI线程传递MESSAGE_POST_RESULT消息，并把执行结果传递到UI线程。

由此可知，AsyncTask在构造的时候已经定义好要异步执行的方法doInBackground(mParams)和任务状态变化后的操作（包括失败和成功）。

当创建完GetIteyeLogoTask对象后，执行

task.execute("http://www.iteye.com/images/logo.gif");

task.execute("http://www.iteye.com/images/logo.gif");

此时会调用AsyncTask的execute(Params...params)方法

public final AsyncTask execute(Params... params) {
if (mStatus != Status.PENDING) {
switch (mStatus) {
case RUNNING:
throw newIllegalStateException("Cannot execute task:"
+ " the taskis already running.");
case FINISHED:
throw newIllegalStateException("Cannot execute task:"
+ " the taskhas already been executed "
+ "(a task canbe executed only once)");
}
}

mStatus = Status.RUNNING;

onPreExecute();//运行在ui线程，在提交任务到线程池之前执行

mWorker.mParams = params;
***ecutor.execute(mFuture);//提交任务到线程池

return this;
}

public final AsyncTask execute(Params... params) { if (mStatus != Status.PENDING) { switch (mStatus) { case RUNNING: throw newIllegalStateException("Cannot execute task:" + " the taskis already running."); case FINISHED: throw newIllegalStateException("Cannot execute task:" + " the taskhas already been executed " + "(a task canbe executed only once)"); } } mStatus = Status.RUNNING; onPreExecute();//运行在ui线程，在提交任务到线程池之前执行 mWorker.mParams = params; ***ecutor.execute(mFuture);//提交任务到线程池 return this; }

当任务正在执行或者已经完成，会抛出IllegalStateException，由此可知我们不能够重复调用execute(Params...params)方法。在提交任务到线程池之前，调用了onPreExecute()方法。然后才执行***ecutor.execute(mFuture)是任务提交到线程池。

前面我们说到，当任务的状态发生改变时（1、执行成功2、取消执行3、进度更新），工作线程会向UI线程的Handler传递消息。我们知道Handler要处理其他线程传递过来的消息。在AsyncTask中，InternalHandler是在UI线程上创建的，它接收来自工作线程的消息，实现代码如下：

private static class InternalHandler extends Handler {
@SuppressWarnings({"unchecked","RawUseOfParameterizedType"})
@Override
public voidhandleMessage(Message msg) {
AsyncTaskResult result =(AsyncTaskResult) msg.obj;
switch (msg.what) {
caseMESSAGE_POST_RESULT:
// There is onlyone result
result.mTask.finish(result.mData[0]);//执行任务成功
break;
caseMESSAGE_POST_PROGRESS:
result.mTask.onProgressUpdate(result.mData);//进度更新
break;
caseMESSAGE_POST_CANCEL:
result.mTask.onCancelled();//取消任务
break;
}
}
}

private static class InternalHandler extends Handler { @SuppressWarnings({"unchecked","RawUseOfParameterizedType"}) @Override public voidhandleMessage(Message msg) { AsyncTaskResult result =(AsyncTaskResult) msg.obj; switch (msg.what) { caseMESSAGE_POST_RESULT: // There is onlyone result result.mTask.finish(result.mData[0]);//执行任务成功 break; caseMESSAGE_POST_PROGRESS: result.mTask.onProgressUpdate(result.mData);//进度更新 break; caseMESSAGE_POST_CANCEL: result.mTask.onCancelled();//取消任务 break; } } }

当接收到消息之后，AsyncTask会调用自身相应的回调方法。

总结：

1、 AsyncTask的本质是一个静态的线程池，AsyncTask派生出的子类可以实现不同的异步任务，这些任务都是提交到静态的线程池中执行。

2、线程池中的工作线程执行doInBackground(mParams)方法执行异步任务

3、当任务状态改变之后，工作线程会向UI线程发送消息，AsyncTask内部的InternalHandler响应这些消息，并调用相关的回调函数