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AsyncTask的实现原理

2015-01-06 15:40 337 查看

概述：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（原理见：《Handler+Looper+MessageQueue深入详解》）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。
例子：

例子：

本章还是以《Handler+Looper+MessageQueue深入详解》中的例子说明AsyncTask的实现原理。

这个例子是在后台下载CSDN的LOGO，下载完成后在UI界面上显示出来。
AsyncTask.java

[java]
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package com.zhuozhuo;

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) {
GetCSDNLogoTask task = new GetCSDNLogoTask();
task.execute("http://csdnimg.cn/www/images/csdnindex_logo.gif");
}
});
}

class GetCSDNLogoTask extends AsyncTask<String,Integer,Bitmap> {//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 com.zhuozhuo;

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) {
GetCSDNLogoTask task = new GetCSDNLogoTask();
task.execute("http://csdnimg.cn/www/images/csdnindex_logo.gif");
}
});
}

class GetCSDNLogoTask extends AsyncTask<String,Integer,Bitmap> {//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。

[java]
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private static final int CORE_POOL_SIZE = 5;//5个核心工作线程

private static final int MAXIMUM_POOL_SIZE = 128;//最多有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<Runnable> sPoolWorkQueue =new LinkedBlockingQueue<Runnable>(10);//等待队列

public static final Executor THREAD_POOL_EXECUTOR= new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE, TimeUnit.SECONDS, sPoolWorkQueue, 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;//最多有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<Runnable> sPoolWorkQueue =new LinkedBlockingQueue<Runnable>(10);//等待队列
public static final Executor THREAD_POOL_EXECUTOR= new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE, TimeUnit.SECONDS, sPoolWorkQueue, 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GetCSDNLogoTask对象：

[java]
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GetCSDNLogoTask task = new GetCSDNLogoTask();

GetCSDNLogoTask task = new GetCSDNLogoTask();

此时会调用父类AsyncTask的构造函数：

AsyncTask.java

[java]
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/**
* 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);
}
}
};
} private void postResultIfNotInvoked(Result result) {
final boolean wasTaskInvoked = mTaskInvoked.get();
if (!wasTaskInvoked) {
postResult(result);
}
}

private Result postResult(Result result) {
@SuppressWarnings("unchecked")
Message message = sHandler.obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(this, result));
message.sendToTarget();
return 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));
}
};

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);
}
}
};
}                                                                                                                  private void postResultIfNotInvoked(Result result) {
        final boolean wasTaskInvoked = mTaskInvoked.get();
        if (!wasTaskInvoked) {
            postResult(result);
        }
    }

    private Result postResult(Result result) {
        @SuppressWarnings("unchecked")
        Message message = sHandler.obtainMessage(MESSAGE_POST_RESULT,
                new AsyncTaskResult<Result>(this, result));
        message.sendToTarget();
        return result;
    }

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GetCSDNLogoTask对象后，执行

[java]
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task.execute("http://csdnimg.cn/www/images/csdnindex_logo.gif");

task.execute("http://csdnimg.cn/www/images/csdnindex_logo.gif");

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

AsyncTask.java

[java]
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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();<SPAN style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; PADDING-BOTTOM: 0px; LINE-HEIGHT: 17.27px; BACKGROUND-COLOR: rgb(248,248,248); MARGIN: 0px; PADDING-LEFT: 0px; PADDING-RIGHT: 0px; FONT-FAMILY: Consolas, 'Courier New', Courier, mono, serif; COLOR: rgb(0,130,0); FONT-SIZE: 11.81px; BORDER-TOP: medium none; BORDER-RIGHT: medium none; PADDING-TOP: 0px" class=comment>//运行在ui线程，在提交任务到线程池之前执行<SPAN style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; PADDING-BOTTOM: 0px; LINE-HEIGHT: 17.27px; BACKGROUND-COLOR: rgb(248,248,248); MARGIN: 0px; PADDING-LEFT: 0px; PADDING-RIGHT: 0px; FONT-FAMILY: Consolas, 'Courier New', Courier, mono, serif; FONT-SIZE: 11.81px; BORDER-TOP: medium none; BORDER-RIGHT: medium none; PADDING-TOP: 0px"> 

mWorker.mParams = params;
exec.execute(mFuture);<SPAN style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; PADDING-BOTTOM: 0px; LINE-HEIGHT: 17.27px; MARGIN: 0px; PADDING-LEFT: 0px; PADDING-RIGHT: 0px; FONT-FAMILY: Consolas, 'Courier New', Courier, mono, serif; COLOR: rgb(0,130,0); FONT-SIZE: 11.81px; BORDER-TOP: medium none; BORDER-RIGHT: medium none; PADDING-TOP: 0px" class=comment>//提交任务到线程池<SPAN style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; PADDING-BOTTOM: 0px; LINE-HEIGHT: 17.27px; MARGIN: 0px; PADDING-LEFT: 0px; PADDING-RIGHT: 0px; FONT-FAMILY: Consolas, 'Courier New', Courier, mono, serif; FONT-SIZE: 11.81px; BORDER-TOP: medium none; BORDER-RIGHT: medium none; PADDING-TOP: 0px"> 

return this;
}

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();<span><span class="comment" style="font-family:Consolas, 'Courier New', Courier, mono, serif;color:#0820;BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; PADDING-BOTTOM: 0px; LINE-HEIGHT: 17px; BACKGROUND-COLOR: rgb(248,248,248); MARGIN: 0px; PADDING-LEFT: 0px; PADDING-RIGHT: 0px; FONT-SIZE: 11px; BORDER-TOP: medium none; BORDER-RIGHT: medium none; PADDING-TOP: 0px">//运行在ui线程，在提交任务到线程池之前执行</span><span style="font-family:Consolas, 'Courier New', Courier, mono, serif;BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; PADDING-BOTTOM: 0px; LINE-HEIGHT: 17px; BACKGROUND-COLOR: rgb(248,248,248); MARGIN: 0px; PADDING-LEFT: 0px; PADDING-RIGHT: 0px; FONT-SIZE: 11px; BORDER-TOP: medium none; BORDER-RIGHT: medium none; PADDING-TOP: 0px"> </span></span>

mWorker.mParams = params;
exec.execute(mFuture);<span><span class="comment" style="font-family:Consolas, 'Courier New', Courier, mono, serif;color:#0820;BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; PADDING-BOTTOM: 0px; LINE-HEIGHT: 17px; MARGIN: 0px; PADDING-LEFT: 0px; PADDING-RIGHT: 0px; FONT-SIZE: 11px; BORDER-TOP: medium none; BORDER-RIGHT: medium none; PADDING-TOP: 0px">//提交任务到线程池</span><span style="font-family:Consolas, 'Courier New', Courier, mono, serif;BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; PADDING-BOTTOM: 0px; LINE-HEIGHT: 17px; MARGIN: 0px; PADDING-LEFT: 0px; PADDING-RIGHT: 0px; FONT-SIZE: 11px; BORDER-TOP: medium none; BORDER-RIGHT: medium none; PADDING-TOP: 0px"> </span></span>
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sExecutor.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。在《Handler+Looper+MessageQueue深入详解》一文中我们提到，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：

AsyncTask.java

[java]
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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]);<SPAN style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; PADDING-BOTTOM: 0px; LINE-HEIGHT: 17.27px; MARGIN: 0px; PADDING-LEFT: 0px; PADDING-RIGHT: 0px; FONT-FAMILY: Consolas, 'Courier New', Courier, mono, serif; COLOR: rgb(0,130,0); FONT-SIZE: 11.81px; BORDER-TOP: medium none; BORDER-RIGHT: medium none; PADDING-TOP: 0px" class=comment>//执行任务成功<SPAN style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; PADDING-BOTTOM: 0px; LINE-HEIGHT: 17.27px; MARGIN: 0px; PADDING-LEFT: 0px; PADDING-RIGHT: 0px; FONT-FAMILY: Consolas, 'Courier New', Courier, mono, serif; FONT-SIZE: 11.81px; BORDER-TOP: medium none; BORDER-RIGHT: medium none; PADDING-TOP: 0px"> 

break;
case MESSAGE_POST_PROGRESS:
result.mTask.onProgressUpdate(result.mData);<SPAN style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; PADDING-BOTTOM: 0px; LINE-HEIGHT: 17.27px; BACKGROUND-COLOR: rgb(248,248,248); MARGIN: 0px; PADDING-LEFT: 0px; PADDING-RIGHT: 0px; FONT-FAMILY: Consolas, 'Courier New', Courier, mono, serif; COLOR: rgb(0,130,0); FONT-SIZE: 11.81px; BORDER-TOP: medium none; BORDER-RIGHT: medium none; PADDING-TOP: 0px" class=comment>//进度更新<SPAN style="BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; PADDING-BOTTOM: 0px; LINE-HEIGHT: 17.27px; BACKGROUND-COLOR: rgb(248,248,248); MARGIN: 0px; PADDING-LEFT: 0px; PADDING-RIGHT: 0px; FONT-FAMILY: Consolas, 'Courier New', Courier, mono, serif; FONT-SIZE: 11.81px; BORDER-TOP: medium none; BORDER-RIGHT: medium none; PADDING-TOP: 0px"> 

break;
}
}
}

 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]);<span><span class="comment" style="font-family:Consolas, 'Courier New', Courier, mono, serif;color:#0820;BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; PADDING-BOTTOM: 0px; LINE-HEIGHT: 17px; MARGIN: 0px; PADDING-LEFT: 0px; PADDING-RIGHT: 0px; FONT-SIZE: 11px; BORDER-TOP: medium none; BORDER-RIGHT: medium none; PADDING-TOP: 0px">//执行任务成功</span><span style="font-family:Consolas, 'Courier New', Courier, mono, serif;BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; PADDING-BOTTOM: 0px; LINE-HEIGHT: 17px; MARGIN: 0px; PADDING-LEFT: 0px; PADDING-RIGHT: 0px; FONT-SIZE: 11px; BORDER-TOP: medium none; BORDER-RIGHT: medium none; PADDING-TOP: 0px">  </span></span>
                    break;
                case MESSAGE_POST_PROGRESS:
                    result.mTask.onProgressUpdate(result.mData);<span><span class="comment" style="font-family:Consolas, 'Courier New', Courier, mono, serif;color:#0820;BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; PADDING-BOTTOM: 0px; LINE-HEIGHT: 17px; BACKGROUND-COLOR: rgb(248,248,248); MARGIN: 0px; PADDING-LEFT: 0px; PADDING-RIGHT: 0px; FONT-SIZE: 11px; BORDER-TOP: medium none; BORDER-RIGHT: medium none; PADDING-TOP: 0px">//进度更新</span><span style="font-family:Consolas, 'Courier New', Courier, mono, serif;BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; PADDING-BOTTOM: 0px; LINE-HEIGHT: 17px; BACKGROUND-COLOR: rgb(248,248,248); MARGIN: 0px; PADDING-LEFT: 0px; PADDING-RIGHT: 0px; FONT-SIZE: 11px; BORDER-TOP: medium none; BORDER-RIGHT: medium none; PADDING-TOP: 0px">  </span></span>
                    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

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