Android异步处理四：AsyncTask的实现原理 .

「爱情、让人受尽委屈。」 2024-04-19 15:08 58阅读 0赞

本博文地址：[http://blog.csdn.net/mylzc/article/details/6774131][http_blog.csdn.net_mylzc_article_details_6774131]，转载请注明出处

[Android异步处理系列文章索引][Android]

[Android异步处理一：使用Thread+Handler实现非UI线程更新UI界面][Android_Thread_Handler_UI_UI]

[Android异步处理二：使用AsyncTask异步更新UI界面][Android_AsyncTask_UI]

[Android异步处理三：Handler+Looper+MessageQueue深入详解][Android_Handler_Looper_MessageQueue]

[Android异步处理四：AsyncTask的实现原理][http_blog.csdn.net_mylzc_article_details_6774131]

在[《Android异步处理二：使用AsyncTask异步更新UI界面》][Android_AsyncTask_UI]一文中，我们介绍了如何使用AsyncTask实现异步下载图片，并且更新图片到UI界面的方法。本篇我们将学习Framework层AsyncTask的实现原理。

**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：[《Android异步处理三：Handler+Looper+MessageQueue深入详解》][Android_Handler_Looper_MessageQueue]）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：**

本章还是以[《Android异步处理二：使用AsyncTask异步更新UI界面》][Android_AsyncTask_UI]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**

**\[java\]** [ view plain][view plain] [copy][view plain] [print][view plain] [?][view plain]

1.  package com.zhuozhuo;
2.  
3.  
4.  import org.apache.http.HttpResponse;
5.  import org.apache.http.client.HttpClient;
6.  import org.apache.http.client.methods.HttpGet;
7.  import org.apache.http.impl.client.DefaultHttpClient;
8.  
9.  import android.app.Activity;
10. import android.graphics.Bitmap;
11. import android.graphics.BitmapFactory;
12. import android.os.AsyncTask;
13. import android.os.Bundle;
14. import android.view.View;
15. import android.view.View.OnClickListener;
16. import android.widget.Button;
17. import android.widget.ImageView;
18. import android.widget.ProgressBar;
19. import android.widget.Toast;
20. 
21. public class AsyncTaskActivity extends Activity \{
22. 
23. private ImageView mImageView;
24. private Button mButton;
25. private ProgressBar mProgressBar;
26. 
27. @Override
28. public void onCreate(Bundle savedInstanceState) \{
29. super.onCreate(savedInstanceState);
30. setContentView(R.layout.main);
31. 
32. mImageView= (ImageView) findViewById(R.id.imageView);
33. mButton = (Button) findViewById(R.id.button);
34. mProgressBar = (ProgressBar) findViewById(R.id.progressBar);
35. mButton.setOnClickListener(new OnClickListener() \{
36. 
37. @Override
38. public void onClick(View v) \{
39. GetCSDNLogoTask task = new GetCSDNLogoTask();
40. task.execute("http://csdnimg.cn/www/images/csdnindex\_logo.gif");
41. \}
42. \});
43. \}
44. 
45. class GetCSDNLogoTask extends AsyncTask<String,Integer,Bitmap> \{ //继承AsyncTask 
46. 
47. @Override
48. protected Bitmap doInBackground(String... params) \{ //处理后台执行的任务，在后台线程执行 
49. publishProgress(0);//将会调用onProgressUpdate(Integer... progress)方法 
50. HttpClient hc = new DefaultHttpClient();
51. publishProgress(30);
52. HttpGet hg = new HttpGet(params\[0\]);//获取csdn的logo 
53. final Bitmap bm;
54. try \{
55. HttpResponse hr = hc.execute(hg);
56. bm = BitmapFactory.decodeStream(hr.getEntity().getContent());
57. \} catch (Exception e) \{
58. 
59. return null;
60. \}
61. publishProgress(100);
62. //mImageView.setImageBitmap(result); 不能在后台线程操作ui 
63. return bm;
64. \}
65. 
66. protected void onProgressUpdate(Integer... progress) \{ //在调用publishProgress之后被调用，在ui线程执行 
67. mProgressBar.setProgress(progress\[0\]);//更新进度条的进度 
68. \}
69. 
70. protected void onPostExecute(Bitmap result) \{ //后台任务执行完之后被调用，在ui线程执行 
71. if(result != null) \{
72. Toast.makeText(AsyncTaskActivity.this, "成功获取图片", Toast.LENGTH\_LONG).show();
73. mImageView.setImageBitmap(result);
74. \}else \{
75. Toast.makeText(AsyncTaskActivity.this, "获取图片失败", Toast.LENGTH\_LONG).show();
76. \}
77. \}
78. 
79. protected void onPreExecute () \{ //在 doInBackground(Params...)之前被调用，在ui线程执行 
80. mImageView.setImageBitmap(null);
81. mProgressBar.setProgress(0);//进度条复位 
82. \}
83. 
84. protected void onCancelled () \{ //在ui线程执行 
85. mProgressBar.setProgress(0);//进度条复位 
86. \}
87. 
88. \}
89. 
90. 
91. \}

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
    	     }
        	
        }
        
    
    }

main.xml

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1.  <?xml version="1.0" encoding="utf-8"?>
2.  <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
3.  android:orientation="vertical" android:layout\_width="fill\_parent"
4.  android:layout\_height="fill\_parent">
5.  <ProgressBar android:layout\_width="fill\_parent" android:layout\_height="wrap\_content" android:id="@+id/progressBar" style="?android:attr/progressBarStyleHorizontal"></ProgressBar>
6.  <Button android:id="@+id/button" android:text="下载图片" android:layout\_width="wrap\_content" android:layout\_height="wrap\_content"></Button>
7.  <ImageView android:id="@+id/imageView" android:layout\_height="wrap\_content"
8.  android:layout\_width="wrap\_content" />
9.  </LinearLayout>

<?xml version="1.0" encoding="utf-8"?>
    <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    	android:orientation="vertical" android:layout_width="fill_parent"
    	android:layout_height="fill_parent">
        <ProgressBar android:layout_width="fill_parent" android:layout_height="wrap_content" android:id="@+id/progressBar" style="?android:attr/progressBarStyleHorizontal"></ProgressBar>
        <Button android:id="@+id/button" android:text="下载图片" android:layout_width="wrap_content" android:layout_height="wrap_content"></Button>
    	<ImageView android:id="@+id/imageView" android:layout_height="wrap_content"
    		android:layout_width="wrap_content" />
    </LinearLayout>

mainifest.xml

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1.  <?xml version="1.0" encoding="utf-8"?>
2.  <manifest xmlns:android="http://schemas.android.com/apk/res/android"
3.  package="com.zhuozhuo"
4.  android:versionCode="1"
5.  android:versionName="1.0">
6.  <uses-sdk android:minSdkVersion="10" />
7.  <uses-permission android:name="android.permission.INTERNET"></uses-permission>
8.  <application android:icon="@drawable/icon" android:label="@string/app\_name">
9.  <activity android:name=".AsyncTaskActivity"
10. android:label="@string/app\_name">
11. <intent-filter>
12. <action android:name="android.intent.action.MAIN" />
13. <category android:name="android.intent.category.LAUNCHER" />
14. </intent-filter>
15. </activity>
16. 
17. </application>
18. </manifest>

<?xml version="1.0" encoding="utf-8"?>
    <manifest xmlns:android="http://schemas.android.com/apk/res/android"
          package="com.zhuozhuo"
          android:versionCode="1"
          android:versionName="1.0">
        <uses-sdk android:minSdkVersion="10" />
    <uses-permission android:name="android.permission.INTERNET"></uses-permission>
        <application android:icon="@drawable/icon" android:label="@string/app_name">
            <activity android:name=".AsyncTaskActivity"
                      android:label="@string/app_name">
                <intent-filter>
                    <action android:name="android.intent.action.MAIN" />
                    <category android:name="android.intent.category.LAUNCHER" />
                </intent-filter>
            </activity>
    
        </application>
    </manifest>

运行结果：

![0_1315914517z11T.gif][]

**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。

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1.  private static final int CORE\_POOL\_SIZE =5;//5个核心工作线程 
2.  private static final int MAXIMUM\_POOL\_SIZE = 128;//最多128个工作线程 
3.  private static final int KEEP\_ALIVE = 1;//空闲线程的超时时间为1秒 
4.  
5.  private static final BlockingQueue<Runnable> sWorkQueue =
6.  new LinkedBlockingQueue<Runnable>(10);//等待队列 
7.  
8.  private static final ThreadPoolExecutorsExecutor = new ThreadPoolExecutor(CORE\_POOL\_SIZE,
9.  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<Runnable> sWorkQueue =
               new LinkedBlockingQueue<Runnable>(10);//等待队列
     
       private static final ThreadPoolExecutorsExecutor = 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GetCSDNLogoTask对象：

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1.  GetCSDNLogoTask task = new GetCSDNLogoTask();

GetCSDNLogoTask task = new GetCSDNLogoTask();

此时会调用父类AsyncTask的构造函数：

AsyncTask.java

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1.  public AsyncTask() \{
2.  mWorker = new WorkerRunnable<Params, Result>() \{
3.  public Result call() throws Exception \{
4.  Process.setThreadPriority(Process.THREAD\_PRIORITY\_BACKGROUND);
5.  return doInBackground(mParams);
6.  \}
7.  \};
8.  
9.  mFuture = new FutureTask<Result>(mWorker) \{
10. @Override
11. protected void done() \{
12. Message message;
13. Result result = null;
14. 
15. try \{
16. result = get();
17. \} catch (InterruptedException e) \{
18. android.util.Log.w(LOG\_TAG, e);
19. \} catch (ExecutionException e) \{
20. throw new RuntimeException("An error occured while executing doInBackground()",
21. e.getCause());
22. \} catch (CancellationException e) \{
23. message = sHandler.obtainMessage(MESSAGE\_POST\_CANCEL,
24. new AsyncTaskResult<Result>(AsyncTask.this, (Result\[\]) null));
25. message.sendToTarget();//取消任务，发送MESSAGE\_POST\_CANCEL消息 
26. return;
27. \} catch (Throwable t) \{
28. throw new RuntimeException("An error occured while executing "
29. \+ "doInBackground()", t);
30. \}
31. 
32. message = sHandler.obtainMessage(MESSAGE\_POST\_RESULT,
33. new AsyncTaskResult<Result>(AsyncTask.this, result));//完成任务，发送MESSAGE\_POST\_RESULT消息并传递result对象 
34. message.sendToTarget();
35. \}
36. \};
37. \}

public AsyncTask() {
            mWorker = new WorkerRunnable<Params, Result>() {
                public Result call() throws Exception {
                    Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
                    return doInBackground(mParams);
                }
            };
    
            mFuture = new FutureTask<Result>(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<Result>(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<Result>(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GetCSDNLogoTask对象后，执行

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1.  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

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1.  public final AsyncTask<Params,Progress, Result> execute(Params... params) \{
2.  if (mStatus != Status.PENDING) \{
3.  switch (mStatus) \{
4.  case RUNNING:
5.  throw newIllegalStateException("Cannot execute task:"
6.  \+ " the taskis already running.");
7.  case FINISHED:
8.  throw newIllegalStateException("Cannot execute task:"
9.  \+ " the taskhas already been executed "
10. \+ "(a task canbe executed only once)");
11. \}
12. \}
13. 
14. mStatus = Status.RUNNING;
15. 
16. onPreExecute();//运行在ui线程，在提交任务到线程池之前执行 
17. 
18. mWorker.mParams = params;
19. sExecutor.execute(mFuture);//提交任务到线程池 
20. 
21. return this;
22. \}

public final AsyncTask<Params,Progress, Result> 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;
            sExecutor.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[《Android异步处理三：Handler+Looper+MessageQueue深入详解》][Android_Handler_Looper_MessageQueue]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

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1.  private static class InternalHandler extends Handler \{
2.  @SuppressWarnings(\{ "unchecked","RawUseOfParameterizedType"\})
3.  @Override
4.  public voidhandleMessage(Message msg) \{
5.  AsyncTaskResult result =(AsyncTaskResult) msg.obj;
6.  switch (msg.what) \{
7.  caseMESSAGE\_POST\_RESULT:
8.  // There is onlyone result 
9.  result.mTask.finish(result.mData\[0\]);//执行任务成功 
10. break;
11. caseMESSAGE\_POST\_PROGRESS:
12. result.mTask.onProgressUpdate(result.mData);//进度更新 
13. break;
14. caseMESSAGE\_POST\_CANCEL:
15. result.mTask.onCancelled();//取消任务 
16. break;
17. \}
18. \}
19. \}

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

本博文地址：[http://blog.csdn.net/mylzc/article/details/6774131][http_blog.csdn.net_mylzc_article_details_6774131]，转载请注明出处

[http_blog.csdn.net_mylzc_article_details_6774131]: http://blog.csdn.net/mylzc/article/details/6774131
[Android]: http://blog.csdn.net/mylzc/article/details/6777767
[Android_Thread_Handler_UI_UI]: http://blog.csdn.net/mylzc/article/details/6736988
[Android_AsyncTask_UI]: http://blog.csdn.net/mylzc/article/details/6772129
[Android_Handler_Looper_MessageQueue]: http://blog.csdn.net/mylzc/article/details/6771331
[view plain]: http://blog.csdn.net/mylzc/article/details/6774131#
[0_1315914517z11T.gif]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/15/3d568be370a84d4684749dfcf802e210.gif