本文共 14603 字，大约阅读时间需要 48 分钟。

api

AsyncTask是个抽象类，需要子类继承，然后调用execute()方法，继承需要设定Params，Progress，Result类型

public class MyActivity extends Activity{
       private Button btn;    private TextView tv;    @Override    public void onCreate(Bundle savedInstanceState)    {        super.onCreate(savedInstanceState);        setContentView(R.layout.main);        btn = (Button) findViewById(R.id.start_btn);        tv = (TextView) findViewById(R.id.content);        btn.setOnClickListener(new Button.OnClickListener(){            public void onClick(View v) {                update();            }        });    }    private void update(){        UpdateTextTask updateTextTask = new UpdateTextTask(this);        updateTextTask.execute();    }    class UpdateTextTask extends AsyncTask
   
    {        private Context context;        UpdateTextTask(Context context) {            this.context = context;        }        /**         * 运行在UI线程中，在调用doInBackground()之前执行         */        @Override        protected void onPreExecute() {            Toast.makeText(context,"开始执行",Toast.LENGTH_SHORT).show();        }        /**         * 后台运行的方法，可以运行非UI线程，可以执行耗时的方法         */        @Override        protected Integer doInBackground(Void... params) {            int i=0;            while(i<10){                i++;                publishProgress(i);                try {                    Thread.sleep(1000);                } catch (InterruptedException e) {                }            }            return null;        }        /**         * 运行在ui线程中，在doInBackground()执行完毕后执行         */        @Override        protected void onPostExecute(Integer integer) {            Toast.makeText(context,"执行完毕",Toast.LENGTH_SHORT).show();        }        /**         * 在publishProgress()被调用以后执行，publishProgress()用于更新进度         */        @Override        protected void onProgressUpdate(Integer... values) {            tv.setText(""+values[0]);        }    }}
   

再来个例子

public class MainActivity extends Activity{
       private Button btn;    private TextView tv;    private ImageView imageView;    private ProgressDialog progressDialog;    @Override    public void onCreate(Bundle savedInstanceState)    {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        btn = (Button) findViewById(R.id.btn);        imageView = findViewById(R.id.iv);        //    弹出要给ProgressDialog        progressDialog = new ProgressDialog(MainActivity.this);        progressDialog.setTitle("提示信息");        progressDialog.setMessage("正在下载中，请稍后......");        //    设置setCancelable(false); 表示我们不能取消这个弹出框，等下载完成之后再让弹出框消失        progressDialog.setCancelable(false);        //    设置ProgressDialog样式为圆圈的形式        progressDialog.setProgressStyle(ProgressDialog.STYLE_SPINNER);        btn.setOnClickListener(new View.OnClickListener() {            @Override            public void onClick(View view) {                update();            }        });    }    private void update(){        MyasyncTast myasyncTast = new MyasyncTast();        myasyncTast.execute("http://developer.android.com/images/home/kk-hero.jpg");    }    /**     * 定义一个类，让其继承AsyncTask这个类     * Params: String类型，表示传递给异步任务的参数类型是String，通常指定的是URL路径     * Progress: Integer类型，进度条的单位通常都是Integer类型     * Result：byte[]类型，表示我们下载好的图片以字节数组返回     * @author xiaoluo     *     */    class MyasyncTast extends AsyncTask
   
    {        @Override        protected void onPreExecute() {            super.onPreExecute();            progressDialog.show();        }        @Override        protected byte[] doInBackground(String... strings) {            //    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(strings[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);                }            }            catch (Exception e)            {                e.printStackTrace();            }            finally            {                httpClient.getConnectionManager().shutdown();            }            return image;        }        @Override        protected void onPostExecute(byte[] bytes) {            super.onPostExecute(bytes);            //    将doInBackground方法返回的byte[]解码成要给Bitmap            Bitmap bitmap = BitmapFactory.decodeByteArray(bytes, 0, bytes.length);            //    更新我们的ImageView控件            imageView.setImageBitmap(bitmap);            //    使ProgressDialog框消失            progressDialog.dismiss();        }    }}
   

缺陷：

优点:简单,快捷过程可控 缺点:在使用多个异步操作和并需要进行Ui变更时,就变得复杂起来.

*  * Copyright (C) 2008 The Android Open Source Project  *  * Licensed under the Apache License, Version 2.0 (the "License");  * you may not use this file except in compliance with the License.  * You may obtain a copy of the License at  *  *      http://www.apache.org/licenses/LICENSE-2.0  *  * Unless required by applicable law or agreed to in writing, software  * distributed under the License is distributed on an "AS IS" BASIS,  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  * See the License for the specific language governing permissions and  * limitations under the License.  */  package android.os;  import java.util.ArrayDeque;  import java.util.concurrent.BlockingQueue;  import java.util.concurrent.Callable;  import java.util.concurrent.CancellationException;  import java.util.concurrent.Executor;  import java.util.concurrent.ExecutionException;  import java.util.concurrent.FutureTask;  import java.util.concurrent.LinkedBlockingQueue;  import java.util.concurrent.ThreadFactory;  import java.util.concurrent.ThreadPoolExecutor;  import java.util.concurrent.TimeUnit;  import java.util.concurrent.TimeoutException;  import java.util.concurrent.atomic.AtomicBoolean;  import java.util.concurrent.atomic.AtomicInteger;  public abstract class AsyncTask
   
     {
         private static final String LOG_TAG = "AsyncTask";      //获取当前的cpu核心数      private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();      //线程池核心容量      private static final int CORE_POOL_SIZE = CPU_COUNT + 1;      //线程池最大容量      private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;      //过剩的空闲线程的存活时间      private static final int KEEP_ALIVE = 1;      //ThreadFactory 线程工厂，通过工厂方法newThread来获取新线程      private static final ThreadFactory sThreadFactory = new ThreadFactory() {          //原子整数，可以在超高并发下正常工作          private final AtomicInteger mCount = new AtomicInteger(1);          public Thread newThread(Runnable r) {              return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());          }      };      //静态阻塞式队列，用来存放待执行的任务，初始容量：128个      private static final BlockingQueue
    
      sPoolWorkQueue =              new LinkedBlockingQueue
     
      (128);      /**      * 静态并发线程池，可以用来并行执行任务，尽管从3.0开始，AsyncTask默认是串行执行任务      * 但是我们仍然能构造出并行的AsyncTask      */      public static final Executor THREAD_POOL_EXECUTOR              = new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,                      TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);      /**      * 静态串行任务执行器，其内部实现了串行控制，      * 循环的取出一个个任务交给上述的并发线程池去执行      */      public static final Executor SERIAL_EXECUTOR = new SerialExecutor();      //消息类型：发送结果      private static final int MESSAGE_POST_RESULT = 0x1;      //消息类型：更新进度      private static final int MESSAGE_POST_PROGRESS = 0x2;      /**静态Handler，用来发送上述两种通知，采用UI线程的Looper来处理消息      * 这就是为什么AsyncTask必须在UI线程调用，因为子线程      * 默认没有Looper无法创建下面的Handler，程序会直接Crash      */      private static final InternalHandler sHandler = new InternalHandler();      //默认任务执行器，被赋值为串行任务执行器，就是它，AsyncTask变成串行的了      private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;      //如下两个变量我们先不要深究，不影响我们对整体逻辑的理解      private final WorkerRunnable
      
        mWorker;      private final FutureTask
       
         mFuture;      //任务的状态 默认为挂起，即等待执行，其类型标识为易变的（volatile）      private volatile Status mStatus = Status.PENDING;      //原子布尔型，支持高并发访问，标识任务是否被取消      private final AtomicBoolean mCancelled = new AtomicBoolean();      //原子布尔型，支持高并发访问，标识任务是否被执行过      private final AtomicBoolean mTaskInvoked = new AtomicBoolean();      /*串行执行器的实现，我们要好好看看，它是怎么把并行转为串行的      *目前我们需要知道，asyncTask.execute(Params ...)实际上会调用      *SerialExecutor的execute方法，这一点后面再说明。也就是说：当你的asyncTask执行的时候，      *首先你的task会被加入到任务队列，然后排队，一个个执行      */      private static class SerialExecutor implements Executor {
             //线性双向队列，用来存储所有的AsyncTask任务          final ArrayDeque
        
          mTasks = new ArrayDeque
         
          (); //当前正在执行的AsyncTask任务 Runnable mActive; public synchronized void execute(final Runnable r) { //将新的AsyncTask任务加入到双向队列中 mTasks.offer(new Runnable() { public void run() { try { //执行AsyncTask任务 r.run(); } finally { //当前AsyncTask任务执行完毕后，进行下一轮执行，如果还有未执行任务的话 //这一点很明显体现了AsyncTask是串行执行任务的，总是一个任务执行完毕才会执行下一个任务 scheduleNext(); } } }); //如果当前没有任务在执行，直接进入执行逻辑 if (mActive == null) { scheduleNext(); } } protected synchronized void scheduleNext() { //从任务队列中取出队列头部的任务，如果有就交给并发线程池去执行 if ((mActive = mTasks.poll()) != null) { THREAD_POOL_EXECUTOR.execute(mActive); } } } /** * 任务的三种状态 */ public enum Status { /** * 任务等待执行 */ PENDING, /** * 任务正在执行 */ RUNNING, /** * 任务已经执行结束 */ FINISHED, } /** 隐藏API：在UI线程中调用，用来初始化Handler */ public static void init() { sHandler.getLooper(); } /** 隐藏API：为AsyncTask设置默认执行器 */ public static void setDefaultExecutor(Executor exec) { sDefaultExecutor = exec; } /** * Creates a new asynchronous task. This constructor must be invoked on the UI thread. */ public AsyncTask() { mWorker = new WorkerRunnable
          
           () { public Result call() throws Exception { mTaskInvoked.set(true); Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); //noinspection unchecked return postResult(doInBackground(mParams)); } }; mFuture = new FutureTask
           
            (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); } } //doInBackground执行完毕，发送消息 private Result postResult(Result result) { @SuppressWarnings("unchecked") Message message = sHandler.obtainMessage(MESSAGE_POST_RESULT, new AsyncTaskResult
            
             (this, result)); message.sendToTarget(); return result; } /** * 返回任务的状态 */ public final Status getStatus() { return mStatus; } /** * 这个方法是我们必须要重写的，用来做后台计算 * 所在线程：后台线程 */ protected abstract Result doInBackground(Params... params); /** * 在doInBackground之前调用，用来做初始化工作 * 所在线程：UI线程 */ protected void onPreExecute() { } /** * 在doInBackground之后调用，用来接受后台计算结果更新UI * 所在线程：UI线程 */ protected void onPostExecute(Result result) { } /** * Runs on the UI thread after {@link #publishProgress} is invoked. /** * 在publishProgress之后调用，用来更新计算进度 * 所在线程：UI线程 */ protected void onProgressUpdate(Progress... values) { } /** * cancel被调用并且doInBackground执行结束，会调用onCancelled，表示任务被取消 * 这个时候onPostExecute不会再被调用，二者是互斥的，分别表示任务取消和任务执行完成 * 所在线程：UI线程 */ @SuppressWarnings({ "UnusedParameters"}) protected void onCancelled(Result result) { onCancelled(); } protected void onCancelled() { } public final boolean isCancelled() { return mCancelled.get(); } public final boolean cancel(boolean mayInterruptIfRunning) { mCancelled.set(true); return mFuture.cancel(mayInterruptIfRunning); } public final Result get() throws InterruptedException, ExecutionException { return mFuture.get(); } public final Result get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException { return mFuture.get(timeout, unit); } /** * 这个方法如何执行和系统版本有关，在AsyncTask的使用规则里已经说明，如果你真的想使用并行AsyncTask， * 也是可以的，只要稍作修改 * 必须在UI线程调用此方法 */ public final AsyncTask
             
               execute(Params... params) { //串行执行 return executeOnExecutor(sDefaultExecutor, params); //如果我们想并行执行，这样改就行了，当然这个方法我们没法改 //return executeOnExecutor(THREAD_POOL_EXECUTOR, params); } /** * 通过这个方法我们可以自定义AsyncTask的执行方式，串行or并行，甚至可以采用自己的Executor * 为了实现并行，我们可以在外部这么用AsyncTask： * asyncTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, Params... params); * 必须在UI线程调用此方法 */ public final AsyncTask
              
                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; //这里#onPreExecute会最先执行 onPreExecute(); mWorker.mParams = params; //然后后台计算#doInBackground才真正开始 exec.execute(mFuture); //接着会有#onProgressUpdate被调用，最后是#onPostExecute return this; } /** * 这是AsyncTask提供的一个静态方法，方便我们直接执行一个runnable */ public static void execute(Runnable runnable) { sDefaultExecutor.execute(runnable); } /** * 打印后台计算进度，onProgressUpdate会被调用 */ protected final void publishProgress(Progress... values) { if (!isCancelled()) { sHandler.obtainMessage(MESSAGE_POST_PROGRESS, new AsyncTaskResult
               (this, values)).sendToTarget(); } } //任务结束的时候会进行判断，如果任务没有被取消，则onPostExecute会被调用 private void finish(Result result) { if (isCancelled()) { onCancelled(result); } else { onPostExecute(result); } mStatus = Status.FINISHED; } //AsyncTask内部Handler，用来发送后台计算进度更新消息和计算完成消息 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 static abstract class WorkerRunnable
                
                  implements Callable
                 
                   { Params[] mParams; } @SuppressWarnings({ "RawUseOfParameterizedType"}) private static class AsyncTaskResult
                   { final AsyncTask mTask; final Data[] mData; AsyncTaskResult(AsyncTask task, Data... data) { mTask = task; mData = data; } } }