本篇文章给大家谈谈android–尝试在后台解析带有AsyncTask的JSON时强制关闭，同时本文还将给你拓展androidasynctask、AndroidAsyncTaskxml解析、Andro
本篇文章给大家谈谈android – 尝试在后台解析带有AsyncTask的JSON时强制关闭，同时本文还将给你拓展android asynctask、Android AsyncTask xml解析、Android AsyncTask 完全解析、Android AsyncTask 源码解析等相关知识，希望对各位有所帮助，不要忘了收藏本站喔。
本文目录一览：- android – 尝试在后台解析带有AsyncTask的JSON时强制关闭
- android asynctask
- Android AsyncTask xml解析
- Android AsyncTask 完全解析
- Android AsyncTask 源码解析
android – 尝试在后台解析带有AsyncTask的JSON时强制关闭
我是Android开发的新手,我正在玩json数据.我设法让解析工作.我想显示一个ProgressDialog,我读到我需要使用AsyncTask.但是出于某种原因,我在doInBackground()中放入相同的工作代码时会立即关闭一个力量,尽管eclipse说一切都很好.
这是源代码：
public class HomeActivity extends Activity {
public class BackgroundAsyncTask extends AsyncTask<Void, Integer, Void> {
ProgressDialog dialog = new ProgressDialog (HomeActivity.this);
@Override
protected void onPreExecute() {
dialog.setMessage("Loading...please wait");
dialog.setIndeterminate(true);
dialog.setCancelable(false);
dialog.show();
}
protected void onPostExecute() {
dialog.dismiss();
}
@Override
protected Void doInBackground(Void... params) {
try {
URL json = new URL("http://www.corps-marchia.de/jsontest.PHP");
URLConnection tc = json.openConnection();
BufferedReader in = new BufferedReader(new InputStreamReader(tc.getInputStream()));
String line;
while ((line = in.readLine()) != null) {
JSONArray ja = new JSONArray(line);
JSONObject jo = (JSONObject) ja.get(0);
TextView txtView = (TextView)findViewById(R.id.TextView01);
txtView.setText(jo.getString("text"));
}
} catch (MalformedURLException e) {
e.printstacktrace();
} catch (IOException e) {
e.printstacktrace();
} catch (JSONException e) {
e.printstacktrace();
}
return null;
}
}
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
new BackgroundAsyncTask().execute();
}
}
这是错误日志：
01-08 12:33:48.225: ERROR/AndroidRuntime(815): FATAL EXCEPTION: AsyncTask #1
01-08 12:33:48.225: ERROR/AndroidRuntime(815): java.lang.RuntimeException: An error occured while executing doInBackground()
01-08 12:33:48.225: ERROR/AndroidRuntime(815): at android.os.AsyncTask$3.done(AsyncTask.java:200)
01-08 12:33:48.225: ERROR/AndroidRuntime(815): at java.util.concurrent.FutureTask$Sync.innerSetException(FutureTask.java:274)
01-08 12:33:48.225: ERROR/AndroidRuntime(815): at java.util.concurrent.FutureTask.setException(FutureTask.java:125)
01-08 12:33:48.225: ERROR/AndroidRuntime(815): at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:308)
01-08 12:33:48.225: ERROR/AndroidRuntime(815): at java.util.concurrent.FutureTask.run(FutureTask.java:138)
01-08 12:33:48.225: ERROR/AndroidRuntime(815): at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1088)
01-08 12:33:48.225: ERROR/AndroidRuntime(815): at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:581)
01-08 12:33:48.225: ERROR/AndroidRuntime(815): at java.lang.Thread.run(Thread.java:1019)
01-08 12:33:48.225: ERROR/AndroidRuntime(815): Caused by: android.view.ViewRoot$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.
01-08 12:33:48.225: ERROR/AndroidRuntime(815): at android.view.ViewRoot.checkThread(ViewRoot.java:2932)
01-08 12:33:48.225: ERROR/AndroidRuntime(815): at android.view.ViewRoot.requestLayout(ViewRoot.java:629)
01-08 12:33:48.225: ERROR/AndroidRuntime(815): at android.view.View.requestLayout(View.java:8267)
01-08 12:33:48.225: ERROR/AndroidRuntime(815): at android.view.View.requestLayout(View.java:8267)
01-08 12:33:48.225: ERROR/AndroidRuntime(815): at android.view.View.requestLayout(View.java:8267)
01-08 12:33:48.225: ERROR/AndroidRuntime(815): at android.view.View.requestLayout(View.java:8267)
01-08 12:33:48.225: ERROR/AndroidRuntime(815): at android.view.View.requestLayout(View.java:8267)
01-08 12:33:48.225: ERROR/AndroidRuntime(815): at android.widget.TextView.checkForRelayout(TextView.java:5521)
01-08 12:33:48.225: ERROR/AndroidRuntime(815): at android.widget.TextView.setText(TextView.java:2724)
01-08 12:33:48.225: ERROR/AndroidRuntime(815): at android.widget.TextView.setText(TextView.java:2592)
01-08 12:33:48.225: ERROR/AndroidRuntime(815): at android.widget.TextView.setText(TextView.java:2567)
01-08 12:33:48.225: ERROR/AndroidRuntime(815): at net.ajzele.demo.andy1.HomeActivity$BackgroundAsyncTask.doInBackground(HomeActivity.java:52)
01-08 12:33:48.225: ERROR/AndroidRuntime(815): at net.ajzele.demo.andy1.HomeActivity$BackgroundAsyncTask.doInBackground(HomeActivity.java:1)
01-08 12:33:48.225: ERROR/AndroidRuntime(815): at android.os.AsyncTask$2.call(AsyncTask.java:185)
01-08 12:33:48.225: ERROR/AndroidRuntime(815): at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:306)
01-08 12:33:48.225: ERROR/AndroidRuntime(815): ... 4 more
01-08 12:33:51.605: ERROR/WindowManager(815): Activity net.ajzele.demo.andy1.HomeActivity has leaked window com.android.internal.policy.impl.PhoneWindow$DecorView@4051d0c0 that was originally added here
01-08 12:33:51.605: ERROR/WindowManager(815): android.view.WindowLeaked: Activity net.ajzele.demo.andy1.HomeActivity has leaked window com.android.internal.policy.impl.PhoneWindow$DecorView@4051d0c0 that was originally added here
01-08 12:33:51.605: ERROR/WindowManager(815): at android.view.ViewRoot.<init>(ViewRoot.java:258)
01-08 12:33:51.605: ERROR/WindowManager(815): at android.view.WindowManagerImpl.addView(WindowManagerImpl.java:148)
01-08 12:33:51.605: ERROR/WindowManager(815): at android.view.WindowManagerImpl.addView(WindowManagerImpl.java:91)
01-08 12:33:51.605: ERROR/WindowManager(815): at android.view.Window$LocalWindowManager.addView(Window.java:424)
01-08 12:33:51.605: ERROR/WindowManager(815): at android.app.Dialog.show(Dialog.java:241)
01-08 12:33:51.605: ERROR/WindowManager(815): at net.ajzele.demo.andy1.HomeActivity$BackgroundAsyncTask.onPreExecute(HomeActivity.java:33)
01-08 12:33:51.605: ERROR/WindowManager(815): at android.os.AsyncTask.execute(AsyncTask.java:391)
01-08 12:33:51.605: ERROR/WindowManager(815): at net.ajzele.demo.andy1.HomeActivity.onCreate(HomeActivity.java:72)
01-08 12:33:51.605: ERROR/WindowManager(815): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
01-08 12:33:51.605: ERROR/WindowManager(815): at android.app.ActivityThread.performlaunchActivity(ActivityThread.java:1586)
01-08 12:33:51.605: ERROR/WindowManager(815): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1638)
01-08 12:33:51.605: ERROR/WindowManager(815): at android.app.ActivityThread.access$1500(ActivityThread.java:117)
01-08 12:33:51.605: ERROR/WindowManager(815): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:928)
01-08 12:33:51.605: ERROR/WindowManager(815): at android.os.Handler.dispatchMessage(Handler.java:99)
01-08 12:33:51.605: ERROR/WindowManager(815): at android.os.Looper.loop(Looper.java:123)
01-08 12:33:51.605: ERROR/WindowManager(815): at android.app.ActivityThread.main(ActivityThread.java:3647)
01-08 12:33:51.605: ERROR/WindowManager(815): at java.lang.reflect.Method.invokeNative(Native Method)
01-08 12:33:51.605: ERROR/WindowManager(815): at java.lang.reflect.Method.invoke(Method.java:507)
01-08 12:33:51.605: ERROR/WindowManager(815): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
01-08 12:33:51.605: ERROR/WindowManager(815): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
01-08 12:33:51.605: ERROR/WindowManager(815): at dalvik.system.NativeStart.main(Native Method)
任何提示？我希望你能帮助我搜索网络并没有找到任何有效的解决方案……提前致谢
解决方法:
检查docs for AsyncTask.您无法直接在doInBackground中操作UI.要进行UI更新,您需要调用publishProgress.您还需要更改AsyncTask的签名以传递String.所以像这样：
public class BackgroundAsyncTask extends AsyncTask<Void, String, Void> {
TextView txtView = (TextView)findViewById(R.id.TextView01);
...
protected void onProgressUpdate(String... message) {
txtView.setText(message[0]);
}
...
@Override
protected Void doInBackground(Void... params) {
try {
URL json = new URL("http://www.corps-marchia.de/jsontest.PHP");
URLConnection tc = json.openConnection();
BufferedReader in = new BufferedReader(new InputStreamReader(tc.getInputStream()));
String line;
while ((line = in.readLine()) != null) {
JSONArray ja = new JSONArray(line);
JSONObject jo = (JSONObject) ja.get(0);
publishProgress(jo.getString("text"));
}
} catch (MalformedURLException e) {
e.printstacktrace();
} catch (IOException e) {
e.printstacktrace();
} catch (JSONException e) {
e.printstacktrace();
}
return null;
}
android asynctask
asynctask<1, 2, 3>
3 doinbackground(1... param)
onprogressbarupdate(2 param)
onpostexcute(3 param)
执行顺序
onpreexecute
doinbackground
onpostexecute
其中可以通过 publishprogress触发onprogressupdate
其中onpreexecute doinbackground运行在非UI主线线程 所以无法更改或者设置UI控件
但是onpostexecute 何onprogressupdate运行在UI主线程
其中onpostexecute接受的参数是doinbackground的返回值
相同的C#中又委托代理机制用于非UI主线程可以修改UI线程中的UI控件
Android AsyncTask xml解析
我尝试在AsyncTask中解析大型xml文档. FeinstaubActivity启动,但是我只看到黑屏,然后从我启动另一个Activity的地方回到RSSReaderActivity.
日志：
DEBUG/SntpClient(58): request time Failed: java.net.socketException: Address family not supported by protocol
WARN/KeyCharacterMap(623): No keyboard for id 0
WARN/KeyCharacterMap(623): Using default keymap: /system/usr/keychars/qwerty.kcm.bin
INFO/ActivityManager(58): Starting activity: Intent { cmp=de.test.testapp/.FeinstaubActivity }
DEBUG/AndroidRuntime(623): Shutting down VM
WARN/dalvikvm(623): threadid=1: thread exiting with uncaught exception (group=0x4001d800)
WARN/dalvikvm(623): threadid=7: thread exiting with uncaught exception (group=0x4001d800)
INFO/Process(623): Sending signal. PID: 623 SIG: 9
INFO/ActivityManager(58): Process de.test.testapp (pid 623) has died.
INFO/WindowManager(58): WIN DEATH: Window{4402be18 AtchDlg:de.test.testapp/de.test.testapp.RSSReaderActivity paused=false}
INFO/WindowManager(58): WIN DEATH: Window{4400dd68 de.test.testapp/de.test.testapp.RSSReaderActivity paused=false}
INFO/ActivityManager(58): Start proc de.test.testapp for activity de.test.testapp/.RSSReaderActivity: pid=631 uid=10035 gids={3003}
DEBUG/dalvikvm(32): GC_EXPLICIT freed 299 objects / 11488 bytes in 162ms
INFO/UsageStats(58): Unexpected resume of de.test.testapp while already resumed in de.test.testapp
DEBUG/dalvikvm(32): GC_EXPLICIT freed 57 objects / 2440 bytes in 138ms
INFO/ActivityManager(58): displayed activity de.test.testapp/.RSSReaderActivity: 411 ms (total 532 ms)
DEBUG/dalvikvm(32): GC_EXPLICIT freed 2 objects / 48 bytes in 126ms
DEBUG/dalvikvm(631): GC_FOR_MALLOC freed 3294 objects / 461928 bytes in 72ms
DEBUG/dalvikvm(631): GC_FOR_MALLOC freed 2750 objects / 557024 bytes in 59ms
DEBUG/dalvikvm(631): GC_FOR_MALLOC freed 2786 objects / 506288 bytes in 60ms
WARN/InputManagerService(58): Got remoteexception sending setActive(false) notification to pid 623 uid 10035
码
Document document = null;
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.feinstaub);
document = (Doxument) new ParseXMLFile().execute();
}
private class ParseXMLFile extends AsyncTask<Integer, Integer, Document>{
@Override
protected Document doInBackground(Integer... params) {
Document parsedXML = null;
try {
parsedXML = builder.parse(getApplicationContext().getResources().openRawResource(R.raw.finedust));
} catch (NotFoundException e) {
// Todo Auto-generated catch block
e.printstacktrace();
} catch (SAXException e) {
// Todo Auto-generated catch block
e.printstacktrace();
} catch (IOException e) {
// Todo Auto-generated catch block
e.printstacktrace();
}
return parsedXML;
}
}
问候
浮动
解决方法:
啊,不确定您能做到这一点：http://developer.android.com/reference/android/os/AsyncTask.html显示execute返回void.
你要这个：
Document document = null;
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.feinstaub);
new ParseXMLFile().execute();
}
void setDocument(Document document){ this.document = document; }
private class ParseXMLFile extends AsyncTask<Integer, Integer, Document>{
@Override public void onPostExecute(Document d){ setDocument(d); }
@Override
protected Document doInBackground(Integer... params) {
Document parsedXML = null;
try {
parsedXML = builder.parse(getApplicationContext().getResources().openRawResource(R.raw.finedust));
} catch (NotFoundException e) {
// Todo Auto-generated catch block
e.printstacktrace();
} catch (SAXException e) {
// Todo Auto-generated catch block
e.printstacktrace();
} catch (IOException e) {
// Todo Auto-generated catch block
e.printstacktrace();
}
return parsedXML;
}
}
您需要做一些范围界定：传递父活动或以某种方式适当地声明setDocument方法,以便您可以从AsyncTask内部对其进行回调,但是应该这样做.
Android AsyncTask 完全解析
我们都知道，Android UI 是线程不安全的，如果想要在子线程里进行 UI 操作，就需要借助 Android 的异步消息处理机制。之前我也写过了一篇文章从源码层面分析了 Android 的异步消息处理机制，感兴趣的朋友可以参考 Android Handler、Message 完全解析，带你从源码的角度彻底理解。不过为了更加方便我们在子线程中更新 UI 元素，Android 从 1.5 版本就引入了一个 AsyncTask 类，使用它就可以非常灵活方便地从子线程切换到 UI 线程，我们本篇文章的主角也就正是它了。AsyncTask 很早就出现在 Android 的 API 里了，所以我相信大多数朋友对它的用法都已经非常熟悉。不过今天我还是准备从 AsyncTask 的基本用法开始讲起，然后我们再来一起分析下 AsyncTask 源码，看看它是如何实现的，最后我会介绍一些关于 AsyncTask 你所不知道的秘密。视频教程：http://www.jikexueyuan.com/study/5/lid/22.html?hmsr=wenku_asynctask
教程下载：http://pan.baidu.com/s/1EkWrk
Android AsyncTask 源码解析
转载请标明出处：http://blog.csdn.net/lmj623565791/article/details/38614699，本文出自：【张鸿洋的博客】
1、概述
相信大家对AsyncTask都不陌生，对于执行耗时任务，然后更新UI是一把利器，当然也是替代Thread + Handler 的一种方式。如果你对Handler机制还不了解，请看：Android 异步消息处理机制 让你深入理解 Looper、Handler、Message三者关系。
2、简单的例子
相信大家都写过这样的代码：
package com.example.zhy_asynctask_demo01;
import android.app.Activity;
import android.app.ProgressDialog;
import android.os.AsyncTask;
import android.os.Bundle;
import android.util.Log;
import android.widget.TextView;
public class MainActivity extends Activity
{
private static final String TAG = "MainActivity";
private ProgressDialog mDialog;
private TextView mTextView;
@Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mTextView = (TextView) findViewById(R.id.id_tv);
mDialog = new ProgressDialog(this);
mDialog.setMax(100);
mDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
mDialog.setCancelable(false);
new MyAsyncTask().execute();
}
private class MyAsyncTask extends AsyncTask<Void, Integer, Void>
{
@Override
protected void onPreExecute()
{
mDialog.show();
Log.e(TAG, Thread.currentThread().getName() + " onPreExecute ");
}
@Override
protected Void doInBackground(Void... params)
{
// 模拟数据的加载,耗时的任务
for (int i = 0; i < 100; i++)
{
try
{
Thread.sleep(80);
} catch (InterruptedException e)
{
e.printStackTrace();
}
publishProgress(i);
}
Log.e(TAG, Thread.currentThread().getName() + " doInBackground ");
return null;
}
@Override
protected void onProgressUpdate(Integer... values)
{
mDialog.setProgress(values[0]);
Log.e(TAG, Thread.currentThread().getName() + " onProgressUpdate ");
}
@Override
protected void onPostExecute(Void result)
{
// 进行数据加载完成后的UI操作
mDialog.dismiss();
mTextView.setText("LOAD DATA SUCCESS ");
Log.e(TAG, Thread.currentThread().getName() + " onPostExecute ");
}
}
}进入某个Activity，Activity中需要的数据来自于网络或者其它耗时操作，可以在AsyncTask中onPreExecute完成一些准备操作，比如上例中显示进度对话框；然后在doInBackground完成耗时操作，在进行耗时操作时还能不时的通过publishProgress给onProgressUpdate中传递参数，然后在onProgressUpdate中可以进行UI操作，比如上例更新进度条的进度；当耗时任务执行完成后，最后在onPostExecute进行设置控件数据更新UI等操作，例如隐藏进度对话框。
效果图：
3、源码解析
注：本篇源码分析基于Andorid-17，因为和3.0之前版本变动较大，有必要标出。
那么大家一定好奇，AsyncTask在Android中是如何实现的，下面进行源码分析：从我们的执行异步任务的起点开始，进入execute方法：
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;
}20行：执行了onPreExecute()，当前依然在UI线程，所以我们可以在其中做一些准备工作。
22行：将我们传入的参数赋值给了mWorker.mParams
23行：exec.execute(mFuture)
相信大家对22行出现的mWorker，以及23行出现的mFuture都会有些困惑。
mWorker找到这个类：
private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> {
Params[] mParams;
}可以看到是Callable的子类，且包含一个mParams用于保存我们传入的参数，下面看初始化mWorker的代码：
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));
}
};
//….
}可以看到mWorker在构造方法中完成了初始化，并且因为是一个抽象类，在这里new了一个实现类，实现了call方法，call方法中设置mTaskInvoked=true，且最终调用doInBackground(mParams)方法，并返回Result值作为参数给postResult方法.可以看到我们的doInBackground出现了，下面继续看：
private Result postResult(Result result) {
@SuppressWarnings("unchecked")
Message message = sHandler.obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(this, result));
message.sendToTarget();
return result;
}可以看到postResult中出现了我们熟悉的异步消息机制，传递了一个消息message, message.what为MESSAGE_POST_RESULT；message.object= new AsyncTaskResult(this,result);
private static class AsyncTaskResult<Data> {
final AsyncTask mTask;
final Data[] mData;
AsyncTaskResult(AsyncTask task, Data... data) {
mTask = task;
mData = data;
}
}AsyncTaskResult就是一个简单的携带参数的对象。
看到这，我相信大家肯定会想到，在某处肯定存在一个sHandler，且复写了其handleMessage方法等待消息的传入，以及消息的处理。
private static final InternalHandler sHandler = new InternalHandler();
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;
}
}
}哈哈，出现了我们的handleMessage，可以看到，在接收到MESSAGE_POST_RESULT消息时，执行了result.mTask.finish(result.mData[0]);其实就是我们的AsyncTask.this.finish(result)，于是看finish方法
private void finish(Result result) {
if (isCancelled()) {
onCancelled(result);
} else {
onPostExecute(result);
}
mStatus = Status.FINISHED;
}可以看到，如果我们调用了cancel()则执行onCancelled回调；正常执行的情况下调用我们的onPostExecute(result);主要这里的调用是在handler的handleMessage中，所以是在UI线程中。如果你对异步消息机制不理解请看： Android 异步消息处理机制 让你深入理解 Looper、Handler、Message三者关系
最后将状态置为FINISHED。
mWoker看完了，应该到我们的mFuture了，依然实在构造方法中完成mFuture的初始化，将mWorker作为参数，复写了其done方法。
public AsyncTask() {
...
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);
}
}好了，到了这里，已经介绍完了execute方法中出现了mWorker和mFurture，不过这里一直是初始化这两个对象的代码，并没有真正的执行。下面我们看真正调用执行的地方。
execute方法中的：
还记得上面的execute中的23行：exec.execute(mFuture)
exec为executeOnExecutor(sDefaultExecutor, params)中的sDefaultExecutor
private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
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);
}
}
}16-17行：判断当前mActive是否为空，为空则调用scheduleNext方法
20行：scheduleNext，则直接取出任务队列中的队首任务，如果不为null则传入THREAD_POOL_EXECUTOR进行执行。
下面看THREAD_POOL_EXECUTOR为何方神圣：
public static final Executor THREAD_POOL_EXECUTOR
=new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,
TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);private static final int CORE_POOL_SIZE = 5;
private static final int MAXIMUM_POOL_SIZE = 128;
private static final int KEEP_ALIVE = 1;
private static final ThreadFactory sThreadFactory = new ThreadFactory() {
private final AtomicInteger mCount = new AtomicInteger(1);
public Thread newThread(Runnable r) {
return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());
}
};
private static final BlockingQueue<Runnable> sPoolWorkQueue =
new LinkedBlockingQueue<Runnable>(10);看到这里，大家可能会认为，背后原来有一个线程池，且最大支持128的线程并发，加上长度为10的阻塞队列，可能会觉得就是在快速调用138个以内的AsyncTask子类的execute方法不会出现问题，而大于138则会抛出异常。
其实不是这样的，我们再仔细看一下代码，回顾一下sDefaultExecutor，真正在execute()中调用的为sDefaultExecutor.execute：
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);
}
}
}可以看到，如果此时有10个任务同时调用execute（s synchronized）方法，第一个任务入队，然后在mActive = mTasks.poll()) != null被取出，并且赋值给mActivte，然后交给线程池去执行。然后第二个任务入队，但是此时mActive并不为null，并不会执行scheduleNext();所以如果第一个任务比较慢，10个任务都会进入队列等待；真正执行下一个任务的时机是，线程池执行完成第一个任务以后，调用Runnable中的finally代码块中的scheduleNext，所以虽然内部有一个线程池，其实调用的过程还是线性的。一个接着一个的执行，相当于单线程。
4、总结
到此源码解释完毕，由于代码跨度比较大，我们再回顾一下：
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;
}18行：设置当前AsyncTask的状态为RUNNING，上面的switch也可以看出，每个异步任务在完成前只能执行一次。
20行：执行了onPreExecute()，当前依然在UI线程，所以我们可以在其中做一些准备工作。
22行：将我们传入的参数赋值给了mWorker.mParams ,mWorker为一个Callable的子类，且在内部的call()方法中，调用了doInBackground(mParams)，然后得到的返回值作为postResult的参数进行执行；postResult中通过sHandler发送消息，最终sHandler的handleMessage中完成onPostExecute的调用。
23行：exec.execute(mFuture)，mFuture为真正的执行任务的单元，将mWorker进行封装，然后由sDefaultExecutor交给线程池进行执行。
5、publishProgress
说了这么多，我们好像忘了一个方法：publishProgress
protected final void publishProgress(Progress... values) {
if (!isCancelled()) {
sHandler.obtainMessage(MESSAGE_POST_PROGRESS,
new AsyncTaskResult<Progress>(this, values)).sendToTarget();
}
}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;
}
}
}在handleMessage中进行了我们的onProgressUpdate(result.mData);的调用。
6、AsyncTask曾经缺陷
记得以前有个面试题经常会问道：AsyncTask运行的原理是什么？有什么缺陷？
以前对于缺陷的答案可能是：AsyncTask在并发执行多个任务时发生异常。其实还是存在的，在3.0以前的系统中还是会以支持多线程并发的方式执行，支持并发数也是我们上面所计算的128，阻塞队列可以存放10个；也就是同时执行138个任务是没有问题的；而超过138会马上出现java.util.concurrent.RejectedExecutionException；
而在在3.0以上包括3.0的系统中会为单线程执行（即我们上面代码的分析）；
空说无凭：下面看测试代码：
package com.example.zhy_asynctask_demo01;
import android.app.Activity;
import android.app.ProgressDialog;
import android.os.AsyncTask;
import android.os.Bundle;
import android.util.Log;
import android.widget.TextView;
public class MainActivity extends Activity
{
private static final String TAG = "MainActivity";
private ProgressDialog mDialog;
private TextView mTextView;
@Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mTextView = (TextView) findViewById(R.id.id_tv);
mDialog = new ProgressDialog(this);
mDialog.setMax(100);
mDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
mDialog.setCancelable(false);
for(int i = 1 ;i <= 138 ; i++)
{
new MyAsyncTask2().execute();
}
//new MyAsyncTask().execute();
}
private class MyAsyncTask2 extends AsyncTask<Void,Void, Void>
{
@Override
protected Void doInBackground(Void... params)
{
try
{
Log.e(TAG, Thread.currentThread().getName());
Thread.sleep(10000);
} catch (InterruptedException e)
{
e.printStackTrace();
}
return null;
}
}
}输出结果为：
AsyncTask#1 - AsyncTask #128同时输出
然后10s后，另外10个任务输出。
可以分析结果，得到结论：AsyncTask在2.2的系统中同时支持128个任务并发，至少支持10个任务等待；
下面将138个任务，改成139个任务：
for(int i = 1 ;i <= 139 ; i++)
{
new MyAsyncTask2().execute();
}简单说一下出现异常的原因：现在是139个任务，几乎同时提交，线程池支持128个的并发，然后阻塞队列数量为10，此时当第11个任务提交的时候则会发生异常。
简单看一下源码：
public static final Executor THREAD_POOL_EXECUTOR
= new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE, TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
if (! isRunning(recheck) && remove(command))
reject(command);
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
else if (!addWorker(command, false))
reject(command);当阻塞队列满的时候workQueue.offer(command)返回false;然后执行addWorker(command,false)方法，如果返回false则执行reject()方法.
private boolean addWorker(Runnable firstTask, boolean core) {
…
int wc = workerCountOf(c);
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
…
}可以看到当任务数目大于容量则返回false，最终在reject()中抛出异常。
上面就是使用2.2模拟器测试的结果；
下面将系统改为4.1.1，也就是我的测试机小米2s
把线程数改为139甚至1000，你可以看到任务一个接一个的在那缓慢的执行，不会抛什么异常，不过线程倒是1个1个的在那执行；
好了，如果现在大家去面试，被问到AsyncTask的缺陷，可以分为两个部分说，在3.0以前，最大支持128个线程的并发，10个任务的等待。在3.0以后，无论有多少任务，都会在其内部单线程执行；
至此，AsyncTask源码分析完毕，相信大家对AsyncTask有了更深的理解~~~
测试代码点击下载
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