AsyncTask（一）AsyncTask的使用
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前言：
虽然现在RxJava非常流行，但是还是觉得应该分析一下Android的原生异步线程实现。
打算用2篇文章来过一下这个知识点。其中第一部分主要为使用例子，第二部分从源码的角度来验证那样使用的原因。
第一篇：
第二篇
AsyncTask。顾名思义，异步的任务。
android.os.AsyncTask是一个抽象类，其类定义如下：
public abstract class AsyncTask<Params, Progress, Result>其中Params对应传入参数，Progress对应执行进度参数，Result对应执行结果参数。需要一一对应。
doInBackground的参数和Params一致，返回结果和Result一致；
onProgressUpdate的参数和Progress一致；
onPostExecute的参数和Result一致。
一个例子如下：
private class AsyncTaskDemo extends AsyncTask<HashMap<String,String>, Integer, String> { @Override protected String doInBackground(HashMap<String,String>... params) { return null; } @Override protected void onProgressUpdate(Integer...progress){ } @Override protected void onPostExecute(String result) { super.onPostExecute(result); } }它有 3个启动入口:
第一个用于串行执行；
第二个用于自定义执行，可以传入AsyncTask.THREAD_POOL_EXECUTOR进行并行计算；第三用于执行制定Runnable。
它提供了5个生命周期方法：
其顺序为：
onPreExecute -> doInBackGround -> onPostExecute
publishProgress(需要手动调用，也就是说进度是自己计算的) -> onProgressUpdateonCacelled
其中，doInBackGround是个抽象方法，是AsyncTask实现类中必须要实现的。
此外，doInBackGround不在UIThread中执行，不能直接操作UI。
串行执行结果：
11-03 15:47:11.285 14236-14236/? D/thm: AsyncTask1:onPreExecute11-03 15:47:11.285 14236-14236/? D/thm: AsyncTask2:onPreExecute11-03 15:47:11.285 14236-14236/? D/thm: AsyncTask3:onPreExecute11-03 15:47:11.285 14236-14236/? D/thm: AsyncTask4:onPreExecute11-03 15:47:11.310 14236-14236/? D/thm: AsyncTask1:2011-03 15:47:11.335 14236-14236/? D/thm: AsyncTask1:4011-03 15:47:11.360 14236-14236/? D/thm: AsyncTask1:6011-03 15:47:11.385 14236-14236/? D/thm: AsyncTask1:8011-03 15:47:11.415 14236-14236/? D/thm: AsyncTask1:10011-03 15:47:11.415 14236-14236/? D/thm: AsyncTask1: is finished11-03 15:47:11.445 14236-14236/? D/thm: AsyncTask2:2011-03 15:47:11.470 14236-14236/? D/thm: AsyncTask2:4011-03 15:47:11.495 14236-14236/? D/thm: AsyncTask2:6011-03 15:47:11.525 14236-14236/? D/thm: AsyncTask2:8011-03 15:47:11.550 14236-14236/? D/thm: AsyncTask2:10011-03 15:47:11.550 14236-14236/? D/thm: AsyncTask2: is finished11-03 15:47:11.575 14236-14236/? D/thm: AsyncTask3:2011-03 15:47:11.605 14236-14236/? D/thm: AsyncTask3:4011-03 15:47:11.630 14236-14236/? D/thm: AsyncTask3:6011-03 15:47:11.660 14236-14236/? D/thm: AsyncTask3:8011-03 15:47:11.685 14236-14236/? D/thm: AsyncTask3:10011-03 15:47:11.685 14236-14236/? D/thm: AsyncTask3: is finished11-03 15:47:11.720 14236-14236/? D/thm: AsyncTask4:2011-03 15:47:11.745 14236-14236/? D/thm: AsyncTask4:4011-03 15:47:11.770 14236-14236/? D/thm: AsyncTask4:6011-03 15:47:11.800 14236-14236/? D/thm: AsyncTask4:8011-03 15:47:11.825 14236-14236/? D/thm: AsyncTask4:10011-03 15:47:11.825 14236-14236/? D/thm: AsyncTask4: is finished
并行执行结果：
11-03 15:47:19.620 14236-14236/? D/thm: AsyncTask1:onPreExecute11-03 15:47:19.625 14236-14236/? D/thm: AsyncTask2:onPreExecute11-03 15:47:19.625 14236-14236/? D/thm: AsyncTask3:onPreExecute11-03 15:47:19.635 14236-14236/? D/thm: AsyncTask4:onPreExecute11-03 15:47:19.655 14236-14236/? D/thm: AsyncTask2:2011-03 15:47:19.655 14236-14236/? D/thm: AsyncTask1:2011-03 15:47:19.660 14236-14236/? D/thm: AsyncTask3:2011-03 15:47:19.680 14236-14236/? D/thm: AsyncTask2:4011-03 15:47:19.680 14236-14236/? D/thm: AsyncTask1:4011-03 15:47:19.680 14236-14236/? D/thm: AsyncTask4:2011-03 15:47:19.685 14236-14236/? D/thm: AsyncTask3:4011-03 15:47:19.705 14236-14236/? D/thm: AsyncTask2:6011-03 15:47:19.705 14236-14236/? D/thm: AsyncTask1:6011-03 15:47:19.710 14236-14236/? D/thm: AsyncTask3:6011-03 15:47:19.715 14236-14236/? D/thm: AsyncTask4:4011-03 15:47:19.730 14236-14236/? D/thm: AsyncTask2:8011-03 15:47:19.735 14236-14236/? D/thm: AsyncTask1:8011-03 15:47:19.735 14236-14236/? D/thm: AsyncTask3:8011-03 15:47:19.740 14236-14236/? D/thm: AsyncTask4:6011-03 15:47:19.755 14236-14236/? D/thm: AsyncTask2:10011-03 15:47:19.755 14236-14236/? D/thm: AsyncTask2: is finished11-03 15:47:19.760 14236-14236/? D/thm: AsyncTask1:10011-03 15:47:19.760 14236-14236/? D/thm: AsyncTask1: is finished11-03 15:47:19.765 14236-14236/? D/thm: AsyncTask4:8011-03 15:47:19.765 14236-14236/? D/thm: AsyncTask3:10011-03 15:47:19.765 14236-14236/? D/thm: AsyncTask3: is finished11-03 15:47:19.790 14236-14236/? D/thm: AsyncTask4:10011-03 15:47:19.790 14236-14236/? D/thm: AsyncTask4: is finished看时间，显然并行执行消耗的总时间远低于串行执行。
但并行执行有其不安全性，即之前的数据可能会覆盖后面的数据。
比如以下MyAsyncTask2 ,同时启动4个Task： static int i =0; private class MyAsyncTask2 extends AsyncTask<Void, Integer, Void> { private String mName = "AsyncTask"; public MyAsyncTask2(int id) { super(); mName += id; } @Override protected void onPreExecute() { Log.d("thm",mName+":onPreExecute"); } @Override protected Void doInBackground(Void... voids) { for(;i<100;){ try { Thread.sleep(25); } catch (Exception e) { e.printStackTrace(); }// Log.d("thm",mName+":"+i); publishProgress(++i); } return null; } @Override protected void onProgressUpdate(Integer...progress){ Log.d("thm", mName+":"+progress[0]); } @Override protected void onPostExecute(Void aVoid) { super.onPostExecute(aVoid); Log.d("thm", mName+":"+" is finished"); } }运行结果如下，显然，这里执行了103次，而我们的预期目标是100次：11-03 16:21:09.499 31561-31561/? D/thm: AsyncTask1:onPreExecute11-03 16:21:09.504 31561-31561/? D/thm: AsyncTask2:onPreExecute11-03 16:21:09.509 31561-31561/? D/thm: AsyncTask3:onPreExecute11-03 16:21:09.509 31561-31561/? D/thm: AsyncTask4:onPreExecute11-03 16:21:09.534 31561-31561/? D/thm: AsyncTask1:111-03 16:21:09.534 31561-31561/? D/thm: AsyncTask2:211-03 16:21:09.534 31561-31561/? D/thm: AsyncTask3:311-03 16:21:09.534 31561-31561/? D/thm: AsyncTask4:411-03 16:21:09.559 31561-31561/? D/thm: AsyncTask2:511-03 16:21:09.559 31561-31561/? D/thm: AsyncTask1:611-03 16:21:09.559 31561-31561/? D/thm: AsyncTask3:711-03 16:21:09.564 31561-31561/? D/thm: AsyncTask4:811-03 16:21:09.584 31561-31561/? D/thm: AsyncTask1:1011-03 16:21:09.584 31561-31561/? D/thm: AsyncTask3:1111-03 16:21:09.584 31561-31561/? D/thm: AsyncTask2:911-03 16:21:09.589 31561-31561/? D/thm: AsyncTask4:1211-03 16:21:09.609 31561-31561/? D/thm: AsyncTask1:1311-03 16:21:09.609 31561-31561/? D/thm: AsyncTask3:1411-03 16:21:09.609 31561-31561/? D/thm: AsyncTask2:1511-03 16:21:09.614 31561-31561/? D/thm: AsyncTask4:1611-03 16:21:09.634 31561-31561/? D/thm: AsyncTask1:1711-03 16:21:09.634 31561-31561/? D/thm: AsyncTask3:1811-03 16:21:09.634 31561-31561/? D/thm: AsyncTask2:1911-03 16:21:09.639 31561-31561/? D/thm: AsyncTask4:2011-03 16:21:09.659 31561-31561/? D/thm: AsyncTask1:2111-03 16:21:09.659 31561-31561/? D/thm: AsyncTask3:2211-03 16:21:09.659 31561-31561/? D/thm: AsyncTask2:2311-03 16:21:09.664 31561-31561/? D/thm: AsyncTask4:2411-03 16:21:09.684 31561-31561/? D/thm: AsyncTask1:2511-03 16:21:09.684 31561-31561/? D/thm: AsyncTask3:2611-03 16:21:09.684 31561-31561/? D/thm: AsyncTask2:2711-03 16:21:09.689 31561-31561/? D/thm: AsyncTask4:2811-03 16:21:09.709 31561-31561/? D/thm: AsyncTask3:3011-03 16:21:09.709 31561-31561/? D/thm: AsyncTask1:2911-03 16:21:09.709 31561-31561/? D/thm: AsyncTask2:3111-03 16:21:09.714 31561-31561/? D/thm: AsyncTask4:3211-03 16:21:09.734 31561-31561/? D/thm: AsyncTask1:3311-03 16:21:09.734 31561-31561/? D/thm: AsyncTask3:3511-03 16:21:09.734 31561-31561/? D/thm: AsyncTask2:3411-03 16:21:09.739 31561-31561/? D/thm: AsyncTask4:3611-03 16:21:09.759 31561-31561/? D/thm: AsyncTask1:3711-03 16:21:09.759 31561-31561/? D/thm: AsyncTask3:3811-03 16:21:09.764 31561-31561/? D/thm: AsyncTask2:3911-03 16:21:09.764 31561-31561/? D/thm: AsyncTask4:4011-03 16:21:09.789 31561-31561/? D/thm: AsyncTask3:4111-03 16:21:09.789 31561-31561/? D/thm: AsyncTask1:4211-03 16:21:09.789 31561-31561/? D/thm: AsyncTask4:4411-03 16:21:09.794 31561-31561/? D/thm: AsyncTask2:4311-03 16:21:09.814 31561-31561/? D/thm: AsyncTask3:4511-03 16:21:09.814 31561-31561/? D/thm: AsyncTask1:4611-03 16:21:09.814 31561-31561/? D/thm: AsyncTask4:4711-03 16:21:09.819 31561-31561/? D/thm: AsyncTask2:4811-03 16:21:09.839 31561-31561/? D/thm: AsyncTask1:4911-03 16:21:09.839 31561-31561/? D/thm: AsyncTask3:5011-03 16:21:09.844 31561-31561/? D/thm: AsyncTask2:5211-03 16:21:09.844 31561-31561/? D/thm: AsyncTask4:5111-03 16:21:09.864 31561-31561/? D/thm: AsyncTask1:5311-03 16:21:09.864 31561-31561/? D/thm: AsyncTask3:5411-03 16:21:09.869 31561-31561/? D/thm: AsyncTask2:5511-03 16:21:09.869 31561-31561/? D/thm: AsyncTask4:5611-03 16:21:09.894 31561-31561/? D/thm: AsyncTask1:5711-03 16:21:09.894 31561-31561/? D/thm: AsyncTask2:5811-03 16:21:09.899 31561-31561/? D/thm: AsyncTask3:5911-03 16:21:09.899 31561-31561/? D/thm: AsyncTask4:6011-03 16:21:09.924 31561-31561/? D/thm: AsyncTask1:6111-03 16:21:09.924 31561-31561/? D/thm: AsyncTask2:6211-03 16:21:09.924 31561-31561/? D/thm: AsyncTask3:6411-03 16:21:09.924 31561-31561/? D/thm: AsyncTask4:6311-03 16:21:09.949 31561-31561/? D/thm: AsyncTask1:6511-03 16:21:09.949 31561-31561/? D/thm: AsyncTask2:6611-03 16:21:09.949 31561-31561/? D/thm: AsyncTask3:6711-03 16:21:09.949 31561-31561/? D/thm: AsyncTask4:6811-03 16:21:09.974 31561-31561/? D/thm: AsyncTask1:6911-03 16:21:09.974 31561-31561/? D/thm: AsyncTask3:7011-03 16:21:09.974 31561-31561/? D/thm: AsyncTask4:7011-03 16:21:09.979 31561-31561/? D/thm: AsyncTask2:7111-03 16:21:09.999 31561-31561/? D/thm: AsyncTask1:7211-03 16:21:10.004 31561-31561/? D/thm: AsyncTask4:7411-03 16:21:10.004 31561-31561/? D/thm: AsyncTask3:7311-03 16:21:10.009 31561-31561/? D/thm: AsyncTask2:7511-03 16:21:10.024 31561-31561/? D/thm: AsyncTask1:7611-03 16:21:10.029 31561-31561/? D/thm: AsyncTask4:7711-03 16:21:10.039 31561-31561/? D/thm: AsyncTask3:7911-03 16:21:10.044 31561-31561/? D/thm: AsyncTask2:7811-03 16:21:10.049 31561-31561/? D/thm: AsyncTask1:8011-03 16:21:10.054 31561-31561/? D/thm: AsyncTask4:8111-03 16:21:10.064 31561-31561/? D/thm: AsyncTask3:8211-03 16:21:10.069 31561-31561/? D/thm: AsyncTask2:8311-03 16:21:10.079 31561-31561/? D/thm: AsyncTask1:8411-03 16:21:10.079 31561-31561/? D/thm: AsyncTask4:8511-03 16:21:10.094 31561-31561/? D/thm: AsyncTask3:8611-03 16:21:10.104 31561-31561/? D/thm: AsyncTask2:8711-03 16:21:10.114 31561-31561/? D/thm: AsyncTask4:8811-03 16:21:10.114 31561-31561/? D/thm: AsyncTask1:8911-03 16:21:10.119 31561-31561/? D/thm: AsyncTask3:9011-03 16:21:10.129 31561-31561/? D/thm: AsyncTask2:9111-03 16:21:10.139 31561-31561/? D/thm: AsyncTask4:9211-03 16:21:10.139 31561-31561/? D/thm: AsyncTask1:9311-03 16:21:10.144 31561-31561/? D/thm: AsyncTask3:9411-03 16:21:10.154 31561-31561/? D/thm: AsyncTask2:9511-03 16:21:10.169 31561-31561/? D/thm: AsyncTask3:9711-03 16:21:10.169 31561-31561/? D/thm: AsyncTask4:9611-03 16:21:10.169 31561-31561/? D/thm: AsyncTask1:9811-03 16:21:10.179 31561-31561/? D/thm: AsyncTask2:9911-03 16:21:10.219 31561-31561/? D/thm: AsyncTask2:10011-03 16:21:10.219 31561-31561/? D/thm: AsyncTask2: is finished11-03 16:21:10.219 31561-31561/? D/thm: AsyncTask1:10111-03 16:21:10.219 31561-31561/? D/thm: AsyncTask1: is finished11-03 16:21:10.219 31561-31561/? D/thm: AsyncTask4:10211-03 16:21:10.219 31561-31561/? D/thm: AsyncTask4: is finished11-03 16:21:10.219 31561-31561/? D/thm: AsyncTask3:10311-03 16:21:10.219 31561-31561/? D/thm: AsyncTask3: is finished两种execute方式各有所长，具体使用那种还需依据实际情况来做。
当然现在的主流已经不再是AsyncTask了，比如现在很火爆的异步框架RxJava，采用观察者+异步的方式，且书写起来十分简便。
除去上面所提到的安全性，使用AsyncTask还需注意下面这些：
1.生命周期AsyncTask不会随着Activity的销毁而销毁，它会一直执行直到doInBackground()方法执行完毕，然后执行onPostExecute/onCancelled。
如果我们的Activity销毁之前，没有取消AsyncTask，这有可能让我们的AsyncTask崩溃(crash)。因为它想要处理的view已经不在了。所以，我们总是必须确保在销毁活动之前取消任务。2.内存泄漏如果AsyncTask被声明为Activity的非静态的内部类，那么AsyncTask会保留一个对Activity的引用。如果Activity已经被销毁，AsyncTask的后台线程还在执行，它将继续在内存里保留这个引用，导致Activity无法被回收，引起内存泄漏。3.结果丢失屏幕旋转或Activity在后台被系统杀掉等情况会导致Activity的重新创建，之前运行的AsyncTask会持有一个之前Activity的引用，这个引用已经无效，这时调用onPostExecute()再去更新界面将不再生效。总之，如果在onPostExecute()中执行UI操作的话，必须在Activity的onDestroy()中做处理。4.AsyncTask的类必须在UI线程加载（从4.1开始系统会帮我们自动完成），AsyncTask对象必须在UI线程创建5.execute方法必须在UI线程调用6.不要在你的程序中去直接调用onPreExecute(), onPostExecute, doInBackground, onProgressUpdate方法7.一个AsyncTask对象只能执行一次，即只能调用一次execute方法，否则会报运行时异常8.AsyncTask不是被设计为处理耗时操作的，耗时上限为几秒钟，如果要做长耗时操作，强烈建议你使用Executor，ThreadPoolExecutor以及FutureTask官方文档如下：
- The AsyncTask class must be loaded on the UI thread. This is done automatically as of
JELLY_BEAN.- The task instance must be created on the UI thread.
execute(Params...)must be invoked on the UI thread.- Do not call
onPreExecute(),onPostExecute(Result),doInBackground(Params...),onProgressUpdate(Progress...)manually.- The task can be executed only once (an exception will be thrown if a second execution is attempted.)
测试代码比较少，这里直接附上：package tian.hangmin.com.asyntaskdemo;import android.app.Activity;import android.content.Context;import android.os.AsyncTask;import android.os.Bundle;import android.util.Log;import android.view.View;import java.util.ArrayList;import java.util.HashMap;import java.util.Map;/** * Created by tianhm on 2016/11/3. */public class AsynTaskActivity extends Activity { @Override protected void onCreate(Bundle onSavedInstanceState) { super.onCreate(onSavedInstanceState); setContentView(R.layout.activity_asyntask_demo); findViewById(R.id.button1).setOnClickListener(listener); findViewById(R.id.button2).setOnClickListener(listener); } View.OnClickListener listener = new View.OnClickListener() { @Override public void onClick(View v) { start(v.getId()); } }; protected void start(int id){ if(id == R.id.button1){ new MyAsyncTask(1).execute(); new MyAsyncTask(2).execute(); new MyAsyncTask(3).execute(); new MyAsyncTask(4).execute(); } else if(id == R.id.button2){ new MyAsyncTask(1).executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR , new Void[1]); new MyAsyncTask(2).executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR , new Void[1]); new MyAsyncTask(3).executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR , new Void[1]); new MyAsyncTask(4).executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR , new Void[1]); } else if( i == R.id.button3){ new MyAsyncTask2(1).executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR , new Void[1]); new MyAsyncTask2(2).executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR , new Void[1]); new MyAsyncTask2(3).executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR , new Void[1]); new MyAsyncTask2(4).executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR , new Void[1]); } } private class MyAsyncTask extends AsyncTask<Void, Integer, Void> { private String mName = "AsyncTask"; public MyAsyncTask(int id) { super(); mName += id; } @Override protected void onPreExecute() { Log.d("thm",mName+":onPreExecute"); } @Override protected Void doInBackground(Void... voids) { for(int i =0;i<100;){ try { Thread.sleep(25); } catch (Exception e) { e.printStackTrace(); }// Log.d("thm",mName+":"+i); publishProgress(++i*100/5); } return null; } @Override protected void onProgressUpdate(Integer...progress){ Log.d("thm", mName+":"+progress[0]); } @Override protected void onPostExecute(Void aVoid) { super.onPostExecute(aVoid); Log.d("thm", mName+":"+" is finished"); } } static int i =0; private class MyAsyncTask2 extends AsyncTask<Void, Integer, Void> { private String mName = "AsyncTask"; public MyAsyncTask2(int id) { super(); mName += id; } @Override protected void onPreExecute() { Log.d("thm",mName+":onPreExecute"); } @Override protected Void doInBackground(Void... voids) { for(;i<100;){ try { Thread.sleep(25); } catch (Exception e) { e.printStackTrace(); }// Log.d("thm",mName+":"+i); publishProgress(++i); } return null; } @Override protected void onProgressUpdate(Integer...progress){ Log.d("thm", mName+":"+progress[0]); } @Override protected void onPostExecute(Void aVoid) { super.onPostExecute(aVoid); Log.d("thm", mName+":"+" is finished"); } }}
xml<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="match_parent" android:layout_height="match_parent" android:orientation="vertical"> <Button android:id="@+id/button1" android:layout_marginTop="8dp" android:text="串行开始" android:layout_gravity="center" android:layout_width="wrap_content" android:layout_height="wrap_content" /> <Button android:id="@+id/button2" android:layout_marginTop="8dp" android:text="并行开始" android:layout_gravity="center" android:layout_width="wrap_content" android:layout_height="wrap_content" /> <Button android:id="@+id/button3" android:layout_marginTop="8dp" android:text="并行不安全" android:layout_gravity="center" android:layout_width="wrap_content" android:layout_height="wrap_content" /></LinearLayout>
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