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出自【zejian的博客】

关联文章:
Android 多线程之HandlerThread 完全详解
Android 多线程之IntentService 完全详解
android多线程-AsyncTask之工作原理深入解析(上)
android多线程-AsyncTask之工作原理深入解析(下)

IntentService

一、IntentService概述

上一篇我们聊到了HandlerThread，本篇我们就来看看HandlerThread在IntentService中的应用，看本篇前建议先看看上篇的HandlerThread，有助于我们更好掌握IntentService。同样地，我们先来看看IntentService的特点：

它本质是一种特殊的Service,继承自Service并且本身就是一个抽象类
它可以用于在后台执行耗时的异步任务，当任务完成后会自动停止
它拥有较高的优先级，不易被系统杀死（继承自Service的缘故），因此比较适合执行一些高优先级的异步任务
它内部通过HandlerThread和Handler实现异步操作
创建IntentService时，只需实现onHandleIntent和构造方法，onHandleIntent为异步方法，可以执行耗时操作

二、IntentService的常规使用套路

大概了解了IntentService的特点后，我们就来了解一下它的使用方式，先看个案例：
IntentService实现类如下：

package com.zejian.handlerlooper;import android.app.IntentService;import android.content.Intent;import android.graphics.Bitmap;import android.graphics.BitmapFactory;import android.os.IBinder;import android.os.Message;import com.zejian.handlerlooper.util.LogUtils;import java.io.BufferedInputStream;import java.io.IOException;import java.net.HttpURLConnection;import java.net.URL;/** * Created by zejian * Time 16/9/3. * Description: */public  class MyIntentService extends IntentService {    public static final String DOWNLOAD_URL="download_url";    public static final String INDEX_FLAG="index_flag";    public static UpdateUI updateUI;    public static void setUpdateUI(UpdateUI updateUIInterface){        updateUI=updateUIInterface;    }    public MyIntentService(){        super("MyIntentService");    }    /**     * 实现异步任务的方法     * @param intent Activity传递过来的Intent,数据封装在intent中     */    @Override    protected void onHandleIntent(Intent intent) {        //在子线程中进行网络请求        Bitmap bitmap=downloadUrlBitmap(intent.getStringExtra(DOWNLOAD_URL));        Message msg1 = new Message();        msg1.what = intent.getIntExtra(INDEX_FLAG,0);        msg1.obj =bitmap;        //通知主线程去更新UI        if(updateUI!=null){            updateUI.updateUI(msg1);        }        //mUIHandler.sendMessageDelayed(msg1,1000);        LogUtils.e("onHandleIntent");    }    //----------------------重写一下方法仅为测试------------------------------------------    @Override    public void onCreate() {        LogUtils.e("onCreate");        super.onCreate();    }    @Override    public void onStart(Intent intent, int startId) {        super.onStart(intent, startId);        LogUtils.e("onStart");    }    @Override    public int onStartCommand(Intent intent, int flags, int startId) {        LogUtils.e("onStartCommand");        return super.onStartCommand(intent, flags, startId);    }    @Override    public void onDestroy() {        LogUtils.e("onDestroy");        super.onDestroy();    }    @Override    public IBinder onBind(Intent intent) {        LogUtils.e("onBind");        return super.onBind(intent);    }    public interface UpdateUI{        void updateUI(Message message);    }    private Bitmap downloadUrlBitmap(String urlString) {        HttpURLConnection urlConnection = null;        BufferedInputStream in = null;        Bitmap bitmap=null;        try {            final URL url = new URL(urlString);            urlConnection = (HttpURLConnection) url.openConnection();            in = new BufferedInputStream(urlConnection.getInputStream(), 8 * 1024);            bitmap= BitmapFactory.decodeStream(in);        } catch (final IOException e) {            e.printStackTrace();        } finally {            if (urlConnection != null) {                urlConnection.disconnect();            }            try {                if (in != null) {                    in.close();                }            } catch (final IOException e) {                e.printStackTrace();            }        }        return bitmap;    }}

通过代码可以看出，我们继承了IntentService，这里有两个方法是必须实现的，一个是构造方法，必须传递一个线程名称的字符串，另外一个就是进行异步处理的方法onHandleIntent(Intent intent) 方法，其参数intent可以附带从activity传递过来的数据。这里我们的案例主要利用onHandleIntent实现异步下载图片，然后通过回调监听的方法把下载完的bitmap放在message中回调给Activity（当然也可以使用广播完成），最后通过Handler去更新UI。下面再来看看Acitvity的代码：

activity_intent_service.xml

<?xml version="1.0" encoding="utf-8"?><LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"    android:orientation="vertical" android:layout_width="match_parent"    android:layout_height="match_parent">    <ImageView        android:id="@+id/image"        android:layout_width="match_parent"        android:layout_height="match_parent" />LinearLayout>

IntentServiceActivity.java

package com.zejian.handlerlooper.util;import android.app.Activity;import android.content.Intent;import android.graphics.Bitmap;import android.os.Bundle;import android.os.Handler;import android.os.Message;import android.widget.ImageView;import com.zejian.handlerlooper.MyIntentService;import com.zejian.handlerlooper.R;/** * Created by zejian * Time 16/9/3. * Description: */public class IntentServiceActivity extends Activity implements MyIntentService.UpdateUI{    /**     * 图片地址集合     */    private String url[] = {            "https://img-blog.csdn.net/20160903083245762",            "https://img-blog.csdn.net/20160903083252184",            "https://img-blog.csdn.net/20160903083257871",            "https://img-blog.csdn.net/20160903083257871",            "https://img-blog.csdn.net/20160903083311972",            "https://img-blog.csdn.net/20160903083319668",            "https://img-blog.csdn.net/20160903083326871"    };    private static ImageView imageView;    private static final Handler mUIHandler = new Handler() {        @Override        public void handleMessage(Message msg) {            imageView.setImageBitmap((Bitmap) msg.obj);        }    };    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_intent_service);        imageView = (ImageView) findViewById(R.id.image);        Intent intent = new Intent(this,MyIntentService.class);        for (int i=0;i<7;i++) {//循环启动任务            intent.putExtra(MyIntentService.DOWNLOAD_URL,url[i]);            intent.putExtra(MyIntentService.INDEX_FLAG,i);            startService(intent);        }        MyIntentService.setUpdateUI(this);    }    //必须通过Handler去更新，该方法为异步方法，不可更新UI    @Override    public void updateUI(Message message) {        mUIHandler.sendMessageDelayed(message,message.what * 1000);    }}

代码比较简单，通过for循环多次去启动IntentService，然后去下载图片，注意即使我们多次启动IntentService，但IntentService的实例只有一个，这跟传统的Service是一样的，最终IntentService会去调用onHandleIntent执行异步任务。这里可能我们还会担心for循环去启动任务，而实例又只有一个，那么任务会不会被覆盖掉呢？其实是不会的，因为IntentService真正执行异步任务的是HandlerThread+Handler，每次启动都会把下载图片的任务添加到依附的消息队列中，最后由HandlerThread+Handler去执行。好~，我们运行一下代码：

每间隔一秒去更新图片，接着我们看一组log：

从Log可以看出onCreate只启动了一次，而onStartCommand和onStart多次启动，这就证实了之前所说的，启动多次，但IntentService的实例只有一个，这跟传统的Service是一样的，最后任务都执行完成后，IntentService自动销毁。以上便是IntentService德使用方式，怎么样，比较简单吧。接着我们就来分析一下IntentService的源码，其实也比较简单只有100多行代码。

三、IntentService源码解析

我们先来看看IntentService的onCreate方法：

@Overridepublic void onCreate() {   // TODO: It would be nice to have an option to hold a partial wakelock   // during processing, and to have a static startService(Context, Intent)   // method that would launch the service & hand off a wakelock.   super.onCreate();   HandlerThread thread = new HandlerThread("IntentService[" + mName + "]");   thread.start();   mServiceLooper = thread.getLooper();   mServiceHandler = new ServiceHandler(mServiceLooper);}

当第一启动IntentService时，它的onCreate方法将会被调用，其内部会去创建一个HandlerThread并启动它，接着创建一个ServiceHandler（继承Handler），传入HandlerThread的Looper对象，这样ServiceHandler就变成可以处理异步线程的执行类了（因为Looper对象与HandlerThread绑定，而HandlerThread又是一个异步线程，我们把HandlerThread持有的Looper对象传递给Handler后，ServiceHandler内部就持有异步线程的Looper，自然就可以执行异步任务了），那么IntentService是怎么启动异步任务的呢？其实IntentService启动后还会去调用onStartCommand方法，而onStartCommand方法又会去调用onStart方法，我们看看它们的源码：

@Overridepublic void onStart(Intent intent, int startId) {    Message msg = mServiceHandler.obtainMessage();    msg.arg1 = startId;    msg.obj = intent;    mServiceHandler.sendMessage(msg);}/** * You should not override this method for your IntentService. Instead, * override {@link #onHandleIntent}, which the system calls when the IntentService * receives a start request. * @see android.app.Service#onStartCommand */@Overridepublic int onStartCommand(Intent intent, int flags, int startId) {    onStart(intent, startId);    return mRedelivery ? START_REDELIVER_INTENT : START_NOT_STICKY;}

从源码我们可以看出，在onStart方法中，IntentService通过mServiceHandler的sendMessage方法发送了一个消息，这个消息将会发送到HandlerThread中进行处理（因为HandlerThread持有Looper对象，所以其实是Looper从消息队列中取出消息进行处理，然后调用mServiceHandler的handleMessage方法），我们看看ServiceHandler的源码：

private final class ServiceHandler extends Handler {   public ServiceHandler(Looper looper) {       super(looper);   }   @Override   public void handleMessage(Message msg) {       onHandleIntent((Intent)msg.obj);       stopSelf(msg.arg1);   }}

这里其实也说明onHandleIntent确实是一个异步处理方法（ServiceHandler本身就是一个异步处理的handler类），在onHandleIntent方法执行结束后，IntentService会通过 stopSelf(int startId)方法来尝试停止服务。这里采用stopSelf(int startId)而不是stopSelf()来停止服务，是因为stopSelf()会立即停止服务，而stopSelf(int startId)会等待所有消息都处理完后才终止服务。最后看看onHandleIntent方法的声明：

protected abstract void onHandleIntent(Intent intent);

到此我们就知道了IntentService的onHandleIntent方法是一个抽象方法，所以我们在创建IntentService时必须实现该方法，通过上面一系列的分析可知，onHandleIntent方法也是一个异步方法。这里要注意的是如果后台任务只有一个的话，onHandleIntent执行完，服务就会销毁，但如果后台任务有多个的话，onHandleIntent执行完最后一个任务时，服务才销毁。最后我们要知道每次执行一个后台任务就必须启动一次IntentService，而IntentService内部则是通过消息的方式发送给HandlerThread的，然后由Handler中的Looper来处理消息，而Looper是按顺序从消息队列中取任务的，也就是说IntentService的后台任务时顺序执行的，当有多个后台任务同时存在时，这些后台任务会按外部调用的顺序排队执行，我们前面的使用案例也很好说明了这点。最后贴一下到IntentService的全部源码，大家再次感受一下：

/* * 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.app;import android.annotation.WorkerThread;import android.content.Intent;import android.os.Handler;import android.os.HandlerThread;import android.os.IBinder;import android.os.Looper;import android.os.Message;/** * IntentService is a base class for {@link Service}s that handle asynchronous * requests (expressed as {@link Intent}s) on demand.  Clients send requests * through {@link android.content.Context#startService(Intent)} calls; the * service is started as needed, handles each Intent in turn using a worker * thread, and stops itself when it runs out of work. * * This "work queue processor" pattern is commonly used to offload tasks * from an application's main thread.  The IntentService class exists to * simplify this pattern and take care of the mechanics.  To use it, extend * IntentService and implement {@link #onHandleIntent(Intent)}.  IntentService * will receive the Intents, launch a worker thread, and stop the service as * appropriate. * * 
All requests are handled on a single worker thread -- they may take as * long as necessary (and will not block the application's main loop), but * only one request will be processed at a time. * *  * 
Developer Guides
 * For a detailed discussion about how to create services, read the * Services developer guide.
 *  * * @see android.os.AsyncTask */public abstract class IntentService extends Service {    private volatile Looper mServiceLooper;    private volatile ServiceHandler mServiceHandler;    private String mName;    private boolean mRedelivery;    private final class ServiceHandler extends Handler {        public ServiceHandler(Looper looper) {            super(looper);        }        @Override        public void handleMessage(Message msg) {            onHandleIntent((Intent)msg.obj);            stopSelf(msg.arg1);        }    }    /**     * Creates an IntentService.  Invoked by your subclass's constructor.     *     * @param name Used to name the worker thread, important only for debugging.     */    public IntentService(String name) {        super();        mName = name;    }    /**     * Sets intent redelivery preferences.  Usually called from the constructor     * with your preferred semantics.     *     * If enabled is true,     * {@link #onStartCommand(Intent, int, int)} will return     * {@link Service#START_REDELIVER_INTENT}, so if this process dies before     * {@link #onHandleIntent(Intent)} returns, the process will be restarted     * and the intent redelivered.  If multiple Intents have been sent, only     * the most recent one is guaranteed to be redelivered.     *     * If enabled is false (the default),     * {@link #onStartCommand(Intent, int, int)} will return     * {@link Service#START_NOT_STICKY}, and if the process dies, the Intent     * dies along with it.     */    public void setIntentRedelivery(boolean enabled) {        mRedelivery = enabled;    }    @Override    public void onCreate() {        // TODO: It would be nice to have an option to hold a partial wakelock        // during processing, and to have a static startService(Context, Intent)        // method that would launch the service & hand off a wakelock.        super.onCreate();        HandlerThread thread = new HandlerThread("IntentService[" + mName + "]");        thread.start();        mServiceLooper = thread.getLooper();        mServiceHandler = new ServiceHandler(mServiceLooper);    }    @Override    public void onStart(Intent intent, int startId) {        Message msg = mServiceHandler.obtainMessage();        msg.arg1 = startId;        msg.obj = intent;        mServiceHandler.sendMessage(msg);    }    /**     * You should not override this method for your IntentService. Instead,     * override {@link #onHandleIntent}, which the system calls when the IntentService     * receives a start request.     * @see android.app.Service#onStartCommand     */    @Override    public int onStartCommand(Intent intent, int flags, int startId) {        onStart(intent, startId);        return mRedelivery ? START_REDELIVER_INTENT : START_NOT_STICKY;    }    @Override    public void onDestroy() {        mServiceLooper.quit();    }    /**     * Unless you provide binding for your service, you don't need to implement this     * method, because the default implementation returns null.      * @see android.app.Service#onBind     */    @Override    public IBinder onBind(Intent intent) {        return null;    }    /**     * This method is invoked on the worker thread with a request to process.     * Only one Intent is processed at a time, but the processing happens on a     * worker thread that runs independently from other application logic.     * So, if this code takes a long time, it will hold up other requests to     * the same IntentService, but it will not hold up anything else.     * When all requests have been handled, the IntentService stops itself,     * so you should not call {@link #stopSelf}.     *     * @param intent The value passed to {@link     *               android.content.Context#startService(Intent)}.     */    @WorkerThread    protected abstract void onHandleIntent(Intent intent);}

此IntentService的源码就分析完了，嗯，本篇完结。

Android 多线程之HandlerThread 完全详解
Android 多线程之IntentService 完全详解
android多线程-AsyncTask之工作原理深入解析(上)
android多线程-AsyncTask之工作原理深入解析(下)

Android(安卓)多线程之IntentService 完全详解

IntentService

一、IntentService概述

二、IntentService的常规使用套路

三、IntentService源码解析

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