【Android应用源码分析】IntentService 源码分析

IntentService是一个带线程的service，用于处理Intent类型的异步任务请求。当客户端调用startService(Intent)发送请求时，Service服务被启动，且在其内部构建一个工作线程来处理Intent请求。当工作线程执行结束，Service服务会自动停止。IntentService继承于Service，它最大的特点是对服务请求逐个进行处理。当我们要提供的服务不需要同时处理多个请求的时候，可以选择继承IntentService。IntentService是一个抽象类，用户必须实现一个子类去继承它，且必须至少要实现两个函数：构造函数和onHandleIntent()函数。

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IntentService源码分析

/*
 * 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.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;
/*ServiceHandler是IntentService的内部类，继承自Handler，在重写消息处理方法handlerMessage里面调用了onHandlerIntent
抽象方法去处理异步任务intent的请求，当异步任务请求结束之后，调用stopSelf方法自动结束IntentService服务。此处handleMessage
方法是在工作线程中调用的，因此我们子类重写的onHandlerIntent
也是在工作线程中实现的。
*/
    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.
     */
     //IntentService构造方法,参数name用于定义工作线程的名称
    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;
    }
    //onCreate方法
    @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.
        //利用HandlerThread类创建了一个循环的工作线程thread，然后将工作线程中的Looper对象作为参数来创建
        ServiceHandler消息执行者。
        super.onCreate();
        HandlerThread thread = new HandlerThread("IntentService[" + mName + "]");
        thread.start();

        mServiceLooper = thread.getLooper();
        mServiceHandler = new ServiceHandler(mServiceLooper);
    }
    /*该方法中通过mServiceHandler获得一个消息对象msg，然后将startId作为该消息的消息码，将异步任务请求intent作为
    消息内容封装成一个消息msg发送到mServiceHandler消息执行者中去处理.*/
    @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
     */
     //Service服务生命周期第二步执行onStartCommand方法。
    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        onStart(intent, startId);
        return mRedelivery ? START_REDELIVER_INTENT : START_NOT_STICKY;
    }
/*  该方法中调用HandlerThread工作线程中Looper对象的quit方法让当前工作线程HandlerThread退出当前Looper循环，进而结束线程。
进而结束当前IntentService服务。
*/
    @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)}.
     */
     /* 该方法用于处理intent异步任务请求，在工作线程中调用该方法。每一个时刻只能处理一个intent请求，当同时又多个intent请求时，
     也就是客户端下一个intent请求。直到所有的intent请求结束之后，IntentService服务会调用stopSelf停止当前服务。
     也就是当intent异步任务处理结束之后，对应的IntentService服务会自动销毁
*/
    protected abstract void onHandleIntent(Intent intent);
}

可以看出来，IntentService的核心就是HandlerThread源码分析，HandlerThread+Handler构建成了一个带有消息循环机制的异步任务处理机制。只要搞明白了HandlerThread，自然而然就明白IntentService了。

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IntentService总结

IntentService有以下特点：
（1） 它创建了一个独立的工作线程来处理所有的通过onStartCommand()传递给服务的intents。
（2） 创建了一个工作队列，来逐个发送intent给onHandleIntent()。
（3） 不需要主动调用stopSelft()来结束服务。因为，在所有的intent被处理完后，系统会自动关闭服务。
（4） 默认实现的onBind()返回null
（5） 默认实现的onStartCommand()的目的是将intent插入到工作队列中。
只要我们的Service继承IntentService，实现onHandleIntent()就可以工作在非主线程，而且还不用担心并发，不用担心关闭service等问题。
IntentService类内部利用HandlerThread+Handler构建了一个带有消息循环处理机制的后台工作线程，这是一个单线程来处理异步任务。客户端只需调用startService(Intent)将Intent任务请求放入后台工作队列中。只要当前IntentService服务没有被销毁，客户端就可以同时投放多个Intent异步任务请求，IntentService服务端这边是顺序执行当前后台工作队列中的Intent请求的，也就是每一时刻只能执行一个Intent请求，直到该Intent处理结束才处理下一个Intent。