【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。

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.
*
* <p>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.
*
* <p>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.
*
* <div class="special reference">
* <h3>Developer Guides</h3>
* <p>For a detailed discussion about how to create services, read the
* <a href="{@docRoot}guide/topics/fundamentals/services.html">Services</a> developer guide.</p>
* </div>
*
* @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.
*
* <p>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.
*
* <p>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。

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。