【Android應用源碼分析】IntentService 源碼分析

IntentService是一個帶線程的service，用于處理Intent類型的異步任務請求。當用戶端調用startService(Intent)發送請求時，Service服務被啟動，且在其内部建構一個工作線程來處理Intent請求。當工作線程執行結束，Service服務會自動停止。IntentService繼承于Service，它最大的特點是對服務請求逐個進行處理。當我們要提供的服務不需要同時處理多個請求的時候，可以選擇繼承IntentService。IntentService是一個抽象類，使用者必須實作一個子類去繼承它，且必須至少要實作兩個函數：構造函數和onHandleIntent()函數。

IntentService源碼分析

/*
 * 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" target="_blank" rel="external nofollow" >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的内部類，繼承自Handler，在重寫消息處理方法handlerMessage裡面調用了onHandlerIntent
抽象方法去處理異步任務intent的請求，當異步任務請求結束之後，調用stopSelf方法自動結束IntentService服務。此處handleMessage
方法是在工作線程中調用的，是以我們子類重寫的onHandlerIntent
也是在工作線程中實作的。
*/
    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構造方法,參數name用于定義工作線程的名稱
    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類建立了一個循環的工作線程thread，然後将工作線程中的Looper對象作為參數來建立
        ServiceHandler消息執行者。
        super.onCreate();
        HandlerThread thread = new HandlerThread("IntentService[" + mName + "]");
        thread.start();

        mServiceLooper = thread.getLooper();
        mServiceHandler = new ServiceHandler(mServiceLooper);
    }
    /*該方法中通過mServiceHandler獲得一個消息對象msg，然後将startId作為該消息的消息碼，将異步任務請求intent作為
    消息内容封裝成一個消息msg發送到mServiceHandler消息執行者中去處理.*/
    @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服務生命周期第二步執行onStartCommand方法。
    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        onStart(intent, startId);
        return mRedelivery ? START_REDELIVER_INTENT : START_NOT_STICKY;
    }
/*  該方法中調用HandlerThread工作線程中Looper對象的quit方法讓目前工作線程HandlerThread退出目前Looper循環，進而結束線程。
進而結束目前IntentService服務。
*/
    @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)}.
     */
     /* 該方法用于處理intent異步任務請求，在工作線程中調用該方法。每一個時刻隻能處理一個intent請求，當同時又多個intent請求時，
     也就是用戶端下一個intent請求。直到所有的intent請求結束之後，IntentService服務會調用stopSelf停止目前服務。
     也就是當intent異步任務處理結束之後，對應的IntentService服務會自動銷毀
*/
    protected abstract void onHandleIntent(Intent intent);
}
           

可以看出來，IntentService的核心就是HandlerThread源碼分析，HandlerThread+Handler建構成了一個帶有消息循環機制的異步任務處理機制。隻要搞明白了HandlerThread，自然而然就明白IntentService了。

IntentService總結

IntentService有以下特點：

（1） 它建立了一個獨立的工作線程來處理所有的通過onStartCommand()傳遞給服務的intents。

（2） 建立了一個工作隊列，來逐個發送intent給onHandleIntent()。

（3） 不需要主動調用stopSelft()來結束服務。因為，在所有的intent被處理完後，系統會自動關閉服務。

（4） 預設實作的onBind()傳回null

（5） 預設實作的onStartCommand()的目的是将intent插入到工作隊列中。

隻要我們的Service繼承IntentService，實作onHandleIntent()就可以工作在非主線程，而且還不用擔心并發，不用擔心關閉service等問題。

IntentService類内部利用HandlerThread+Handler建構了一個帶有消息循環處理機制的背景工作線程，這是一個單線程來處理異步任務。用戶端隻需調用startService(Intent)将Intent任務請求放入背景工作隊列中。隻要目前IntentService服務沒有被銷毀，用戶端就可以同時投放多個Intent異步任務請求，IntentService服務端這邊是順序執行目前背景工作隊列中的Intent請求的，也就是每一時刻隻能執行一個Intent請求，直到該Intent處理結束才處理下一個Intent。