Android IntentService 源碼分析

IntentService簡介：

IntentService是一個通過Context.startService(Intent)啟動可以處理異步請求的Service,使用時你只需要繼承IntentService和重寫其中的onHandleIntent(Intent)方法接收一個Intent對象,該服務會在異步任務完成時自動停止服務. 所有的請求的處理都在IntentService內部工作線程中完成,它們會順序執行任務(但不會阻塞主線程的執行),某一時刻只能執行一個異步請求。

IntnetService特點：

1.無需自己在Service中開啟一個線程去處理耗時任務。 2.無需自己手動的去停止Service。

IntentService使用示例：

服務端代碼如下：

package com.xjp.broadcast;

import android.app.IntentService;
import android.content.Intent;
import android.util.Log;

/**
 * Description:
 * User: xjp
 * Date: 2015/5/4
 * Time: 15:47
 */

public class MyIntentService extends IntentService {

    private static final String TAG = "MyIntentService";

    /**
     * Creates an IntentService.  Invoked by your subclass's constructor.
     *
     * @param name Used to name the worker thread, important only for debugging.
     */
    public MyIntentService() {
        super("TEST");
    }

    @Override
    public void onCreate() {
        Log.e(TAG, "====onCreate==");
        super.onCreate();
    }

    @Override
    public void onDestroy() {
        Log.e(TAG, "====onDestroy==");
        super.onDestroy();
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        Log.e(TAG, "====onStartCommand==");
        Log.e(TAG, "====Current Thread Id==" + Thread.currentThread().getId());
        return super.onStartCommand(intent, flags, startId);
    }

    @Override
    protected void onHandleIntent(Intent intent) {
        Log.e(TAG, "====onHandleIntent==");
        Log.e(TAG, "====Current Thread Id==" + Thread.currentThread().getId());

        /**
         * 此處模擬耗時任務執行
         */
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        int key = intent.getIntExtra("key", 0);

        Log.e(TAG, "====the key is ==" + key);
    }
}

客戶端代碼如下：

package com.xjp.broadcast;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;


public class MainActivity extends Activity {



    private Button startService;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);


        startService = (Button) findViewById(R.id.startService);
        startService.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent service2 = new Intent(MainActivity.this, MyIntentService.class);
                service2.putExtra("key", 3);
                startService(service2);
            }
        });

    }


    @Override
    protected void onResume() {
        super.onResume();

        Intent service = new Intent(this, MyIntentService.class);
        service.putExtra("key", 1);
        startService(service);

        Intent service1 = new Intent(this, MyIntentService.class);
        service1.putExtra("key", 2);
        startService(service1);
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
    }


}

啟動應用程序之後，打印如下：


我們發現：啟動了兩次服務，執行了一次onCreate， 兩次 onStartCommand 且Thread id=1，說明服務是在UI線程執行， 執行了兩次 onHandleIntent 且Thread id = 234，說明抽象方法 onHandleIntent 在子線程中執行，因此耗時任務可以在這裡方法裡去執行。且你會發現，當第二個任務執行完之後，打印輸出 onDestory 說明該服務自動停止，無需人為去停止。
為什麼 onHandleIntent 是在子線程中執行呢？什麼時候創建了子線程？ 為什麼IntentService 可以在 onHandleIntent 執行異步耗時任務？還有任務執行完之後為什麼會自動停止Service？？接下來我們從源碼角度解開神秘面紗吧！！！

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.
 *
 * 

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)}. */ protected abstract void onHandleIntent(Intent intent); }

我們看源碼的第 107--111行： IntentService內部實現了一個 HandlerThread 帶有循環消息處理機制的線程。關於 HandlerThread 的原理和使用方法，請參考Android HandlerThread 源碼分析 。因此，IntentService 內部其實也實現了一個帶有循環消息處理機制的線程。看看我們客戶端的耗時任務是怎麼傳給 IntentService 的內部 HandlerThread 線程執行的呢？ 我們每啟動一次服務就執行一次 onStartCommand。
我們看源碼的130行： 調用了 onStart方法。
我們看源碼的115--120行： onStart方法的實現， 在這個方法中，我們封裝客戶端傳遞過來的 Intent，將之通過 Handler + Message 消息處理機制 mServiceHandler.sendMessage(msg); 傳遞給 HandlerThread子線程去執行客戶端的耗時任務。 我們看看 mServiceHandler 的實現
源碼第58--68行： 實現了 mServiceHandler，構成了一個 Handler + Message + Looper 循環消息處理機制。
源碼第65行： 調用了 帶客戶端的Intent消息參數的 onHandlerIntent 抽象方法，該抽象方法留給子類去實現相應的 異步耗時任務。因此，我們繼承的IntentService 子類 必須實現 onHandlerIntent抽象方法。
源碼第 66 行： 調用了stopSelf（）方法，當異步任務執行完之後，自動停止Servvice。因此，我們客戶端無需自己去手動停止Service。

IntentService 總結：

IntentService 分析完畢， 1.其實內部就是實現了一個 HandlerThread+Handler 帶有循環消息處理機制的線程去處理後台的耗時任務，無需用戶去實現一個線程執行耗時任務。 2.並且任務執行之後 會自動 調用stopSelf停止服務，無需客戶端去手動管理服務，客戶端只需在需要執行異步後台耗時任務時，再次啟動任務即可。 3.如果對 HandlerThread有所了解的童鞋就會知道，為什麼 IntentService 執行異步任務都是順序的，且一個時刻只能執行一個任務。因為HandlerThread + Handler 是一個單一的工作線程，也就是說，當客戶端同時投放倆個異步任務時，只有前一個任務完成之後才能執行下一次任務，第二個任務是會阻塞等待的。不過是在子線程中執行，不會影響UI線程，也就不會出現ANR.