IntentService源码分析

一、IntentService诞生

我们在做产品需求的时候，经常需要在Service里做一些耗时操作(比如文件下载)，而Service却是运行在主线程的，这时就需要我们自己去创建一个子线程和编写子线程相关的代码，这时你会不会想如果有人帮我把线程创建了该多好，这样只需要把精力放在业务相关的代码上，这多棒！！！现实中Google已经为我们考虑到这一点IntentService就因此而诞生。

ps: IntentService是继承自Service，所以多次启动服务，同一时间也只会有一个IntentService实例，但onStartCommand会调用多次，同时不必担心多次调用onStartCommand会覆盖掉之前执行的任务，因为IntentService中维护一个消息列表MessageQueue，而每个任务就是一个消息Message。最后说一下，在所有消息执行完毕后，IntentService会自动结束掉，不需要我们手动结束。

二、IntentService使用小例子

public class IntentServiceTest {
    private static final String TAG = IntentServiceTest.class.getSimpleName();
    private static final String TAGLife = IntentServiceTest.class.getSimpleName() + "—Life";
    private static final String INTENT_KEY = "intent_key";
    private Context mCtx;

    public IntentServiceTest(Context context) {
        mCtx = context;
    }

    public void excute() {
        for (int i = 0; i < 5; i++) {
            // 快速启动5次
            Intent intent = new Intent(mCtx, MyIntentService.class);
            intent.putExtra(INTENT_KEY, "intent" + i);
            mCtx.startService(intent);
        }
    }


    public static final class MyIntentService extends IntentService {
        public MyIntentService() {
            super("MyIntentService");
        }
        @Override
        public void onCreate() {
            super.onCreate();
            Log.d(TAGLife, "MyIntentService -------------- onCreate()");
        }
        @Override
        public int onStartCommand(@Nullable Intent intent, int flags, int startId) {
            Log.d(TAGLife, "MyIntentService -------------- onStartCommand()");
            return super.onStartCommand(intent, flags, startId);
        }
        @Override
        public void onDestroy() {
            Log.d(TAGLife, "MyIntentService -------------- onDestroy()");
            super.onDestroy();
        }
        @Override
        protected void onHandleIntent(@Nullable Intent intent) {
            Log.d(TAG, "主线程id=" + Looper.getMainLooper().getThread().getId());
            Log.d(TAG, "当前运行线程id=" + Thread.currentThread().getId());
            try {
                // 模拟耗时操作
                Thread.sleep(3000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            if (intent != null) {
                String value = intent.getStringExtra(INTENT_KEY);
                Log.d(TAG, "intent_key=" + value + "  threadId=" + Thread.currentThread().getId());
            }
        }
    }
}

在上面的代码中，我们连续启动五次MyIntentService，同时在MyIntentService的onHandleIntent()中让线程休息3秒钟，防止MyIntentService在3秒内销毁，以测试多次start只创建一个实例，调用多次onStartCommand。

别忘了maniofest中要注册下

<service android:name=".mj.IntentServiceTest$MyIntentService" />

运行结果：

02-08 18:53:11.557 19198-19198/com.ktln.must D/IntentServiceTest—Life: MyIntentService -------------- onCreate()
02-08 18:53:11.557 19198-19198/com.ktln.must D/IntentServiceTest—Life: MyIntentService -------------- onStartCommand()
02-08 18:53:11.557 19198-19198/com.ktln.must D/IntentServiceTest—Life: MyIntentService -------------- onStartCommand()
02-08 18:53:11.557 19198-19198/com.ktln.must D/IntentServiceTest—Life: MyIntentService -------------- onStartCommand()
02-08 18:53:11.557 19198-19198/com.ktln.must D/IntentServiceTest—Life: MyIntentService -------------- onStartCommand()
02-08 18:53:11.557 19198-19198/com.ktln.must D/IntentServiceTest—Life: MyIntentService -------------- onStartCommand()
02-08 18:53:11.567 19198-19216/com.ktln.must D/IntentServiceTest: 主线程id=1
02-08 18:53:11.567 19198-19216/com.ktln.must D/IntentServiceTest: 当前运行线程id=23182
02-08 18:53:14.567 19198-19216/com.ktln.must D/IntentServiceTest: intent_key=intent0  threadId=23182
02-08 18:53:14.567 19198-19216/com.ktln.must D/IntentServiceTest: 主线程id=1
02-08 18:53:14.567 19198-19216/com.ktln.must D/IntentServiceTest: 当前运行线程id=23182
02-08 18:53:17.567 19198-19216/com.ktln.must D/IntentServiceTest: intent_key=intent1  threadId=23182
02-08 18:53:17.567 19198-19216/com.ktln.must D/IntentServiceTest: 主线程id=1
02-08 18:53:17.567 19198-19216/com.ktln.must D/IntentServiceTest: 当前运行线程id=23182
02-08 18:53:20.567 19198-19216/com.ktln.must D/IntentServiceTest: intent_key=intent2  threadId=23182
02-08 18:53:20.567 19198-19216/com.ktln.must D/IntentServiceTest: 主线程id=1
02-08 18:53:20.567 19198-19216/com.ktln.must D/IntentServiceTest: 当前运行线程id=23182
02-08 18:53:23.567 19198-19216/com.ktln.must D/IntentServiceTest: intent_key=intent3  threadId=23182
02-08 18:53:23.567 19198-19216/com.ktln.must D/IntentServiceTest: 主线程id=1
02-08 18:53:23.567 19198-19216/com.ktln.must D/IntentServiceTest: 当前运行线程id=23182
02-08 18:53:26.567 19198-19216/com.ktln.must D/IntentServiceTest: intent_key=intent4  threadId=23182
02-08 18:53:26.567 19198-19198/com.ktln.must D/IntentServiceTest—Life: MyIntentService -------------- onDestroy()

三、IntentService源码分析

public abstract class IntentService extends Service {
    private volatile Looper mServiceLooper; // 内置消息队列，并提供消息循环遍历
    private volatile ServiceHandler mServiceHandler; // 处理Looper中取出的消息
    private String mName;
    // IntentService意外中断后处理方式：
    // true 返回 START_REDELIVER_INTENT，使用保留的Intent重启IntentService
    // false 返回 START_NOT_STICKY ， 不重启
    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.
     *
     * <p>If enabled is true,
     * 如果设置为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.  
     * onStartCommand(Intent, int, int)方法返回START_REDELIVER_INTENT，因此当Service在执行
     * onHandleIntent(Intent)之前挂掉，则线程会使用最近发送的Intent，来重新启动Service
     * If multiple Intents have been sent, only the most recent one is guaranteed to be redelivered.
     * 如果发生了多个Intent，只有最后一个才能被重新发送。
     *
     * <p>If enabled is false (the default),
     * 如果设置为false
     * {@link #onStartCommand(Intent, int, int)} will return
     * {@link Service#START_NOT_STICKY}, and if the process dies, the Intent
     * dies along with it.
     * onStartCommand(Intent, int, int)方法返回START_NOT_STICKY，
     * 并且如果Service挂掉，Intent也同样会被回收
     */
    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();
        // 创建一个HandlerThread对象，HandlerThread调用start方法后，会帮助我们创建一个Looper对象
        HandlerThread thread = new HandlerThread("IntentService[" + mName + "]");
        thread.start();
        // 获得HandlerThread中的Looper对象
        mServiceLooper = thread.getLooper();
        // 使用HandlerThread的Looper对象，构建一个Handler，这个Handler是运行在子线程中（即HandlerThread）
        mServiceHandler = new ServiceHandler(mServiceLooper);
    }

    // 废弃方法，现在使用onStartCommand来替代
    @Override
    public void onStart(@Nullable 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(@Nullable Intent intent, int flags, int startId) {
        onStart(intent, startId); // startId系统生成的，可以理解为对当前Intent的标记
        return mRedelivery ? START_REDELIVER_INTENT : START_NOT_STICKY;
    }

    @Override
    public void onDestroy() {
        // 销毁时，释放Looper的资源
        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
    @Nullable
    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)}.
     *               This may be null if the service is being restarted after
     *               its process has gone away; see
     *               {@link android.app.Service#onStartCommand}
     *               for details.
     */
    @WorkerThread
    protected abstract void onHandleIntent(@Nullable Intent intent); // 处理耗时操作，运行在工作线程（即：子线程）
}

代码分析都已经放到源码注释上了，直接看源码就好。IntentService的源码比较简单，主要是对HandlerThread的使用，不熟悉HandlerThread的可以看我的这篇文章【HandlerThread源码分析】，这里额外提一下stopSelf(msg.arg1);调用此方法并不一定会停止Service，具体源码可以看这篇文章【详细剖析IntentService的运作机理】


顺便给一下Android中framework层源码下载地址，方便小伙伴们学习
github: https://github.com/android/platform_frameworks_base
google 官方: https://android.googlesource.com/platform/frameworks/base.git