【安卓学习积累】IntentService的源码分析

今天主要总结一下IntentService的源码，里面是如何实现的，为什么IntentService在执行完耗时操作后会自动停止。

1.the service is started as needed, handles each Intent in turn using a worker thread, and stops itself when it runs out of work.

2.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. 同一时间只会执行一个进程。

一、IntentService的源码分析

我们知道在实现IntentService只需要把我们自己Service继承IntentService并实现里面的onHandleIntent方法就可以了。

我们带着两个小问题来看源码，

1.是否可以通过bindService来启动IntentService？（不行）

2.多次调用startService方法会启动多个IntentService吗？（不会）

3.为什么IntentService在执行完耗时操作后会自动停止？（主要在handleMessage里面有停止操作）

ps：源码还是比较简单的，为了方便学习我把源码都粘贴出来了。(*^▽^*)

package android.app;

import android.annotation.WorkerThread;
import android.annotation.Nullable;
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.
 *
 * 
Note: IntentService is subject to all the
 * background execution limits
 * imposed with Android 8.0 (API level 26). In most cases, you are better off
 * using {@link android.support.v4.app.JobIntentService}, which uses jobs
 * instead of services when running on Android 8.0 or higher.
 * 
 *
 * 

 * 
Developer Guides
 * 
For a detailed discussion about how to create services, read the
 * Services developer
 * guide.
 * 
 *
 * @see android.support.v4.app.JobIntentService
 * @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) {
            //这里我们可以看到收到消息后会先执行onHandleIntent，执行完后会调用stopSelf来停止。
            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();
        //创建一个HandlerThread
        HandlerThread thread = new HandlerThread("IntentService[" + mName + "]");
        thread.start();
        //获取当前thread的Lopper对象
        mServiceLooper = thread.getLooper();
        mServiceHandler = new ServiceHandler(mServiceLooper);
    }

    @Override
    public void onStart(@Nullable Intent intent, int startId) {
        //多次调用startService并不会启动多个Service，只会发送多的消息到消息队列中。
        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) {
        //启动IntentService会直接调用onStartCommand，在这个里面会调用onStart方法去发送消息
        //这也就是为什么调用startService方法来启动IntentService，因为会调用onStartCommand
        //当然上面也写了，这个方法不用你重写。
        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
    @Nullable
    public IBinder onBind(Intent intent) {
        return null;
    }
    //下面的方法也说了，onHandleIntent在worker线程中工作，一次只能一个，当所有发送的请求都执行完，就是停止自己。

    /**
     * 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);
}

二、总结

其实Intentservice源码相对比较简单，但是我们也需要了解里面是如果执行的。这样也方便我们被别人问到可以更流畅的回答。

2.多次调用startService方法会启动多个IntentService吗？（不会）

由于onCreate（）方法只会调用一次，所以只会创建一个工作线程，当多次调用startService的时候onStartCommand和onStart会调用很多次）但不会创建新的工作线程，只是会把消息发送到消息队列中等待执行，所以多次调用IntentService会按照顺序执行的。当然了如果服务停止了会调用OnDestory会把所有消息移除，后续的时间也不会被执行了。