【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.** <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.** <p class="note"><b>Note:</b> IntentService is subject to all the* <a href="/preview/features/background.html">background execution limits</a>* 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.* </p>** <div class="special reference">* <h3>Developer Guides</h3>* <p>For a detailed discussion about how to create services, read the* <a href="{@docRoot}guide/components/services.html">Services</a> developer* guide.</p>* </div>** @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);}@Overridepublic 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.** <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;}@Overridepublic 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HandlerThreadHandlerThread thread = new HandlerThread("IntentService[" + mName + "]");thread.start();//获取当前thread的Lopper对象mServiceLooper = thread.getLooper();mServiceHandler = new ServiceHandler(mServiceLooper);}@Overridepublic 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*/@Overridepublic 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;}@Overridepublic 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@Nullablepublic 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.*/@WorkerThreadprotected 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。