最近的面试中有面试官问到这个问题，由于项目业务问题service这个东西用的比较少，今天就好好探究一下。由于源码比较少，咱们就直接上源码进行分析。
我们都知道Service是默认运行在主线程中的，如果我们进行一些耗时操作就很容易会出现ANR，所以我们就需要通过多线程来进行实现。而IntentService类就是Android用来处理service里面进行多线程操作的封装类。
由于IntentService的源码不多，咱们直接贴源码如下：
public abstract class IntentService extends Service {
private volatile Looper mServiceLooper;
@UnsupportedAppUsage
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.
*
* <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;
}
@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(@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);
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;
}
/**
* 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);
}
从源码中我们可以知道IntentService继承了Service所以它包含了Service的所有特性及生命周期。
当我们启动一个IntentService的时候它会创建一个HandlerThread线程并且启动它，同时还会创建一个ServiceHandler对象。
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);
return mRedelivery ? START_REDELIVER_INTENT : START_NOT_STICKY;
}
当执行到onStart或onStartCommand时候会会将intent数据通过ServiceHandler发送到主线程进行处理。
而ServiceHandler接收消息后并没有直接处理而是调用了onHandleIntent方法让他去进行处理，并且停止当前子线程(ps:所以只适用一次性消费场景)
@Override
public void handleMessage(Message msg) {
onHandleIntent((Intent)msg.obj);
stopSelf(msg.arg1);
}
我们从源码中可以知道onHandleIntent是一个抽象方法，需要子类去进行实现。也就是最终的intent数据处理
class MyIntentService: IntentService("MyIntentService") {
override fun onHandleIntent(intent: Intent?) {
TODO("intent逻辑处理")
}
}
总结：回归主题，从上述一顿分析可知，Service和IntentService是父子关系service有的功能和生命周期IntentService都继承了，并且它的每一次调用都开启了一个子线程且执行完毕会自己终止掉。所以当有需要在service进行耗时操作且只进行一次的可以选择IntentService。
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