Android Service（三） IntentService详解
通过前面两篇文章的介绍，我们对Service有了更深入的了解了！如果你还未了解，详情请看 Android Service（一） Service初识 和 Android Service（二） Service使用 。今天我们就来介绍一个有关Service的类IntentService，IntentService是什么呢？怎么用呢？请看下文。
一.概述。
IntentService本质上还是一个Service，只不过，它的内部维护了线程，用于异步处理，当处理结束后，会清理线程，并且关掉服务。简单来说，就是我们只需要使用IntentService就可以实现一个异步耗时任务，
下面依旧通过代码来演示它的具体使用。
二.实例。
1.新建一个类，继承自IntentService，
package cn.xinxing.service;
import android.app.IntentService;
import android.content.Intent;
import android.util.Log;
/**
* Created by apple on 2016/12/14.
*/
public class MyIntentServic extends IntentService{
/**
* Creates an IntentService. Invoked by your subclass's constructor.
*
* @param name Used to name the worker thread, important only for debugging.
*/
private static final String TAG = "MyIntentServic";
public MyIntentServic() {
super(TAG);
}
@Override
protected void onHandleIntent(Intent intent) {
String tag= intent.getStringExtra("tag");
Log.e(TAG, "ag-------->"+tag);
Log.e(TAG, "Thread.id----->"+Thread.currentThread());
try {
Thread.sleep(5*1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
@Override
public void onCreate() {
super.onCreate();
Log.e(TAG, "-------->onCreate:");
}
@Override
public int onStartCommand(Intent intent, int flags, int startId) {
Log.e(TAG, "-------->onStartCommand:");
return super.onStartCommand(intent, flags, startId);
}
@Override
public void onDestroy() {
super.onDestroy();
Log.e(TAG, "-------->onDestroy:");
}
}
下面再看具体是如何调用的。
当点击按钮时，启动服务，
void startService() {
Intent intent = new Intent(this,MyIntentServic.class);
tag++;
intent.putExtra("tag",tag+"");
startService(intent);
}手动点击了5次启动按钮，截图如下，
调用了一次onCreate()，调用了5次onStartCommand()，并且打印了执行任务的线程(是子线程)，最后服务自己主动销毁了！
这么简单，到底内部是如何实现的呢！下面就通过源码来深入解析IntentService。
三.源码解析。
/*
* 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);
}
1.首先看它的onCreate()方法。
在onCreate()方法中，创建了一个线程HandlerThread对象，接着启动了该线程，然后将该线程的Looper对象赋值给内部类ServiceHandler。
ServiceHandler顾名思义，就是一个Handler，在它的handleMessage()方法中，调用了onHandleIntent((Intent)msg.obj)和stopSelf(msg.arg1)方法，而onHandleIntent((Intent)msg.obj)方法是一个抽象方法，需要用户自己实现；stopSelf(msg.arg1)方法，这不是停止Service的方法嘛！
onStartCommand()方法内部调用了onStart()方法，那么还是看看onStart()方法。
2.onStart()方法。
IntentService仅仅是通过mServiceHandler发送了一个消息，这个消息会在HandlerThread中被处理。而消息的obj属性值是onStart()方法的参数intent，消息arg1属性值是startId，也是onStart()方法的参数。当mServiceHandler收到消息后，会将intent对象传递给onHandleIntent()方法去处理。注意这个intent对象的内容和外界的startService(intent)中的intent内容时完全一致的。通过这个intent对象即可解析出外界启动IntentService()时所传递的参数，通过这些参数就可以区分具体的后台任务。这样在onHandleIntent()方法中就可以对不同的后台任务做处理了。当onHandleIntent()方法执行结束后，IntentService()会通过stopSelf(int startId)方法来尝试停止服务。这里之所以采用stopSelf(int startId)而不是stopSelf()来停止服务，那是因为 stopSelf()会立刻停止服务，而这个时候可能还有其他消息未处理，stopSelf(int startId)在尝试停止服务之前会判断最近启动服务的次数是否和startId相等，如果相等就立刻停止服务，不相等则不停止服务。
从这里，我们可以看出，如果多次调用onStart()方法，会发送多条消息，但是Service还是只有一个。这个我们之前讲解Service的是一样的。
IntentService的onHandleIntent()方法是一个抽象方法，它需要我们在子类中实现，它的作用是从Intent参数中区分具体的任务并执行这些任务。如果目前只存在一个后台任务，那么onHandleIntent()方法执行这个任务后，stopSelf(int startId)就会直接停止服务；如果目前存在多个后台任务，那么当onHandleIntent()方法执行完最后一个任务时，stopSelf(int startId)才会直接停止服务。另外，由于每执行一个后台任务就必须启动一次IntentService，而IntentService内部则通过消息的方式向IntentService请求执行任务，Handler中的Looper是顺序处理消息的，这就意味着IntentService也是顺序执行后台任务的，当有多个后台任务存在时，这些后台任务会按照外界发起的顺序排队执行的。
通过源码分析，我们可以看出，IntentService内部维持了一个消息循环，当多次调用startService()时，相当于添加了多次任务，由于内部只有一个线程，所以是按顺序执行这多个任务，当执行完所有任务后，会自动关闭服务。
PS:
HandlerThread源码，
/*
* Copyright (C) 2006 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.os;
/**
* Handy class for starting a new thread that has a looper. The looper can then be
* used to create handler classes. Note that start() must still be called.
*/
public class HandlerThread extends Thread {
int mPriority;
int mTid = -1;
Looper mLooper;
public HandlerThread(String name) {
super(name);
mPriority = Process.THREAD_PRIORITY_DEFAULT;
}
/**
* Constructs a HandlerThread.
* @param name
* @param priority The priority to run the thread at. The value supplied must be from
* {@link android.os.Process} and not from java.lang.Thread.
*/
public HandlerThread(String name, int priority) {
super(name);
mPriority = priority;
}
/**
* Call back method that can be explicitly overridden if needed to execute some
* setup before Looper loops.
*/
protected void onLooperPrepared() {
}
@Override
public void run() {
mTid = Process.myTid();
Looper.prepare();
synchronized (this) {
mLooper = Looper.myLooper();
notifyAll();
}
Process.setThreadPriority(mPriority);
onLooperPrepared();
Looper.loop();
mTid = -1;
}
/**
* This method returns the Looper associated with this thread. If this thread not been started
* or for any reason is isAlive() returns false, this method will return null. If this thread
* has been started, this method will block until the looper has been initialized.
* @return The looper.
*/
public Looper getLooper() {
if (!isAlive()) {
return null;
}
// If the thread has been started, wait until the looper has been created.
synchronized (this) {
while (isAlive() && mLooper == null) {
try {
wait();
} catch (InterruptedException e) {
}
}
}
return mLooper;
}
/**
* Quits the handler thread's looper.
*
* Causes the handler thread's looper to terminate without processing any
* more messages in the message queue.
*
* Any attempt to post messages to the queue after the looper is asked to quit will fail.
* For example, the {@link Handler#sendMessage(Message)} method will return false.
*
* Using this method may be unsafe because some messages may not be delivered
* before the looper terminates. Consider using {@link #quitSafely} instead to ensure
* that all pending work is completed in an orderly manner.
*
*
* @return True if the looper looper has been asked to quit or false if the
* thread had not yet started running.
*
* @see #quitSafely
*/
public boolean quit() {
Looper looper = getLooper();
if (looper != null) {
looper.quit();
return true;
}
return false;
}
/**
* Quits the handler thread's looper safely.
*
* Causes the handler thread's looper to terminate as soon as all remaining messages
* in the message queue that are already due to be delivered have been handled.
* Pending delayed messages with due times in the future will not be delivered.
*
* Any attempt to post messages to the queue after the looper is asked to quit will fail.
* For example, the {@link Handler#sendMessage(Message)} method will return false.
*
* If the thread has not been started or has finished (that is if
* {@link #getLooper} returns null), then false is returned.
* Otherwise the looper is asked to quit and true is returned.
*
*
* @return True if the looper looper has been asked to quit or false if the
* thread had not yet started running.
*/
public boolean quitSafely() {
Looper looper = getLooper();
if (looper != null) {
looper.quitSafely();
return true;
}
return false;
}
/**
* Returns the identifier of this thread. See Process.myTid().
*/
public int getThreadId() {
return mTid;
}
}
从HandlerThread的实现来看，它和普通的Thread有显著的不同。普通的Thread主要在run()方法中执行一个耗时任务，而HandlerThread在内部创建了消息队列，外界需要通过Handler的消息方式来通知HandlerThread执行一个具体的任务。由于HandlerThread的run()方法是一个无限循环，因此当明确不需要再使用HandlerThread时，可以通过它的quit ()和quitSafely()方法来终止线程的执行。
四.总结。
1.继承IntentService的类至少要实现两个方法：构造方法和onHandleIntent()方法；
2.它的内部维持了一个独立的工作线程来处理所有的通过onStartCommand()传递给服务的intents；
3.它的内部维持一个异步消息，拥有消息队列，通过消息处理来逐个发送intent给onHandleIntent()；
4. 不需要主动调用stopSelft()来结束服务。因为，在所有的intent被处理完后，系统会自动关闭服务。
5. 由于它的内部只有一个线程，如果有多个任务时，会按顺序执行，所以如果你想同时执行多个任务的话(并发)，IntentService那么可能并不适合！