Android Service（三） IntentService详解

素颜马尾好姑娘i 2022-07-14 15:15 203阅读 0赞

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 ][Android Service_ Service_]和[ Android Service（二） Service使用 ][Android Service_ Service_ 1]。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:");
        }
    }

需要实现一个无参的构造函数以及实现onHandleIntent()方法。并且还在其他生命周期方法中打印了log日志。

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);
        }

好了，接着，就可以运行该项目了！

手动点击了5次启动按钮，截图如下，

![Center][]

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.
     *
     * <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.
     *
     * <div class="special reference">
     * <h3>Developer Guides</h3>
     * <p>For a detailed discussion about how to create services, read the
     * <a href="{@docRoot}guide/topics/fundamentals/services.html">Services</a> developer guide.</p>
     * </div>
     *
     * @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.
         *
         * <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(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);
    }

我们按照的生命周期方法来介绍每个方法。  
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！![安静][quiet.gif]

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.
         * <p>
         * Causes the handler thread's looper to terminate without processing any
         * more messages in the message queue.
         * </p><p>
         * 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.
         * </p><p class="note">
         * 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.
         * </p>
         *
         * @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.
         * <p>
         * 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.
         * </p><p>
         * 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.
         * </p><p>
         * 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.
         * </p>
         *
         * @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;
        }
    }

HandlerThread继承自Thread,它是一种可以使用Handler的Thread，它的实现也很简单，就是在run()方法中通过Looper.prepare()方法来创建了一个消息队列，并通过Looper.loop()来开启消息循环，这样在实际的使用中就允许在HandlerThread中创建Handler了。

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！

[Android Service_ Service_]: http://blog.csdn.net/zxw136511485/article/details/52836390
[Android Service_ Service_ 1]: http://blog.csdn.net/zxw136511485/article/details/53537993
[Center]: /images/20220714/fa0c908327d64687bb22fc20bfe10b8a.png
[quiet.gif]: /images/20220714/237883815a5041118a75b8899a4ddf45.png