Let's first introduce the difference between intentService and Service.

IntentService inheritance has the same declaration cycle and method as Service and Service. The difference is that IntentService is a google engineer who sees that we often do some time-consuming operations in Service to help us encapsulate it. If we don't encapsulate it, we can use Service+Thread+Handler or Service + Handler Thread. Unlike service, IntentService runs an internal thread to execute your time-consuming operation when it performs onCreate.

Basic usage method

package com.sym.service;

import android.app.IntentService;
import android.content.Intent;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.IBinder;
import android.os.Message;

import com.zejian.handlerlooper.util.LogUtils;

import java.io.BufferedInputStream;
import java.io.IOException;
import java.net.HttpURLConnection;
import java.net.URL;

/**
 * Created by zejian
 * Time 17/3/18.
 * Description:
 */
public  class MyIntentService extends IntentService {
    public static final String DOWNLOAD_URL="download_url";
    public static final String INDEX_FLAG="index_flag";
    public static UpdateUI updateUI;


    public static void setUpdateUI(UpdateUI updateUIInterface){
        updateUI=updateUIInterface;
    }

    public MyIntentService(){
        super("MyIntentService");
    }

    /**
     * The Method of Achieving Asynchronous Tasks
     * @param intent Activity Intent passed in, data encapsulated in intent
     */
    @Override
    protected void onHandleIntent(Intent intent) {

        //Network requests in sub-threads
        Bitmap bitmap=downloadUrlBitmap(intent.getStringExtra(DOWNLOAD_URL));
        Message msg1 = new Message();
        msg1.what = intent.getIntExtra(INDEX_FLAG,0);
        msg1.obj =bitmap;
        //Notify the main thread to update the UI
        if(updateUI!=null){
            updateUI.updateUI(msg1);
        }
        //mUIHandler.sendMessageDelayed(msg1,1000);

        LogUtils.e("onHandleIntent");
    }
    @Override
    public void onCreate() {
        LogUtils.e("onCreate");
        super.onCreate();
    }

    @Override
    public void onStart(Intent intent, int startId) {
        super.onStart(intent, startId);
        LogUtils.e("onStart");
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        LogUtils.e("onStartCommand");
        return super.onStartCommand(intent, flags, startId);

    }

    @Override
    public void onDestroy() {
        LogUtils.e("onDestroy");
        super.onDestroy();
    }

    @Override
    public IBinder onBind(Intent intent) {
        LogUtils.e("onBind");
        return super.onBind(intent);
    }


    public interface UpdateUI{
        void updateUI(Message message);
    }


    private Bitmap downloadUrlBitmap(String urlString) {
        HttpURLConnection urlConnection = null;
        BufferedInputStream in = null;
        Bitmap bitmap=null;
        try {
            final URL url = new URL(urlString);
            urlConnection = (HttpURLConnection) url.openConnection();
            in = new BufferedInputStream(urlConnection.getInputStream(), 8 * 1024);
            bitmap= BitmapFactory.decodeStream(in);
        } catch (final IOException e) {
            e.printStackTrace();
        } finally {
            if (urlConnection != null) {
                urlConnection.disconnect();
            }
            try {
                if (in != null) {
                    in.close();
                }
            } catch (final IOException e) {
                e.printStackTrace();
            }
        }
        return bitmap;
    }

}

Layout file

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="match_parent"
    android:layout_height="match_parent">

    <ImageView
        android:id="@+id/image"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />
</LinearLayout>

Using examples

public class IntentServiceActivity extends Activity implements MyIntentService.UpdateUI{
    /**
     * Picture Address Set
     */
    private String url[] = {
            "http://img.blog.csdn.net/20160903083245762",
            "http://img.blog.csdn.net/20160903083252184",
            "http://img.blog.csdn.net/20160903083257871",
            "http://img.blog.csdn.net/20160903083257871",
            "http://img.blog.csdn.net/20160903083311972",
            "http://img.blog.csdn.net/20160903083319668",
            "http://img.blog.csdn.net/20160903083326871"
    };

    private static ImageView imageView;
    private static final Handler mUIHandler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            imageView.setImageBitmap((Bitmap) msg.obj);
        }
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_intent_service);
        imageView = (ImageView) findViewById(R.id.image);

        Intent intent = new Intent(this,MyIntentService.class);
        for (int i=0;i<7;i++) {//Loop Start Task
            intent.putExtra(MyIntentService.DOWNLOAD_URL,url[i]);
            intent.putExtra(MyIntentService.INDEX_FLAG,i);
            startService(intent);
        }
        MyIntentService.setUpdateUI(this);
    }

    //It must be updated through Handler, which is an asynchronous method and cannot update the UI.
    @Override
    public void updateUI(Message message) {
        mUIHandler.sendMessageDelayed(message,message.what * 1000);
    }
}

Principle analysis

Let's first look at IntentService's onCreate() method

@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);
}

Find out, it creates a HandlerThread inside Service, and creates a ServiceHandler object and associates the Handler object with the internal Looper of HandlerThread, as we did in the previous article. Handler Thread for Android Advancement The use of Handler Thread is the same, if you are not familiar with the use of Handler Thread, you can first read this article to consolidate.

Let's look again at onStart() and onStartCommand() methods

@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;
}

Review the basic use of Service
startService(Intent intent) opens a Service and transfers data through Intent. Looking at the source code, you will find that the official document of google has such a sentence.

 * <p>If you need your application to run on platform versions prior to API
     * level 5, you can use the following model to handle the older {@link #onStart}
     * callback in that case.  The <code>handleCommand</code> method is implemented by
     * you as appropriate:

The onStart() method is called in api 5 and previous versions, and the onStartCommand() method is called later, so the logic above is sorted out. If there is a multi-task time-consuming operation (image download above), we write a loop that calls the startService(Intent intent) method several times to pass the downloaded url address to IntentService and then call IntentService's S. The tartCommand method then adds tasks to the message queue of HandlerThread through mServiceHandler.

Check out the ServiceHandler source code

 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);
        }
    }

We found that we did time-consuming operations in the onHandleIntent method.

Attach IntentService source code

/*
 * 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.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.
 *
 * <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.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(@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);
}