Create Spring REST service for Google App Engine in 15 minutes

Here's how you setup a REST service deployed in the Google App Engine cloud in 15 minutes. The use case in this example is a highscore backend service for my Android game Othello Legends.

Requirements:
We want to create a REST interface for these resources representing a highscore service.


GET http://myapp.appspot.com/api/highscores/
Fetch all applications backed by the highscore service since we want to reuse this for multiple games.

GET http://myapp.appspot.com/api/highscores/myappname
Fetch a sorted listed of highscores for a particular application myappname.

POST http://myapp.appspot.com/api/highscores/myappname
Post a potential new highscore to service. If it makes it to the highscore list it will be saved in database. The data will be sent as query parameters.

Ingredients of the solution:
Google App Engine runs Java and Python. This example will use the Java infrastructure.
So what we'll do is to create a standard Java J2EE web application built for deployment in App Engine backed by a simple DAO to abstract the Google BigTable databases. By using Spring REST together with Jackson we can communicate with JSON in a RESTful manner with minimum effort.

Sounds complicated? Not at all, here's how you do it!

Prerequisities:

• Java knowledge
• Eclipse and some basic skills of working in it.
• A Google account
• The Eclipse plugin for Google App Engine. If you ain't got it, the install is easy, follow the instruction at http://code.google.com/intl/sv-SE/appengine/docs/java/tools/eclipse.html

REST Implementation:

So to create an App Engine web app, click the New Web Application Project icon. Deselect Google Web Toolkit if you don't intend to use it.

Now, we're going to use Spring REST for the REST heavy weight lifting. Download Spring Framework 3 or later from http://www.springsource.org/download. While at it, download the Jackson JSON library from http://jackson.codehaus.org/. Put the downloaded jars in the /war/WEB-INF/lib/ folder and add them to the classpath of your web application.

Now, to bootstrap Spring to handle your incoming servlet requests you should edit the web.xml file of your web application found in war/WEB-INF/.

   api
   
      org.springframework.web.servlet.DispatcherServlet
   
   1

  

   api
   /api/*



   index.html

That will put Spring in charge of everything coming in under path /api/*. Spring must now which packages to scan for Spring annotated classes. We add a Spring configuration file for this and also add some Spring/Jackson config for specifying how to convert from our Java POJOs to JSON. Put this stuff in a file called api-servlet.xml in war/WEB-INF.

Without going into detail, this config pretty much tells Spring to convert POJOs to JSON as default using Jackson for servlet responses. If you're not interested in the details just grab it, but you must adjust the <context:component-scan base-package="se.noren.othello" /> to match your package names.

Now to the fun part, mapping Java code to the REST resources we want to expose. We need a controller class to annotate how our Java methods should map to the exposed HTTP URIs. Create something similar to

import java.util.Date;
import java.util.List;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.validation.BindingResult;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.servlet.ModelAndView;

/**
 * Controller for Legends app family highscore services.
 */
@Controller
@RequestMapping("/highscores")
public class LegendsHighScoreController {
 private static final long serialVersionUID = 1L;

 @Autowired
 HighScoreService highScoresService;

 /**
  * @return Fetch all registered applications in the highscore database.
  */
 @RequestMapping(value = "/", method = RequestMethod.GET)
 public ModelAndView getAllApplications() {
  List<String> allApplications = highScoresService.getAllApplications();
  return new ModelAndView("highScoresView", BindingResult.MODEL_KEY_PREFIX + "applications", allApplications);
 }
 
 /**
  * Fetch all highscores for a particular application.
  * @param application Name of application
  * @return
  */
 @RequestMapping(value = "/{application}", method = RequestMethod.GET)
 public ModelAndView getAllHighScores(@PathVariable String application) {
  List<HighScore> allHighScores = highScoresService.getAllHighScores(application);
  return new ModelAndView("highScoresView", BindingResult.MODEL_KEY_PREFIX + "scores", allHighScores);
 }
 
 /**
  * Add a new highscore to the database if it makes it to the high score list.
  * @param application Name of application
  * @param owner Owner of the highscore
  * @param score Score as whole number
  * @param level Level of player reaching score.
  * @return The created score.
  */
 @RequestMapping(value = "/{application}", method = RequestMethod.POST)
 public ModelAndView addHighScores(@PathVariable String application,
                             @RequestParam String owner,
                             @RequestParam long score,
                             @RequestParam long level
                             ) {
  
  HighScore highScore = new HighScore(owner, score, application, new Date().getTime(), level);
  highScoresService.addHighScores(highScore);
  return new ModelAndView("highScoresView", BindingResult.MODEL_KEY_PREFIX + "scores", highScore);
 }
}

So what's the deal with all the annotations? They're pretty self explanatory once you start matching the Java methods to the three HTTP REST URIs we wanted to create, but in short:

• @Controller - The usual Spring annotation to tell Spring that this is a controller class that should be managed by the Spring container. All RESTful stuff is contained within the this class.
• @RequestMapping("/highscores") - This means that this controller class should accept REST calls under the path /highscores. Since we deployed the servlet under servlet mapping /api in the web.xml this means all REST resources resides under http://host.com/api/highscores
• @Autowired HighScoreService highScoresService - Our backing service class to do real business logic. Agnostic that we're using a RESTful front.
• @RequestMapping(value = "/{application}", method = RequestMethod.GET) public ModelAndView getAllHighScores(@PathVariable String application) -  A method annotated like this creates a REST resource /api/highscores/dynamicAppName where the value given for dynamicAppName is given via the path variable application. The request method specifies that this Java method will be called if this URI is requested via HTTP GET. All ordinary HTTP verbs are supported.
• @RequestParam String owner - If you wish to pass query parameters like myvar1=foo&myvar2=bar you can use the request param annotation.
• The Java class returned in the ModelAndView response will be automatically marshalled to JSON by Jackson on the same structure as the Java POJO.

Database

Google App Engine uses the Google BigTables behind the scenes to store data. You can abstract this by using the standard JPA annotations on your POJOs. The similar JDO standard can be used as well. I've used JDO in previous projects and it works very well. For this simple server application we will however use the query language to directly access the document database. Here's the code for the first method to fetch all highscores for a particular Legends application. The database can filter and sort via API methods in the query.

@Service
public class HighScoreServiceImpl implements HighScoreService {

 @Override
 public List<HighScore> getAllHighScores(String application) {
  ArrayList<HighScore> list = new ArrayList<HighScore>();
  DatastoreService datastore = DatastoreServiceFactory
    .getDatastoreService();

  // The Query interface assembles a query
  Query q = new Query("HighScore");
  q.addFilter("application", Query.FilterOperator.EQUAL, application);
  q.addFilter("score", FilterOperator.GREATER_THAN_OR_EQUAL, 0);
  q.addSort("score", SortDirection.DESCENDING);

  // PreparedQuery contains the methods for fetching query results
  // from the datastore
  PreparedQuery pq = datastore.prepare(q);

  for (Entity result : pq.asIterable()) {
   String owner = (String) result.getProperty("owner");
   Long date = (Long) result.getProperty("date");
   Long score = (Long) result.getProperty("score");
   Long level = (Long) result.getProperty("level");
   list.add(new HighScore(owner, score, application, date, level));
  }

  return list;
 }

That's pretty much it. Run the project locally by right-clicking it and choose Run As -> Web application. Once you are ready to go live create a cloud application by going to https://appengine.google.com/ and Create new application

Now in Eclipse, right click on your project and choose Google -> Deploy to Google App Engine.

You will be asked to supply the name you created in the App Engine administration interface. Wait a few seconds and the application will be deployed in the cloud.

Othello Legends 1.0 in Android Market!

A first version of Othello Legends is now published to the Android Market, check out https://market.android.com/details?id=se.noren.android.othello! The game is the classic Othello/Reversi with a twist for unlocking harder levels when beating opponents. Compete with other players by increasing your total scores.

The aim of the project was to learn how to build an Android application of some complexity with features like OpenGL rendering, integration with backend server, Google AdMob for serving ads, Google Analytics tracking of application usage, SD storage and possibility to run on both phones and tablets with appealing layout and performance.

Lesson learned 1 - 3D
Accelerated 3D graphics is hard even if you're experienced with desktop OpenGL. To make OpenGL ES work on all devices requires a lot of testing which you can't do without devices. And generally debugging accelerated graphics in the Android emulator is no fun. Therefore, use a library to leverage yourself from the low level details.
I looked into jMonkeyEngine, libgdx which are general and large frameworks with quite massive APIs which probably would have worked out great but seemed to have some threshold for a newcomer to overcome.

In the end I decided to work with the more limited jPCT which has worked out very well. A stable and reliable library with an active community. jPCT handles 3DS-models well which makes it easy to create environments via some tooling.
I used the open source modeller Blender which is free and has support for all you would need as sculpt modelling and texture UV coordinate tooling. Antother appealing feature of jPCT is that is developed both for Android and as standard Java for desktop so you can port your apps between them without great effort.


Lesson 2 - Revenue model
If you haven't decided whether charging for your app or using ads I can only say that Ads are easy! If you're familiar with Google AdSense for creating ads on your websites you'll find it intuitive to work with Google AdMob. If you have an Android Activity made up of standard Android layouts you can simply add an AdView view to your layout and the Ad library will populate the container with ads.

Compared to the standard Google AdSense interfaces for managing and following up your ad reports AdMob is more limited and not as well polished but who cares? Will revenues be larger with mobile app ads than with ordinary web ads? I'll come back later on that.

Lesson 3 - Mobile is not desktop
Memory is scarse when you go down the 3D pathway. I early discovered that you must be cheap on your textures and the polygon levels of your meshes. The devices have no problem with rendering polygon heavy meshes with impressive framerates, but you soon run out of memory if you don't do clever texture unloads when you don't need them. My lesson here was: Create a game engine with strict modules for each game state so that you can be sure to deallocate all resources when you change state and use more low res textures than you usually would.

Lesson 4 - Tune your app after how your users use it
So in this game each level becomes more difficult and it seems like a good tuning approach to make the  difficulties of the first two levels easy enough to to make sure all players passes them. After that it should be exponentially more difficult. How to know how your app users are doing? I notices that the game was way too hard when I tried it on people. Some sort of surveillance would be nice without intruding in the users' privacy. Lesson here is to not invent anything new. By using Google Analytics you can track how users travel around in your application by marking different states as you would use Google Analytics to mark web pages in a web site to follow traffic around your site and adapt your game to how users respond.

Lesson 5 - Android is not Java

Another more depressive lesson learned is that when you plan to reuse some Java library first make a Google search whether anyone has had difficulties using it on the Android platform. For example the JSON marshaller Jackson proved to be hard to use.

Android - track down memory leaks

My current Android application project is starting to make sense. Unfortunately it crasches after a few levels of playing due to java.lang.OutOfMemoryError. Up to that point I hadn't put much thinking into the memory model of Android applications and simply consumed memory without hesitations. I've now been forced to rewrite some critical parts of the application and i thought I'll write a few words to remember the most useful tools I came across.

First of all, Android apps have small heaps. And with different sizes, it's up to the vendor of the device to decide. Here's a few numbers I came across:

• G1 = 16 Mb
• Droid = 24 Mb
• Nexus One = 32 Mb
• Xoom = 48 Mb
• GalaxyTab = 64 Mb

So you see that allocated heaps are far from using the entire RAM of the devices since no application should be able to clog the system. The natural approach to solving a memory problem would be to increase the heap but that is not so easy. If you have a rooted phone you may edit

/system/build.props
and set the heap size via
dalvik.vm.heapsize=24m


Or, if you're running on a tablet (3.x) Android version there is a manifest setting to ask for a large heap

<application android:label="@string/app_name" android:hardwareAccelerated="true" android:largeHeap="true" android:debuggable="true">

but that is no guarantee and you will instead be punished with longer GC cycle times.

On the other hand, changing the VM heap size in your emulator is easy, and could be a good thing in order to verify that your app works on devices with smaller heaps. To do that, fire up your Android SDK and AVD Manager and click edit on your virtual device. Under hardware, there is a setting Max VM application heap size.

So the conclusion is that you have to live with small heaps and limited memory. How to get an estimate of  your consumed memory and how much there is available then?
Run your application in the emulator or connect your real device via USB and use the Android Debug Bridge (adb). It's located in your Android SDK tools folder.

To dump memory info for all your running applications use
$>adb shell dumpsys meminfo

or for your specific application


$>adb shell dumpsys meminfo se.noren.android.othello


Applications Memory Usage (kB):
Uptime: 8979886 Realtime: 8979886


** MEMINFO in pid 1073 [se.noren.android.othello] **
                    native   dalvik    other    total
            size:    24648    10119      N/A    34767
       allocated:    10869     7335      N/A    18204
            free:        2     2784      N/A     2786
           (Pss):     2857     8568     9385    20810
  (shared dirty):     1508     4092     2556     8156
    (priv dirty):     2656     6020     7732    16408


 Objects
           Views:        0        ViewRoots:        0
     AppContexts:        0       Activities:        0
          Assets:        2    AssetManagers:        2
   Local Binders:        6    Proxy Binders:       10
Death Recipients:        0
 OpenSSL Sockets:        0


 SQL
            heap:        0       memoryUsed:        0
pageCacheOverflo:        0  largestMemAlloc:        0


To understand this table we must know that you have a managed heap, dalvik, and a native heap. For example some graphics are stored in native heap. But important, it is the sum of these heaps that can not exceed the VM heap size. so you can't fool the runtime by putting more stuff in either native or managed heap. So to me, the most important numbers are the number under dalvik and total above. The dalvik heap is the managed VM heap and the native numbers are memory allocated by native libraries (malloc).
You'll probably see these numbers fluctating each time you run the command, that is because objects are allocated by the runtime all the time but GCs are not run particularly often. So, in order to know that you really have garbage collected all unused objects you must either wait for the Android debug log in logcat to say something like
GC_FOR_MALLOC or GC_EXTERNAL_MALLOC or similar to that which indicates that the GC has been invoked. Still, this does not mean that all unused memory has been released since the inner workings of the GC might not have done a complete sweep.

You can of course ask for a GC programmatically by System.gc();
But that is never a good option. You should have trust in the VM to garbage collect for you. If you for example want to allocate a large memory chunk the gc will be invoked if necessary.

You can force a gc using the Dalvik Debug Monitor (DDMS). Either start it from Eclipse or from the ddms tool in the Android SDK installation folders.

If you can't see your process right away, go to menu Actions and Reset adb. After that you can turn on heap updates via the green icon Show heap updates. To force a GC, click on Cause GC.

If you wish to monitor the memory usage programmatically there are a few APIs you can use.

ActivityManager.getMemoryInfo() can be used to get an idea of how the memory situation is for the whole Android system. If running low on the gauges you can expect background processes to be killed off soon.

To start inspecting your process in particular use the Debug APIs http://developer.android.com/intl/de/reference/android/os/Debug.html#getMemoryInfo(android.os.Debug.MemoryInfo. There's an excellent explanation of the data you can retrieve from this here http://stackoverflow.com/questions/2298208/how-to-discover-memory-usage-of-my-application-in-android

For example, to see how much memory is allocated in the native heap, use:
Debug.getNativeHeapAllocatedSize()

So back to DDMS. This tool can also create heap dumps which are particulary useful when tracing down memory leaks. To dump the heap, click on the icon Dump HPROF file. There are many tools for analyzing heap dumps but the one I'm most familiar is the Eclipse Memory Analyzer (MAT). Download it from http://www.eclipse.org/mat/.

MAT can't handle the DDMS heap dumps right away, but there is a converter tool in the Android SDK.
So simply run this command.

C:\Temp\hprof>hprof-conv rawdump.hprof converteddump.hprof

Then you can open the converted heap dump in MAT. An important concept here is retained size. The retained size of an object found in the heap is how much memory could be freed if this object could be garbage collected. That includes the object itself, but also child objects which no other objects outside of the retained set has references to.

MAT gives you an overview of where your memory is allocated and has some good tooling on finding suspicious allocations that could be memory leaks.

So to find my memory leak, I used the dominator tree tab which sorts the allocated objects by retained heap
and I soon discovered that the GLRendered object held far too many references to a large 512x512 texture.

The tool becomes even more valuable when the leaking objects are small but many. The dominator tree tell you right away that you have a single object holding a much larger retained heap than you would expect it to.

If you want to learn more, check out this speech by Patrick Dubroy on Android memory management from Google IO 2011 where he explains the Android memory model in more detail.

adb - getting inside your Android simulator

I've been working with persistent state between launches of my Android application and wanted to easily inspect my application data between launches. I discovered the power of the Android Debug Bridge or adb tool. In short a dev tool that lets you hook up against a running Android simulator.

You'll find it in your sdk folder under platform-tools. On my machine:
C:\Program Files (x86)\Android\android-sdk\platform-tools\adb.exe

A few handy commands:

- List currently running devices:
>adb devices
List of devices attached
emulator-5554   device

- Launch a terminal shell against a running device
>adb -s emulator-5554 shell

Then you can do your ordinary linux stuff like cd, ls, cat etcetera to get to know your Android device.
My happiest discovery was that the preference file you fetch and save via the SharedPreference API:
SharedPreferences settings = getSharedPreferences("OthelloLegendsPrefs", 0);

are located under the path
/data/data/se.noren.android.othello/shared_prefs/OthelloLegendsPrefs.xml

and is a simple XML file which you can inspect and edit.

So, to pull a file from the simulator to your development computer
>adb -s emulator-5554 pull <remotefile> <localfile>

Similarly, to upload a file to the device
>adb -s emulator-5554 push <localfile> <remotefile>

For more info on adb, http://developer.android.com/guide/developing/tools/adb.html.

How to get character encoding correct on Google App Engine

Character encodings can be a primary trigger for stomach ulcers. My Swedish web applications deployed on Google App Engine have had great difficulties to behave when presented with user input containing for example Swedish characters Å, Ä and Ö.

So here's a recipe for treating such characters with respect.

• Don't use ISO-8859-1 as character encoding. Just don't.
• Instead specify you JSP's to use UTF-8 with something like 

<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>

However, this might not be enough unfortunately. Current browsers might not set a character encoding even if specified in the HTML page or form.

So if you aren't already using Spring, add spring-web to your application and add one of the Spring filters first in your filter chain.

<filter>

   <filter-name>SetCharacterEncoding</filter-name>

   <filter-class>org.springframework.web.filter.CharacterEncodingFilter</filter-class>

   <init-param>

       <param-name>encoding</param-name>

       <param-value>UTF-8</param-value>

   </init-param>

   <init-param>

      <param-name>forceEncoding</param-name>

      <param-value>true</param-value>

   </init-param>

</filter>

<filter-mapping>

   <filter-name>SetCharacterEncoding</filter-name>

   <url-pattern>/*</url-pattern>

</filter-mapping>

That's all folks.

Mapping DNS domain to Google App Engine application

I have been working on a small application for promoting and selling books on popular science. I made a prototype and bought a domain name so I can setup all trackers and Amazon Affiliate accounts. I deployed the prototype as a Google App Engine application. Using GAE you can have a site up in 3 minutes.

When you deploy your GAE app, it will automatically get DNS mapped as yourappid.appspot.com. So in my case the app resides as http://popularsciencemedia.appspot.com in the Google cloud. From the Swedish DNS supplier Loopia I had bought the domain popularsciencemedia.com and now simply wanted to DNS remap popularsciencemedia.appspot.com to my own http://www.popularsciencemedia.com.

This is not as easy as you might think! The obvious solution would be to create a DNS CNAME entry with an alias to reroute traffic to www.popularsciencemedia.com to http://popularsciencemedia.appspot.com but but Google won't allow that. So I thought I'd write a few lines on how you do it to avoid you from the same misery. The purpose here is to map www.popularsciencemedia.com since Google App Engine only can be mapped to sub domains. www is good enough for me.

• You need to sign up for Google Apps for you application. Go to https://www.google.com/a/cpanel/domain/new and enter your registrered domain name and fill in the forms. You will in this process create a system user for this Google App. In my case I created the user admin@popularsciencemedia.com. Google Apps let's you create emails, calenders and much more for your app.
• This process will need you to verify that you own the domain. There are several ways to do this, I thought the easiest way was to add a Google verification code in the DNS registry as a TXT entry. The DNS record now looks like this. If you can't create TXT records with your DNS provider Google has other mechanisms for verifying that you own the domain.

$ORIGIN popularsciencemedia.com.
$TTL 300
@ SOA ns1.loopia.se. registry.loopia.se. (
    1312117911
    3H ; Refresh after three hours
    1H ; Retry after one hour
    1W ; Expire after one week
    1D ) ; Minimum one day TTL

@    IN 3600 NS ns1.loopia.se.
@    IN 3600 NS ns2.loopia.se.

@    IN 3600 A 194.9.94.85
@    IN 3600 A 194.9.94.86
@    IN 3600 TXT google-site-verification=Uy4magKHIasdeEOasdgs6b7qYt8tR8

*    IN 3600 CNAME ghs.google.com.

www    IN 3600 CNAME ghs.google.com.

• Notice the additions. The TXT entry maps against the value you get from the Google App sign up. This makes it possible for Google Apps to verify that you own the domain you claim to own. Then, add also a CNAME mapping to ghs.google.com for the subdomain www since this is the sub domain we want to use.
• Now to to your Google App account, go to something like https://www.google.com/a/cpanel/popularsciencemedia.com. Under Sites-> Services -> YourAppId (App Engine) there should be a possibility to add a new address under the domain you have registrered. 
• I add www so my app is mapped as www.popularsciencemedia.com.
• After a waiting a very short while all DNS changes seems to be working and I can surf to http://www.popularsciencemedia.com/

Profiling an Android application tutorial

I'm spending some spare time on an Android Reversi game which could need some performance tuning. After figuring out how the tooling works for Android profiling it works like a charm.

There are two ways to profile an application, using the debugging server DDMS or manually decide which parts of the code base are interesting for inspection. DDMS could be useful if you are inspecting code you might not be able to recompile. DDMS can also be used to inspect memory usage and more.

The easiest approach however is to use the debug interface provided by the Android API in your sources to specify when to start generating profiling information and when to end.

public int[] alphabeta(Board b, int maxDepth, long maxTime) {
      
      Debug.startMethodTracing("othello_profiling");

      // Here goes code to profile
  
      Debug.stopMethodTracing();
      return result;
   }

Run your program and you'll see in the VM logs when the profiler kicks in. (As usual the performance of your app in the emulator will sink to the bottom when profiling is enabled)

Now you got your profiling info written to the SD card of your Android emulator device. If you run into permission issues when writing to the SD card, add something like this to your Android Manifest.

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />     

To fetch the file to your development computer use the adb tool that comes with the Android SDK. On my Windows machine I did something like this.


C:\Program Files (x86)\Android\android-sdk\platform-tools>adb pull /sdcard/othello_profiling.trace c:\temp\othello_profiling.trace
126 KB/s (2266911 bytes in 17.491s)


The tool traceview can interprete the file.

C:\Program Files (x86)\Android\android-sdk\tools>traceview.bat c:\Temp\othello_profiling.trace


Voila! You get a profiling view similar to what you get from common profilers like JProfiler, hprof etcetera. Here you can see each methods execution time and which parents and children methods it has connection to and much more.

Theres more you can do with the trace file. Traceview can also show you each threads exectution and calls in chronological order. You can simple zoom in on the interesting parts.

You may also want to try the tool dmtracdedump to create graphs over your call stack. See the Android documentation for more information.