Sunday, October 6, 2013

Apache Shiro with Spring framework, Java config and WebApplicationInitializer

Recently I was adding Apache Shiro security framework to Spring based web application which is using Java config and doesn't have xml configuration at all, not even web.xml

Apache Shiro documentation is mostly using xml examples so it took some time to put it all together in Java config based application.

Central part of Shiro security is  a realm. Here is how official Shiro documentation defines realms:

"A Realm is a component that can access application-specific security data such as users, roles, and permissions. The Realm translates this application-specific data into a format that Shiro understands so Shiro can in turn provide a single easy-to-understand Subject programming API no matter how many data sources exist or how application-specific your data might be."

Shiro comes with number of out-of-the-box Realm implementations that connects directly to database, to LDAP, etc, but in this example we will use custom Realm implementation since we want to access user data via our own user manager.

First, we have SecurityConfig. java where all security related beans are defined.
package com.xxx.yyy.config;

import com.xxx.yyy.security.CustomSecurityRealm;
import org.apache.shiro.spring.LifecycleBeanPostProcessor;
import org.apache.shiro.spring.security.interceptor.AuthorizationAttributeSourceAdvisor;
import org.apache.shiro.web.mgt.DefaultWebSecurityManager;
import org.apache.shiro.web.mgt.WebSecurityManager;
import org.springframework.aop.framework.autoproxy.DefaultAdvisorAutoProxyCreator;
import org.springframework.beans.factory.config.MethodInvokingFactoryBean;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.DependsOn;

@Configuration
public class SecurityConfig {

    @Bean
    public CustomSecurityRealm customSecurityRealm(){
        return new CustomSecurityRealm();
    }

    @Bean
    public WebSecurityManager securityManager(){
        DefaultWebSecurityManager securityManager = new DefaultWebSecurityManager();
        securityManager.setRealm(customSecurityRealm());
        return securityManager;
    }

    @Bean
    public LifecycleBeanPostProcessor lifecycleBeanPostProcessor(){
        return new LifecycleBeanPostProcessor();
    }

    @Bean
    public MethodInvokingFactoryBean methodInvokingFactoryBean(){
        MethodInvokingFactoryBean methodInvokingFactoryBean = new MethodInvokingFactoryBean();
        methodInvokingFactoryBean.setStaticMethod("org.apache.shiro.SecurityUtils.setSecurityManager");
        methodInvokingFactoryBean.setArguments(new Object[]{securityManager()});
        return methodInvokingFactoryBean;
    }

    @Bean
    @DependsOn(value="lifecycleBeanPostProcessor")
    public DefaultAdvisorAutoProxyCreator defaultAdvisorAutoProxyCreator(){
        return new DefaultAdvisorAutoProxyCreator();
    }

    @Bean
    public AuthorizationAttributeSourceAdvisor authorizationAttributeSourceAdvisor(){
        AuthorizationAttributeSourceAdvisor authorizationAttributeSourceAdvisor = new AuthorizationAttributeSourceAdvisor();
        authorizationAttributeSourceAdvisor.setSecurityManager(securityManager());
        return authorizationAttributeSourceAdvisor;
    }

}

First bean defined is our custom security realm implementation. We will take a look at it in a moment, but for now let just look where is it used. And we don't have to go far, it is used by shiro security manager defined as second bean in SecurityConfig.java. We use DefaultWebSecurityManager since we plan to use Shiro for securing our applications URLs.
We just create an instance and inject our custom securtity realm bean to it.

After that we have few Shiro beans and we just inject our security manager bean wherever required.

Let's look how our custom security realm implementation looks like.
package com.xxx.yyy.security;

import com.xxx.yyy.security.Role;
import com.xxx.yyy.security.Permission;
import com.xxx.yyy.User;
import com.xxx.yyy.UserManager;
import org.apache.shiro.authc.*;
import org.apache.shiro.authc.credential.SimpleCredentialsMatcher;
import org.apache.shiro.authz.AuthorizationInfo;
import org.apache.shiro.authz.SimpleAuthorizationInfo;
import org.apache.shiro.authz.permission.WildcardPermission;
import org.apache.shiro.realm.AuthorizingRealm;
import org.apache.shiro.subject.PrincipalCollection;
import org.springframework.beans.factory.annotation.Autowired;

public class CustomSecurityRealm extends AuthorizingRealm {

    @Autowired
    private UserManager userManager;

    @Override
    protected AuthorizationInfo doGetAuthorizationInfo(PrincipalCollection principals) {

        Set roles = new HashSet<>();
        Set permissions = new HashSet<>();

        Collection principalsList = principals.byType(User.class);
        for (User user : principalsList) {
            for (Role role : user.getRoles()) {
                roles.add(role.getName());
                for (Iterator iterator = role.getPermissions().iterator(); iterator.hasNext(); ) {
                    Permission permission = iterator.next();
                    permissions.add(new WildcardPermission(permission.name()));
                }
            }
        }

        SimpleAuthorizationInfo info = new SimpleAuthorizationInfo(roles);
        info.setRoles(roles);
        info.setObjectPermissions(permissions);

        return info;
    }

     @Override
    protected AuthenticationInfo doGetAuthenticationInfo(AuthenticationToken token) throws AuthenticationException {

        UsernamePasswordToken upat = (UsernamePasswordToken) token;
        User user = userManager.getByUsername(upat.getUsername());
        if(user != null && user.getPassword().equals(new String(upat.getPassword()))) {
            return new SimpleAuthenticationInfo(user, user.getPassword(), getName());
        }
        else {
            throw new AuthenticationException("Invalid username/password combination!");
        }
    }
}

Our security  realm implementation will be used both for authentication and authorization so we extend  AuthorizingRealm which extends AuthenticatingRealm.
Autowired UserManager is our application service for accessing users and their roles and permissions.

doGetAuthenticationInfo method is used to authenticate user, and it has one argument - AuthenticationToken which holds username and password entered by user in login form.
Inside this method we check if user for given username exists and if password matches the password enetered by user. If those conditions are satisfied, we return AuthenticationInfo object with our user object as principal. We use Shiro's SimpleAuthenticationInfo implementation of AuthenticationInfo interface.
If user doesn't exist or password doesn't match we throw Authentication exception.
This is very simple example, in real project we will probably use Shiro's HashedCredentialsMatcher for checking username/password combination since we probably want to use encoded passwords.

doGetAuthorizationInfo method is used by Shiro to get roles and permissions for specific principal(s) so it has PrincipalCollection as argument. For every principal in given collection (usually there will be only one) we will get roles and permissions and set them to AuthorizationInfo which will be returned by this method. We use Shiro's SimpleAuthorizationInfo implementation for this purpose. The code should be pretty self-explanatory.

Now that we have our basic security infrastructure defined, we need to integrate it with our web application.
In order to protect urls we need to add Shiro filter to our web app descriptor.  As I mentioned in the beginning of this post, we don't use web.xml but instead we have WebApplicationInitializer.
here is how it looks like:

package com.xxx.yyy;

import com.xxx.yyy.config.DataConfig;
import com.xxx.yyy.config.SecurityConfig;
import com.xxx.yyy.config.WebConfig;
import org.springframework.web.WebApplicationInitializer;
import org.springframework.web.context.ContextLoaderListener;
import org.springframework.web.context.support.AnnotationConfigWebApplicationContext;
import org.springframework.web.filter.DelegatingFilterProxy;
import org.springframework.web.servlet.DispatcherServlet;

public class WebInitializer implements WebApplicationInitializer {

    @Override
    public void onStartup(ServletContext container) {

        // Create the 'root' Spring application context
        AnnotationConfigWebApplicationContext rootContext = new AnnotationConfigWebApplicationContext();
        rootContext.register( DataConfig.class, SecurityConfig.class);

        // Manage the lifecycle of the root application context
        container.addListener(new ContextLoaderListener(rootContext));


        // Create the dispatcher servlet's Spring application context
        AnnotationConfigWebApplicationContext dispatcherContext = new AnnotationConfigWebApplicationContext();
        dispatcherContext.setServletContext(container);
        dispatcherContext.setParent(rootContext);
        dispatcherContext.register(WebConfig.class);


        // Register and map the dispatcher servlet
        ServletRegistration.Dynamic dispatcher = container.addServlet("dispatcher", new DispatcherServlet(dispatcherContext));
        dispatcher.setLoadOnStartup(1);
        dispatcher.addMapping("/");


        container.addFilter("shiroFilter", new DelegatingFilterProxy("shiroFilterBean", dispatcherContext))
                           .addMappingForUrlPatterns(null, false, "/*");

    }
}
The code speaks for itself. We have root context with DataConfig (which contains JPA configuration, but this is not relevant for this story) and our SecurityConfig explained earlier.

In order to configure our Spring MVC  we added dispatcher context,and registered WebConfig class which contains required beans.

Last bean is most relevant since it defines Shiro filter which is configured to intercept all URLs.
We use DelegatingFilterProxy as filter implementation, and we provide "shiroFilterBean" for bean name.
This bean is defined in our WebConfig class, so let's take a look at it:
package com.xxx.yyy;

import org.apache.shiro.spring.web.ShiroFilterFactoryBean;
import org.apache.shiro.web.mgt.WebSecurityManager;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.MessageSource;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.support.ReloadableResourceBundleMessageSource;
import org.springframework.web.servlet.LocaleResolver;
import org.springframework.web.servlet.config.annotation.*;
import org.springframework.web.servlet.i18n.CookieLocaleResolver;

@Configuration
@EnableWebMvc
@ComponentScan(basePackages = {"com.xxx.yyy.web"})
public class WebConfig extends WebMvcConfigurerAdapter {


    @Autowired
    private WebSecurityManager securityManager;


    @Bean
    public VelocityConfigurer velocityConfig() {
        VelocityConfigurer configurer = new VelocityConfigurer();
        configurer.setResourceLoaderPath("/WEB-INF/templates");
        Properties props = new Properties();
        props.put("output.encoding", "UTF-8");
        props.put("input.encoding", "UTF-8");
        configurer.setVelocityProperties(props);
        return configurer;
    }

    @Bean
    public VelocityViewResolver viewResolver() {
        VelocityViewResolver resolver = new VelocityLayoutViewResolver();
        resolver.setExposeSpringMacroHelpers(true);
        resolver.setContentType("text/html;charset=UTF-8");
        resolver.setSuffix(".vm");
        return resolver;
    }

    @Bean
    public MessageSource messageSource() {
        ReloadableResourceBundleMessageSource messageSource = new ReloadableResourceBundleMessageSource();
        messageSource.setBasenames("/WEB-INF/localization/messages");
        messageSource.setDefaultEncoding("UTF-8");
        messageSource.setCacheSeconds(10);
        return messageSource;
    }

    @Bean
    public LocaleResolver localeResolver() {
        CookieLocaleResolver localeResolver = new CookieLocaleResolver();
        localeResolver.setCookieName("LOCALE");
        return localeResolver;
    }


    @Bean
    public ShiroFilterFactoryBean shiroFilterBean(){
        ShiroFilterFactoryBean shiroFilter = new ShiroFilterFactoryBean();
        Map<String, String> definitionsMap = new HashMap<>();
        definitionsMap.put("/login.jsp", "authc");
        definitionsMap.put("/admin/**", "authc, roles[admin]");
        definitionsMap.put("/**", "authc");
        shiroFilter.setFilterChainDefinitionMap(definitionsMap);
        shiroFilter.setLoginUrl("/login.jsp");
        shiroFilter.setSecurityManager(securityManager);
        return shiroFilter;
    }
}


This is typical java config based Spring MVC configuration.
Beside usual Spring MVC beans, we added ShiroFilterFactoryBean (at the end) which will be referenced  from our WebApplicationInitializer , remember ?
ShiroFilterFactoryBean requires WebSecurityManager and since we defined it in SecurityConfig.java , all we need to do here is to autowire it to private field (line 12) and inject it to shiroFilterBean (line 60)

In between we just have velocity template engine configuration beans as well as localization beans which are not relevant for security framework.

Now, when user attempts to  access any application URL, Shiro filter will intercept it, and delegate security checking to shiroFilterBean which will use securityManager bean to determine if the user has right to access this specific URL.
If user is not authenticated yet, Shiro filter will redirect user to login page. Here is simple example of login.jsp:

<form action="" method="post" name="loginform">
  <table align="left" border="0" cellpadding="3" cellspacing="0">
    <tr>
 <td>Username:</td>
 <td><input maxlength="30" name="username" type="text" /></td>
    </tr>
    <tr>
 <td>Password:</td>
 <td><input maxlength="30" name="password" type="password" /></td>
    </tr>
    <tr>
 <td align="left" colspan="2">
   <input name="rememberMe" type="checkbox" /><span style="font-size: x-small;">Remember Me</span>
 </td>
    </tr>
    <tr>
 <td align="right" colspan="2"><input name="submit" type="submit" value="Login" /></td>
    </tr>
  </table>
</form>


And that's it... we have basic Shiro security setup. The way how application persists users, their roles and permission is application specific. All that Shiro has to know about this will get it from our security realm implementation.


UPDATE: Added /login.jsp to  ShiroFilterFactoryBean definitionsMap.  Without it Shiro will not handle login form submit correctly.  Although it looks like we are restricting anonymous access to it, Shiro will know that it should allow it.


UPDATE 2: Added imports to code snippets .

Sunday, September 15, 2013

Building native android app with Sencha Cmd on linux

Recently I started to work on project with  Sencha Touch framework (client requirement) and my first task  was to build simple screen with some GUI components and make it run on android as native app.

I was required to use Sencha Architect for development and app building but since I use Linux  and Sencha Architect supports app packaging only on windows and Mac, I had to use Sencha Cmd utility for building native app.

Before I go any further let me throw some info on my development environment:

OS: Ubuntu 13.04
Java 1.7
Ruby 1.9.3
Sencha Arcitect 2.2.2 build 991
Sencha Cmd v3.1.3.372


After reading quick start docs it all looked easy, but as it often happens when using new tools, I quickly run into some frustrating problems.

I made simple app, tested it in browser, and I thought I was ready to package it for android. Before building native app, I configured packager file (which contains mobile app configuration, such as app name, package id, android API level, android sdk path etc...)

Fired up terminal, navigated to project folder, and as Sencha documantation instructs, I run build command : sencha app build native

And... after few seconds my apk is generated... NOOOT!



First bump, tool complained that it misses nimblekit.jar.

I haven't found anything about this in official Sencha documentation, but some people out there on the net  experienced same problem and found the solution: copy st-res folder from $sencha_cmd_install/stbuild to your project folder.
I am not sure why Sencha Cmd doesn't know where to find st-res folder, documentation doesn't says anything about it (AFAIK). Same problem happens on windows. However, copying st-res folder solves the problem both on linux and windows.




OK, problem solved, let's continue...

sencha app build native ...  and... after few seconds my apk is generated...  NOOOT!

Second problem caused me saying "WTF" quite a few times.

Command runs for a long time (minute or two) and then reports "Zip add failed" error on Icon.png file.
Since I haven't change icon configuration in packager component, I expected default icons to be used.
After unsuccessful googling for the problem and running the command few times, I noticed that Icon.png file in build/native/res/drawable-mdpi grows up to 2 G !!! Same goes for drawable-mdpi.
But in drawable-hdpi , Icon size is normal, about 3.5 K.  I checked packager icon config and realized that default icon config doesn't define low dpi and medium dpi at all.
On windows this problem doesn't appear event though icon config is same.
Obviously, there is some bug on linux version of Sencha Cmd that causes the problem.
I managed to solve the problem by defining low and medium dpi icons (36 and 48 px).


Since I 'm pretty new to Sencha, it's probably to early to judge, but my first impression is not very positive.
Besides the problems I described what's bothering me  the most is that Sencha Architect doesn't have code completion.

Feel free to throw a comment, especially if you had similar experiences with Sencha.

Sunday, August 18, 2013

Developing 3d game for Android - part 2

Well, it is done!

I finished version 1.0 of my 3D space arcade game based on  Rajawali framework and published it on  Google Play.

You can check it out here: https://play.google.com/store/apps/details?id=com.mabuga.gravity.android.before4&hl=en

And here is gameplay video on youtube:





I enjoyed developing with Rajawali, it was very fun and the issues I run into were not critical.
Issue that I spent most time dealing with is rendering explosions. I wanted to use simple animated sprites for explosions and Rajawali supports that via Particle primitive. Basically, you set spritesheet texture to a Particle, define how many tiles spritesheet has, and Rajawali shader will render the particle using given spritesheet creating animation from it's tiles.

What I wanted is just one big animated sprite to render explosion. And that was where the problem appears - the size of single particle.  Actually there are two problems related to particle size.

1. There is a bug in Particle code that is causing particle look smaller as as the distance from world origin gets bigger, regardless of camera position.

2. Particle is rendered as textured point and unfortunately OpenGL implementations limit the size of the rendered point. Even worse, different implementation has different values for point size limit. OpenGL specification doesn't guarantee point size bigger then 1 px.

My workaround for first problem was to update particle code (GL shader and java) to fit my needs. Unfortunately, my solution did not solve the bug in generic way (since the problem is not just bug in calculation but conceptual as well) so I could not commit it back to the project.

Second problem actually can not be really solved since it it hardware specific, so I just accepted imposed point size limit. Quick test on few devices I own showed me that point size limit is around 100 px on my HTC Explorer with 480x320 px screen and around 500 px on my Samsung Galaxy Tab 2 with 1280x800 px screen. That was ok for my explosions, however, there are probably devices with big screen and small point size limit and my explosions probably look bad on those.

Also, I experienced some weird rendering issues when returning from RajawaliActivity with RajawaliRendered to normal Activity, but that was solved by setting android:hardwareAccellerated to false on activity elements in android manifest.

So, there were actually very few problems with Rajawali framework.
Besides the issues, I would like to share some thoughts about challenges of optimizing the game for low CPU powered devices.

Crucial part of almost any game is collision detection. Rajawali already has built in collision detection features, but checking collisions too often can have significant CPU cost causing glitches in gameplay.
In every frame (about 30 times per second) we have to check collision of every moving object with all other objects on the scene. Let's say that we have 30 moving objects (ships + bullets) and about 20 - 30 static objects (meteors) on the scene at the same time, this means we have to handle ~ 30 * 50 = 1500 checks per frame which makes 45 000 per second. And that's a lot.
In order to minimize number of collision checks per frame, I divided game space into "sectors". For static objects I only calculate in which sector they are in game init phase, and for moving objects I had to calculate in which sector they are every time they move.
Also, I added global sector map that maps sector name to a list of objects currently in that sector. This map is also updated on every object sector calculation.
So, now for every moving object we only need to check collisions with objects currently in same sector , and this is usually 0-5 objects. This reduces CPU usage big time. This way number of collision checks doesn't increase at all if we add more static (non moving) objects.

Another thing I optimized in order to achieve better game performance is to handle object creation little bit smarter then just create them when ever I need them. Actually, I recycle them.
Whenever new bullet is fired or new ship should appear, it should be added to scene, and whenever ship is destroyed or bullet hits something, it should be removed from the scene.  But adding and removing new objects can also be costly, especially if it happens often. If we instantiate new bullet every time ship fires, we spend CPU cycles on initiation of the bullet. After that we have to add it to a scene (renderer), and this also takes some CPU time since renderer must register new object to it's list of objects in synchronized way. Rajawally uses synchronized "copy on write" list for this purpose.

So, instead of creating and destroying new objects all the time, I used well known technique called "object pooling" . It boils down to creating certain number of objects and store them in a pool (a list or a map) , then take it from that pool when we need new object and return it when the object should be destroyed/removed.
In my case, I had two different pools, one for bullets, another for explosions. Using two pools for specific object types instead using one generic pool also makes things faster since we don't have to do any type checking.
If new object is required and the pool is empty, we simple create new object and add it to the pool, so our pool will grow depending on demands.

When object need to be removed from the scene, I don't actually remove it, I just hide it by setting visibility to false and by setting its position far out of the camera view. I also remove it from my internal list of movable objects, so I don't move it or check collisions for it while it is in the pool (kind of object passivation).

When taken from the pool, object is set to visible, its position/rotation is updated as needed, and it is added to movable objects list, so it can be managed by game logic (moving and collision checking).
And that's it. Simple, but effective.

OpenGL handles rendering in most optimized way it can, and besides the few tricks described above there are several other things developer can do to make the game run smooth:
- use low poly 3D models
- keep number of objects on the scene as low as you can
- keep objects static (non movable) whenever possible

I covered some basic and common things related to game optimization, but every game is specific and it has specific things that can be optimized.

Friday, July 5, 2013

Developing 3d game for Android

To develop real 3D game is not trivial task no matter what platform you target.

Five or six  years ago I was experimenting with JOGL - Java Binding for the OpenGL API trying to develop 3D game in java for PC.
In that time it was common opinion that 3D games can only be devleoped with C++ since java is too slow. Even then that was not true. JOGL delegates all rendering to OpenGL , and in initialization phase you can create complex objects in OpenGL memory so later you can reference it from java and give short instructions to OpenGL in order to move or rotate that object. That means that java only have to manage positions and rotations of the objects, check collisions and of course game logic.

Since rendering is most demanding part of 3D game and it is handled by OpenGL ,  C++ being faster then java doesn't matter so much. Of course , in Java you will probably have to optimize more, but it is definitely fast enough.

I was very happy how my experiment turned out. Although I never finished and published the game, I managed to make nice little space combat with real LAN multiplayer so players could fight in space. It had laser guns, missiles that follow the targets, radar, etc. It was running smooth both on Windows and Linux.

Today, even cheap smartphones (like my HTC Explorer) have similar CPU power and memory my PC had at time I was playing with JOGL.

Smartphones support OpenGLES - The Standard for Embedded Accelerated 3D Graphics. Most phones support OpenGL ES 2.0, only older models still use OpenGL ES 1.x.

Working with JOGL, developer actually has to know much about OpenGL since JOGL is not a 3D engine, it is just a bridge between Java and OpenGL which allows us to call OpenGL commands from Java code.

In recent years several good 3D libraries emegred, making 3D apps development much easier. Since they manage low level OpenGL calls, you can focus more on your game logic.

There is a great OpenGL ES 2.0 based 3D engine for Android platform called Rajawali, developed by Dennis Ippel (great job Dennis). There is also Android Demo app that demonstrate Rajawali engine features.

Basic feature that every 3D engine provides is loading external 3D model, adding it to 3D scene and manipulating it's position and rotation. Most 3D engines provide much more features including collision checking, physics, lighting, various visual effects, etc.

Rajawali's set of features amazed me so I decided to try it out by  making simple 3D game that focuses on playability  rather then on visual attraction. As a kid a loved games like Galaga and Phoenix because they were dynamic and addictive. My goal is to achieve similar gameplay dynamic in 3D world.  Although I limited ships freedom to move to a plane (can't go up or down), 3D perspective gives me more "space" (player can see distant enemy ships much earlier that it would be possible if the view was from above) and of course 3D models of ships and meteors look more realistic then it can be achieved in 2D.

Here are first screenshots of the game :




So far everything works fine on HTC Explorer (Android 2.3.5) and HTC Desire C (Android 4.0). 
I'll keep you posted about my progress here.

Sunday, June 23, 2013

Developing android apps with Kotlin

For those who haven't heard yet, Kotlin is new JVM language developed by JetBrains, creators of IntelliJ IDEA, which many people consider the best java IDE.

Kotlin is still in development, beta should be released in following months, according to Kotlin forum, but it is already stable enough to do some real work in it.

Being java programmer for a long time, I always liked it for simplicity of the language itself, stability, large ecosystem of frameworks and libraries, cross-platform support etc...

But sometimes I missed few things that other languages have and Java doesn't.  Mostly things that would boost my productivity and make me write less code that is still elegant and readable.

One of those things is passing a function as an argument to another function (feature known as higher order functions or closures).  Writing listeners every time you need to use some kind of callback function can be really tedious. If you can pass callback function as an argument to another function/method it can really save time and lines of code. Javascript and Actionscript, languages that I used for some client side development, and that I considered inferior to java in many ways , have this for a long time.
Kotlin, as almost every other modern language has this feature.

Another thing that can save code and time is having default values of method arguments, so passing those arguments can be omitted in method call. This reduces, if not removes, the need for method overloading since it is more flexible way of having different combinations of arguments. 
I liked this feature in ActionScript , but Kotlin goes one step further: it supports default argument values, but it also supports passing arguments by name. This means, for example,  that you can pass only first and fourth argument, skipping second and third if they have default values. In action script if you want to pass forth argument you also need to pass all arguments before it.

There are lots of other features that make Kotlin code shorter and more elegant then Java code. 

Elegance and ease of use is not  Kotlin's only advantage over Java,   there are also features that make Kotlin more powerful, although with greater power comes more responsibility so in general it requires programmer to be more careful. 
To mention just few of them: null-safety, immutability, extension functions, lambdas, inline functions, operator overloading, range expressions... etc.
To see full list of Kotlin features check out docs page and  don't miss comparison to Java.

Many modern languages have very similar feature list like Kotlin: Java 8, Scala, Ceylon, Clojure (clojure is little bit different animal since it's Lisp dialect). So why to choose Kotlin?

For Java programmer Kotlin should be the easiest to switch to (except of course Java 8), since one of main goals of Kotlin is to be Java-compatible. This means that Kotlin should provide much better interoperability with Java then Scala, Clojure, Ceylon... 
Also, Kotlin compiles as fast as java , that is another goal of Kotlin authors.

Scala is Kotlin's main competitor so it is good to know what are similarities and differences between them.

Since Android apps run on Java and Kotlin is highly interoperable with Java it is possible to write android apps using Kotlin.

Here are some useful links on how to start with it:



For now, using IntelliJ IDEA is the easiest way to develop with Kotlin. 

I look forward to make myself more familiar with Kotlin using it on my new android app and I will share my experience here soon.








Friday, January 25, 2013

How much memory do images in android apps really use?

Recently I had some OutOfMemory issues in one my apps.
After analyzing logcat I could see that one image used as background is taking more then 2MB even though the image in png format was less then 200KB.

At the first look it was confusing, but it is actually only logical when you make yourself think about it little bit more.
Image size in compressed format such as (jpg,png) is one thing , and Bitmap decoded from it and used for actually drawing on screen is something completely different.

After quick research I found exact formula to calculate memory required for bitmap.

Memory required by bitmap image depends on image dimensions and bitmap type.

M =  WIDTH * HEIGHT * 4    (for ARGB_8888 Bitmap -  if you need transparency)
M =  WIDTH * HEIGHT * 2    (for RGB_565 Bitmap - no transparency)

where result M is in bytes and WIDTH and HEIGHT are in pixels.

So using few large images as backgrounds in your layouts can quickly lead to OutOfMemory exception on devices with low memory per app limit.

Tuesday, October 2, 2012

Can android app be protected from piracy?

If you are android developer , you know how much skill, effort, creativity and innovation is required to make competitive android application.

After investing your knowledge, creativity and time, you probably don't want your app to be copied, repackaged, and sold by unauthorized parties.

So, what can we do to protect our work?

There are many methods,like those suggested in this presentation, that could be used for this purpose.
Unfortunately, there are tools available that allow people to crack many of this protection methods, even people with very little or no knowledge at all of android development and java programming in general.

In my apps , I use simple technique that can be described in one sentence.
Check if app was tampered with, and if so, make it stop working properly.

I prefer this method since it doesn't require connection to google market server or any other server on the  net.

Checking the signature is trivial, but it will not be of much use if the attacker can easy find the part of your code that does the checking and remove it.
So the hardest part of protecting application this way is to hide the code that does the checking best we can.

Using obfuscator is basic stuff, so we use it as a first line of defence. It will make decompiled code very hard to read and to follow program flow. The problem is that attacker doesn't actually need to understand how your app works in general, he just needs to find your protection and disable it.
Since all methods for checking app authenticity (signature checking that we use is one of these methods) are well known, attacker will probably focus on finding them in your code. Obfuscated code will rename your variables and methods to some meaningless names, throw in bunch of junk stuff etc., but calls to android framework methods will remain unchanged and easy to find in decompiled code.

Here is an example of the code for getting app signature:

Signature[] myAppSignatures = ctx.getPackageManager().getPackageInfo(ctx.getPackageName(), PackageManager.GET_SIGNATURES).signatures;

More advanced obfuscators such as Dexguard can help hiding this code by using reflection to call getPackageInfo method instead of directly calling this. You also want to encrypt method name strings like "getPackageInfo" so that attacker can not simply search decompiled code for it.
Dexguard even comes with configuration template that already contains settings for doing this, you only need to uncomment few lines to use it.

After using reflection in combination with string encryption it is very hard for attacker to find this method call in decompiled code.

However, this level of protection is still not good enough. Why?
Because  tools like anti-lvl intercept getPackageInfo() method even if you use reflection with encrypted strings.

It  does it by replacing all method.invoke(...) calls with it's own code (hook) that checks method name after the method object is already initiated. If method name equals "getPackageInfo" hook returns PackageInfo with fake signatures (your original signatures instead of repackaged app signature). Otherwise, if the method is not relevant for checking app authenticity, hook will just call method.invoke method as you originally intended.

Can we do something about this?

We can encrypt whole class instead of only encrypting string used for method invocation via reflection. With encryption applied,  Method.invoke(...) calls become invisible for cracking tool so it can not replace it with hook.
So, encrypting the class containing signature checking will stop anti-lvl tool (current version at least) to crack the app.

You should be aware of the fact that  encrypted classes can be decrypted, actually intercepted after decryption, but as far as I know anti-lvl doesn't yet do that.

This means that with some additional manual work of decrypting classes before applying anti-lvl, signature checking method still can be cracked.