Meeting 5

When: Monday, October 13th, 2008 - 19:00

Where: Freihaus HS4

Inversion of Control

Download the slides (pdf, 38KB).

Florian Motlik introduced todays topic IoC, which follows the slogan "don't call us, we'll call you". Your code just implements the specific logic but the actual control itself is passed to a certain controller class. A trivial example would be iterating over a collection.

Instead of writing ...

var someList = [ "Larry", "Curly", "Moe" ];
for(var i = 0; i < someList.length; i++) {
  var name = someList[i];
  println(name.length);
}

... one could just say ...

[ "Larry", "Curly", "Moe" ].each { name | println(name.length)}

... and that's all. By passing a function to a certain method provided by the datastructure itself (make use of closures), the actual iterating over each field is hidden and you don't have to worry about it anymore; your could will be invoked for each item, and therefore control is inverted.

Dependency injection (originally introduced by Martin Fowler) is a specific form of IoC. Dependencies of a class will be injected for you (constructor/setter/field injections are possible) and therefore the coupling is not necessarily reduced, but its shifted outside the code (in case of Spring into a XML file).

Guice

Download the slides (pdf, 115KB).
Download the sourcecode (pdf, 13KB).

Basic facts

Of course theory is nice, but it becomes useful when you start using a concret technology which enables dependency injection for you, such the Google Guice framework presented by Jan Zarnikov. It provides a pure Java style (no external XML configuration which makes it possible to misstype a name; refactorings also changes configuration!) with all Java5 features such as generics (type safety) and annotations (injecting is realized via Guice own annotations).

Simple example

The following chapter shows a simple application with a database connection and a mocked connection for its unit tests:

The UserController class is subject of the JUnit test and requires a database (in fact, just something that implements the DAO interface). Because unit tests should only test a certain unit (and not the database management system or anything else), the controller class will get another implementation of DAO injected if it is run by JUnit (namely TestDAO instead of JdbcDAO).

Injecting objects is just as easy as annotating the constructor with the @Inject annotation (you could also have used it at the field itself, or provided a setter method): <source lang="java"> public class UserController {

 private final DAO dao;

 @Inject
 public UserController(DAO dao) {
   this.dao = dao;
 }

 public boolean login(String username, String password) {
   List<User> users = this.dao.getUsers();
   // ...
 }

} </source>

Of course you also have to configure your dependencies somewhere: <source lang="java"> public class AppModule extends AbstractModule {

 public void configure() {
   bind(DAO.class).to(JdbcDAO.class);
 }

}

public class TestModule extends AbstractModule {

 public void configure() {
   bind(DAO.class).to(TestDAO.class);
 }

} </source>

To startup the Guice framework, you have to do following first: <source lang="java"> public class MyApplication {

 public static void main(String[] args) {
   // configure Guice with AppModule
   Injector injector = Guice.createInjector(new AppModule());

   // userController already got its proper DAO instance injected
   UserController userController = injector.getInstance(UserController.class);

   boolean isLoggedIn = userController.login("Foo", "Bar");
 }

} </source>

Or use your special configuration for unit test: <source lang="java"> public class UserControllerTest {

 private UserController userController;

 @Before
 public void setup() {
   // configure Guice with TestModule, instead with AppModule
   Injector injector = Guice.createInjector(new TestModule());
   this.userController = injector.getInstance(UserController.class);

´ }

 // some tests...

} </source>

Links

• http://code.google.com/p/google-guice/ ... Official Guice website
• http://docs.google.com/View?docid=dd2fhx4z_5df5hw8 ... Official userguide
• http://crazybob.org/2007/06/introduction-to-guice-video-redux.html ... Presentation about Guice
• http://www.youtube.com/watch?v=FFXhXZnmEQM ... Other presentation about Guice

Springframework

Download the slides (pdf, 0.6MB).

Content

Held by Christoph Pickl.

Links

• springframework.org - Official Website
• Comparing Dependency Injection Frameworks - Short presentation at parelys.com

Gallery

• 

  Sun representative Ivan and Florian Motlik

• 

  Christoph Pickl talking about Spring's mission statement

• 

  The excited Crowd

Thanks to Jan for providing these pictures.