Meeting 5
When: Monday, October 13th, 2008 - 19:00
Where: Freihaus HS4
In what follows we provide a simple formal model of this process. ,
Inhaltsverzeichnis
Guice
Download the slides (pdf, 115KB).
You can also view the slides online.
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).
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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>
Wow, you're unnecessarily angry about this. Aren't there walhes being poached in the ocean, people dying in Darfur, or other causes better served by your unmitigated fury? With regard to XML config: At first I thought this was bad because, I believed the config file was used to be able to change things out without having to recompile the jar, but then I realized that was ridiculous and not a viable use case. That doesn't sound accurate. Of course you can change an XML file without having to recompile anything, and there are totally viable use cases for this specifically IMO. I've seen a few in action.On a side note, I also agree that XML config is kind of a pain in the ass and I like the annotation-based config for many use cases, but on this point I'm not sure I agree with you (or maybe I just don't understand what your exact point is).As for this one: now I realize their are passionate developers out there like Bob who want to bring Java back to it’s roots and give it frameworks that work within the language instead of around the language . Assuming you are describing Spring, I'm not sure I agree with this either. If you ask me, Spring works around JEE APIs more than it works around the Java language itself. In fact, Java's introspection capabilities and dynamic class instantiation facilitate Spring's IoC model, so there's a symbiotic relationship in there somewhere.*dons flame-retardant jodhpurs in preparation for response* - max
Springframework
Download the slides (pdf, 0.6MB).
You can also view the slides online.
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Content
Spring should be part of every Java application today; at least so says the lecturer Christoph Pickl. If it is not Spring, it should be any other framework which enables dependency injection. Spring ...
- ... is a leightweight inversion of control and aspect oriented container framework.
- ... makes developing JEE applications easier and more fun!
Spring's mission statements are:
- JEE should be easier to use.
- It is best to program to interfaces, rather than classes [...]
- JavaBeans offer a great way of configuring applications.
- OO is more important than any implementation technology [...]
- Checked exceptions are overused in Java [...]
- Testability is essential [...]
Each and every bean declarations got a beans root node declaring some namespaces:
<source lang="xml">
<?xml version="1.0" encoding="UTF-8"?>
<beans
xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.0.xsd"
>
</beans>
</source>
One could, for example, define a single dao-instance, and reuse that instance by injecting it into different controller classes:
<source lang="xml">
<bean id="myDao" class="jsug.MyDao" />
<bean id="myCtrl1" class="jsug.MyController1">
<property name="dao"> <ref bean="myDao" /> </property>
</bean>
<bean id="myCtrl2" class="jsug.MyController2">
<property name="dao"> <ref bean="myDao" /> </property>
</bean> </source>
Using the springframework today is very non-intrusive. Just load your application context and that's it:
<source lang="java">
// single code dependency to spring throughout our whole application
import org.springframework.context.support.ClassPathXmlApplicationContext;
public class App {
public static void main(String[] args) {
String[] beanDefinitions = { "/beans.xml" };
new ClassPathXmlApplicationContext(beanDefinitions);
}
public App() {
// spring will invoke this constructor for us
}
public void init() {
// and spring will also invoke this initialize method for us
}
} </source>
Sometimes, your beans have to be initialized via a certain method. To do this, declare your bean as follows:
<source lang="xml">
<bean id="myBean" class="jsug.MyBean" init-method="init">
<constructor-arg index="0" value="someStringValue" /> <constructor-arg index="1" ref="someOtherBean" /> <property name="firstName" value="Christoph" />
</bean> </source>
The corresponding class: <source lang="java"> public class MyBean {
private final String someString;
private String firstName;
// #1 first of all, the bean will be created with proper constructor args
public MyBean(String someString, Bean b) {
this.someString = someString;
}
// #2 afterwards, all properties will be set via setter methods
public void setFirstName(String firstName) {
this.firstName = firstName;
}
// #3 finally, the initialize method will get invoked
public void init() {
// initialize bean in here
}
} </source>
Links
- http://www.springframework.org ... Official Website
- http://parleys.com/display/...Injection%20Frameworks ... Short presentation "Comparing Dependency Injection Frameworks"
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.
