Inversion of Control, Dependency Injection in Spring

1. The Spring Framework:
A brief introduction to
Inversion of Control
Snehal Masne
www.iSnehal.com

2. What is Spring?
2 Things:
◦ An Inversion of Control (IoC) Container
◦ Utilities that provide a consistent (and simple!)
API to many other technologies (JDBC, ORM,
AOP, Declarative Transactions, etc)

3. Guiding Principles of Spring
◦ Minimize dependencies on Spring
◦ Minimize dependencies between all layers of an
application.
◦ All application code should be testable, without an
application server or other complex environment
◦ Fully factor the APIs (the 90% case should be
accomplished in one line of code!)

4. What is Inversion of Control (IoC)?
(besides yet another confusing term for a simple concept)
IoC is all about Object dependencies.
Traditional "Pull" approach:
◦ Direct instantiation
◦ Asking a Factory for an implementation
◦ Looking up a service via JNDI
"Push" approach:
◦ Something outside of the Object "pushes" its dependencies
into it. The Object has no knowledge of how it gets its
dependencies, it just assumes they are there.
The "Push" approach is called "Dependency Injection".

5. The BookDemo Application:
(A simple CRUD app)
User Interface
Domai
n
Service Layer
Model
Data Access Layer
DB

6. Pull Example
public class BookDemoServicePullImpl implements BookDemoService {
public void addPublisherToBook(Book book) {
BookDemoFactory factory = BookDemoFactory.getFactory();
BookDemoDao dao = factory.getBookDemoDao();
String isbn = book.getIsbn();
if (book.getPublisher() == null && isbn != null) {
Publisher publisher = dao.findPublisherByIsbn(isbn);
book.setPublisher(publisher);
}
}
}

7. Push Example
(Dependency Injection)
public class BookDemoServiceImpl implements BookDemoService {
private BookDemoDao dao;
public void addPublisherToBook(Book book) {
String isbn = book.getIsbn();
if (book.getPublisher() == null && isbn != null) {
Publisher publisher = dao.findPublisherByIsbn(isbn);
book.setPublisher(publisher);
}
}
public void setBookDemoDao(BookDemoDao dao) {
this.dao = dao;
}
}

8. BookDemoService Unit Test

9. Definitions
Dependency Injection is the act of injecting dependencies into an
Object.
Inversion of Control is the general style of using Dependency
Injection to wire together application layers.
Hence Spring is an Inversion of Control container. That is, it is a
container that handles Dependency Injection for you.

10. Why is Dependency Injection better?
2 reasons:
◦ Loose Coupling
◦ Testability
Loose Coupling is improved because you don't hard-code
dependencies between layers and modules. Instead you
configure them outside of the code. This makes it easy to swap
in a new implementation of a service, or break off a module and
reuse it elsewhere.
Testability is improved because your Objects don't know or care
what environment they're in as long as someone injects their
dependencies. Hence you can deploy Objects into a test
environment and inject Mock Objects for their dependencies
with ease.

11. How Spring does Inversion of Control
◦ Write a configuration file in which you name
concrete "beans" for the interfaces between your
layers.
◦ "Wire" the application together by stating which
beans are dependent on each other.
◦ Instantiate a Spring object called an
ApplicationContext. This is a type of bean factory
that will instantiate all your other beans and handle
dependency injection.

12. Example Spring applicationContext.xml
<beans>
<bean id="bookDemoDao"
class="com.bookdemo.dao.hibernate.BookDemoDaoHibernateImpl">
<property name="sessionFactory">
<ref local="sessionFactory"/>
</property>
</bean>
<bean id="bookDemoService"
class="com.bookdemo.service.impl.BookDemoServiceImpl">
<property name="bookDemoDao">
<ref bean="bookDemoDao"/>
</property>
</bean>
</bean>

13. Bootstapping the IoC container
To start an app using IoC:
◦ Create an ApplicationContext object and tell it
where applicationContext.xml is.
ApplicationContext appContext =
new ClassPathXmlApplicationContext("classpath*:applicationContext.xml");
This just has to be done once on startup, and can be done
in the main() method or whatever code bootstraps the
application.

14. Bootstapping the IoC container
The appContext holds a single copy of each bean
declared in applicationContext.xml, so you could ask the
Context for any bean by name:
MyService service = (MyService)appContext.getBean("myService");
But don't! That is a "Pull" technique that treats the
ApplicationContext like a Factory.
Instead, make sure that applicationContext.xml connects
every bean to every other bean that needs it. None of the
beans thus have a dependency on spring.jar

15. Bootstapping the IoC container
For web applications the situation is simpler:
Web applications are bootstrapped by the web
container based on the web.xml file. Hence creating
an ApplicationContext on startup is as simple as a
single declaration in web.xml:
<listener>
<listener-class>
org.springframework.web.context.ContextLoaderListener
</listener-class>
</listener>

16. A Spring app with no dependencies
on Spring?
When bootstrapping spring from web.xml, there are
only two pieces:
◦ The applicationContext.xml file
◦ A single tag within web.xml
Not a single line of Java code!
Therefore, not one of your custom classes has a
dependency on spring.jar

17. The BookDemo App

18. What else has Spring got?
Spring provides either implementations or fully-factored API
wrappers over these technologies:
◦ JDBC and DAOs
◦ ORM: Hibernate, iBatis, TopLink and others
◦ Declarative Transaction Support (without a full J2EE
app server)
◦ Aspect-Oriented Programming
◦ Remote calls and Web Services (Axis)
◦ EJBs

19. Resources
Spring Website:
http://www.springframework.org/
Download Spring with sample applications:
http://prdownloads.sourceforge.net/springframework/spring-framework-1.2.8-with-dependencies.zip?
download
Rod Johnson's book on Spring:
http://www.powells.com/biblio?PID=719&cgi=product&isbn=0764574833
Wikipedia:
http://en.wikipedia.org/wiki/Spring_Framework_%28Java%29
EasyMock:
http://www.easymock.org/
Echo2 (Rich Web Inteface framework)
http://www.nextapp.com/platform/echo2/echo/