Dependency injection

Dependency Injection
With Spring
Presenter: Pranav Kumar, Mindfire Solutions

Agenda
 What is Dependency Injection ?
 Dependency Inversion Principle
 Types of Dependency Injection
 Why DI ?
 Understand DI with StudentSystem Example
 DI with Spring Configuration
 Benefits of DI
 DI Using Annotations

DI: What it is ?
 Dependency Injection is a software design pattern that allows removing hard
coded dependencies and making it possible to change them at run-time.
 Dependencies are injected into objects by a container.
 Also Known as Inversion Of Control (IoC).

DI Principle
 The dependency inversion principle was postulated by Robert C. Martin
 The principle states:
• Always use abstractions (Interface)
• Objects should define their dependencies only through
 Constructor arguments
 Arguments to a factory method.
 Properties which are set on the object instance

DI: Types
 Setter injection - Dependent module exposes a setter method that the framework uses to
inject the dependency.
 Constructor injection - Dependencies are provided through the class constructor.

DI: Why ?
 Dependency Injection enables 'POJO programming'
 POJO programming facilitate
- Simplicity
- Effective separation of concerns
- Proper unit testing
 Well-factored and well-tested code
- Tends to be well-designed code
- Evolves well into maintainable systems

Example: The Student System
 To improve your skills in Java development, you decide to develop a student system.
 You decide to use a file to store student information.
 You create a class FileStudentDAO responsible for writing and reading to the file
 You create a class StudentSystem responsible for performing the logic of the system.
 You’ve learned that it’s a good thing to program to interfaces

DI:

The DefaultStudentSystem

Works! Or .. ?
The system is a big success – University of Oslo wants to adopt it!

Works! Or .. ?
You make a new implementation of the StudentDAO for University of Oslo, a
MySQLStudentDAO:

Works! Or .. ?
 Won’t work! The FileStudentDAO is hard-coded into the StudentSystem:
 The DefaultStudentSystem implementation is responsible for obtaining a StudentDAO.
 Dependent both on the StudentDAO interface and the implementation
public class DefaultStudentSystem implements StudentSystem {
private StudentDAO studentDAO = new FileStudentDAO();
...

Problems: Student System
How to deploy the StudentSystem at different locations?
 Develop various versions for each location?
• Time consuming
• Confusing and error-prone
• Requires more efforts for versioning
Solution:
 Use Dependency Injection!
• More specific term derived from the term Inversion of Control

Traditional Vs DI

DI: With Spring
 Bean: A class that is managed by a Spring IoC container
 Setter based DI: Provide a public set-method for the dependency reference.

DI: Spring Configuration
 Configuration: How to instantiate, configure, and assemble the objects in your application
• The Spring container accepts many configuration formats
• XML based configuration most common.

studentDAO
Injected into
studentSystem
Refers to the
studentDAO
bean
Refers to the
studentDAO attribute in Java
class

Summary

DI: Benefits
 Easier to swap implementations of dependencies
 The system can be re-configured without changing the compiled code
 Easier testing
 Improved re-usability of your classes
 Cleaner code

DI: Using Annotations
 Annotation is a meta-tag – applied to classes, methods...
• Associated with program behaviour
 @Component
• Defines a class as a Spring container managed component
 @Autowired
• Tells Spring to inject a component
 Classpath scanning for components
• <context:component-scan base-package="org.example"/>

@Component
public class StudentService
{
@Autowired
StudentDAO studentDAO;
//constructors
//Getter setter methods
}
@Component
public class StudentDAO
{
@Override
public String toString() {
return "This is StudentDAO";
}
}
StudentService.java StudentDAO.java

<bean id="studentDAO" class="com.mindfire.spring.student.dao.StudentDAO"/>

<bean id="studentService" class="com.mindfire.spring.student.service.StudentService">
<property name="studentDAO" ref="studentDAO" /> 
<constructor-arg ref="studentDAO"> </constructor-arg> 
</bean>

<context:component-scan base-package="com.mindfire.spring.student" />
Annotation configuration
XML Configuration

Resources
 Books
• Rod Johnson, Juergen Hoeller: Expert One-on-One J2EE
• Craig Walls and Ryan Breidenbach: Spring in Action
 The Spring reference documentation
• www.springframework.org

Questions ?

Dependency injection

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