Dependency Injection Basics

Dependency Injection Basics

Topics

• Dependency Injection (DI)
  • BeanFactory
  • Autowiring
  • ApplicationContext

DI Basic Concept

• A kind of Inversion of Control (IoC)
• Hollywood Principle:
  • Don't call me, I'll call you
• Container resolves (injects) dependencies of components by setting implementation object (push)
• Martin Fowler calls Dependency Injection

DI Basic Concept

• Flexible
  • Avoid adding lookup code in business logic
• Testable
  • No need to depend on external resources or containers for testing
• Maintainable
  • Allows reuse in different application environments by changing configuration files instead of code

DI Basic Concept

Two Dependency Injection Variants

• Constructor DI
  • Dependencies are provided through the constructors of the component
• Setter DI
  • Dependencies are provided through the JavaBean style setter methods of the component
  • More popular than Constructor dependency injection

DI Basic Concept

Constructor Dependency Injection

public class ConstructorBean {

	private String valueToInject;
	
	public ConstructorBean(final String valueToInject){
		this.valueToInject = valueToInject;
	}
}

DI Basic Concept

Setter Dependency Injection

public class SetterBean {

	private String valueToInject;

	public void setValueToInject(String valueToInject) {
		this.valueToInject = valueToInject;
	}		
}

DI in Spring

Sub-topics

• BeanFactory Interface
• XmlBeanFactory implementation
• Bean Configuration file
  • Setter DI
  • Constructor DI
• Beans
• Injection parameters

DI in Spring

BeanFactory

• BeanFactory object is responsible for managingbeans and their dependencies
• Your application interacts with Springs DI container through BeanFactory interface
  • BeanFactory object has to be created by the application typically XmlBeanFactory

+++

• BeanFactory object, when it gets created, reads bean configuration file and performs the wiring
• Once created, the application can access the beans via BeanFactory interface

DI in Spring

BeanFactory Implementations

• XmlBeanFactory
  • Convenience extension of DefaultListableBeanFactory that reads bean definitions from an XML document

DI in Spring

Reading Xml Config file via XmlBeanFactory class

public class XmlConfigWithBeanFactory {
  
	public static void main(String[] args) {
    
		XmlBeanFactory factory = 
			new XmlBeanFactory(new FileSystemResource("beans.xml"));
		SomeBeanInterface b = 
			(SomeBeanInterface) factory.getBean("nameOftheBean";
  
	}
}

DI in Spring

BeanFactory Configuration File

• Each bean is defined using bean tag under the root of the beans tag
• The id attribute is used to give the bean its default name
• The class attribute specifies the type of the bean

DI in Spring

Example: Setter DI

public class SetterBean {

	private String valueToInject;

	public void setValueToInject(String valueToInject) {
		this.valueToInject = valueToInject;
	}		
}

// <bean id="setterBean" 
//	class="io.fractus.springframework.tutorial.dependency.injection.setterdi.SetterBean">    	
//	 <property name="valueToInject" value="value injected via setter"></property>
// </bean>

DI in Spring

Example: Constructor DI

public class ConstructorBean {

	private String valueToInject;
	
	public ConstructorBean(final String valueToInject){
		this.valueToInject = valueToInject;
	}
}

// <bean id="constructorBean" 
//  class="io.fractus.springframework.tutorial.dependency.injection.constructordi.ConstructorBean">
//     <constructor-arg value="value injected via constructor"/>
// </bean>

DI in Spring

Beans

• The term "bean" is used to refer any component managed by the BeanFactory
• The "beans" are in the form of JavaBeans (in most cases)
• no arg constructor
• getter and setter methods for the properties

+++

• Beans are singletons by default
• Properties the beans may be simple values or references to other beans
• Beans can have multiple names

DI in Spring

Injection ParameterTypes

• Spring supports various kinds of injection parameters
• Simple Values
• Beans in same factory
• Beans in another factory

+++

• Collections
• Externally defined properties
• You can use these types for both setter and constructor injections

DI in Spring

Injecting Simple Values

//    <bean id="injectSimpleValues" 
//	 class="io.fractus.springframework.tutorial.dependency.injection.injectsimplevalues.InjectSimpleValues">	 
//        <property name="name">
//            <value>dstar55</value>
//        </property>
//        <property name="year">
//            <value>2017</value>
//        </property>
//        <property name="height">
//            <value>9.99</value>
//        </property>
//        <property name="springFun">
//            <value>true</value>
//        </property>
//        <property name="yearInSeconds">
//            <value>1103760000</value>
//        </property>
...

DI in Spring

Bean Naming

• Each bean must have at least one name that is unique within the containing BeanFactory
• A bean can have multiple names
• Specify comma or semicolon-separated list of names in the name attribute

+++

• Name resolution procedure
• If a <bean> tag has an id attribute, the value of the id attribute is used as the name
• If there is no id attribute, Spring looks for name attribute
• If neither id nor name attribute are defined, Spring use the class name as the name

DI in Spring

Bean Naming Example

DI in Spring

Autowiring Properties

• Beans may be auto-wired (rather than using <ref>)
  • Per-bean attribute autowire
  • Explicit settings override
• autowire="byName"
• Bean identifier matches property name

+++

• autowire="byType"
• Type matches other defined bean
• autowire="constructor"
• Match constructor argument types
• autowire="autodetect"
• Attempt by constructor, otherwise "type"

DI in Spring

Benefits of Dependency Injection

• Frees your object from resolving dependencies
• Simplifies your code
• Improves code reusability
• Promotes programming to interfaces
• Improves testability
• Opens the door for new possibilities(control over life cycle)