spring

Question

reason of di

Concepts

Container

core of Spring Framework
create, wire, configure and manage objects(Beans)
types: BeanFactory container and ApplicationContext container

IoC

Inversion of Control

Dependency Injection

Types
- Constructor-based dependency injection
- Setter-based dependency injection
example

// Normal way
public class TextEditor {
   private SpellChecker spellChecker;

   public TextEditor() {
      spellChecker = new SpellChecker();
   }
}

// IoC way, constructor-based
public class TextEditor {
   private SpellChecker spellChecker;

   public TextEditor(SpellChecker spellChecker) {
      this.spellChecker = spellChecker;
   }
}

// IoC way, setter-based
public class TextEditor {
   private SpellChecker spellChecker;

   // a setter method to inject the dependency.
   public void setSpellChecker(SpellChecker spellChecker) {
      this.spellChecker = spellChecker;
   }
}

// requeried bean configuration
<?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-3.0.xsd">

   <!-- Definition for textEditor bean using inner bean -->
   <bean id = "textEditor" class = "com.tutorialspoint.TextEditor">
      <property name = "spellChecker">
         <bean id = "spellChecker" class = "com.tutorialspoint.SpellChecker"/>
      </property>
   </bean>

</beans>

Beans

object that is instantiated, assembled, and otherwise managed by a Spring IoC Container
use context.getBean(<bean id>) to create a bean
example:

// HelloWorld.java
public class HelloWorld {
    private String message;

    public void getMessage() { return message; }
}

// MainApp.java
public class MainApp {
   public static void main(String[] args) {
      ApplicationContext context = new ClassPathXmlApplicationContext("Beans.xml");
      HelloWorld obj = (HelloWorld) context.getBean("helloWorld");
      obj.getMessage();
   }
}

// Beans.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-3.0.xsd">

   <bean id = "helloWorld" class = "com.tutorialspoint.HelloWorld">
      <property name = "message" value = "Hello World!"/>
   </bean>

</beans>

scope
- singleton, single instance per container
- prototype, any number of object instances
- request, session, global-session
support life cycle hooks and inheritance
support definition through java Annotation

@Configuration
public class HelloWorldConfig {
   @Bean
   public HelloWorld helloWorld(){
      return new HelloWorld();
   }
}

// equivalent to
<beans>
   <bean id = "helloWorld" class = "com.tutorialspoint.HelloWorld" />
</beans>