Inner Beans

In Spring framework, whenever a bean is used for only one particular property, it’s advise to declare it as an inner bean. And the inner bean is supported both in setter injection ‘property‘ and constructor injection ‘constructor-arg‘.

See a detail example to demonstrate the use of Spring inner bean.

 DrawingApp.java

public class DrawingApp {
    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext("spring.xml");
        Circle circle = (Circle) context.getBean("circle");
        circle.draw();
    }
}

Circle.java

public class Circle { 

    private Point center;
    public Point getCenter() {
        return center;
    }
    public void setCenter(Point center) {
        this.center = center;
    }
    public void draw() {
        System.out.println("Center is  (" +center.getX()+","+center.getY()+")");
    }
}

Point.java 

public class Point {
    private int x;
    private int y;
   
    public int getX() {
        return x;
    }
    public void setX(int x) {
        this.x = x;
    }
    public int getY() {
        return y;
    }
    public void setY(int y) {
        this.y = y;
    }
   
}

Often times, you may use ‘ref‘ attribute to reference the “Point” bean into “Circle” bean, person property as following :

spring.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN 2.0//EN" "http://www.springframework.org/dtd/spring-beans-2.0.dtd">

<beans>
    <bean id="circle" class="com.example2.Circle">
        <property name="center" ref="centerpoint" />
    </bean>
    <bean id="centerpoint" class="com.example2.Point">
        <property name="x" value="0" />
        <property name="y" value="0" />
    </bean>
</beans>

In general, it’s fine to reference like this, but since the  point bean is only used for Circle bean only, it’s better to declare this Point Bean as an inner bean as following

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN 2.0//EN" "http://www.springframework.org/dtd/spring-beans-2.0.dtd">
<beans>
    <bean id="circle" class="com.example2.Circle">
        <property name="center">
           <bean class="com.example2.Point">     
                    <property name="x" value="0" />
                    <property name="y" value="0" />
            </bean>
        </property>
    </bean>
</beans>


Output :  Center is (0,0)