Inner beans in spring are those beans which are under the scope of another beans. In spring framework normally to inject a bean, we define it first on container with some id and pass the reference of that bean wherever dependency is needed, using the ref attribute.

Spring inner bean Example:

beans.xml

<bean id="CustomerBean" class="com.evon.example.Customer">
    <property name="person" ref="PersonBean" />
</bean>
<bean id="PersonBean" class="com.evon.example.Person">
    <property name="name" value="Sumit" />
    <property name="address" value="address1" />
    <property name="age" value="28" />
</bean>

But in case where bean is used only by the one bean, then we don't require to define it seperately. We can define inner bean as follows:

Example code of Spring inner bean

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"
  xmlns:c="http://www.springframework.org/schema/c"
  xsi:schemaLocation="http://www.springframework.org/schema/beans
      http://www.springframework.org/schema/beans/spring-beans.xsd">

  <bean id="college" class="com.evon.example.innerBeanExample.College">
    <property name="collegeName" value="MIT"/> 
    <property name="student">
        <bean class="com.evon.example.innerBeanExample.Student">
            <property name="studentName" value="Sumit"/>
            <property name="age" value="22"/>
            <property name="course" value="MCA"/>
        </bean>
    </property>
  </bean>  
 </beans>

In above configuration dependency is injected through setter, in case of constructor replace the above bean config from following code:

  <bean id="college" class="com.evon.example.innerBeanExample.College">
    <constructor-arg value="MIT"/>
    <constructor-arg>
        <bean class="com.evon.example.innerBeanExample.Student">
            <property name="studentName" value="Sumit"/>
            <property name="age" value="22"/>
            <property name="course" value="MCA"/>
        </bean>
    </constructor-arg>
  </bean>

College.java

package com.evon.example.innerBeanExample;
public class College {
    private String collegeName;
    private Student student;

    College(String collegeName, Student student){
        this.collegeName=collegeName;
        this.student = student;
    }
    College(Student student){
        this.collegeName="MIT";
        this.student = student;
    }    
    public String getCollegeName() {
        return collegeName;
    }    
    public void setCollegeName(String collegeName) {
        this.collegeName = collegeName;
    }    
    public Student getStudent() {
        return student;
    }    
    public void setStudent(Student student) {
        this.student = student;
    }
}

Student.java

package com.evon.example.innerBeanExample;
public class Student {
    private String studentName;
    private int age;
    private String course;
    public String getStudentName() {
        return studentName;
    }
    public void setStudentName(String studentName) {
        this.studentName = studentName;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public String getCourse() {
        return course;
    }
    public void setCourse(String course) {
        this.course = course;
    }
}

Main.java

package com.evon.example.innerBeanExample;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.FileSystemXmlApplicationContext;
import com.evon.example.innerBeanExample.Student;
public class Main {
    public static void main(String[] args) {
        ApplicationContext context = new FileSystemXmlApplicationContext("/src/main/java/com/evon/example/innerBeanExample/bean.xml");
        College college = (College) context.getBean("college");
        Student student = college.getStudent();
        System.out.println("College name : "+college.getCollegeName());
        System.out.println("Student name : "+student.getStudentName());
        System.out.println("Student age : "+student.getAge());
        System.out.println("Student course : "+student.getCourse());
    }
}

Output:

College name : MIT
Student name : Sumit
Student age : 22
Student course : MCA