How to fix ''Error creating bean with name defined in class path resource"?

I took a bean class and inside this class I took an inner bean. I've configured them both in configuration file. But When I'm trying to run my Java application it is throwing me exception saying "Error creating bean with name 'id1' defined in class path resource [spconfig.xml]: Error setting property values; nested exception is org.springframework.beans.NotWritablePropertyException: Invalid property 'db' of bean class [SampleBean]: Bean property 'db' is not writable or has an invalid setter method. Did you mean 'DB'?" why I'm not getting welcome to inner bean? what to do?

Property defined correctly in configuration file.

SampleBean.java

public class SampleBean {

    private DemoBean db;

    public void setDB(DemoBean db) {
        this.db=db;
    }

    public void show() {
        db.m1();
    }
}

DemoBean.java

public class DemoBean {

    public void m1() {
        System.out.println("Welcome to inner bean");
    }
}

spconfig.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="id1" class = "SampleBean">
 <property name="db">
 <bean class = "DemoBean"/>
 </property>
 </bean>
 </beans>

Client.java

import org.springframework.beans.factory.*;
import org.springframework.beans.factory.xml.*;
import org.springframework.core.io.*;
public class Client {
    public static void main(String[] args) {


    Resource res = new ClassPathResource("spconfig.xml");
    @SuppressWarnings("deprecation")
    BeanFactory factory = new XmlBeanFactory(res);
    Object o = factory.getBean("id1");
    SampleBean sb = (SampleBean)o;
    sb.show();
    }

}

Spring uses the JavaBeans naming convention.

With a field like

private DemoBean db;

and a declaration like

<property name="db">

Spring expects a setter named setDb, not setDB like you have in your code. Change it to setDb and your main method will correctly print

Welcome to inner bean