Java Spring Mvc Overview

This is my notes about Java Spring MVC. Content is about 30 minutes read, its purpose is:

• To refresh your memory if you already worked on it.
• Or if you are totally new, it will give you an overview, so you can kick start your first Java Spring MVC application.

# Installation

Windows:

• JDK
• Download Spring libraries (then add to build path)

# ClassPathXmlApplicationContext

At first, in Java Spring Core module, there is a Bean Factory (an objects container). You can imagine the Bean is like Object in OOP. BeanFactory allows you to access objects defined in Xml bean configuration file.

So why using object (Bean) defined in XML Bean Configuration file but not instantiate the objects in source code directly?

Using this approach, you can de-couple dependencies

You mentioned about BeanFactory, but what is its relation with ClassPathXmlApplicationContext?

The ClassPathApplicationContext (another objects container) provides additional functionalities over the BeanFactory. For example, you can use getBean function just like with BeanFactory -

./src/bean-config.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="beanId" class="Object">
       <property name="message" value="Application Context Example"/>
   </bean> 
</beans> 

./src/test_application_context.java

ApplicationContext context = new ClassPathXmlApplicationContext("bean-config.xml");
Object obj = (Object) context.getBean("beanId");
obj.publicMethod();

About advantages of ApplicationContext over the BeanFactory, beside the additional functionalities, any other bold differences?

BeanFactory is lazy loading, it only instantiate the object after calling getBean() method and call a method in the object. ApplicationContext instantiate the object as soon as the ApplicationContext is created.

About ApplicationContext, as my observation in your example, an ApplicationContext created by ClassPathXmlApplicationContext method. Is there other methods to instantiate an ApplicationContext?

Yes, you are correct, the most popular methods:

• ClassPathXmlApplicationContext: Load the Bean configuration xml file from class path (e.g. folder ./src).
• FileSystemXmlApplicationContext: Load from the system file (e.g. folder C:\beans.xml)
• XmlWebApplicationContext: Load over the internet.

In my imagination, each Bean defined in the XML file has an ID. Is it Singleton Bean (Object)?

It is about scope of Beans. The Spring framework has predefined the following scopes:

• singleton (default) (different for each ApplicationContext, i.e., different xml bean configuration file)
• prototype (instantiate new object every time getBean method is called)
• request (different for each request)
• session (different for each session)
• global_session (different for each session) Example: ``` xml

> Type of Beans is defined in "class" attribute, I guess that we have to cast the result to desired class when using getBean method (of ApplicationContext). 
>Is it correct?

Yes, you are correct, following is an example (notice the PrototypeBean casting):
```java
public class TestPrototypeBean {
      public static void main(String[] args) {
	      ApplicationContext context = new ClassPathXmlApplicationContext("beans.xml");
	      PrototypeBean prototypeBeanA = (PrototypeBean)context.getBean("prototypeBean");
	      System.out.println(prototypeBeanA); 
	      
	      PrototypeBean prototypeBeanB = (PrototypeBean)context.getBean("prototypeBean");
	      System.out.println(prototypeBeanB);
	         
	      System.out.println("Is Prototype Bean A and Prototype B are same ? " +  (prototypeBeanA==prototypeBeanB));
      }
}

<bean id="beanID" init-method="myCustomInitMethod" destroy-method="myCustomDestroyMethod" class="MyClass" />

<bean id="humanBean" class="Human">
    <property name="name" value="TBD"/>
    <property name="dateOfBirth" value="1900/01/01"/>
</bean>

<bean id="studentBean" parent="humanBean">

class Room{
    private int roomSize;
    private String teacherName;
    
    Room(int roomSize) {
        this.roomSize = roomSize;    
    }
    public int getRoomSize() {
        return roomSize;
    }
    public int getTeacherName() {
        return teacherName;
    }
    public int setTeacherName(String teacherName) {
        this.teacherName = teacherName;
    }
}

<bean id="roomBeanId" class="Room">
    <constructor-arg value="10"/>
    <property name="teacherName" value="Ramond"/>
</bean>

public Object getBean(String beanId) {
     ...
     Object object = new Room(10);
     ((Room)object).setTeacherName("Ramond");
     return object;
}

<bean id="roomBeanId" class="Room">
    <constructor-arg value="10"/>
    <property ref="teacherBeanId" value=""/>
</bean>

<bean id="teacherBeanId" class="Teacher">
    <constructor-arg value="Raymond"/>
</bean>

public class ConnectionUtil{
   private Connection instance;
   private ConnectionUtil() {
   }
   public Connection getInstance() {
       return instance;   
   }
   public String justAnotherMethod() {
       return "String";
   }
}

<bean id="beanId" class="ConnectionUtil" factory-method="getInstance"/>

public class Car {
    private String name;
    private String color;
}

public class CarFactory {
    private String name;
    CarFactory(String name) {
        this.name = name;
    }
    public Car getCar() {
        return new Car();
    }
}

<bean id="carFactoryBeanId" class="CarFactory"> 
    <constructor-arg value="CarFast"/>
</bean>

<bean id="carBeanId" class="Car" factory-bean="carFactoryBeanId" factory-method="getCar" />

<bean id="countryBeanId" class="Country">
     <property name="stateNameList" >
        <list>
            <value>State 1</value>
            <value>State 2</value>
            <value ref="stateBeanId3" />
            <value ref="stateBeanId4" />
        </list>
     </property>
     <property name="stateNameSet">
         <set>
            <value>State 1</value>
            <value>State 2</value>
            <value ref="stateBeanId3" />
            <value ref="stateBeanId4" />
         </set>
     </property>
     <property name="stateCode">
         <map>
            <entry key="stateName" value="+84"/>
            <entry key-ref="stateBeanId" value-ref="stateBeanId2"/>
         </map>
     </property>
</bean> 

public class DataSource {
    private Properties dbProperties;
    public Properties getDbProperties() {
        return dbProperties;
    }
    public void setDbProperties(Properties properties) {
        this.dbProperties = properties;
    }
}

<?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="datasource" class="DataSource">             
              <property name="dbProperties">
                     <props>
                           <prop key="username">root</prop>
                           <prop key="password"></prop>
                           <prop key="host">localhost</prop>
                           <prop key="port">3306</prop>
                     </props>            
              </property>
        </bean>
</beans>