Spring MVC

What is MVC?

• MVC Pattern stands for Model-View-Controller Pattern.
• It helps in separating the business logic, presentation logic, and navigation logic. 
  • Models are responsible for encapsulating the application data.
  • The Views render a response to the user with the help of the model object. 
  • Controllers are responsible for receiving the request from the user and calling the back-end services.

Spring vs Spring MVC

Spring Framework is an open source application framework and inversion of control container for the Java platform. Spring MVC (Model–view–controller) is one component within the whole Spring Framework, to support development of web applications.

Why Spring MVC?

• Spring MVC is a Java framework which is used to build web applications.
• It implements all the basic features of a core spring framework like Inversion of Control, Dependency Injection.
• Spring MVC provides an elegant solution to use MVC in spring framework by the help of DispatcherServlet.

Spring MVC request flow

• When user sends a request, the DispatcherServlet first receives the request.
• The DispatcherServlet consults the HandlerMapping and invokes the Controller associated with the request.
• The Controller processes the request by calling the appropriate service methods and returns a ModelAndView object to the DispatcherServlet. The ModelAndView object contains the model data and the view name.
• The DispatcherServlet sends the view name to a ViewResolver to find the actual View to invoke.
• Now, the DispatcherServlet will pass the model object to the View to render the result.
• The View, with the help of the model data, will render the result back to the user.

Spring MVC – Typical Project structure

• The DispatcherSevlet, as the name indicates, is a single servlet or the front controller that manages the entire request-handling process. When a request is sent to the DispatcherServlet, it delegates the job by invoking the appropriate controllers to process the request. Like any other servlet, the DispatcherServlet needs to be configured in the web deployment descriptor as shown below. The name of the dispatcher servlet configuration comes from web.xml.

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" id="WebApp_ID" version="2.5">
  <servlet>
        <servlet-name>mvc-dispatcher</servlet-name>
        <servlet-class> org.springframework.web.servlet.DispatcherServlet </servlet-class>
        <load-on-startup>1</load-on-startup>
    </servlet>
    <servlet-mapping>
        <servlet-name>dispatcher</servlet-name>
        <url-pattern>*.htm</url-pattern>
    </servlet-mapping>
    <welcome-file-list>
        <welcome-file>index.jsp</welcome-file>
    </welcome-file-list>
</web-app>

• Here, the servlet name is mvc-dispatcher. By default, the DispatcherServlet will look for a file name mvc-dispatcher-servlet.xml to load the Spring MVC configuration. This filename is formed by concatenating the servlet name (“mvc-dispatcher“) with “-servlet.xml“. Here, we use the the url-pattern as “.htm” in order to hide the implementations technology to the users.

• As we have listed index.jsp as the welcome file, the index.jsp will be invoked first when we execute the Spring web application. This is the only jsp file outside the WEB-INF directory, and it is here to provide a redirect to the DispatcherServlet. All the other views should be stored under the WEB-INF directory so that they can be invoked only through the controller process.
• The mvc-dispatcher-servlet.xml is shown below. It shows that we have configured annotation driven processing and all the components should be scanned from the base package “com.test”. Here, the InternalResourceViewResolver is used to resolve the view name to the actual view. The prefix value + view name + suffix value will give the actual view location. 

<beans xmlns="http://www.springframework.org/schema/beans"
	xmlns:context="http://www.springframework.org/schema/context"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xmlns:mvc="http://www.springframework.org/schema/mvc"
	xsi:schemaLocation="
        http://www.springframework.org/schema/beans     
        http://www.springframework.org/schema/beans/spring-beans-3.2.xsd
        http://www.springframework.org/schema/mvc 
        http://www.springframework.org/schema/mvc/spring-mvc-3.2.xsd
        http://www.springframework.org/schema/context 
        http://www.springframework.org/schema/context/spring-context-3.2.xsd">

	<context:component-scan base-package="com.test" />
        <bean class="org.springframework.web.servlet.view.InternalResourceViewResolver">
		<property name="prefix">
			<value>/WEB-INF/pages/</value>
		</property>
		<property name="suffix">
			<value>.jsp</value>
		</property>
	</bean>
	<mvc:resources mapping="/resources/**" location="/resources/" />
	<mvc:annotation-driven />
</beans>