Spring MVC

The Spring web MVC framework provides model-view-controller architecture and ready components that can be used to develop flexible and loosely coupled web applications.
 MVC:

• The Model encapsulates the application data and in general they will consist of POJO.
• The View is responsible for rendering the model data and in general it generates HTML output that the client's browser can interpret.
• The Controller is responsible for processing user requests and building appropriate model and passes it to the view for rendering
  
  
  

  DispatcherServlet:

  The Spring Web model-view-controller (MVC) framework is designed around a DispatcherServlet that handles all the HTTP requests and responses.  The following diagram wll explain the flow.
  
  
  
  
  The User request will be sent to the DispatcherServlet. This will process the request as follows
  
  
  1. After receiving an HTTP request, DispatcherServlet consults the HandlerMapping to call the appropriate Controller.
  2. The Controller takes the request and calls the appropriate service methods based on used GET or POST method. The service method will set model data based on defined business logic and returns view name to the DispatcherServlet.
  3. The DispatcherServlet will take help from ViewResolver to pickup the defined view for the request.
  4. Once view is finalized, The DispatcherServlet passes the model data to the view which is finally rendered on the browser.
  
  Next We will sea how we can define this with an example
  Using URL mapping you need to map the request that the DispatcherServlet to handle.Following example shows how we can map.
  
  

  <web-app id="WebApp_ID" version="2.4"
      xmlns="http://java.sun.com/xml/ns/j2ee" 
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee 
      http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
   
      <display-name>Spring MVC Application</display-name>
  
     <servlet>
        <servlet-name>HelloWeb</servlet-name>
        <servlet-class>
           org.springframework.web.servlet.DispatcherServlet
        </servlet-class>
        <load-on-startup>1</load-on-startup>
     </servlet>
  
     <servlet-mapping>
        <servlet-name>HelloWeb</servlet-name>
        <url-pattern>*.jsp</url-pattern>
     </servlet-mapping>
  
  </web-app>

  
  
  The web.xml file will be kept WebContent/WEB-INF directory of your web application. After initialization of DispatcherServlet , the framework will look for the application context  from the file named [servlet-name]-servlet.xml located in the application's WebContent/WEB-INF directory by default.
  If you want to give your own name for the application context file and if u want to place that file in some other location then  by adding the servlet listener ContextLoaderListener in your web.xml file as follows:.
  
  

  <web-app...>
  
  <!-------- DispatcherServlet definition goes here----->
  ....
  <context-param>
     <param-name>contextConfigLocation</param-name>
     <param-value>/WEB-INF/HelloWeb-servlet.xml</param-value>
  </context-param>
  
  <listener>
     <listener-class>
        org.springframework.web.context.ContextLoaderListener
     </listener-class>
  </listener>
  </web-app>

 Lets create HelloWeb-servlet.xml file, placed in your web application's WebContent/WEB-INF directory:

<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"
   xsi:schemaLocation="
   http://www.springframework.org/schema/beans     
   http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
   http://www.springframework.org/schema/context 
   http://www.springframework.org/schema/context/

            spring-context-3.0.xsd">

   <context:component-scan base-package="com.examples" />

   <bean class="org.springframework.web.servlet.view.

              InternalResourceViewResolver">
      <property name="prefix" value="/WEB-INF/jsp/" />
      <property name="suffix" value=".jsp" />
   </bean>

</beans>

• The [servlet-name]-servlet.xml file will be used to create the beans defined, overriding the definitions of any beans defined with the same name in the global scope.
• The <context:component-scan...> tag will be use to activate Spring MVC annotation scanning capability which allows to make use of annotations like @Controller and @RequestMapping etc.
• The InternalResourceViewResolver will have rules defined to resolve the view names. As per the above defined rule, a logical view named hello is delegated to a view implementation located at /WEB-INF/jsp/hello.jsp .
  
  DispatcherServlet delegates the request to the controllers to execute the functionality specific to it. The @Controller annotation indicates that a particular class serves the role of a controller. The @RequestMapping annotation is used to map a URL to either an entire class or a particular handler method.
  

  @Controller
  @RequestMapping("/hello")
  public class HelloController{
   
     @RequestMapping(method = RequestMethod.GET)
     public String printHello(ModelMap model) {
        model.addAttribute("message", 

   "Hello Spring MVC Framework!");
        return "hello";
     }
  
  }

  The @Controller annotation defines the class as a Spring MVC controller. Here, the first usage of @RequestMapping indicates that all handling methods on this controller are relative to the /hello path. Next annotation @RequestMapping(method = RequestMethod.GET) is used to declare the printHello() method as the controller's default service method to handle HTTP GET request. You can define another method to handle any POST request at the same URL.
  You can write above controller in another form where you can add additional attributes in @RequestMapping as follows:
  

  @Controller
  public class HelloController{
   
     @RequestMapping(value = "/hello", method = RequestMethod.GET)
     public String printHello(ModelMap model) {
        model.addAttribute("message", 

   "Hello Spring MVC Framework!");
        return "hello";
     }
  
  }

  The value attribute indicates the URL to which the handler method is mapped and the method attribute defines the service method to handle HTTP GET request. There are following important points to be noted about the controller defined above:
  
• You will defined required business logic inside a service method. You can call another methods inside this method as per requirement.
• Based on the business logic defined, you will create a model within this method. You can setter different model attributes and these attributes will be accessed by the view to present the final result. This example creates a model with its attribute "message".
• A defined service method can return a String which contains the name of the view to be used to render the model. This example returns "hello" as logical view name.

Creating JSP Views

Spring MVC supports many types of views for different presentation technologies. These include - JSPs, HTML, PDF, Excel worksheets, XML, Velocity templates, XSLT, JSON, Atom and RSS feeds, JasperReports etc. But most commonly we use JSP templates written with JSTL. So let us write a simple hello view in /WEB-INF/hello/hello.jsp:

<html>
   <head>
   <title>Hello Spring MVC</title>
   </head>
   <body>
   <h2>${message}</h2>
   </body>
</html>

Here ${message} is the attribute which we have setup inside the Controller. You can have multiple attributes to be displayed inside your view.


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