Spring DispatcherServlet – how it works?

Learn about Spring’s DispatcherServlet class, it’s responsibilities and how to configure it with examples.

Table of Contents

1. What is DispatcherServlet
2. How it uses WebApplicationContext
3. DispatcherServlet XML based Configuration
4. DispatcherServlet Java based Configuration
5. Beans supporting DispatcherServlet
6. DispatcherServlet Demo

1. What is Spring DispatcherServlet

DispatcherServlet acts as front controller for Spring based web applications. It provides a mechanism for request processing where actual work is performed by configurable, delegate components. It is inherited from javax.servlet.http.HttpServlet, it is typically configured in the web.xml file.

A web application can define any number of DispatcherServlet instances. Each servlet will operate in its own namespace, loading its own application context with mappings, handlers, etc. Only the root application context as loaded by ContextLoaderListener, if any, will be shared. In most cases, applications have only single DispatcherServlet with the context-root URL(/), that is, all requests coming to that domain will be handled by it.

DispatcherServlet uses Spring configuration classes to discover the delegate components it needs for request mapping, view resolution, exception handling etc.

2. How it uses WebApplicationContext

Let’s understand how dispatcher servlet works internally? In a Spring-based application, our application objects live within an object container. This container creates objects and associations between objects, and manages their complete life cycle. These container objects are called Spring-managed beans (or simply beans), and the container is called an application context (via class ApplicationContext) in the Spring world.

WebApplicationContext is an extension of a plain ApplicationContext. it is web aware ApplicationContext i.e it has Servlet Context information. When DispatcherServlet is loaded, it looks for the bean configuration file of WebApplicationContext and initializes it.

By having access to Servlet context, any spring bean which implement ServletConextAware interface – can get access to ServletContext instance and do many things with it. For example, it can get context init parameters, get context root information and get resources location inside web application folders.

3. DispatcherServlet XML Configuration

Let’s see how a typical DispatcherServlet declaration and initialization looks like.

<web-app>

  <listener>
    <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
  </listener>

  <context-param>
    <param-name>contextConfigLocation</param-name>
    <param-value>/WEB-INF/dispatcher-servlet-context.xml</param-value>
  </context-param>

  <servlet>
    <servlet-name>dispatcher-servlet</servlet-name>
    <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
    <init-param>
      <param-name>contextConfigLocation</param-name>
      <param-value></param-value>
    </init-param>
    <load-on-startup>1</load-on-startup>
  </servlet>

  <servlet-mapping>
    <servlet-name>dispatcher-servlet</servlet-name>
    <url-pattern>/*</url-pattern>
  </servlet-mapping>

</web-app>

In above code, dispatcher-servlet-context.xml file will contain all beans definitions and associations which will be available to DispatcherServlet. These bean definitions will override the definitions of any beans defined with the same name in the global scope. e.g.

<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-2.5.xsd">

    <bean id="viewResolver"
    	class="org.springframework.web.servlet.view.InternalResourceViewResolver" >
        <property name="prefix">
            <value>/WEB-INF/views/</value>
        </property>
        <property name="suffix">
            <value>.jsp</value>
        </property>
    </bean>

</beans>

4. DispatcherServlet Java Configuration

Starting from Servlet 3.0, in addition to declarative configuration in the web.xml file, DispatcherServlet can be configured programmatically by implementing or extending either of these three support classes provided by Spring –

• WebAppInitializer interface
• AbstractDispatcherServletInitializer abstract class
• AbstractAnnotationConfigDispatcherServletInitializer abstract class

4.1. WebAppInitializer Example

In below class, WebApplicationInitializer ensures that class ApplicationInitializer is detected by SpringServletContainerInitializer (which itself is bootstrapped automatically) and used to initialize any Servlet 3 container.

An example of spring boot dispatcherservlet mapping.

public class ApplicationInitializer implements WebApplicationInitializer 
{
	@Override
	public void onStartup(ServletContext servletContext) throws ServletException 
	{
		XmlWebApplicationContext appContext = new XmlWebApplicationContext();
		appContext.setConfigLocation("/WEB-INF/dispatcher-servlet-context.xml");

		ServletRegistration.Dynamic registration = servletContext
					.addServlet("rootDispatcher", new DispatcherServlet(appContext));
		registration.setLoadOnStartup(1);
		registration.addMapping("/");
	}
}

4.2. Complete Java Based Initialization

public class ApplicationInitializer implements WebApplicationInitializer 
{
	@Override
	public void onStartup(ServletContext container) 
	{
		// Create the 'root' Spring application context
		AnnotationConfigWebApplicationContext rootContext = new AnnotationConfigWebApplicationContext();
		rootContext.register(AppConfig.class);

		// Manage the lifecycle of the root application context
		container.addListener(new ContextLoaderListener(rootContext));

		// Create the dispatcher servlet's Spring application context
		AnnotationConfigWebApplicationContext dispatcherContext = new AnnotationConfigWebApplicationContext();
		dispatcherContext.register(DispatcherConfig.class);

		ServletRegistration.Dynamic dispatcher = container.addServlet("dispatcher", 
						new DispatcherServlet(dispatcherContext));
		dispatcher.setLoadOnStartup(1);
		dispatcher.addMapping("/");
	}
}

In above code, AppConfig and DispatcherConfig classes define the spring managed beans which will be in web application context.

4.3. AbstractDispatcherServletInitializer Example

This is base class for WebApplicationInitializer implementations that register a DispatcherServlet in the servlet context.

public class ApplicationInitializer extends AbstractDispatcherServletInitializer {

	@Override
	protected WebApplicationContext createRootApplicationContext() {
	        return null;
	}

	@Override
	protected WebApplicationContext createServletApplicationContext() {
	        XmlWebApplicationContext cxt = new XmlWebApplicationContext();
	        cxt.setConfigLocation("/WEB-INF/dispatcher-servlet-context.xml");
	        return cxt;
	}

	@Override
	protected String[] getServletMappings() {
	        return new String[] { "/" };
	}

	//Register filters
	@Override
    protected Filter[] getServletFilters() {
        return new Filter[] { new HiddenHttpMethodFilter(), new CharacterEncodingFilter() };
    }
}

Please note that if you need to customize the DispatcherServlet, you can override the createDispatcherServlet() method.

4.4. AbstractAnnotationConfigDispatcherServletInitializer Example

This class extends AbstractDispatcherServletInitializer and does few things implicitly which otherwise you could be doing yourself. An added advantage is that you now can use the convenience classes provided by Spring instead of manually configuring the DispatcherServlet and/or ContextLoaderListener.

It is the preferred approach for applications that use Java-based Spring configuration. It enables you to start servlet application context as well as root application context.

public class AppInitializer extends AbstractAnnotationConfigDispatcherServletInitializer {

   @Override
   protected Class<?>[] getRootConfigClasses() {
      return new Class[] { RootConfig.class };
   }

   @Override
   protected Class<?>[] getServletConfigClasses() {
      return new Class[] { WebMvcConfig.class };
   }

   @Override
   protected String[] getServletMappings() {
      return new String[] { "/" };
   }
}

Here RootConfig and WebMvcConfig classes are used to configure beans in root and servlet context scope.

Read More: Spring 5 MVC Example

5. Beans supporting DispatcherServlet

Upon receiving a web request, DispatcherServlet performs a set of operations for request processing. It used a set of supporting beans for this. This table lists these default configured beans and their responsibilities –

If you want to change any particular behavior of any bean, then you need to override it.

6. Spring DispatcherServlet Example

To demo the use of DispatcherServlet, I have written a very minimum application which just configure the dispatcher servlet and override the view resolver bean.

6.1. Project Structure

6.2. AppInitializer.java

package com.howtodoinjava.demo.spring.config;

public class AppInitializer extends 
	AbstractAnnotationConfigDispatcherServletInitializer {

   @Override
   protected Class<?>[] getRootConfigClasses() {
      return new Class[] { };
   }

   @Override
   protected Class<?>[] getServletConfigClasses() {
      return new Class[] { WebMvcConfig.class };
   }

   @Override
   protected String[] getServletMappings() {
      return new String[] { "/" };
   }
}

6.3. WebMvcConfig.java

@Configuration
@EnableWebMvc
@ComponentScan(basePackages = { "com.howtodoinjava.demo.spring"})
public class WebMvcConfig implements WebMvcConfigurer {

   @Bean
   public InternalResourceViewResolver resolver() {
      InternalResourceViewResolver resolver = new InternalResourceViewResolver();
      resolver.setViewClass(JstlView.class);
      resolver.setPrefix("/WEB-INF/views/");
      resolver.setSuffix(".jsp");
      return resolver;
   }
}

6.4. HomeController.java

@Controller
public class HomeController 
{
	@GetMapping("/")
	public String homeInit(Locale locale, Model model) {
		return "home";
	}
}

home.jsp

<html>
<head>
	<title>Spring 5 Web MVC Example</title>
</head>
<body>
	<h1>HowToDoInJava.com</h1>
	<h2>Spring 5 Web MVC DispatcherServlet Example</h2>
</body>
</html>

6.5. pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd;
	<modelVersion>4.0.0</modelVersion>
	<groupId>com.howtodoinjava.spring5.mvc</groupId>
	<artifactId>spring5-webmvc-demo</artifactId>
	<packaging>war</packaging>
	<version>0.0.1-SNAPSHOT</version>
	<name>spring5-webmvc-demo Maven Webapp</name>
	<url>http://maven.apache.org</url>
	<properties>
		<failOnMissingWebXml>false</failOnMissingWebXml>
		<spring.version>5.2.0.RELEASE</spring.version>
		<jstl.version>1.2.1</jstl.version>
		<tld.version>1.1.2</tld.version>
		<servlets.version>3.1.0</servlets.version>
		<jsp.version>2.3.1</jsp.version>
	</properties>
	<dependencies>
		<!-- Spring MVC Dependency -->
		<dependency>
			<groupId>org.springframework</groupId>
			<artifactId>spring-webmvc</artifactId>
			<version>${spring.version}</version>
		</dependency>
		
		<!-- JSTL Dependency -->
		<dependency>
			<groupId>javax.servlet.jsp.jstl</groupId>
			<artifactId>javax.servlet.jsp.jstl-api</artifactId>
			<version>${jstl.version}</version>
		</dependency>
		
		<dependency>
			<groupId>taglibs</groupId>
			<artifactId>standard</artifactId>
			<version>${tld.version}</version>
		</dependency>

		<!-- Servlet Dependency -->
		<dependency>
			<groupId>javax.servlet</groupId>
			<artifactId>javax.servlet-api</artifactId>
			<version>${servlets.version}</version>
			<scope>provided</scope>
		</dependency>

		<!-- JSP Dependency -->
		<dependency>
			<groupId>javax.servlet.jsp</groupId>
			<artifactId>javax.servlet.jsp-api</artifactId>
			<version>${jsp.version}</version>
			<scope>provided</scope>
		</dependency>
	</dependencies>
	<build>
		<sourceDirectory>src/main/java</sourceDirectory>
		<resources>
			<resource>
				<directory>src/main/resources</directory>
			</resource>
		</resources>
		<plugins>
			<plugin>
				<artifactId>maven-compiler-plugin</artifactId>
				<version>3.5.1</version>
				<configuration>
					<source>1.8</source>
					<target>1.8</target>
				</configuration>
			</plugin>
			<plugin>
				<groupId>org.apache.tomcat.maven</groupId>
				<artifactId>tomcat7-maven-plugin</artifactId>
				<version>2.2</version>
				<configuration>
					<path>/</path>
				</configuration>
			</plugin>
		</plugins>
	</build>
</project>

6.6. Run the application

To run the application execute maven goal : tomcat7:run. Now open http://localhost:8080 in browser.

Drop me your questions in comments section.

Happy Learning !!