Spring - Configure Dispatcher Servlet in Three Different Ways

The DispatcherServlet is at the heart of any Spring web application. It's the front controller that manages the flow of the application. Configuring the DispatcherServlet correctly is essential for a Spring web application to function.

Here are three ways to configure DispatcherServlet:

Using web.xml Configuration (Traditional Servlet Configuration)

Before Spring 3.1 and the introduction of the Servlet 3.0 spec, this was the most common way of configuring DispatcherServlet.

web.xml:

<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee" 
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee
         http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"
         version="3.1">

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

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

</web-app>

Here, the DispatcherServlet will load the configuration from spring-dispatcher-servlet.xml.

Using Java Configuration (Servlet 3.0+)

With Servlet 3.0+ and Spring 3.1+, you can set up the DispatcherServlet without any XML by implementing WebApplicationInitializer.

public class MyWebAppInitializer implements WebApplicationInitializer {
    
    @Override
    public void onStartup(ServletContext container) {
        AnnotationConfigWebApplicationContext context = new AnnotationConfigWebApplicationContext();
        context.setConfigLocation("com.example.config");
        
        ServletRegistration.Dynamic registration = container.addServlet("dispatcher", new DispatcherServlet(context));
        registration.setLoadOnStartup(1);
        registration.addMapping("/");
    }
}

Here, the configuration classes for the DispatcherServlet will be searched for in the com.example.config package.

Using Spring Boot
Spring Boot offers an opinionated way to configure Spring, which greatly simplifies the setup process. With Spring Boot, you typically won't manually configure the DispatcherServlet as it's auto-configured for you.
Maven dependency for Spring Boot web starter:
```
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>
```
SpringBootApplication class:
```
@SpringBootApplication
public class Application {
    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }
}
```
In the background, Spring Boot automatically sets up the DispatcherServlet for you and makes reasonable default decisions based on the libraries present in the classpath.

For most new projects, Spring Boot is the recommended approach because of its simplicity and convention-over-configuration philosophy. However, understanding the traditional configurations provides a deeper understanding of the underlying workings of a Spring MVC application.

Setting up DispatcherServlet using XML in Spring framework:

Use XML configuration to set up DispatcherServlet in Spring.

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

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

JavaConfig approach to configure DispatcherServlet in Spring:

Configure DispatcherServlet using JavaConfig.

// AppConfig.java
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;
import org.springframework.web.servlet.config.annotation.EnableWebMvc;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurerAdapter;

@Configuration
@EnableWebMvc
@ComponentScan("com.example")
public class AppConfig extends WebMvcConfigurerAdapter {
    // Configuration details...
}

Using annotations to configure DispatcherServlet in Spring:

Leverage annotations to configure DispatcherServlet.

// WebAppInitializer.java
import org.springframework.web.servlet.support.AbstractAnnotationConfigDispatcherServletInitializer;

public class WebAppInitializer extends AbstractAnnotationConfigDispatcherServletInitializer {

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

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

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