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1. Overview

In this article, we’ll focus on the various handler adapters implementations available in the Spring framework.

2. What is a HandlerAdapter?

The HandlerAdapter is basically an interface which facilitates the handling of HTTP requests in a very flexible manner in Spring MVC.

It’s used in conjunction with the HandlerMapping, which maps a method to a specific URL.

The DispatcherServlet then uses a HandlerAdapter to invoke this method. The servlet doesn’t invoke the method directly – it basically serves as a bridge between itself and the handler objects, leading to a loosely coupling design.

Let’s take a look at various methods available in this interface:

public interface HandlerAdapter {
    boolean supports(Object handler);
    
    ModelAndView handle(
      HttpServletRequest request,
      HttpServletResponse response, 
      Object handler) throws Exception;
    
    long getLastModified(HttpServletRequest request, Object handler);
}

The supports API is used to check if a particular handler instance is supported or not. This method should be called first before calling the handle() method of this interface, in order to make sure whether the handler instance is supported or not.

The handle API is used to handle a particular HTTP request. This method is responsible for invoking the handler by passing the HttpServletRequest and HttpServletResponse object as the parameter. The handler then executes the application logic and returns a ModelAndView object, which is then processed by the DispatcherServlet.

3. Maven Dependency

Let’s start with the Maven dependency that needs to be added to pom.xml:

<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-webmvc</artifactId>
    <version>4.3.4.RELEASE</version>
</dependency>

The latest version of the spring-webmvc artifact can be found here.

4. Types of HandlerAdapter

4.1. SimpleControllerHandlerAdapter

This is the default handler adapter registered by Spring MVC. It deals with classes implementing Controller interface and is used to forward a request to a controller object.

If a web application uses only controllers then we don’t need to configure any HandlerAdapter as the framework uses this class as the default adapter for handling a request.

Let’s define a simple controller class, using the older style of controller (implementing the Controller interface):

public class SimpleController implements Controller {
    @Override
    public ModelAndView handleRequest(
      HttpServletRequest request, 
      HttpServletResponse response) throws Exception {
        
        ModelAndView model = new ModelAndView("Greeting");
        model.addObject("message", "Dinesh Madhwal");
        return model;
    }
}

The similar XML configuration:

<beans ...>
    <bean name="/greeting.html"
      class="com.baeldung.spring.controller.SimpleControllerHandlerAdapterExample"/>
    <bean id="viewResolver"
      class="org.springframework.web.servlet.view.InternalResourceViewResolver">
        <property name="prefix" value="/WEB-INF/" />
        <property name="suffix" value=".jsp" />
    </bean>
</beans>

The BeanNameUrlHandlerMapping class is the mapping class for this handler adapter.

Note: If a custom handler adapter is defined in BeanFactory, then this adapter is not automatically registered. Thus, we need to define it explicitly in the context. If it is not defined and we have defined a custom handler adapter, then we will get an exception that says that no adapter for a handler is specified.

4.2. SimpleServletHandlerAdapter

This handler adapter allows the use of any Servlet to work with DispatcherServlet for handling the request. It forwards the request from DispatcherServlet to the appropriate Servlet class by calling its service() method.

The beans which implement the Servlet interface are automatically handled by this adapter. It is not registered by default and we need to register it like any other normal bean in the configuration file of DispatcherServlet:

<bean name="simpleServletHandlerAdapter" 
  class="org.springframework.web.servlet.handler.SimpleServletHandlerAdapter" />

4.3. AnnotationMethodHandlerAdapter

This adapter class is used to execute the methods that are annotated with @RequestMapping annotation. It is used to map the methods based on HTTP methods and HTTP paths.

The mapping class for this adapter is DefaultAnnotationHandlerMapping, which is used to process the @RequestMapping annotation at the type level and AnnotationMethodHandlerAdaptor is used to process at a method level.

These two classes are already registered by the framework when the DispatcherServlet is initialized. However, if the other handler adapters are already defined, then we need to define it as well in the configuration file.

Let’s define a controller class:

@Controller
public class AnnotationHandler {
    @RequestMapping("/annotedName")
    public ModelAndView getEmployeeName() {
        ModelAndView model = new ModelAndView("Greeting");        
        model.addObject("message", "Dinesh");       
        return model;  
    }  
}

The @Controller annotation indicates that this class serves the role of controller.

The @RequestMapping annotation maps the getEmployeeName() method to the URL /name.

There are 2 different ways of configuring this adapter depending on whether the application uses Java-based configuration or XML based configuration. Let us look at the first way using Java configuration:

@ComponentScan("com.baeldung.spring.controller")
@Configuration
@EnableWebMvc
public class ApplicationConfiguration implements WebMvcConfigurer {
    @Bean
    public InternalResourceViewResolver jspViewResolver() {
        InternalResourceViewResolver bean = new InternalResourceViewResolver();
        bean.setPrefix("/WEB-INF/");
        bean.setSuffix(".jsp");
        return bean;
    }
}

If the application uses XML configuration, then there are two different approaches for configuring this handler adapter in web application context XML. Let us take a look at the first approach defined in the file spring-servlet_AnnotationMethodHandlerAdapter.xml:

<beans ...>
    <context:component-scan base-package="com.baeldung.spring.controller" />
    <bean 
      class="org.springframework.web.servlet.mvc.annotation.DefaultAnnotationHandlerMapping"/>
    <bean 
      class="org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter"/>
    <bean id="viewResolver"
      class="org.springframework.web.servlet.view.InternalResourceViewResolver">
        <property name="prefix" value="/WEB-INF/" />
        <property name="suffix" value=".jsp" />
    </bean>
</beans>

The <context:component-scan /> tag is used to specify the package to scan for controller classes.

Let us take a look at the second approach:

<beans ...>
    <mvc:annotation-driven/>
    <context:component-scan base-package="com.baeldung.spring.controller" />
    <bean id="viewResolver"
      class="org.springframework.web.servlet.view.InternalResourceViewResolver">
        <property name="prefix" value="/WEB-INF/" />
        <property name="suffix" value=".jsp" />
    </bean>
</beans>

The <mvc:annotation-driven> tag will automatically register these two classes with spring MVC. This adapter was deprecated in Spring 3.2 and a new handler adapter called RequestMappingHandlerAdapter was introduced in Spring 3.1.

4.4. RequestMappingHandlerAdapter

This adapter class was introduced in Spring 3.1, deprecating the AnnotationMethodHandlerAdaptor handler adapter in Spring 3.2.

It’s used with RequestMappingHandlerMapping class, which executes methods annotated with @RequestMapping.

The RequestMappingHandlerMapping is used to maintain the mapping of the request URI to the handler. Once the handler is obtained, the DispatcherServlet dispatches the request to the appropriate handler adapter, which then invokes the handlerMethod().

The type-level and method-level mappings were processed in two different stages in the Spring version prior to 3.1.

The first stage was to select the controller by DefaultAnnotationHandlerMapping and the second stage was to invoke the actual method by AnnotationMethodHandlerAdapter.

From Spring version 3.1, there is only one stage, which involves identifying the controller as well as which method needs to be invoked to process the request.

Let’s define a simple controller class:

@Controller
public class RequestMappingHandler {
    
    @RequestMapping("/requestName")
    public ModelAndView getEmployeeName() {
        ModelAndView model = new ModelAndView("Greeting");        
        model.addObject("message", "Madhwal");        
        return model;  
    }  
}

There are 2 different ways of configuring this adapter depending on whether the application uses Java-based configuration or XML based configuration.

Let’s look at the first way using Java configuration:

@ComponentScan("com.baeldung.spring.controller")
@Configuration
@EnableWebMvc
public class ServletConfig implements WebMvcConfigurer {
    @Bean
    public InternalResourceViewResolver jspViewResolver() {
        InternalResourceViewResolver bean = new InternalResourceViewResolver();
        bean.setPrefix("/WEB-INF/");
        bean.setSuffix(".jsp");
        return bean;
    }
}

If the application uses XML configuration, then there are two different approaches for configuring this handler adapter in web application context XML. Let us take a look at the first approach defined in the file spring-servlet_RequestMappingHandlerAdapter.xml:

<beans ...>
    <context:component-scan base-package="com.baeldung.spring.controller" />
    
    <bean 
      class="org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping"/>
    
    <bean
      class="org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter"/>
    
    <bean id="viewResolver"
      class="org.springframework.web.servlet.view.InternalResourceViewResolver">
        <property name="prefix" value="/WEB-INF/" />
        <property name="suffix" value=".jsp" />
    </bean>
</beans>

And here’s the second approach:

<beans ...>
    <mvc:annotation-driven />
    
    <context:component-scan base-package="com.baeldung.spring.controller" />
    
    <bean id="viewResolver"
      class="org.springframework.web.servlet.view.InternalResourceViewResolver">
        <property name="prefix" value="/WEB-INF/" />
        <property name="suffix" value=".jsp" />
    </bean>
</beans>

This tag will automatically register these two classes with Spring MVC.

If we need to customize the RequestMappingHandlerMapping, then we need to remove this tag from the application context XML and manually configure it in the application context XML.

4.5. HttpRequestHandlerAdapter

This handler adapter is used for the handlers that process HttpRequests. It implements the HttpRequestHandler interface, which contains a single handleRequest() method for processing the request and generating the response.

The return type of this method is void and it doesn’t generate ModelAndView return type as produced by other handler adapters. It’s basically used to generate binary responses and it doesn’t generate a view to render.

5. Running the Application

If the application is deployed on localhost with the port number 8082 and the context-root is spring-mvc-handlers:

http://localhost:8082/spring-mvc-handlers/

6. Conclusion

In this article, we discussed various types of handler adapters available in Spring framework.

Most developers will probably stick to the defaults, but it’s well worth understanding just how flexible the framework when we need to go beyond the basics.

The source code of this tutorial can be found in the GitHub project.

I just announced the new Spring Boot 2 material, coming in REST With Spring:

>> CHECK OUT THE LESSONS