Any Spring @RestController in a Spring Boot application should render JSON response by default as long as Jackson2 is on the classpath. For example:

@RestController
public class MyController {

    @RequestMapping("/thing")
    public MyThing thing() {
            return new MyThing();
    }

}

As long as MyThing can be serialized by Jackson2 (e.g. a normal POJO or Groovy object) then localhost:8080/thing will serve a JSON representation of it by default. Sometimes in a browser you might see XML responses because browsers tend to send accept headers that prefer XML.

If you have the Jackson XML extension (jackson-dataformat-xml) on the classpath, it will be used to render XML responses and the very same example as we used for JSON would work. To use it, add the following dependency to your project:

<dependency>
    <groupId>com.fasterxml.jackson.dataformat</groupId>
    <artifactId>jackson-dataformat-xml</artifactId>
</dependency>

You may also want to add a dependency on Woodstox. It’s faster than the default Stax implementation provided by the JDK and also adds pretty print support and improved namespace handling:

<dependency>
    <groupId>org.codehaus.woodstox</groupId>
    <artifactId>woodstox-core-asl</artifactId>
</dependency>

If Jackson’s XML extension is not available, JAXB (provided by default in the JDK) will be used, with the additional requirement to have MyThing annotated as @XmlRootElement:

@XmlRootElement
public class MyThing {
    private String name;
    // .. getters and setters
}

To get the server to render XML instead of JSON you might have to send an Accept: text/xml header (or use a browser).

Spring MVC (client and server side) uses HttpMessageConverters to negotiate content conversion in an HTTP exchange. If Jackson is on the classpath you already get the default converter(s) provided by Jackson2ObjectMapperBuilder.

The ObjectMapper (or XmlMapper for Jackson XML converter) instance created by default have the following customized properties:

Spring Boot has also some features to make it easier to customize this behavior.

You can configure the ObjectMapper and XmlMapper instances using the environment. Jackson provides an extensive suite of simple on/off features that can be used to configure various aspects of its processing. These features are described in six enums in Jackson which map onto properties in the environment:

For example, to enable pretty print, set spring.jackson.serialization.indent_output=true. Note that, thanks to the use of relaxed binding, the case of indent_output doesn’t have to match the case of the corresponding enum constant which is INDENT_OUTPUT.

If you want to replace the default ObjectMapper completely, define a @Bean of that type and mark it as @Primary.

Defining a @Bean of type Jackson2ObjectMapperBuilder will allow you to customize both default ObjectMapper and XmlMapper (used in MappingJackson2HttpMessageConverter and MappingJackson2XmlHttpMessageConverter respectively).

Another way to customize Jackson is to add beans of type com.fasterxml.jackson.databind.Module to your context. They will be registered with every bean of type ObjectMapper, providing a global mechanism for contributing custom modules when you add new features to your application.

Finally, if you provide any @Beans of type MappingJackson2HttpMessageConverter then they will replace the default value in the MVC configuration. Also, a convenience bean is provided of type HttpMessageConverters (always available if you use the default MVC configuration) which has some useful methods to access the default and user-enhanced message converters.

See also the Section 68.4, “Customize the @ResponseBody rendering” section and the WebMvcAutoConfiguration source code for more details.

Spring uses HttpMessageConverters to render @ResponseBody (or responses from @RestController). You can contribute additional converters by simply adding beans of that type in a Spring Boot context. If a bean you add is of a type that would have been included by default anyway (like MappingJackson2HttpMessageConverter for JSON conversions) then it will replace the default value. A convenience bean is provided of type HttpMessageConverters (always available if you use the default MVC configuration) which has some useful methods to access the default and user-enhanced message converters (useful, for example if you want to manually inject them into a custom RestTemplate).

As in normal MVC usage, any WebMvcConfigurerAdapter beans that you provide can also contribute converters by overriding the configureMessageConverters method, but unlike with normal MVC, you can supply only additional converters that you need (because Spring Boot uses the same mechanism to contribute its defaults). Finally, if you opt-out of the Spring Boot default MVC configuration by providing your own @EnableWebMvc configuration, then you can take control completely and do everything manually using getMessageConverters from WebMvcConfigurationSupport.

See the WebMvcAutoConfiguration source code for more details.

Spring Boot embraces the Servlet 3 javax.servlet.http.Part API to support uploading files. By default Spring Boot configures Spring MVC with a maximum file of 1Mb per file and a maximum of 10Mb of file data in a single request. You may override these values, as well as the location to which intermediate data is stored (e.g., to the /tmp directory) and the threshold past which data is flushed to disk by using the properties exposed in the MultipartProperties class. If you want to specify that files be unlimited, for example, set the multipart.maxFileSize property to -1.

The multipart support is helpful when you want to receive multipart encoded file data as a @RequestParam-annotated parameter of type MultipartFile in a Spring MVC controller handler method.

See the MultipartAutoConfiguration source for more details.

Spring Boot wants to serve all content from the root of your application / down. If you would rather map your own servlet to that URL you can do it, but of course you may lose some of the other Boot MVC features. To add your own servlet and map it to the root resource just declare a @Bean of type Servlet and give it the special bean name dispatcherServlet (You can also create a bean of a different type with that name if you want to switch it off and not replace it).

The easiest way to take complete control over MVC configuration is to provide your own @Configuration with the @EnableWebMvc annotation. This will leave all MVC configuration in your hands.

A ViewResolver is a core component of Spring MVC, translating view names in @Controller to actual View implementations. Note that ViewResolvers are mainly used in UI applications, rather than REST-style services (a View is not used to render a @ResponseBody). There are many implementations of ViewResolver to choose from, and Spring on its own is not opinionated about which ones you should use. Spring Boot, on the other hand, installs one or two for you depending on what it finds on the classpath and in the application context. The DispatcherServlet uses all the resolvers it finds in the application context, trying each one in turn until it gets a result, so if you are adding your own you have to be aware of the order and in which position your resolver is added.

WebMvcAutoConfiguration adds the following ViewResolvers to your context:

Check out WebMvcAutoConfiguration, ThymeleafAutoConfiguration, FreeMarkerAutoConfiguration, GroovyTemplateAutoConfiguration and VelocityAutoConfiguration