SpringMVC and @Configuration: register ConversionService and HttpMessageConverter

Question

I really like spring and the @Configuration style to get rid of xml based configuration. I successfully use it for the service and repository layer. What I also love is the dependency injection feature and the JDO/JPA/Jdbc utilities!

What I don't really get is, how the Spring WebMVC works. There is too much uncontrollable magic for me. (and with @EnableAutoConfiguration there is even more magic introduced. Good for easy prototyping, difficult to maintain).

That is how I configure my webapp:

public class SpringWebBooter implements WebApplicationInitializer {

    @Override
    public void onStartup(ServletContext servletContext) throws ServletException {
        AnnotationConfigApplicationContext rootContext = new AnnotationConfigApplicationContext();
        rootContext.register(SpringConfiguration.class); //main configuration class for all beans
        rootContext.refresh();

        AnnotationConfigWebApplicationContext ctx = new AnnotationConfigWebApplicationContext();
        ctx.setParent(rootContext);
        ctx.register(SpringWebConfiguration.class); //web context configuration class

        ServletRegistration.Dynamic api = servletContext.addServlet("api", new DispatcherServlet(ctx));
        api.setLoadOnStartup(1);
        api.addMapping("/api/*");
    }

}

Now I want to add type converters and httpMessageConverters, so in the SpringWebConfiguration class I tried:

@EnableWebMvc
@Configuration
@ComponentScan
public class SpringWebConfiguration {

    //works but feels very *magic*
    @Autowired
    public void configureConversionService(FormattingConversionService conversionService) {
        conversionService.addConverter(new PointConverter(GEOMETRY_FACTORY));
        conversionService.addConverterFactory(new StringToEnumConverterFactory());
    }

    //not working yet
    @Bean
    public MappingJackson2HttpMessageConverter createJsonMessageConverter() {
        ObjectMapper o = new ObjectMapper();
        o.enable(SerializationFeature.INDENT_OUTPUT);
        MappingJackson2HttpMessageConverter c = new MappingJackson2HttpMessageConverter();
        c.setObjectMapper(o);
        return c;
    }
}

What I'd rather find intuitive is to add type and message converters when I construct the dispatcher servlet. That would be much clearer than some dubious autowiring or bean creation. I always "hope" that the Dispatcher Servlet internally pics up my beans, but it is often just trial and error. Is it possible to set up spring Mvc in a directer way? With less magic and more concrete instantiations and #addHttpMessageConverter(...) calls for example?

Same basically for the ExceptionResolvers, RequestHandler and RequestAdapter.

Jan

Answer 1

The most direct way would be to extend WebMvcConfigurationSupport. You can setup almost everything that way by overriding the methods.

But be aware that it is a very direct way to setup stuff. It gives you much more control than you have now or than even WebMvcConfigurerAdapter would give you. From the docs:

If the customization options of {@link WebMvcConfigurer} do not expose
something you need to configure, consider removing the {@code @EnableWebMvc}
annotation and extending directly from {@link WebMvcConfigurationSupport}
overriding selected {@code @Bean} methods

Custom (or customized) message converters can be added by overriding configureMessageConverters.

Answer 2

If you extend the WebMvcConfigurerAdapter for your web configuration, it should feel a little less magic and also give you the hook to configure your message converters as well as a number of other components.

    @Configuration
    @ComponentScan
    @EnableWebMvc
    public class WebConfiguration extends WebMvcConfigurerAdapter
    {
      @Autowired
      private CustomObjectMapper domainMapper;

      @Override
      public void configureMessageConverters(List<HttpMessageConverter<?>> converters)
      {
        MappingJackson2HttpMessageConverter converter = new MappingJackson2HttpMessageConverter();
        converter.setObjectMapper(domainMapper);
        converters.add(converter);
        super.configureMessageConverters(converters);
      }

    }