Master Spring MVC Customization: From WebConfig to YAML Message Converters

1. Enable Spring MVC

Activate Spring MVC by adding @EnableWebMvc in a configuration class.

<code>@Configuration
@EnableWebMvc
public class WebConfig {
}
</code>

2. Type Conversion Configuration

Register a custom ConverterFactory to convert String to Number (example shown for Integer ).

<code>@Configuration
public class WebConfig implements WebMvcConfigurer {
    @Override
    public void addFormatters(FormatterRegistry registry) {
        registry.addConverterFactory(new ConverterFactory<String, Number>() {
            @Override
            public <T extends Number> Converter<String, T> getConverter(Class<T> targetType) {
                return new Converter<String, T>() {
                    public T convert(String source) {
                        return (T) Integer.valueOf(source);
                    }
                };
            }
        });
    }
}
</code>

Converters are stored in a ConvertersForPair queue where newer converters are added to the front:

<code>private static class ConvertersForPair {
    private final Deque<GenericConverter> converters = new ConcurrentLinkedDeque<>();
    public void add(GenericConverter converter) {
        this.converters.addFirst(converter);
    }
}
</code>

3. Data Validation

If a Bean Validation implementation (e.g., Hibernate Validator) is on the classpath, Spring registers LocalValidatorFactoryBean as the global Validator :

<code>@Configuration
public class WebConfig implements WebMvcConfigurer {
    public Validator getValidator() {
        return new LocalValidatorFactoryBean();
    }
}
</code>

4. Request Interceptor

Example interceptor that checks for a token header before allowing the request to proceed.

<code>@Configuration
public class WebConfig implements WebMvcConfigurer {
    public void addInterceptors(InterceptorRegistry registry) {
        registry.addInterceptor(new HandlerInterceptor() {
            @Override
            public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
                if (request.getHeader("token") == null) {
                    return false;
                }
                return true;
            }
        }).addPathPatterns("/**");
    }
}
</code>

Interceptors are not a security layer; for robust security use Spring Security with MvcRequestMatcher .

5. Content Negotiation

Configure Spring MVC to recognize a custom yaml media type.

<code>@Configuration
public class WebConfig implements WebMvcConfigurer {
    @Override
    public void configureContentNegotiation(ContentNegotiationConfigurer configurer) {
        // map "yaml" extension to "application/yaml"
        configurer.mediaType("yaml", new MediaType("application", "yaml"));
    }
}
</code>

Register the ContentNegotiationManager bean when needed:

<code>@Bean
public ContentNegotiationManager mvcContentNegotiationManager() {
    if (this.contentNegotiationManager == null) {
        ContentNegotiationConfigurer configurer = new ContentNegotiationConfigurer(this.servletContext);
        configurer.mediaTypes(getDefaultMediaTypes());
        configureContentNegotiation(configurer);
        this.contentNegotiationManager = configurer.buildContentNegotiationManager();
    }
    return this.contentNegotiationManager;
}
protected ContentNegotiationManager buildContentNegotiationManager() {
    this.factory.addMediaTypes(this.mediaTypes);
    return this.factory.build();
}
</code>

6. Custom YAML Message Converter

Define a HttpMessageConverter that writes objects as YAML.

<code>public class YamlHttpMessageConverter implements HttpMessageConverter<Object> {
    @Override
    public boolean canWrite(Class<?> clazz, MediaType mediaType) {
        return User.class.isAssignableFrom(clazz);
    }
    @Override
    public List<MediaType> getSupportedMediaTypes() {
        return Arrays.asList(new MediaType("application", "yaml"));
    }
    @Override
    public void write(Object t, MediaType contentType, HttpOutputMessage outputMessage) throws IOException, HttpMessageNotWritableException {
        StreamUtils.copy(new org.yaml.snakeyaml.Yaml().dump(t), StandardCharsets.UTF_8, outputMessage.getBody());
    }
}
</code>

Register the converter with highest priority:

<code>@Configuration
public class WebConfig implements WebMvcConfigurer {
    public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
        // add at position 0 to override default JSON converter
        converters.add(0, new YamlHttpMessageConverter());
    }
}
</code>

Test endpoint:

<code>@GetMapping("/yaml")
public Object yaml() {
    return new User(10, "zhangsan");
}
</code>

Resulting YAML output is shown below:

7. View Controller

Map a URL directly to a view without a controller method.

<code>@Configuration
public class WebConfig implements WebMvcConfigurer {
    @Override
    public void addViewControllers(ViewControllerRegistry registry) {
        // /index renders the "test" view
        registry.addViewController("/index").setViewName("test");
    }
}
</code>

Define the view bean:

<code>@Component("test")
public class PackView implements View {
    @Override
    public String getContentType() {
        return "text/html";
    }
    @Override
    public void render(Map<String, ?> model, HttpServletRequest request, HttpServletResponse response) throws Exception {
        response.getWriter().print("View Controllers");
    }
}
</code>

Rendered result:

8. View Resolver

Register the custom YamlViewResolver so that views ending with .yaml are resolved.

<code>@Configuration
public class WebConfig implements WebMvcConfigurer {
    public void configureViewResolvers(ViewResolverRegistry registry) {
        registry.viewResolver(new YamlViewResolver());
    }
}
</code>

The resolver is added to ViewResolverComposite , which aggregates all resolvers.

9. Static Resource Configuration

Expose static resources under /resources/** from /public or classpath:/static/ directories.

<code>@Configuration
@EnableWebMvc
public class WebConfig implements WebMvcConfigurer {
    @Override
    public void addResourceHandlers(ResourceHandlerRegistry registry) {
        registry.addResourceHandler("/resources/**").addResourceLocations("/public", "classpath:/static/");
    }
}
</code>

The article concludes with a hope that these configurations help you customize Spring MVC effectively.