This rule flags usage of the javax.xml.transform.Source interface when the SourceHttpMessageConverter class is not found. In Spring Framework 6.0, the SourceHttpMessageConverter class is no longer registered by default in Spring MVC or RestTemplate.

As a consequence, Spring web applications using javax.xml.transform.Source now need to configure SourceHttpMessageConverter explicitly. It is important to note that the order of converter registration is important. For example, SourceHttpMessageConverter should be registered before "catch-all" converters like MappingJackson2HttpMessageConverter.

The following code snippet shows usage of javax.xml.transform.Source:

                @RestController
                public class TransformController {
                    @GetMapping(value = "/transform", produces = MediaType.TEXT_HTML_VALUE)
                    public String transformXml() throws Exception{
                        Source xmlSource = new StreamSource(new ClassPathResource("sample.xml").getInputStream());
                        Source xsltSource = new StreamSource(new ClassPathResource("transform.xslt").getInputStream());
                        StringWriter writer = new StringWriter();
                        Result result = new StreamResult(writer);
                        TransformerFactory factory = TransformerFactory.newInstance();
                        Transformer transformer = factory.newTransformer(xsltSource);
                        transformer.transform(xmlSource, result);
                        return writer.toString();
                    }
                }  
             

In Spring Framework 6.0, the controller code remains unchanged, but additional configuration steps are required to ensure it functions correctly. You can Register SourceHttpMessageConverter manually in Configuration code. The following is an example of this use case:

            @Configuration
            public class WebConfig implements WebMvcConfigurer{
                @Override
                public void extendMessageConverters(List<HttpMessageConverter<?>> converters) {
                    converters.add(new SourceHttpMessageConverter<>());
                }
            }
            

Alternatively, you can configure the SourceHttpMessageConverter within a RestTemplate if you're using it to retrieve XML from external services. Here is an example of this use case:

            @Bean
            public RestTemplate restTemplate() {
                RestTemplate restTemplate = new RestTemplate();
                restTemplate.getMessageConverters().add(new SourceHttpMessageConverter<>());
                return restTemplate;
            }       
            

For more information, see the following resources:

• Spring Framework 6.0 Release Notes
• Spring GitHub Issue: Make SourceHttpMessageConverter optional