Spring Rest Template creating multipart form/data client working like a postman throws non convertable exception

Question

I want to turn this postman client with multipart/form-data header request to a spring template client.

Right now, I have a basic rest controller which works well.

@RestController
@RequestMapping("/api")
public class MainConroller {

    private static final Logger log = LoggerFactory.getLogger(MainConroller.class);
    @Autowired
    private MainService mainService;

    public MainConroller(MainService mainService) {
        this.mainService = mainService;
    }

    @PostMapping("/mails/send")
    public void send(
            @RequestParam("usertoken") String usertoken,
            @RequestParam("sendTo") String sendTo,
            @RequestParam("subject") String subject,
            @RequestParam("content") String content,
            @RequestParam(required = false, name = "files") List<MultipartFile> multipartFiles) {
        log.debug("{}, {}, {}, {}", usertoken, sendTo, subject, content);

        mainService.processMessage(usertoken, sendTo, subject, content, multipartFiles);

    }

}

I need to, however, create a rest client so I used a rest template, it right now looks like this:

ArrayList<HttpMessageConverter<?>> converters = new ArrayList<>(
            Arrays.asList(new MappingJackson2HttpMessageConverter(), new ResourceHttpMessageConverter(), new FormHttpMessageConverter()));
    @Bean
    public RestTemplate restTemplate() {
        return new RestTemplate(converters);
    }
    File file = new File("*********");

            HttpHeaders httpHeaders = new HttpHeaders();
            httpHeaders.setContentType(MediaType.MULTIPART_FORM_DATA);

            MultiValueMap<String, Object> fileValueMap = new LinkedMultiValueMap<>();
            fileValueMap.add(file.getName(), file);
            fileValueMap.add(file.getName(), file);
            fileValueMap.add(file.getName(), file);

            HttpEntity<MultiValueMap<String, Object>> filesPart = new HttpEntity<>(fileValueMap, httpHeaders);

    //        String key values part

            MultiValueMap<String, String> stringPart = new LinkedMultiValueMap<>();
            stringPart.add("usertoken", "test");
            stringPart.add("sendTo", "test");
            stringPart.add("subject", "test");
            stringPart.add("content", "test");

            HttpEntity<MultiValueMap<String, String>> objectPart = new HttpEntity<>(stringPart, httpHeaders);


            MultiValueMap<String, Object> multiPartRequest = new LinkedMultiValueMap<>();
            multiPartRequest.add("ObjectPart", objectPart);
            multiPartRequest.add("FilesPart", filesPart);


            HttpEntity<MultiValueMap<String, Object>> requestEntity = new HttpEntity<>(multiPartRequest, httpHeaders);

            String serverUrl = "****";

            restTemplate().postForEntity(serverUrl, requestEntity, String.class);

The problem is when I try to send the post request, It throws

Exception in thread "main" org.springframework.web.client.RestClientException: Could not write request: no suitable HttpMessageConverter found for request type [org.springframework.util.LinkedMultiValueMap] and content type [multipart/form-data]

Update

The solution on the client side is very simple, you can just send String values in the object values, which are going to be automatically casted to Strings via generics.

Also, the files cannot be send just as files but you have to create FileSystemResource instead, here is the complete code of the client side:

@Service
public class RestTemplatePost {

    @Bean
    public RestTemplate restTemplate() {
        return new RestTemplate();
    }

    public void prepareMessage() throws Exception {


        HttpHeaders httpHeaders = new HttpHeaders();
        httpHeaders.setContentType(MediaType.MULTIPART_FORM_DATA);

        MultiValueMap<String, Object> form = new LinkedMultiValueMap<>();
        form.add("usertoken", "test");
        form.add("sendTo", "test");
        form.add("subject", "test");
        form.add("content", "test");
        form.add("files", getTestFile());
        form.add("files", getTestFile());

        HttpEntity<MultiValueMap<String, Object>> requestEntity = new HttpEntity<>(form, httpHeaders);

        String serverUrl = "******";

        restTemplate().postForEntity(serverUrl, requestEntity, String.class);


    }

    public static Resource getTestFile() throws IOException {
    Path testFile = Paths.get("C****");
    System.out.println("Creating and Uploading Test File: " + testFile);
    Files.write(testFile, "Hello World !!, This is a test file.".getBytes());
    return new FileSystemResource(testFile.toFile());
}

}

Answer 1

You are making things too complex. You should use a single map to hold the form values not a map of maps. Next to that Spring Boot already provides a RestTemplate so you don't need to configure your own again.

File file = new File("*********");

HttpHeaders httpHeaders = new HttpHeaders();
httpHeaders.setContentType(MediaType.MULTIPART_FORM_DATA);

MultiValueMap<String, Object> form = new LinkedMultiValueMap<>();
form.add("files", file);
form.add("files", file);
form.add("files", file);

form.add("usertoken", "test");
form.add("sendTo", "test");
form.add("subject", "test");
form.add("content", "test");

HttpEntity<MultiValueMap<String, Object>> requestEntity = new HttpEntity<>(form, httpHeaders);

String serverUrl = "****";

restTemplate().postForEntity(serverUrl, requestEntity, String.class);

The RestTemplate (or actually the FormHttpMessageConverter) will transform it into a correct request.