I have to use Spring’s RestTemplate to call an external API that accepts POST requests with Content-Type: multipart/form-data. The input data is just key-value pairs, no attachments, but the server forces me to use multipart/form-data.

The following is the original request that worked fine.

POST http://the-api:8080 HTTP/1.1
Content-Type: multipart/form-data; boundary=--Eh0oKOHPOSEIJTzFevDxHhPNKhQl7AP6kQL
Accept: */*
Host: the-api:8080
accept-encoding: gzip, deflate
content-length: 680
Connection: keep-alive

--Eh0oKOHPOSEIJTzFevDxHhPNKhQl7AP6kQL
Content-Disposition: form-data; name="param1"

value1
--Eh0oKOHPOSEIJTzFevDxHhPNKhQl7AP6kQL
Content-Disposition: form-data; name="param2"

value2
--Eh0oKOHPOSEIJTzFevDxHhPNKhQl7AP6kQL--

Here is my original request to extract and rearrange from RestTemplate’s logs, and it didn’t work because the server mistook headers for values.

POST http://the-api:8080 HTTP/1.1
Content-Type: multipart/form-data; boundary=--Eh0oKOHPOSEIJTzFevDxHhPNKhQl7AP6kQL
Accept: */*
Host: the-api:8080
accept-encoding: gzip, deflate
content-length: 680
Connection: keep-alive

--Eh0oKOHPOSEIJTzFevDxHhPNKhQl7AP6kQL
Content-Disposition: form-data; name="param1"
Content-Type: text/plain; charset=UTF-8
Content-Length: 29

value1
--Eh0oKOHPOSEIJTzFevDxHhPNKhQl7AP6kQL
Content-Disposition: form-data; name="param2"
Content-Type: text/plain; charset=UTF-8
Content-Length: 14

value2
--Eh0oKOHPOSEIJTzFevDxHhPNKhQl7AP6kQL--

The code is as follows

HttpHeaders headers = new HttpHeaders();
headers.setContentType(MediaType.MULTIPART_FORM_DATA);
MultiValueMap<String, String> params = new LinkedMultiValueMap<>();
params.add("param1", "value1);
params.add("param2", "value2);
HttpEntity<MultiValueMap<String, String>> request = new HttpEntity<>(params, headers);

URI uri = UriComponentsBuilder.fromHttpUrl("http://the-api:8080")
        .build().encode(Charset.forName("UTF-8")).toUri();

return restTemplate.postForObject(uri, request, KKPMailResponse.class);

Question

How to prevent Spring’s RestTemplate from automatically adding header Content-Type: text/plain for each parameter; charset=UTF-8 and Content-Length: xx

I didn’t find a way to stop Spring from generating entries, but you can use an interceptor to remove them before sending the request. To do this, you must manipulate the request body in the interceptor as follows:

public class MultiPartFormDataCleaningInterceptor implements ClientHttpRequestInterceptor {

@Override
    public ClientHttpResponse intercept(HttpRequest request, byte[] body, ClientHttpRequestExecution execution) throws IOException {
        final MediaType contentType = request.getHeaders().getContentType();
        if (contentType != null
                && MediaType.MULTIPART_FORM_DATA.getType().equals(contentType.getType())
                && MediaType.MULTIPART_FORM_DATA.getSubtype().equals(contentType.getSubtype())) {
            return execution.execute(request, stripContentTypeAndLength(body));
        }
        return execution.execute(request, body);
    }

private byte[] stripContentTypeAndLength(byte[] body) {
        final String bodyStr = new String(body);
        final StringBuilder builder = new StringBuilder();
        try (final Scanner scanner = new Scanner(bodyStr)) {
            while (scanner.hasNextLine()) {
                final String line = scanner.nextLine();
                if (!line.startsWith("Content-Type:")
                        && !line.startsWith("Content-Length:")) {
                    builder.append(line).append("\r\n");
                }
            }
        }
        final String newBodyStr = builder.toString();
        return newBodyStr.getBytes();
    }
}