Error in org.springframework.web.client.RestClientException: No HttpMessageConverter for java.util.HashMap

Question

I'm getting the following error in org.springframework.web.client.RestClientException: No HttpMessageConverter for java.util.HashMapwhen using rest template. Does anyone have any idea whats wrong?

Using java 1.8, spring 5.2.2, commons-logging 1.2.

import java.util.HashMap;
import java.util.Map;

import org.springframework.web.client.RestTemplate;

public class testapi {
   private static final String API_BASE_URL = "https://xyz/rest/ng";
   private static RestTemplate template = new RestTemplate();

public static void main(String[] args)
   throws Exception {
String token = login();
}

 private static String login() {

       Map<String, Object> payload = new HashMap<>();
       payload.put("loginName", "abc.com");
          payload.put("password", "xyz");
          Map<String, Object> resp = template.postForObject(getUrl("/sessions"), payload, Map.class);
          return (String) resp.get("token");

            }
}

Error

Exception in thread "main" org.springframework.web.client.RestClientException: No HttpMessageConverter for java.util.HashMap
    at org.springframework.web.client.RestTemplate$HttpEntityRequestCallback.doWithRequest(RestTemplate.java:964)
    at org.springframework.web.client.RestTemplate.doExecute(RestTemplate.java:740)
    at org.springframework.web.client.RestTemplate.execute(RestTemplate.java:677)
    at org.springframework.web.client.RestTemplate.postForObject(RestTemplate.java:421)
    at TestFormApis.login(testapis.java:117)
    at TestFormApis.main(testapis.java:61)

Answer 1

You are using Spring's web client RestTemplate in the non-Spring environment (at least, your example demonstrates that you run this test in the standard Java way).

If you start a Spring application, it takes most configuration to itself, including adding converters (default converters from package org.springframework.http.converter.*: ByteArrayHttpMessageConverter, StringHttpMessageConverter, ResourceHttpMessageConverter, AllEncompassingHttpMessageConverter, MappingJackson2XmlHttpMessageConverter, MappingJackson2HttpMessageConverter). Last one - for your case, RestTemplate uses inner Jackson for converting.

So, if you want to use your sample, just add the specific converter.

More easier way - to add Jackson library and convert manually:

var mapper = ObjectMapper();
var strPayload = mapper.writeValueAsString(payload);

And use strPayload in RestTempate calls.

Or just start your application as Spring one.

Answer 2

Run the app as Spring one and do the conversion using the code below. I had to include the jackson-core-2.2.0-rc1.jar libs to my project as well.

RestTemplate template = new RestTemplate();
MappingJackson2HttpMessageConverter mappingJackson2HttpMessageConverter = new MappingJackson2HttpMessageConverter();


public String login() {

mappingJackson2HttpMessageConverter.setSupportedMediaTypes(Arrays.asList(MediaType.APPLICATION_JSON, MediaType.APPLICATION_OCTET_STREAM));
       template.getMessageConverters().add(mappingJackson2HttpMessageConverter);

       Map<String, Object> payload = new HashMap<>();
       payload.put("loginName", "xyz");
          payload.put("password", "abc");
Map<String, Object> resp = template.postForObject(getUrl("/abc"), payload, Map.class);
          return (String) resp.get("token");
    }