How do I send List<T> with HTTPEntity in rest Template - resttemplate

Question

Exception in thread "main"
org.springframework.web.client.HttpClientErrorException: 400 Bad
Request
And my code is
RestTemplate restTemplate = new RestTemplate();
UriComponentsBuilder builder = UriComponentsBuilder.fromHttpUrl(Constants.ADD_EVENT_URL)
.queryParam("dataMap", eventsList);
String paramUrl = builder.build().encode().toString();
HttpHeaders requestHeaders = new HttpHeaders();
requestHeaders.setContentType(MediaType.APPLICATION_JSON);
HttpEntity<List<T>> httpEntity = new HttpEntity<List<T>>(eventsList, requestHeaders);
ResponseEntity<String> eventsAdded = restTemplate.exchange (paramUrl, HttpMethod.POST, httpEntity,String.class);

Answer 1

I realize this is a little old, but I presume the client is set up to receive a collection of some type?
An HTTP 400 happens from when your request is malformed, so either your entity is not correct, or you have not provided the correct path parameters, request parameters, and / or entity information.
From the URI being constructed there it looks like you are trying to pass the list as a querystring parameter and as a part of the request body, which is redundant.
Maybe try something like
UriComponentsBuilder builder = UriComponentsBuilder.fromUrlString(Constants.ADD_EVENT_URL);
...
HttpEntity<List<T>> httpEntity = new HttpEntity<List<T>>(eventsList, requestHeaders);
ResponseEntity<String> eventsAdded = restTemplate.exchange (builder.toUriString(), HttpMethod.POST, httpEntity, String.class);
OR
UriComponentsBuilder builder = UriComponentsBuilder.fromHttpUrl(Constants.ADD_EVENT_URL)
.queryParam("dataMap", eventsList);
...
ResponseEntity<String> eventsAdded = restTemplate.exchange (builder.toUriString(), HttpMethod.POST, null, String.class);