Apex REST API GET method with request body?

Question

How do you do a HttpRequest for a GET method with a request body? For some reason this GET method is getting converted to POST and is not successful. This is working via postman.

 Http http = new Http();
 HttpRequest request = new HttpRequest();
 HttpResponse response;

 request.setEndpoint('http://www.someendpoint.com/getdata');
 request.setMethod('GET');

 String requestBody = '{ "accountId": "123345" }';

 request.setBody(requestBody);
 response = http.send(request);

We are getting the following

 {
    "errors" : [ {
       "message" : "Request method 'POST' not supported",
       "type" : "HttpRequestMethodNotSupportedError"
    } ]
 }

Answer 1

Some info about GET method with body Http GET methods with body are kind of grey area. Per spec you can include body in GET and server must parse it but they are not supposed to use it to serve GET.

See this thread for more info.

About your question Salesforce is upgrading the method to POST because you included request body. If you comment out code where you are setting body, sfdc leaves the method as GET. So either you need to work with server to support POST for this endpoint or support query params for data you are posting.

This is working in Postman because it is not upgrading the method like sfdc.

Btw, httpbin.org is an excellent resource to test what sfdc is sending to server.