"Unexpected end of JSON input" on a Void SpringMVC Ajax Controller - ajax

Question

I have a SpringMVC/Thymeleaf applications where the following Ajax handling works perfectly fine if I return a boolean. However, as soon as the method is void, then I get the error
Unexpected end of JSON input in Firebug. This is a POST request.
#ResponseBody
#PostMapping("/addOrUpdate")
public void /*boolean works!*/ addOrUpdate(#RequestBody String json) throws Exception {
service.addOrUpdateUserRoles(json);
/*boolean works - return true;*/
}
JS
$.ajax({
type : "post",
dataType : 'json',
contentType : 'text/plain',
url : 'addOrUpdate',
data : id
})
.then(function() {
//...
})
.fail(function(jqXHR, textStatus, errorThrown) {
//...
});
If I just remove #ResponseBody from the method definition, Thymeleaf complains,
org.thymeleaf.exceptions.TemplateInputException: Error resolving template [addOrUpdate], template might not exist or might not be accessible by any of the configured Template Resolvers
I followed the ResponseEntity example here, but it didn't help -- same error, JS goes into the Error section with Unexpected End of Input.
#ResponseBody
#PostMapping("/addOrUpdate")
public ResponseEntity addOrUpdate(#RequestBody String json) throws Exception {
service.addOrUpdate(json);
return new ResponseEntity(HttpStatus.OK);
}

Answer 1

With dataType : 'json', you are telling jQuery that you're expecting JSON as a response. An empty response is no valid JSON, and the error message Unexpected end of JSON input is telling you exactly that.
If you intend not to return anything from the addOrUpdate controller method, remove the #ResponseBody annotation, as there is no response body, and stick to the ResponseEntity but use HttpStatus.NO_CONTENT instead to inform clients in your response that there's no content to be expected. Also, change your dataType to something that may be empty, like 'text'.

Answer 2

As the exception says, the point of failure is input.
You need to send json format input.
$.ajax({
type : "post",
dataType : 'json',
contentType : 'text/plain',
url : 'addOrUpdate',
data : {id: id}
...

Answer 3

FINAL SOLUTION based on digitalbreed's answer
Controller
#PostMapping("/addOrUpdate")
public ResponseEntity<String> addOrUpdate(#RequestBody String json) throws Exception {
try {
service.addOrUpdate(json);
return new ResponseEntity<String>(HttpStatus.OK); // No exceptions
}
catch (Exception e) {
log.error("Error", e);
return new ResponseEntity<String>(HttpStatus.BAD_REQUEST); // This will enable JS to catch the Exception from Ajax
}
}
JS
$.ajax({
type : "post",
dataType : 'text', // Returns a ResponseEntity, not JSON (Void method)
contentType : 'text/plain',
url : 'addOrUpdate',
data : somedata
})
.then(function() {
//...
})
.fail(function(jqXHR, textStatus, errorThrown) {
//... - will come here for a ResponseEntity of 'Bad Request'
});