HttpServletResponse#sendError How to change ContentType

Question

I'm creating a simple Servlet to answer a form submition. This servlet receive POST request and should response Application/JSON datas. This is working well. I want now to add errors managment to my servlet: If the servlet receive bad request parameters, I want it to answer the client a 400 — "Bad Request" response with a simple {"error":"ERROR MESSAGE"} JSON response.

I'm using the HttpServletResponse#sendError(int,String) to correctly send the error but the ContentType of the response is always set to text/html, even though I used HttpServletResponse#setContentType("application/json") before.

Is this the good way to send error using Servets ? How can I force the ContentType of the response to application/json?

Sample code of the doPost() method of my test Servlet :

public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("application/json;charset=UTF-8");
        PrintWriter out = response.getWriter();
        out.print("{\"error\":1}");
        response.sendError(HttpServletResponse.SC_BAD_REQUEST);
}

Answer 1

That works as specified:

Sends an error response to the client using the specified status and clears the buffer. The server defaults to creating the response to look like an HTML-formatted server error page containing the specified message, setting the content type to "text/html".

HttpServletResponse.sendError(int)

on the other hand does not have this restriction/feature.

Sends an error response to the client using the specified status code and clearing the buffer.

This means: set the content type, write to the buffer, call sendError(400).