Question

I’m writing my first JAVA servlet and I have a question.

May be it’s important to say that my servlet will be called from Google Web Toolkit (AJAX)

First thing that I do is to create PrintWriter and start to write in it my JSON output

PrintWriter out = response.getWriter();
...
out.println("[");
out.println("  {");
out.println("    \"validation\" : {");
...

but what will happened if meanwhile I get an error condition?

What is the right way to return an error to client? (AJAX client)

Do I have to buffer my output (HOW?) and return error as JSON (instead of output)
or
I have to throw ServletException?

Answer 1

Just build the string in memory using for example StringBuilder. Do not write any character to the response yet until you’re finished building the string. That’s “buffering”.

StringBuilder builder= new StringBuilder();
builder.append("[");
builder.append("  {");
builder.append("    \"validation\" : {");
// ...

// When finished:
response.getWriter().write(builder.toString());

When something fails in meanwhile, either throw ServletException (which will end up in a server default error page with status code 500), or use HttpServletResponse#sendError() to send a more specific error status. But generally, status code 500 is enough sign for XMLHttpRequest client that something failed at the server side.

try {
    // Build and write JSON.
} catch (Exception e) {
    throw new ServletException(e);
}