Question

There is an ajax js function which send request & get response and it often get successful but i smell somthing messy when i checked using firbug. please take a look.
when request success: ajaxRequest.getResponseHeader('content-Length'); returns null
But in actual the content-length is in digits. following is the firbug console output

HeadersResponseXML

Server  Apache-Coyote/1.1
Content-Type    text/xml
Transfer-Encoding   chunked
Date    Thu, 15 Sep 2011 07:23:36 GMT

when request failed : it returns 0 (zero),and it’s correct.
Headers
Server Apache-Coyote/1.1
Content-Length 0
Date Thu, 15 Sep 2011 07:23:53 GMT

I don’t see any content-length in response header when it succeed why? please compare success & failure firebug console output. How can I get actual content-length.

Answer 1

Read the javadoc of the doGet() (or doPost()) method of HttpServlet:

…

Where possible, set the Content-Length header (with the ServletResponse.setContentLength(int) method), to allow the servlet container to use a persistent connection to return its response to the client, improving performance. The content length is automatically set if the entire response fits inside the response buffer.

When using HTTP 1.1 chunked encoding (which means that the response has a Transfer-Encoding header), do not set the Content-Length header.

…

In other words, if you do not set the content length yourself inside your servlet and the response body is larger than the default response buffer size (often 2KB, configureable in server settings), then the servlet will send it using chunked encoding (technically, to prevent memory problems if it had to buffer large responses entirely in memory before being able to set the content length).

If you really need the content length in the response header for some reason, then you need to buffer the entire response body yourself before writing. E.g.

StringBuilder builder = new StringBuilder();
builder.append(someXml);
// ...

response.setContentType("text/xml");
response.setCharacterEncoding("UTF-8");
response.setContentLength(builder.length());
response.getWriter().write(builder.toString());

You only need to keep in mind that this can be a potential memory hog whenever the response body is extraordinary large.