Tomcat Servlet - How your Servlet or Filter can tell if the client closed its end of the HTTP connection

When the client disconnects, and your servlet tries to write to the output stream, Tomcat will throw a ClientAbortException (you may have already seen this):

http://tomcat.apache.org/tomcat-6.0-doc/api/org/apache/catalina/connector/ClientAbortException.html

Tomcat appears to throw this exception only when you write some bytes to the output stream, or when you flush the output stream, and the client already closed its end of the connection. You can try flushing the output stream periodically and see if doing so throws this exception. You can flush at any time, even if you have written zero bytes. Try it like this:

try {
    response.getOutputStream().flush();
} catch (ClientAbortException e) {
    // The client already closed its end of the TCP connection.
}

The first time you call flush, it will send the HTTP response headers to the client, so you must set the headers before flushing.