[ACCEPTED]-How to set limit to the number of concurrent request in servlet?-servlets

Accepted answer
Score: 19

I'd suggest writing a simple servlet Filter. Configure 8 it in your web.xml to apply to the path that you 7 want to limit the number of concurrent requests. The 6 code would look something like this:

public class LimitFilter implements Filter {
    private int limit = 5;
    private int count;
    private Object lock = new Object();

    public void doFilter(ServletRequest request, ServletResponse response,
            FilterChain chain) throws IOException, ServletException {
        try {
            boolean ok;
            synchronized (lock) {
                ok = count++ < limit;
            if (ok) {
                // let the request through and process as usual
                chain.doFilter(request, response);
            } else {
                // handle limit case, e.g. return status code 429 (Too Many Requests)
                // see http://tools.ietf.org/html/rfc6585#page-3
        } finally {
            synchronized (lock) {

Or alternatively 5 you could just put this logic into your 4 HttpServlet. It's just a bit cleaner and more reusable 3 as a Filter. You might want to make the limit 2 configurable through the web.xml rather than hard 1 coding it.

Check definition of HTTP status code 429.

Score: 3

You can use RateLimiter. See this article for 1 explanation.

Score: 2

I've thought about using a static counter 10 to keep track of the number of request, but 9 it would raise a problem of race condition.

If 8 you use a AtomicInteger for the counter, you 7 will not have the problem of race conditions.

An 6 other way would be using the Java Executor Framework (comes with 5 Java 1.5). There you are able to limit the 4 number of running threads, and block new 3 once until there is a new free thread.

But 2 I think the counter would work and be the 1 easyest solution.

Attention: put the counter relese in a finally block!

//psydo code
final AtomicInteger counter;
while(true) {
  int v = counter.getValue()
  if (v > max) return FAILURE;
  if(counter.compareAndSet(v, v+1)) break;  
} finally{
Score: 2

You might want to have a look on Semaphore.

Semaphores 5 are often used to restrict the number of 4 threads than can access some (physical or 3 logical) resource.

Or even better try to 2 figure it out with the server settings. That 1 would of course be server-dependant.

Score: 0

If you are serving static files, it's unlikely 12 that the server will crash. The bottleneck 11 would be the network throughput, and it 10 degrades gracefully - when more requests 9 come in, each still get served, just a little 8 bit slower.

If you set a hard limit on total 7 requests, remember to set a limit on requests 6 per IP. Otherwise, it's easy for one bad 5 guy to issue N requests, deliberately read 4 the responses very slowly, and totally clog 3 your service. This works even if he's on 2 a dialup and your server network has a vast 1 throughput.

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