Semaphore filter

SemaphoreFilter.java

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.concurrent.Semaphore;

public class SemaphoreFilter implements Filter {

    private Semaphore semaphore;

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        semaphore = new Semaphore(20);
    }

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        if (semaphore.tryAcquire()) {
            try {
                filterChain.doFilter(servletRequest, servletResponse);
            }
            finally {
                semaphore.release();
            }
        }
        else {
            ((HttpServletResponse) servletResponse).sendError(503);
        }
    }

    @Override
    public void destroy() {

    }
}