__example-use.sh

:➜ curl http://localhost:8080/booking/bigip.status                                        
online
active sessions: 0
... Start 2 nettlesere mot appen (som bruker sessions) ...
:➜ curl -X POST http://localhost:8080/booking/bigip.status\?secret\=ABCDEFG\&state\=offline
offline
active sessions: 2
:➜ curl http://localhost:8080/booking/bigip.status                                         
offline
active sessions: 2
➜ date
Wed Apr 13 20:45:16 CEST 2011
... vent 5 min ...
➜ date
Wed Apr 13 20:51:21 CEST 2011
:➜ curl http://localhost:8080/booking/bigip.status                                         
offline
active sessions: 0

BigIPStatusHandlerFilter.java

package bekkopen.servlet;

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.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.PrintWriter;

public class BigIPStatusHandlerFilter implements Filter {

    private final String secretToken = "ABCDEFG";
    private boolean available = true;

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {}

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        if (request instanceof HttpServletRequest && response instanceof HttpServletResponse) {
            handle((HttpServletRequest) request, (HttpServletResponse) response);
        }
        // don't care about chain.doFilter()
    }

    private void handle(HttpServletRequest request, HttpServletResponse response) throws IOException {
        if ("POST".equals(request.getMethod()) && secretToken.equals(request.getParameter("secret"))) {
            available = !("offline".equals(request.getParameter("state")));
        }

        response.setContentType("text/plain");
        response.setStatus(available ? 200 : 226);

        PrintWriter out = response.getWriter();

        out.println(available ? "online" : "offline");
        out.println("active sessions: " + HttpSessionCountListener.numberOfSessions() );
        out.close();
    }

    @Override
    public void destroy() {}
}

HttpSessionCountListener.java

package bekkopen.servlet;

import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;

public class HttpSessionCountListener implements HttpSessionListener {
    private static volatile int numberOfSessions;

    public void sessionCreated(HttpSessionEvent se) {
        synchronized (HttpSessionCountListener.class) {
            numberOfSessions++;
        }
    }

    public void sessionDestroyed(HttpSessionEvent se) {
        synchronized (HttpSessionCountListener.class) {
            numberOfSessions--;
        }
    }

    public static int numberOfSessions() {
        return numberOfSessions;
    }

}

web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://java.sun.com/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
         metadata-complete="false"
         version="2.5">

    <!-- ... -->

    <filter>
        <filter-name>BigIPStatusHandlerFilter</filter-name>
        <filter-class>bekkopen.servlet.BigIPStatusHandlerFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>BigIPStatusHandlerFilter</filter-name>
        <url-pattern>/bigip.status</url-pattern>
    </filter-mapping>

    <!-- ... -->

    <listener>
        <listener-class>bekkopen.servlet.HttpSessionCountListener</listener-class>
    </listener>

    <session-config>
      <session-timeout>5</session-timeout> 
    </session-config>

</web-app>