This is really useful when you’re trying to determine if your Java application running on a Tomcat server is currently serving any clients. For example, if your server is under use, you can avoid taking it offline for maintenance or upgrade. Or, you can use it to determine the load on a server.
Essentially Tomcat has some internal statistics that can be accessed using JMX (Java Management Extension). More information and details for that are available here: https://tomcat.apache.org/tomcat-8.0-doc/monitoring.html
The Tomcat JMX stats can be accessed in several ways. This article will cover how to access them from within the tomcat serer, which was our requirement. Since the application deployed to the tomcat server will be running locally on the JVM running tomcat, JMX can be accessed locally from the Java code, and no remote setup is required.
The code is as simple as this:
MBeanServer mBeanServer = ManagementFactory.getPlatformMBeanServer(); ObjectName objectName = new ObjectName("Catalina:type=Manager,context=/,host=localhost"); int activeSessions = (Integer) mBeanServer.getAttribute(objectName, "activeSessions");
And that’s it! You’ll get the number of active sessions tomcat is reporting. And since this is running within the application deployed to tomcat, no remote setup or custom JVM arguments for tomcat are required.