Java EE - find session size

Question

Is there a way to programatically track the size of a particular session on a Java EE app server, or do I have to resort to the app server's vendor specific instrumentation to do this?

Two scenarios:

1. Track from within the application (a sort of JMX-type interface)
2. Track from without (outside) - a generic piece of code that works on all app servers.

Answer 1

There are two approaches:

• if your session is Serializable (as should be), there are some tools that calculate the length of the serialized version of the Session object. e.g. LambdaProbe for Apache Tomcat

• if your session is not Serializable, then it's harder. The solution we've taken is to follow the sizeofagent technique. It makes use of the [Instrumentation.getObjectSize() method](http://java.sun.com/j2se/1.5.0/docs/api/java/lang/instrument/Instrumentation.html#getObjectSize(java.lang.Object)). It requires you to start the JVM with special agent:

  java -javaagent:sizeofag.jar

Answer 2

There isn't a standard way todo this. In fact there isn't actually a particularly good way to weigh an Object, assuming it's more than just primatives. One way to is to serialise the object to a byte array and take that as an indicator of the size.

An option would be to use a profiler like YourKit switch makes a pretty good stab at calculating the retained size of a reference.

There maybe Vendor specific API's for this as most SessionManager's have to serialise the session data for replication and persistence.