Is there any effective opensource alternative available for New Relic for the Java platform (particularly for monitoring J2EE application servers like JBoss)?
Even though it does not have all the features of new relic, you can take a look at stagemonitor. It is a open source java web application performance monitor. It captures response time metrics, JVM metrics, request details (including a call stack captured by the request profiler) and more. The overhead is very low.
Optionally, you can use the great timeseries database graphite with it to store a long history of datapoints that you can look at with fancy dashboards.
Take a look at the project website to see screenshots, feature descriptions and documentation.
Note: I am the developer of stagemonitor