JMX API

The most basic way to get GC-related information from the running JVM is via the standard JMX API. This is a standardized way for the JVM to expose internal information regarding the runtime state of the JVM. You can access this API either programmatically from your own application running inside the very same JVM, or using JMX clients.

Two of the most popular JMX clients are JConsole and JVisualVM (with a corresponding plugin installed). Both of these tools are part of the standard JDK distribution, so getting started is easy. If you are running on JDK 7u40 or later, a third tool bundled into the JDK called Java Mission Control is also available.

All the JMX clients run as a separate application connecting to the target JVM. The target JVM can be either local to the client if running both in the same machine, or remote. For the remote connections from client, JVM has to explicitly allow remote JMX connections. This can be achieved by setting a specific system property to the port where you wish to enable the JMX RMI connection to arrive:

java -Dcom.sun.management.jmxremote.port=5432 com.yourcompanyYourApp

In the example above, the JVM opens port 5432 for JMX connections.

After connecting your JMX client to the JVM of interest and navigating to the MBeans list, select MBeans under the node “java.lang/GarbageCollector”. See below for two screenshots exposing information about GC behavior from JVisualVM and Java Mission Control, respectively:

JMX GC tuning

JMX MBean view attributes

As the screenshots above indicate, there are two garbage collectors present. One of these collectors is responsible for cleaning the young generation and one for the old generation. The names of those elements correspond to the names of the garbage collectors used. In the screenshots above we can see that the particular JVM is running with ParallelNew for the young generation and with Concurrent Mark and Sweep for the old generation.

For each collector the JMX API exposes the following information:

  • CollectionCount – the total number of times this collector has run in this JVM,
  • CollectionTime – the accumulated duration of the collector running time. The time is the sum of the wall-clock time of all GC events,
  • LastGcInfo – detailed information about the last garbage collection event. This information contains the duration of that event, and the start and end time of the event along with the usage of different memory pools before and after the last collection,
  • MemoryPoolNames – names of the memory pools that this collector manages,
  • Name – the name of the garbage collector
  • ObjectName – the name of this MBean, as dictated by JMX specifications,
  • Valid – shows whether this collector is valid in this JVM. I personally have never seen anything but “true” in here

In my experience this information is not enough to make any conclusions about the efficiency of the garbage collector. The only case where it can be of any use is when you are willing to build custom software to get JMX notifications about garbage collection events. This approach can rarely be used as we will see in the next sections, which give better ways of getting beneficial insight into garbage collection activities.