This page describes how to configure and use object instance tracking for Java applications. For more information about why you may need to configure this, see Java Memory Thrash

When object instance tracking is enabled, AppDynamics tracks the top 20 application and top 20 system (core Java) classes in the heap by default. You can also track specific classes.

AppDynamics does not perform allocation tracking for core Java classes by default since doing so would add significant system overhead. However, it is possible to track core classes on a short-term basis (for example, while troubleshooting) or in pre-production environments. For more information, see Enabling Allocation Tracking for Core Java Classes.

Activating object instance tracking increases the amount of information captured by the agents, resulting in additional overhead. AppDynamics recommends using object instance tracking only while troubleshooting potential memory leaks. It does not normally need to be enabled during normal operation. 

You need the Configure Agent Properties permission to set object instance tracking. You need the Configure Memory Monitoring permission to configure the custom classes to track. See Manage Custom Roles.

Prerequisites for Object Instance Tracking

  • Review Java Supported Environments for information about platform support for object instance tracking.  
  • For JVMs prior to Java 9, object instance tracking uses tools.jar. If your application runs with the JDK, tools.jar should be already available. If you are running with the JRE, you must add tools.jar to <JRE_HOME>/lib/ext and restart the JVM. You can find tools.jar in <JAVA_HOME>/lib/tools.jar. For Java 9 onwards JEP220 moved the capabilities needed by OIT to the Core Java runtime, therefore tools.jar is no longer necessary.
  • In some cases, you might also need to copy (Linux) or attach.dll (Windows) from your JDK to your JRE. 
  • Depending on the JDK version, you may also need to specify the classpath as shown below (along with other -jar options).

jdk.jcmd is no longer a required module for the custom runtime. However, it is required for the proper functioning of object instance tracking.

Specify the Classpath

When using the JDK runtime environment, set the classpath using the -classpath option for the application. For example:

  • On Windows:
    java -classpath <complete-path-to-tools.jar>;%CLASSPATH% -jar myApp.jar
  • On Unix:
    java -Xbootclasspath/a:<complete-path-to-tools.jar> -jar myApp.jar

Enable Object Instance Tracking

To start an object instance tracking session, follow these steps:

  1. In the left navigation pane, click Tiers & Nodes
  2. In the right pane, expand the tier node and open the Node Dashboard for the node on which you want to enable object instance tracking. 
  3. Click the Memory tab.
  4. Click the Object Instance Tracking subtab.
  5. Click ON.

Tracked classes now appear in the Object Instance Tracking table. You can drill down to the tracked classes to see details. 

Track Specific Classes

For performance reasons, only the top 20 application classes and the top 20 system (core Java) classes in the heap are tracked automatically. 

Use the Configure Custom Classes to Track option on the Object Instance Tracking subtab to specify instances of specific classes to track. Note that the classes you configure here are tracked only if their instance count is among the top 1000 instance counts in the JVM. 

To track instances of custom classes:

  1. On the Object Instance Tracking tab, click Configure Custom Classes to Track.
  2. On the Instrumentation page that displays, click the tier you want to customize. 
  3. In the Object Instance Tracking section for the tier, click Add
  4. Enter the fully-qualified class name of the class to track and click Save.

The class you added is now tracked during object instance tracking sessions.