What is JVM startup parameter: -XX:+ [+|-]PrintClassHistogram?

Usage:                                                    

The option “-XX:[+|-]PrintClassHistogram” print a histogram of class instances.

Since:

Starting from JDK 6.

Syntax:

java -XX:[+|-]PrintClassHistogram MainClass

Description:

The flag “PrintClassHistogram” is a diagnostic flag for the hotspot JVM that allows us to enable/disable printing of a class instance histogram after one of the following events:

• Oracle Solaris, Linux, and OS X: Control+Break
• Windows: Control+C (SIGTERM).

Setting this option is equivalent to running the “jmap -histo” command, or the “jcmd pid GC.class_histogram” command, where pid is the current Java process identifier.

The following picture display a sample histogram of a class:

As we can see, the histogram can summary all informations about the heap objects used.

Num: is the order of the instances sorted by bytes

Instances: Number of instances of that class

Bytes: heap allocation of those instances

Class name: The class of the instance, [I represent an array of integers (int[]), [Ljava.lang.String represent a list of String (String[])

Default Value:

This option is disabled by default.

Errors:

None.

Related Posts:

• jmap - Memory Map
• The jcmd Utility
• Java