jinfo - VM Option Value Checker

Java Tools Tutorials - Herong's Tutorial Examples

∟jinfo - VM Option Value Checker

This section provides a tutorial example on how to use the 'jinfo' tool to view the current value of a given HotSpot VM option.

The first JVM troubleshooting tool I want to try is the "jinfo" tool.

The "jinfo" tool supports functions to view and modify HotSpot VM options of the specified JVM process. Here is the "jinfo" command syntax:

herong> jinfo

Usage:
    jinfo [option] <pid>
        (to connect to running process)
    jinfo [option] <executable <core>
        (to connect to a core file)
    jinfo [option] [server_id@]<remote server IP or hostname>
        (to connect to remote debug server)

where <option> is one of:
    -flag <name>         to print the value of the named VM flag
    -flag [+|-]<name>    to enable or disable the named VM flag
    -flag <name>=<value> to set the named VM flag to the given value
    -flags               to print VM flags
    -sysprops            to print Java system properties
    <no option>          to print both of the above
    -h | -help           to print this help message

HotSpot VM supports many options as described on the "Java HotSpot VM Options" Website at oracle.com/technetwork/java/javase/tech/vmoptions-jsp-140102.html. You can change the default value of any HotSpot VM option using the "-XX:..." command option when running the "java" command.

Here is a tutorial example of how to use the "jinfo" tool get the current value of a given VM option:

herong> java -XX:MaxHeapSize=123456789 PrimeNumberSeeker 10 200
Period, Current int, # primes
1, 2, 0
2, 10, 4
...

(Start another command window.)
herong> jps -l -m
14084 PrimeNumberSeeker 10 200
10620 jdk.jcmd/sun.tools.jps.Jps -l -m

herong> jinfo -flag MaxHeapSize 14084
-XX:MaxHeapSize=123731968

Note that:

The "jps" tool is used to get the process ID (pid) of the PrimeNumberSeeker JVM: 14084.
The output of the "jinfo" command shows that the current value of the MaxHeapSize option is 123731968 bytes in the PrimeNumberSeeker JVM.
The reported value, 123731968 bytes, of MaxHeapSize seems to match what I specified with the "-XX:MaxHeapSize=123456789" option when launching the PrimeNumberSeeker JVM.