JVM Parameter Types and Hands‑On Experiments

The purpose of this lab is to explain the three major JVM parameter types and demonstrate how to view and set them, which is essential knowledge for JVM tuning.

Standard Parameters

-version – get JDK version

-help – get help information

-showversion – display version and help

Experiment 1 – View Standard Parameters

Run the following commands in the console:

java -version

Result shows JDK version 1.8.0_131.

java -help

Displays the help document.

java -showversion

Shows both version and help.

X Parameters

X Parameter Overview

-Xint – pure interpretation mode

-Xcomp – compile to native code before execution

-XMixed – mixed mode (both compilation and interpretation)

Experiment 2 – View and Configure X Parameters

Check current mode:

java -version

Switch to interpretation mode:

java -Xint -version

Switch to compilation‑only mode:

java -Xcomp -version

XX Parameters

XX Parameter Overview

XX parameters come in two forms:

Boolean type: -XX:+SomeFlag (enable) or -XX:-SomeFlag (disable). Example: -XX:-PrintGCDetails disables GC detail output.

Key‑value type: -XX:Key=Value . Example: -XX:MetaspaceSize=2000000 sets metaspace size.

Experiment 3 – Check If a Parameter Is Enabled

Create a long‑running Java program (demoXXparam.java) and run it, then use:

jps -l

Find the process ID, then query the flag:

jinfo -flag PrintGCDetails

The output -XX:-PrintGCDetails indicates the flag is disabled.

Experiment 4 – Enable a Parameter

Terminate the program, then restart it with the flag enabled:

java -XX:+PrintGCDetails demoXXparam

Now jinfo -flag PrintGCDetails shows -XX:+PrintGCDetails , meaning the flag is active.

Experiment 5 – Key‑Value Parameter Value

Check metaspace size:

jinfo -flag MetaspaceSize

Set metaspace to 128 M:

java -XX:MetaspaceSize=128m demoXXparam

Verify the new value with the same jinfo command.

Experiment 6 – Set InitialHeapSize and MaxHeapSize

Set initial heap size:

java -XX:InitialHeapSize=200m demoXXparam

Set maximum heap size:

java -XX:MaxHeapSize=200m demoXXparam

Experiments 7‑10 – List JVM Flags

Print default flags: java -XX:+PrintFlagsInitial -version

Print final flags (including user‑defined): java -XX:+PrintFlagsFinal -version

Print command‑line flags: java -XX:+PrintCommandLineFlags -version

Print all flags of a running process: jinfo -flags

Experiment Summary

The lab covered how to view basic, X, and XX JVM parameters, how to modify them, and how to use jps and jinfo to inspect running Java processes. These skills form the foundation for JVM performance tuning.