Read Java JVM startup parameters (eg -Xmx)

Question

I'm trying to figure out if there's a way to determine the JVM startup properties from within a running java process. Specifically I'm trying to find out where parameters such as -Xmx (max heap size) and -XX:MaxPermSize are stored. I'm running Sun's 1.6 jvm.

If you're wondering why I want to do this, I have a number of JVM webservers that may or may not be configured correctly and I want to add this to the startup code check. It's much easier for me to check in a piece of java code that gets deployed everywhere than to manually find and check all of the jvm startup files. Right now the jvm configuration files for better or worse are not part of our build process or checked into source control.

Ursala

Answer 1

Try:

import java.lang.management.ManagementFactory;
import java.lang.management.RuntimeMXBean;

import java.util.List;

public void runtimeParameters() {
  RuntimeMXBean bean = ManagementFactory.getRuntimeMXBean();
  List<String> aList = bean.getInputArguments();

  for (int i = 0; i < aList.size(); i++) {
    System.out.println( aList.get( i ) );
  }
}

That should show all JVM parameters.

Note: we do not have JVM parameter in VCS either, but in a database, read by our own launchers in productions. That way, we can change those values remotely, without having to redeploy JVM parameter file settings.

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You will find a good sumary of various JVM tools to use in this article (from the "Dustin's Software Development Cogitations and Speculations"), including Java Application Launcher links to :

• ManagementFactory.getRuntimeMXBean() call
• getInputArguments() javadoc
• Accessing JVM Arguments from Java (to determine, for instance, if JVM is running in debug mode, in order to alter the "grid initialization" logic of an application)
• Annotation Type MXBean
• MXBean Java Tutorial

This technique takes advantage of Platform MXBeans available since J2SE 5 (custom MXBeans support was added in Java SE 6).

Two useful sources of information on the JVM parameters available when using Sun's JVM are:

• A Collection of JVM Options and
• Charles Nutter's Favorite Hotspot JVM Flags.

Both of these resources list and describe some/all of the not-recommended-for-the-casual-developer double X arguments (-XX) that are available.

Olly

Works like a charm! I clearly don't know java.lang.management as well as I should. – Ursala

Answer 2

With Java 7 or later it's as easy as

java -XshowSettings:all

Osmium