Option '-X' - Specifying Non-Standard Options

Java Tools Tutorials - Herong's Tutorial Notes - Version 5.32, by Dr. Herong Yang

Java Tools Tutorials - Herong's Tutorial Notes

∟Option '-X' - Specifying Non-Standard Options

This section provides a tutorial example on how to use the '-X' option for the 'java' tool to specify non-standard options. The tutorial example shows how to control the memory size of the JVM.

"java" can also take "non-standard" options by using the "-X" option.

Two of the "non-standard" options are very useful to control the memory size of the JVM to be launched.

-Xmsn 
-Xmxn

where "ms" specifies the initial size of the JVM, "mx" specifies the maximum size of the JVM, and "n" specifies the memory size in bytes, kilo bytes or mega bytes. For example: -Xms32M and -Xmx64M.

Here is a program to show you how to use these options:

/* ShowMemory.java
#- Copyright (c) 2015, HerongYang.com, All Rights Reserved.
 */
class ShowMemory {
   public static void main(String[] a) {
      Runtime rt = Runtime.getRuntime();
      System.out.println(" Free memory: " + rt.freeMemory());
      System.out.println("Total memory: " + rt.totalMemory());
      while (true);
   }
}

I launched the program with different options:

C:\herong>javac ShowMemory.java

C:\herong>java ShowMemory
 Free memory: 15709392
Total memory: 16252928

C:\herong>java -Xms32M -Xmx128M ShowMemory
 Free memory: 31727064
Total memory: 32440320

C:\herong>java -Xms64M -Xmx128M ShowMemory
 Free memory: 63806136
Total memory: 64880640

Note that ShowMemory is running an infinite loop at the end. You need to stop the program by "Ctrl-C"

Last update: 2015.

Table of Contents

About This Book

Java Tools Terminology

Installing Java 8 on Windows

'javac' - The Java Program Compiler

►'java' - The Java Program Launcher

'java' - Java Launching Command and Options

Launching Hello.java - My First Java Program

Option "-classpath" - Specifying Class Path

Option '-jar' - Specifying Executable JAR Files