This section provides a tutorial example on how to access system properties provided by the JVM instance and how to add your own properties.
The java.lang.System class also allows you to manage a collection of properties
stored at the JVM instance level. Whey your application is started, JVM will add many built-in properties
into the collection. While your application is running, you can add, retrieve, and remove any properties.
You can also reset or add any properties when invoking the JVM using the -D command line option.
Here is tutorial example program showing you how to manage system properties:
/**
* SystemJvmProperty.java
* Copyright (c) 2010 by Dr. Herong Yang, herongyang.com
*/
class SystemJvmProperty {
public static void main(String[] a) {
java.io.PrintStream out = System.out;
// adding my own entries to JVM properties
System.setProperty("hy.computer.maker","HP");
System.setProperty("hy.computer.os","Windows");
// copying all JVM properties into a Properties object
java.util.Properties props = System.getProperties();
java.util.Enumeration e = props.propertyNames();
out.println("JVM Properties:");
while (e.hasMoreElements()) {
String k = (String) e.nextElement();
String v = props.getProperty(k);
out.println(" "+k+" = "+v);
}
}
}
When executed on my Windows XP system with JDK 1.6.0,
I got this result:
C:\herong\jvm>java -Dhy.computer.color=Black SystemJvmProperty
JVM Properties:
java.runtime.name = Java(TM) SE Runtime Environment
sun.boot.library.path = C:\local\jdk\jre\bin
java.vm.version = 10.0-b22
hy.computer.maker = HP
java.vm.vendor = Sun Microsystems Inc.
java.vendor.url = http://java.sun.com/
path.separator = ;
java.vm.name = Java HotSpot(TM) Client VM
file.encoding.pkg = sun.io
sun.java.launcher = SUN_STANDARD
sun.os.patch.level = Service Pack 3
java.vm.specification.name = Java Virtual Machine Specification
user.dir = C:\herong\jvm
hy.computer.color = Black
java.runtime.version = 1.6.0_06-b02
java.awt.graphicsenv = sun.awt.Win32GraphicsEnvironment
os.arch = x86
java.io.tmpdir = C:\Temp\
line.separator =
java.vm.specification.vendor = Sun Microsystems Inc.
user.variant =
os.name = Windows XP
sun.jnu.encoding = Cp1252
java.library.path = C:\local\jdk\bin;.;C:\WINDOWS\system32;C:\WINDOWS
java.specification.name = Java Platform API Specification
java.class.version = 50.0
hy.computer.os = Windows
sun.management.compiler = HotSpot Client Compiler
os.version = 5.1
user.home = C:\Documents and Settings\herong
user.timezone =
java.awt.printerjob = sun.awt.windows.WPrinterJob
java.specification.version = 1.6
file.encoding = Cp1252
user.name = herong
java.class.path = .
java.vm.specification.version = 1.0
sun.arch.data.model = 32
java.home = C:\local\jdk\jre
java.specification.vendor = Sun Microsystems Inc.
user.language = en
awt.toolkit = sun.awt.windows.WToolkit
java.vm.info = mixed mode, sharing
java.version = 1.6.0_06
java.ext.dirs = C:\local\jdk\jre\lib\ext
sun.boot.class.path = C:\local\jdk\jre\lib\resources.jar;...
java.vendor = Sun Microsystems Inc.
file.separator = \
java.vendor.url.bug = http://java.sun.com/cgi-bin/bugreport.cgi
sun.cpu.endian = little
sun.io.unicode.encoding = UnicodeLittle
sun.desktop = windows
...
The test result tells me that:
The JVM indeed provided many built-in properties for me.
The "-Dhy.computer.color=Black" option did insert a new property for me.
The System.addProperty() method worked correctly.
Some built-in properties controls default behaviors of some system classes.
For example, "file.encoding = Cp1252" sets the default encoding when
reading from files or writing to files as characters.
