Getting and Adding System Properties

JVM Tutorials - Herong's Tutorial Examples

∟java.lang.System Class - The Operating System

∟Getting and Adding System Properties

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) HerongYang.com. All Rights Reserved.
 */
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 computer with JDK 10, I got this result:

herong> java -Dhy.computer.color=Black SystemJvmProperty

   java.runtime.name = Java(TM) SE Runtime Environment
   java.vm.version = 10.0.1+10
   sun.boot.library.path = C:\Progra~1\Java\jdk-10.0.1\bin
   java.vm.vendor = "Oracle Corporation"
   java.vendor.url = http://java.oracle.com/
   path.separator = ;
   java.vm.name = Java HotSpot(TM) 64-Bit Server VM
   file.encoding.pkg = sun.io
   sun.os.patch.level = Service Pack 1
   user.script =
   user.country = US
   sun.java.launcher = SUN_STANDARD
   java.vm.specification.name = Java Virtual Machine Specification
   hy.computer.color = Black
   java.vm.compressedOopsMode = 32-bit
   java.runtime.version = 10.0.1+10
   java.awt.graphicsenv = sun.awt.Win32GraphicsEnvironment
   os.arch = amd64
   java.io.tmpdir = C:\Users\herong\AppData\Local\Temp\
   line.separator =

   java.vm.specification.vendor = Oracle Corporation
   user.variant =
   os.name = Windows 7
   sun.jnu.encoding = Cp1252
   java.library.path = C:\Progra~1\Java\jdk-10.0.1\bin;...
   jdk.debug = release
   java.class.version = 54.0
   java.specification.name = Java Platform API Specification
   hy.computer.os = Windows
   sun.management.compiler = HotSpot 64-Bit Tiered Compilers
   os.version = 6.1
   user.home = C:\Users\herong
   user.timezone =
   java.awt.printerjob = sun.awt.windows.WPrinterJob
   file.encoding = Cp1252
   java.specification.version = 10
   user.name = herong
   java.class.path = .
   java.vm.specification.version = 10
   sun.arch.data.model = 64
   sun.java.command = SystemJvmProperty
   java.home = C:\Progra~1\Java\jdk-10.0.1
   user.language = en
   java.specification.vendor = Oracle Corporation
   awt.toolkit = sun.awt.windows.WToolkit
   java.vm.info = mixed mode
   java.version = 10.0.1
   java.vendor = Oracle Corporation
   sun.stderr.encoding = cp437
   file.separator = \
   java.version.date = 2018-04-17
   java.vendor.url.bug = http://bugreport.java.com/bugreport/
   sun.io.unicode.encoding = UnicodeLittle
   sun.cpu.endian = little
   java.vendor.version = 18.3
   sun.stdout.encoding = cp437
   sun.desktop = windows
   sun.cpu.isalist = amd64
   ...

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.