Java Platform System Properties

The Java platform itself uses a Properties object to maintain its own configuration. The System class maintains a Properties object that describes the configuration of the current working environment. System properties include information about the current user, the current version of the Java runtime, and the character used to separate components of a file path name.

Important system properties

The following table describes some of the most important system properties

Key	Meaning
"file.separator"	Character that separates components of a file path. This is"/" on UNIX and "\" on Windows.
"java.class.path"	Path used to find directories and JAR archives containing class files. Elements of the class path are separated by a platform-specific character specified in the path.separator property.
"java.home"	Installation directory for Java Runtime Environment (JRE)
"java.vendor"	JRE vendor name
"java.vendor.url"	JRE vendor URL
"java.version"	JRE version number
"line.separator"	Sequence used by operating system to separate lines in text files
"os.arch"	Operating system architecture
"os.name"	Operating system name
"os.version"	Operating system version
"path.separator"	Path separator character used in java.class.path
"user.dir"	User working directory
"user.home"	User home directory
"user.name"	User account name

Reading System Propertie

The System class has two methods used to read system properties: getProperty and getProperties.
The System class has two different versions of getProperty. Both retrieve the value of the property named in the argument list. The simpler of the two getProperty methods takes a single argument, a property key For example, to get the value of path.separator, use the following statement:

System.getProperty("user.dir");

The getProperty method returns a string containing the value of the property. If the property does not exist, this version of getProperty returns null.

The other version of getProperty requires two String arguments: the first argument is the key to look up and the second argument is a default value to return if the key cannot be found or if it has no value. For example, the following invocation of getProperty looks up the System property called user.name. This is not a valid system property, so instead of returning null, this method returns the default value provided as a second argument: "Guest User"

System.getProperty("user.name", "Guest User");

The last method provided by the System class to access property values is the getProperties method, which returns a Properties object. This object contains a complete set of system property definitions.

From Java Tutorials: System Properties