Java reference

System.getProperty()

Returns a JVM system property: the Java version, the OS name, or your own property set with -D.

Available since: Java 1.0

Syntax

public static String getProperty(String key, String def)

Parameters

Parameter	Type	Description
key	String	The property name, for example java.version or app.mode.
def	String	An optional default value used when the property is not set.

Return value

StringThe property value, the default value, or null.

Examples

Standard JVM properties (the values depend on the machine):

void main() {
    IO.println(System.getProperty("java.version"));
    IO.println(System.getProperty("os.name"));
}

Output:

25.0.4.1
Windows 10

Pass your own property with -D before the file name: java -Dapp.mode=test Main.java. The second argument is the default value:

void main() {
    IO.println(System.getProperty("app.mode", "dev"));
    IO.println(System.getProperty("app.debug", "false"));
}

Output:

test
false

Notes

Properties belong to the JVM, environment variables belong to the operating system; they are read by different methods. Useful keys: java.version, os.name, user.home, user.dir (the current folder), line.separator. System.setProperty() changes a property while the program runs. Ready-made helpers: Integer.getInteger("app.port", 8080) and Boolean.getBoolean("app.debug") read a property and convert it to a number or a boolean.

Related recipes

How to read an environment variable with a default value

In the tutorial

Arguments, environment and exit codes — How a Java program gets settings from outside — command-line arguments, environment variables with System.getenv, system properties with System.getProperty and -D, and exit codes with System.exit.

Thanks for reading!