How can I get System variable value in Java?

Use the System.getenv(String) method, passing the name of the variable to read.

To clarify, system variables are the same as environment variables. User environment variables are set per user and are different whenever a different user logs in. System wide environment variables are the same no matter what user logs on.

To access either the current value of a system wide variable or a user variable in Java, see below:

String javaHome = System.getenv("JAVA_HOME");

For more information on environment variables see this wikipedia page.

Also make sure the environment variable you are trying to read is properly set before invoking Java by doing a:

echo %MYENVVAR%

You should see the value of the environment variable. If not, you may need to reopen the shell (DOS) or log off and log back on.

There are a few details of interest when getting system/environment properties.

First, System.getenv(String) was introduced way-back-when, then deprecated. The deprecation (foolishly, IHMO) continued all the way into JSE 1.4.

It got re-introduced in JSE 5.

Those are set using the Environment Variables panel in Windows. Changes to the variables may not get picked up until your current VM is shutdown, and the CMD.exe instance is exited.

In contrast to the environment properties, Java also has Java system properties, accessible through System.getProperties(). These variables can be initialized when the VM is started using a series -Dname=value command line arguments. For example, the values for the properties maxInMemory and pagingDirectory are set in the command below:

C:\> java.exe -DmaxInMemory=100M -DpagingDirectory=c:\temp -jar myApp.jar

These properties can be modified at runtime, barring security policy restrictions.