The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.
See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.
See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.
Many operating systems use environment variables to pass configuration information to applications. Like properties in the Java platform, environment variables are key/value pairs, where both the key and the value are strings. The conventions for setting and using environment variables vary between operating systems, and also between command line interpreters. To learn how to pass environment variables to applications on your system, refer to your system documentation.
On the Java platform, an application uses
System.getenv
to retrieve environment variable values. Without an argument, getenv
returns a read-only instance of java.util.Map
, where the map keys are the environment variable names, and the map values are the environment variable values. This is demonstrated in the
example:EnvMap
import java.util.Map; public class EnvMap { public static void main (String[] args) { Map<String, String> env = System.getenv(); for (String envName : env.keySet()) { System.out.format("%s=%s%n", envName, env.get(envName)); } } }
With a String
argument, getenv
returns the value of the specified variable. If the variable is not defined, getenv
returns null
. The
example uses Env
getenv
this way to query specific environment variables, specified on the command line:
public class Env { public static void main (String[] args) { for (String env: args) { String value = System.getenv(env); if (value != null) { System.out.format("%s=%s%n", env, value); } else { System.out.format("%s is" + " not assigned.%n", env); } } } }
When a Java application uses a
ProcessBuilder
object to create a new process, the default set of environment variables passed to the new process is the same set provided to the application's virtual machine process. The application can change this set using ProcessBuilder.environment
.
There are many subtle differences between the way environment variables are implemented on different systems. For example, Windows ignores case in environment variable names, while UNIX does not. The way environment variables are used also varies. For example, Windows provides the user name in an environment variable called USERNAME
, while UNIX implementations might provide the user name in USER
, LOGNAME
, or both.
To maximize portability, never refer to an environment variable when the same value is available in a system property. For example, if the operating system provides a user name, it will always be available in the system property user.name
.