How to get Environment Variables in Java - Example Tutorial

Environment variables in Java

There are two ways to get the environment variable in Java, by using System properties or by using System.getEnv(). System properties provide only a limited set of predefined environment variables like java.classpath, for retrieving Java Classpath or java.username  to get User Id which is used to run Java program, etc but a more robust and platform-independent way of getting environment variable in Java program on the other hand System.getEnv() method provide access to all environment variables inside Java program but subject to introduce platform dependency if the program relies on a particular environment variable.

The System.getEnv() is an overloaded method in Java API and if invoked without a parameter it returns an unmodifiable String map that contains all environment variables and their values available to this Java process while System.getEnv(String name) returns the value of the environment variable if exists or is null. 

In our earlier posts, we have seen How to get the current directory in Java and  How to run shell commands from the Java program, and in this Java tutorial, we will see how to access environment variables in Java.

How to get environment variables in Java - Example

Here is a quick example of How to get environment variables in Java using System.getEnv() and System.getProperty(). Remember System.getEnv() return String map of all environment variables while System.getEnv(String name) only return the value of a named environment variable like JAVA_HOME will return PATH of your JDK installation directory.

/**
 * Java program to demonstrate How to get the value of environment variables in Java.
 * Don't confuse between System property and Environment variable and there is separate
 * way to get the value of System property than environment variable in Java, as shown in this
 * example.
 *
 * @author Javin Paul
 */

public class EnvironmentVariableDemo {  

    public static void main(String args[]){
   
      //getting username using System.getProperty in Java
       String user = System.getProperty("user.name") ;
       System.out.println("Username using system property: "  + user);
   
     //getting username as an environment variable in java only works in windows
       String userWindows = System.getenv("USERNAME");
       System.out.println("Username using environment variable in windows : "  + userWindows);
   
     
     //name and value of all environment variables in Java  program
      Map<String, String> env = System.getenv();
        for (String envName : env.keySet()) {
            System.out.format("%s=%s%n", envName, env.get(envName));
        }

    }
     
}

Output:
Username using system property: harry
Username using environment variable in windows: harry
USERPROFILE=C:\Documents and Settings\harry
JAVA_HOME=C:\Program Files\Java\jdk1.6.0_20\
TEMP=C:\DOCUME~1\harry\LOCALS~1\Temp

Getting environment variable in Java – Things to remember

Java is a platform-independent language but there are many things that can make a Java program platform-dependent e.g. using a native library. Since environment variables also vary from one platform to another e.g. from windows to Unix you need to be a bit careful while directly accessing environment variables inside the Java program. Here are few points which are worth noting :

1) Use system properties if the value of environment variable is available via system property e.g. Username which is available using "user.name" system property. If you access it using environment variable directly you may need to ask for different variables as it may be different in Windows e.g. USERNAME and Unix as USER.

2) Environment variables are case sensitive in Unix while case insensitive in Windows so relying on that can again make your Java program platform dependent.

3) System.getEnv() was deprecated in release JDK 1.3 in support of using System.getProperty() but reinstated again in JDK 1.5.

That's all on how to get the environment variable in Java. Though you have a convenient method like System.getEnv() which can return the value of the environment variable, its better to use System.getProperty() to get that value in a platform-independent way if that environment variable is available as a system property in Java.

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