Java: Get Environment Variable

In Java, accessing environment variables is a common task for configuring applications based on the runtime environment. This guide will cover how to retrieve environment variables using the System class.

Table of Contents

1. Introduction
2. Using System.getenv Method
3. Listing All Environment Variables
4. Handling Non-Existent Environment Variables
5. Conclusion

Introduction

Environment variables are key-value pairs set in the operating system environment. They are commonly used to configure applications without changing the code. Java provides a straightforward way to access these environment variables through the System class.

Using System.getenv Method

The System.getenv method allows you to retrieve the value of a specific environment variable.

Example

public class EnvironmentVariableExample {
    public static void main(String[] args) {
        // Replace "MY_ENV_VAR" with the name of your environment variable
        String envVar = System.getenv("MY_ENV_VAR");

        if (envVar != null) {
            System.out.println("Value of MY_ENV_VAR: " + envVar);
        } else {
            System.out.println("Environment variable MY_ENV_VAR is not set.");
        }
    }
}

Explanation

• System.getenv("MY_ENV_VAR"): Retrieves the value of the environment variable named "MY_ENV_VAR".
• Checks if the environment variable is null (i.e., not set) before printing its value.

Output:

Value of MY_ENV_VAR: <value>

Listing All Environment Variables

You can also list all environment variables available in the current runtime environment.

Example

import java.util.Map;

public class EnvironmentVariablesExample {
    public static void main(String[] args) {
        Map<String, String> env = System.getenv();

        for (String envName : env.keySet()) {
            System.out.println(envName + "=" + env.get(envName));
        }
    }
}

Explanation

• System.getenv(): Returns a map of all environment variables.
• Iterates over the map and prints each environment variable and its value.

Output:

<ENV_VAR_1>=<value>
<ENV_VAR_2>=<value>
...

Handling Non-Existent Environment Variables

When accessing environment variables, it is important to handle cases where the variable might not be set.

Example

public class EnvironmentVariableExample {
    public static void main(String[] args) {
        String envVar = System.getenv("MY_ENV_VAR");

        if (envVar != null) {
            System.out.println("Value of MY_ENV_VAR: " + envVar);
        } else {
            System.out.println("Environment variable MY_ENV_VAR is not set. Using default value.");
            envVar = "default_value";
        }

        System.out.println("Using value: " + envVar);
    }
}

Explanation

• Checks if the environment variable is null.
• If it is null, uses a default value instead.

Output:

Environment variable MY_ENV_VAR is not set. Using default value.
Using value: default_value

Conclusion

Accessing environment variables in Java can be accomplished using the System.getenv method. This method allows you to retrieve the value of a specific environment variable, list all environment variables, and handle cases where variables might not be set. By understanding these methods, you can effectively manage environment-based configurations in your Java applications.