Java Cookbook, 4th Edition

Chapter 2. Interacting with the Environment

2.0 Introduction

This chapter describes how your Java program can deal with its immediate surroundings with what we call the runtime environment. In one sense, everything you do in a Java program using almost any Java API involves the environment. Here we focus more narrowly on things that directly surround your program. Along the way we’ll be introduced to the System class, which knows a lot about your particular system.

Two other runtime classes deserve brief mention. The first, java.lang.Runtime, lies behind many of the methods in the System class. System.exit(), for example, just calls Runtime.exit(). Runtime is technically part of the environment, but the only time we use it directly is to run other programs, which is covered in Recipe 18.1.

2.1 Getting Environment Variables

Problem

You want to get the value of environment variables from within your Java program.

Solution

Use System.getenv().

Discussion

The seventh edition of Unix, released in 1979, had a new feature known as environment variables. Environment variables are in all modern Unix systems (including macOS) and in most later command-line systems, such as the DOS or Command Prompt in Windows, but they are not in some older platforms or other Java runtimes. Environment variables are commonly used for customizing an individual computer user’s runtime environment, hence the name. To take one familiar example, on Unix or DOS the environment variable PATH determines ...