Reading system properties and environment variables in Java

This page will be of particular interest to you if you have one of the following questions:

• how do I find out the system temporary directory in Java?
• how do I find out the local machine's default file format (character encoding, line separator) in Java?
• how do I find out processor information (e.g. CPU type, endianness) in Java?
• how do I find out the Java VM version from a running Java program?

Information such as this can be found in two places:

• environment variables, defined and controlled by the operating system;
• system properties, essentially local to the Java system, which are defined by the JVM and can potentially be specified on the command line when your Java application is run.

In general, system properties are more useful for multi-platform applications because they are more consistent across platforms. Indeed, some system properties are actually set by the VM on the basis of OS environment variables in order to provide a more consistent way of reading the relevant information. However, we will look at both types of properties here: the "raw" environment variables, and Java's system properties.

OS environment variables: System.getenv()

Remember that environment variables are in some sense more "raw" variables defined and controlled by the operating system. Here are some examples of environment variables typically set on Windows systems:

You can retrieve a particular environment variable as follows:

String value = System.getenv("USERNAME");

On the next page, we will look at how to list the environment variables that are set on a given system.

Java system properties: System.getProperty()

As mentioned above, system properties are conceptually similar to environment variables. But they are essentially local to, and controlled by, the Java system, not by the operating system. On the surface, individual system properties are retrieved in a very similar way to environment variables, but using the System.getProperty() method:

String value = System.getProperty("user.dir");

Potentially useful properties that are typically defined include the following. In general, you can reasonably expect these properties to be defined, irrespective of the operating system (unlike environment variables, which are OS dependent):

It should be noted that many of these properties can be implicitly accessed via other API methods. For example, when you construct a File object with no directory specified, it implicitly refers to a file inside user.dir. Simiarly, java.io.tmpdir is implicitly— and more conveniently— read by the API call File.createTemporaryDir().

As with environment variables, on the next page see how to list the Java system properties that are set in a given JVM instance.