System Properties
The Java platform uses a Properties object to provide information about the local system and configuration, and we call it System Properties.
System Properties include information such as the current user, the current version of the Java runtime, and the file path-name separator.
In the below code, we use System.getProperty("log_dir") to read the value of the property log_dir. We also make use
of the default value parameter, so if the property doesn't exist, getProperty returns /tmp/log:
String log_dir = System.getProperty("log_dir","/tmp/log");
To update System Properties at runtime, we use the System.setProperty method:
System.setProperty("log_dir", "/tmp/log");
We can pass our own properties or configurations values to the application using the propertyName command line argument:
java -jar jarName -DpropertyName=value
For example
java -jar app -Dfoo="bar"
Environment Variables
Environment Variables are key/value pairs like Properties. Many Operating Systems use Environment Variables to allow configuration information to be passed into applications.
The way to set an environment variable differs from one operating system to another. For example, on Linux, we use
export MY_ENV_VAR=foo
When creating a process, it inherits a clone environment of its parent process by default.
To obtain a single environment variable programmatically, we can call getenv with the variable name:
String log_dir = System.getenv("log_dir");
getenv() returns a read-only Map. Trying to add values to the map throws an UnsupportedOperationException.
The Differences
Although both are essentially maps that provide String values for String keys, let's look at a few differences:
- We can update Properties at runtime, while Environment Variables are an immutable copy of the Operating System's variables.
- Properties are contained only within the Java platform, while Environment Variables are global at the Operating System level, available to all applications running on the same machine.
- Properties must exist when packaging the application, but we can create Environment Variables on the Operating System at almost any point.