Classpath variables

The build path for a Java project can include source code files, other Java projects, and JAR files. JAR files can be specified using file system paths, or by using variables that refer to locations on the network. 

Classpath variables allow you to avoid references to the location of a JAR file on your local file system. By using a classpath variable, you can specify a JAR file or library using only a variable name, such as JRE_LIB, rather than specifying the location of the JRE on your workstation. In this way, you can share build paths across teams and define the variables to refer to the correct location for your particular computer.

Java development tools (JDT)
Build classpath

Adding a variable classpath entry
Attaching source to a classpath variable
Defining a classpath variable
Deleting a classpath variable

Classpath Variables preferences
Java Build Path properties