The default JRE is used for compiling and launching Java programs in all projects unless you specifically override the default JRE. The default JRE is the installed JRE to which JRE_LIB, JRE_SRC and JRE_SRCROOT are bound. A project is not compiled against the default JRE if the JRE_LIB variable has been removed from its build path. A program is not launched with the default JRE if a custom runtime JRE has been set for its project.
Here is how you can change the default JRE:
Adding a new JRE definition
Choosing a JRE for launching a project
Working with build paths
Working with JREs