Licensed under GNU GPLv3.
Using this fork.
(from the official documentation)
JEL is the library for evaluating a simple single line expressions in Java. There is certain class of tasks where user should be given a possibility to enter an expression of his own into the program. Typical examples include : plotting user-defined functions, fitting the data by the arbitrary user-defined functions, calculation of integrals and solving differential equations involving the user-defined functions... and many many more.... The key term above is the "evaluation of the user-defined functions", which is exactly what JEL is supposed to do for You.
The key feature of JEL is the fact that it is a compiler ! To write the interpreter for expressions is the task, close to trivial but incurs a HUGE perfomance penalty due to the fact that Java is interpreted language itself. JEL, in contrary, compiles expressions directly to Java bytecodes, allowing their fast evaluation. Moreover, if user's Java virtual mashine has JIT compiler, expressions are transparently compiled into the native machine code, resulting in performance HIGHER that for most C written interpreters (plus JEL do not require recompilation when transferred from platform to platform).