Janino For Mac

Janino is a small, easy to use compiler that reads a Java expression, block, or source file, and generates Java bytecode that is loaded and executed directly.

Janino is not intended to be a development tool, but an embedded compiler for run-time compilation purposes, such as expression evaluators or "server pages" engines like JSP.

Say you build an e-commerce system, which computes the shipping cost for the items that the user put into his/her shopping cart.

Because you don't know the merchant's shipping cost model at implementation time, you could implement a set of shipping cost models that come to mind (flat charge, by weight, by number of items, ...) and select one of those at run-time.

In practice, you will most certainly find that the shipping cost models you implemented will rarely match what the merchant wants, so you must add custom models, which are merchant-specific. If the merchant's model changes later, you must change your code, re-compile and re-distribute your software.

Because this is so unflexible, the shipping cost expression should be specified at run-time, not at compile-time. This implies that the expression must be scanned, parsed and evaluated at run-time, which is why you need an expression evaluator.

A simple expression evaluator would parse an expression and create a "syntax tree". The expression "a + b * c", for example, would compile into a "Sum" object who's first operand is parameter "a" and who's second operand is a "Product" object who's operands are parameters "b" and "c".

Such a syntax tree can evaluated relatively quickly. However, the run-time performance is about a factor of 100 worse than that of native Java code.