Running user entered code

[doubleoseven]

I have an application that allows the user to place boxes of text on a page with special placeholders and then output a pdf where the placeholders are replaced but values from a database. Example: "My name is %NAME%" when composed results in "My name is Bob". Now I need a way to allow the user to enter some form of simple code to create a conditional statement which will be parsed when the document is composed. Example: if (%NAME% == "Joe") { return "Bob" } I have no idea how to even begin to approach this. Does anyone know of a way to turn a user entered string of text into actual Java code that the application will run?

[graviton]

there are several possibilities.
one elegant version is using a compiler of Java code at run-time, like http://www.janino.net, which also supports expression evaluation.
another elegant approach would be to include a scripting language, like the ones listed at http://pnuts.org/~tomatsu/embedding.html.
the third elegant approach is writing your own little scripting language with jflex, jcup or antlr. but this will assume knowledge on building languages and compilers.

[major]

graviton is right, we will need to look into making a compiler, and you need to be able to detect syntax errors. I cant say that this is easy.