Macros

Macro support in MVEL is a basic facility to allow the replacement of a token with an expanded source output. The functionality was introduced as a way of creating specialized keywords that enclose an Interceptor.

A macro can consist of of any legal identifier for example: modify

Consider the following code:

This is not a legal MVEL expression. There is, after all, no modify keyword in MVEL. However, in JBoss Drools this functionality is used to implement a form of change listener by replacing the modify identifier with the string: @Modify with.

The org.mvel.Macro Interface

The interface is very simple. Upon execution, the doMacro() method returns a String to replace the representative token. For example:

Using the MacroProcessor

The MacroProcessor is a lightweight and fast text parser that uses the provided macros to replace all the matching identifiers.

The defined macros are passed via a Map with the key representing the identifier to match, and the value being an instance of the Macro. For example:

The returned String can be simply passed to the MVEL compiler.

Using Macros with the Interpreter If performance is an important consideration while using MVEL in interpreted mode, you should avoid using macros or instead consider switching to using pre-compiled expressions which only have a one-time overhead to parse and compile.