Groovy 闭包 $v

Question

@CompileDynamic
ClassNode buildTemplateClass(ClassNode reference) {
    def methodCount = constX(reference.methods.size())
    def fieldCount = constX(reference.fields.size())

    return  new MacroClass() {

        class Statistics {

            java.lang.Integer getMethodCount() {
                return $v { methodCount }
            }

            java.lang.Integer getFieldCount() {
                return $v { fieldCount }
            }
        }
    }
}

I am not able to get my head around this construct $v { fieldCount } . What does it mean and why is this needed here ? For a full reference of what the code is doing refer here .

In case it is not clear what the constX method does refer here .

Answer 1

As it's written in your doc , $v used inside a macro for substituting an expression

In order to substitute any expression inside the macro we need to use the $v method. $v receives a closure as an argument, and the closure is only allowed to substitute expressions, meaning classes inheriting org.codehaus.groovy.ast.expr.Expression.