为什么我不能通过Groovy中构造函数内的闭包初始化最终的类变量？

public class Person {
    public /*final*/ String firstName,lastName

    Person(Map parameters) {
        // This does *not* work with "final":
        parameters.each { name,value ->
            this."$name" = value
        }

        // This *does* work with "final":
        this.lastName = parameters['lastName']
    }
}

Person p = new Person(firstName: 'Joe',lastName: 'Doe')
println p.firstName + ' ' + p.lastName

换句话说,为什么我在闭包内或构造函数的顶层初始化最终变量有什么不同？