Private fields and methods are not private in groovy

Thursday, July 12, 2012

I used to code in Java before I met groovy. Like most of you, groovy attracted me with many enhancements. This was to my surprise to discover that method visibility in groovy is handled different than Java!

Consider this example:

class Person {
    private String name
    public  String surname

    private Person() {}

    private String signature() { "${name?.substring(0, 1)}. $surname" }

    public String toString() { "I am $name $surname" }

How is this class interpreted with Java?

  1. Person has private constructor that cannot be accessed
  2. Field "name" is private and cannot be accessed
  3. Method signature() is private and cannot be accessed

Let's see how groovy interpretes Person:

public static void main(String[] args) {
    def person = new Person()   // constructor is private - compilation error in Java

    person.@name = 'Mike'       // access name field directly - compilation error in Java
    println(person.toString()) = 'John'        // there is a setter generated by groovy

    person.@surname = 'Foo'     // access surname field directly

    person.surname = 'Bar'      // access auto-generated setter

    println(person.signature()) // call private method - compilation error in Java

I was really astonished by its output:

I am null null
I am Mike null
I am John null
I am John Foo
I am John Bar
J. Bar

As you can see, groovy does not follow visibility directives at all! It treats them as non-existing. Code compiles and executes fine. It's contrary to Java. In Java this code has several errors, pointed out in comments.

I've searched a bit on this topic and it seems that this behaviour is known since version 1.1 and there is a bug report on that: It is not resolved even with groovy 2 release. As Tim Yates mentioned in this Stackoverflow question: "It's not clear if it is a bug or by design". Groovy treats visibility keywords as a hint for a programmer.

I need to keep that lesson in mind next time I want to make some field or method private!


  1. Jochen added a comment near the end of that JIRA with a link to the new tasks that will be fixed in 3.0: