What means "public static {}" in bytecode generated by clojure compiler?

Question

When I generate a class in clojure (through gen-class), I get the following definition upon inspection via javap:

public class foo.bar extends java.lang.Object implements java.io.Serializable{
    public final java.lang.Object state;
    public static {};
    public foo.bar();
    ...
}

I wonder what the construct public static {} means as I never saw something like this before …

Can someone please enlighten me?

Answer 1

The static section contains code which runs during static class initialization (before any instances of the class are created).

Think of having namespace-level code with side effects in Clojure -- those side effects take place as soon as anyone requires or uses the namespace, even if they don't actually call any functions. This is a comparable situation.

Answer 2

If you take a look at full output of javap (with javap -c ...) you'll see that it's just a bunch of code that clojure compiler put to be executed ahead of first class access. Usually it is interning of vars which are used later and so on.

Answer 3

sounds like a static initialization block on the class.

You can use that block to initialize all static variables of the class. However, I had never seen the "public" qualifier before it.

Try this in Java so you see the order of invocation

public class StaticTest {

  { 
    System.out.println("Anonymous Block.");
  }

  static {
    //probably equivalent to that public {} you see on your code.
    System.out.println("Static Block.");
  }

  public StaticTest() {
     System.out.println("Constructor.");
  }

  public static void main(String[] args) {

    StaticTest test = new StaticTest() {
            {
                System.out.println("Anonymous block in instance.");
            }
    };
  }
}

when you execute this it prints the following:

> $ java StaticTest 
> Static Block. 
> Anonymous Block. 
> Constructor.
> Anonymous block in instance.