Can EventActionDispatcher publish "this" before the constructor finishes?

Question

The recommended way to use the EventActionDispatcher is as follows (per the API docs @ http://struts.apache.org/1.2.9/api/org/apache/struts/actions/EventActionDispatcher.html )

   public class MyCustomAction extends Action {

       protected ActionDispatcher dispatcher = new EventActionDispatcher(this);

       public ActionForward execute(ActionMapping mapping,
                                    ActionForm form,
                                    HttpServletRequest request,
                                    HttpServletResponse response)
                           throws Exception {
           return dispatcher.execute(mapping, form, request, response);
       }
   }

Does doing this publish the reference to "this" before the constructor exits? What are the rules governing field assignments outside of methods.

Thanks in advance.

Sincerely, LES

Answer 1

In this case (I hope) this does not get published. It's still only reachable through the MyCustomAction instance.

In Java (I believe C# does the opposite), instance field initialisation and instance initialisers are called directly before the (implicit or explicit) call to the super constructor. Therefore you can use this in field initialisation, although your object may not have finished construction.

Publishing this so it it reachable from outside the object during construction is generally a bad idea.

Answer 2

That took 3 people a week to track down once... "this" can be null if the EventActionDispatcher starts a thread or does anything with a thread that causes the "this" to be used.

NEVER pass "this" before the constructor has completed or you run the risk of "this" being null in the case of threading.

What I do is add an "init()" method to my classes that need to do things like that and call it after I create the object.

There are also other subtleties, such as this example:

public abstract class Foo
{
    protected Foo()
    {
        car();
    }

    public abstract void car();
}

public class Bar 
    extends Foo
{
    private final String value;

    public Bar(final String str)
    {
        value = str;
    }

    public void car()
    {
        // this line will crash because value is null
        System.out.println(value.charAt(0));
    }
}

public class Main
{
    public static void main(final String[] argv)
    {
        final Foo foo;

        foo = new Bar("Hello");
    }
}

The safest thing to do is:

• never use "this" before the constructor has returned
• never call any of the classes own non-static methods from a constructor unless the class is final.

You can can call final methods, but you have to be sure that they do not call overrideable methods... and that can mean things break down the road... so safer not to do it.