Java - Leaking this in constructor

Question

I'd like to avoid (most of the) warnings of Netbeans 6.9.1, and I have a problem with the 'Leaking this in constructor' warning.

I understand the problem, calling a method in the constructor and passing "this" is dangerous, since "this" may not have been fully initialized.

It was easy to fix the warning in my singleton classes, because the constructor is private and only called from the same class.

Old code (simplified):

private Singleton() {
  ...
  addWindowFocusListener(this);
}

public static Singleton getInstance() {

  ...
  instance = new Singleton();
  ...
}

New code (simplified):

private Singleton() {
  ...
}

public static Singleton getInstance() {

  ...
  instance = new Singleton();
  addWindowFocusListener( instance );
  ...
}

This fix is not working if the constructor is public and can be called from other classes. How is it possible to fix the following code:

public class MyClass {

  ...
  List<MyClass> instances = new ArrayList<MyClass>();
  ...

  public MyClass() {
    ...
    instances.add(this);
  }

}

Of course I want a fix which does not require to modify all my codes using this class ( by calling an init method for instance).

Answer 1

Since you make sure to put your instances.add(this) at the end of the constructor you should IMHO be save to tell the compiler to simply suppress the warning. A warning, by its nature, doesn't necessarily mean that there's something wrong, it just requires your attention.

If you know what you're doing you can use a

@SuppressWarnings(...)

annotation. However I googled for the string to use to denote this kind of warnings and unfortunately didn't find any, e.g. in http://www.thebuzzmedia.com/supported-values-for-suppresswarnings/.

You could still do a

@SuppressWarning("all")

But that's quite unsafe then of course, because it'll also suppress warnings that would actually need your attention/correction and don't relate to the use of "this" at all.

Answer 2

The best options you have :

• Extract your WindowFocusListener part in another class (could also be inner or anonymous) . The best solution, this way each class has a specific purpose.
• Ignore the warning message.

Using a singleton as a workaround for a leaky constructor is not really efficient.

Answer 3

This is a good case of where a Factory that created instances of your class would helpful. If a Factory was responsible for creating instances your class, then you would have a centralized location where the constructor is called, and it would be trivial to add a required init() method to your code.

Regarding your immediate solution, I would suggest that you move any calls that leak this to the last line of your constructor, and then suppress them with an annotation once you've "proved" that it is safe to do so.

In IntelliJ IDEA, you can suppress this warning with the following comment right above the line:
//noinspection ThisEscapedInObjectConstruction