javascopewarningscompiler-warningsthis-pointer

What is a "this-escape" warning, and how do I deal with it?

I struggle to find resources on this, and yet, so many of my classes are running into this error when I compile my code on the latest Java (21).

Here is a code example.

public class ThisEscapeExample
{
        public Object o;

        public ThisEscapeExample()
        {
                this.overridableMethod();
        }

        public void overridableMethod()
        {
                this.o = new Object();
        }
}

And here is my compilation command.

javac -Xlint:all ThisEscapeExample.java
ThisEscapeExample.java:9: warning: [this-escape] possible 'this' escape before subclass is fully initialized
                this.overridableMethod();
                                      ^
1 warning
Solution

  • Here is the JDK Bug System entry that introduces this new warning - https://bugs.openjdk.org/browse/JDK-8299995

    Long story short, the this-escape warning is to warn you when your constructor might expose the this that it is creating, before the constructor has finished.

    This is dangerous because your instance may be in a (temporarily) invalid state. Obviously, once the constructor finishes, your this should be in a valid state. But since you are exposing it pre-maturely, it could be caught "with its pants down", so to speak.

    Exposing your instance in an invalid state can result in unpredictable behaviour. And once that happens, integrity has been violated, and you can treat this the same as C's Undefined Behaviour. Anything goes, at that point.

    There are a few ways to remedy this.

    1. Only use methods (FROM THE SAME CLASS) in the constructor that cannot be overridden.

      • static methods.

      • final methods.

      • private methods.

    2. Only use methods (FROM THE SAME CLASS) in the constructor when your (SAME) class itself is final.

    3. Don't pass in/use this to begin with - instead, pass in the particular component of this that you needed.

      • Basically, quit being lazy and be explicit with what you need. Don't just pass in your God object -- pass in only the specific attributes you need.

    (And, again, when I say SAME CLASS, I literally mean it has to be a method declared within the same .java file. Methods inherited from a parent type WILL TRIGGER A THIS-ESCAPE. The only way to appease this is by only allowing your constructor to use methods created within the literal same .java file that your constructor is in, plus the rules above.)

    Here is a code example to help demonstrate the above rules.

    import javax.swing.*;

public class GUI
{

   private final JFrame frame;

   public GUI()
   {
   
      this.frame = new JFrame();
   
      this.frame.add(this.createBottomPanel());
   
   }

   //final! Does that mean we are safe?
   final JPanel createBottomPanel()
   {
   
      final JButton save = new JButton();
   
      save
         .addActionListener
         (
            /*
             * No. We get the warning here at the start of this lambda.
             * The reason is because we have given this lambda the
             * ability to call this.toString(), and we don't know when
             * it will do that. Maybe now, maybe later. But if it does
             * it now, then we could end up doing things before the
             * object is fully created. And if that were to happen, then
             * that would be a this-escape. So, the warning occurs here,
             * to let you know that it is possible.
             */
            actionEvent ->
            {
           
               this.toString();
           
            }
           
         )
         ;
   
      return null;
   
   }

}

    This question also has some useful examples, in case mine above does not make much sense -- Why does 'this-escape' warning trigger when calling final methods from the superclass

    Now, if none of the solutions above are an option for you, consider the tactic of lazy loading your data. Lazy loading is when you load your data only as needed -- meaning, NOT in your constructor. For example, if your class needs a database connection, don't make the connection happen in the constructor, do it in the getter call. Like this.

    public class SomeClass
{

    private DbConnection connection = null;

    //More fields here.

    public SomeClass()
    {

        //Don't set the db connection here.

    }

    public DbConnection getConnection()
    {

        if (this.connection == null)
        {

            this.connection = createAConnection(this);

        }

        return this.connection;

    }

}

    And finally, if none of this works, or there is just some entirely unescapable situation, there are 2 very hacky, undesirable ways to just silence the error. You really should NOT depend on this, but if you are CERTAIN that it can't hurt you, here are 2 ways to silence it.