Is there any reason lazy initialization couldn't be built into Java?

Question

Since I'm working on a server with absolutely no non-persisted state for users, every User-related object we have is rolled out on every request.

Consequently I often find myself doing lazy initialization of properties of objects that may go unused.

protected EventDispatcher dispatcher = new EventDispatcher();

Becomes...

protected EventDispatcher<EventMessage> dispatcher;

public EventDispatcher<EventMessage> getEventDispatcher() {
    if (dispatcher == null) {
        dispatcher = new EventDispatcher<EventMessage>();
    }
    return dispatcher;
}

Is there any reason this couldn't be built into Java?

protected lazy EventDispatcher dispatcher = new EventDispatcher();

---

As mentioned below in the comments, I realize a language could theoretically evolve to include most anything you want. I'm looking for a practical measurement of possibility. Would this conflict with other features? Is the implementation simple enough to work well with the JVM as it exists? And even, is it a good idea?

Answer 1

Here is an eight page answer to your question: http://tinlizzie.org/~awarth/papers/fool07.pdf

If i can try and coarsely summarize the problems with adding laziness, it's the corner cases. There are a lot of caveats around side-effects. Consider, in your example, if the constructor had visible side-effects like bumping a global counter or doing I/O... It's hard to reason about when that would happen. Or consider even uglier side-effects about exceptions (they're thrown ... when you reference the lazy object?)

Just skip to section 6 in the above paper. (And admire all the type system formal logic on the pages you skip ...)

Answer 2

Of course it is eminently possible. In fact, scala already has exactly this feature! (Scala is a JVM language and compiles down to bytecode). Here is a piece of scala source:

class Foo {
  lazy val bar = "Hello World"
}

And here is what an intermediate form of the compiled code looks like:

scalac -Xprint:icode Foo.scala

[[syntax trees at end of icode]]// Scala source: Foo.scala
package <empty> {
  class Foo extends java.lang.Object with ScalaObject {
    @volatile protected var bitmap$0: Int = 0;
    lazy private[this] var bar: java.lang.String = _;
    <stable> <accessor> lazy def bar(): java.lang.String = {
      if (Foo.this.bitmap$0.&(1).==(0))
        {
          Foo.this.synchronized({
            if (Foo.this.bitmap$0.&(1).==(0))
              {
                Foo.this.bar = "Hello World";
                Foo.this.bitmap$0 = Foo.this.bitmap$0.|(1);
                ()
              };
            scala.runtime.BoxedUnit.UNIT
          });
          ()
        };
      Foo.this.bar
    };
    def this(): Foo = {
      Foo.super.this();
      ()
    }
  }

}

Answer 3

I think you first need to add real properties to the Java langauge, rather than relying on the getX/setX idiom. That way you could just mark the property as lazy (and synchronized, readonly etc...).

Sort of what is asked for here (Objective-C, but concept applies).

Answer 4

Of course this could be added to Java, the keyword lazy could be implemented as syntactic sugar. However, wether it will be implemented depends on the vision of the compiler builders.