Comment by cowlevel

2 days ago

It's wrong. The caller doesn't get the reference until the constructor runs. Although it looks like two operations in bytecode, the only thing you can legally do with an uninitialised object reference is call its constructor.

2 comments

cowlevel

hedora 1 day ago

In java, the caller gets the reference before the constructor runs. It's in the language spec. It's terrible. You can prove it yourself with something like:

   this.dangerous = new Lol();

and then have Lol()'s constructor sleep for 10 sec then set done = true at the end of the contstructor. In another thread:

   while(!this.dangerous) { }
   assert(this.done) // will throw.

I guess you could use double-checked locking as a class author by having every public method on your class do a if (!this.done) { throw; }, and make sure there are no public fields.

That's pretty lame though.

cowlevel 16 hours ago

That's a data race. The caller gets the reference after the constructor runs, but writes to non-final fields from the constructor might not be visible to other threads.