<init> and factories

[Dan Smith]

The plan of record for compiling the constructors of inline classes is to generate static methods named "<init>" with an appropriate return type, and invoke them with 'invokestatic'.

This requires relaxing the existing restrictions on method names and references. Historically, the special names "<init>" and "<clinit>" have been reserved for special-purpose JVM rules (for example, 'invokespecial' is treated like a distinct instruction if it invokes a method named '<init>'); for convenience, we've also prohibited all other method names that include the characters '<' or '>' (JVMS 4.2.2).

Equivalently, we might say that, within the space of method names, we've carved out a reserved space for special purposes: any names that include '<' or '>'.

A few months ago, I put together a tentative specification that effectively cedes a chunk of the reserved space for general usage [1]. The names "<init>" and "<clinit>" are no longer reserved, *unless* they're paired with descriptors of a certain form ("(.*)V" and "()V", respectively). Pulling on the thread, we could even wonder whether the JVM should have a reserved space at all—why can't I name my method "bob>" or "<janet>", for example?

In retrospect, I'm not sure this direction is such a good idea. There is value in having well-known names that instantly indicate important properties, without having more complex tests. (Complex tests are likely to be a source of bugs and security exploits.) Since the JVM ecosystem is already accustomed to the existence of a reserved space for special method names, we can keep that space for free, while it's potentially costly to give it up.

So here's a alternative design:

- "<init>" continues to indicate instance initialization methods; "<clinit>" continues to indicate class initialization methods

- A new reserved name, "<new>", say, can be used to declare factories

- To avoid misleading declarations, methods named "<new>" must be static and have a return type that matches their declaring class; only 'invokestatic' instructions can reference them

- The rest of the "<.*>" space of names (plus ".*<.*" and ".*>.*") is held in reserve, available for special purposes as we discover them

The Java compiler would only use "<new>" methods for inline class construction, for now; perhaps in the future we'll find other use cases that make sense (like surfacing some sort of factory mechanism).

Does this seem promising? Any particular reason it's better to overload "<init>" than just come up with a new special name?

[1] http://cr.openjdk.java.net/~dlsmith/lw2/lw2-20190628/specs/init-methods-jvms.html