<init> and factories
daniel.smith at oracle.com
Thu Oct 17 18:22:38 UTC 2019
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 . 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?
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