<init> and factories

Brian Goetz brian.goetz at oracle.com
Thu Oct 17 19:19:52 UTC 2019

I think the choice to keep the reserved space is a good one.

For the <init> vs <new> distinction, it makes me want to ask: what's a 
factory?  Obviously, inline classes have constrained-form constructors 
that are translated to factory methods, but is this the end of the 
factory story, or the beginning?

As has been discussed, javac could well generate _both_ a <init> method 
and a factory, where the latter is derived from the former via 
new/dup/init.  Whether this is desirable depends on what we get for this.

But, the question that this plan leaves me wondering is whether there 
should be a notion of a factory in the language (such a concept would 
warrant a novel translation strategy, if for no other reason than not 
being lossy.)  Currently, we use the word "factory" quite loosely 
(basically, any this-class-returning static method), and there's no type 
checking that, for example, prevents a factory from returning null.

So I think much of the value of having a factory concept in the VM is 
coupled to whether we have a factory concept in the language.

(If we asked the personification of records, he/she would definitely 
want factory methods, because then we could be justified in making 
constructors private and instead exposing a factory, as this would 
actually have linguistic meaning.)

On 10/17/2019 2:22 PM, Dan Smith wrote:
> 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

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