Small question about Dependence.Modifier.SYNTHETIC

[Alex Buckley]

On 2/6/2012 9:40 AM, Jonathan Gibbons wrote:
> There are two flags, ACC_SYNTHETIC and ACC_SYNTHESIZED. According to the
> latest spec, you should be getting ACC_SYNTHESIZED. But, that change may
> not have made it to the main repos yet.

Good time to explain these modifiers.

Historically, too many class file artifacts have been marked synthetic, 
and thus hidden from reflection, when in fact they represent "user-land" 
constructs which the Java SE specification (notably the Java Language 
Spec) defines as present in classes.

The canonical example is Enum.values(), which is mandated by the JLS to 
exist in enum types as a completely ordinary method. It can only be 
generated by a compiler but it should not be marked synthetic because it 
is not javac-specific and must be exposed via reflection.

So, the Java team has been aware for some time of the need to 
distinguish "synthesized" content (compiler-generated but user-visible 
and in some sense "standardized") from "synthetic" content 
(compiler-generated but invisible and implementation-specific). In 
neither case does the content map to constructs that were physically 
present in source. Synthesized content maps to constructs that were 
logically present in source.

The Jigsaw class file structures are the first to introduce a 
first-class ACC_SYNTHESIZED flag to complement ACC_SYNTHETIC, but I 
expect other structures will adopt it over time. The JVM Spec will say 
something like:

- If an entity has the synthetic flag, then the entity represents a 
source-level artifact that is specific to the implementation of the host 
system which generated the class file. The entity may not exist in all 
class files generated from the same source code, and should not appear 
in language-level models of reflection.

- If an entity has the synthesized flag, then the entity represents a 
source-level artifact that is defined by the Java SE platform despite 
not appearing in source code. The entity is not specific to the 
implementation of the host system which generated the class file. The 
entity must exist in all class files generated from the same code, and 
should appear in language-level models of reflection.

To clarify, 
http://openjdk.java.net/projects/jigsaw/doc/lang-vm.html#jigsaw-1.2 
ought to mandate a _synthesized_ dependency:

"If a module declaration does not express a dependency on the java.base 
module, and the module declaration is not of java.base itself, then the 
module declaration automatically has a _synthesized_ dependency on a 
version of the java.base module chosen by the host system."

Alex