Re: Annotation scanning with java 9 multi version jars

Mark Thomas

On 13/09/2017 01:02, Greg Wilkins wrote:


We are a bit concerned with providing reasonable support for annotation
scanning in java 9 multi version jars.

We are already seeing these jars in our java 8 deployments (from some
logging frameworks), so we are having to update our scanning to ignore
the versioned classes (as they can't be java 8).

However, we want to update our scanning to support running on java 9,
but just cannot work out a good algorithm for doing so, specially when
confronted with inner classes.

A multi versioned jar might contain something like:

* org/example/Foo.class
* org/example/Foo$Bar.class
* META-INF/versions/9/org/example/Foo.class

So it is clear that there is a java 9 version of Foo. But what is
unclear is the inner class Foo$Bar? Is that only used by the base Foo
version? or does the java 9 version also use the Foo$Bar inner class,
but it didn't use any java 9 features, so the base version is able to be

So it looks like we are going to need to analyse the actual Foo class
version used to see if Foo$Bar is referenced and only then scan Foo$Bar
for annotations (and recursive analysis for Foo$Bar$Bob class )! It
means that given the index only of a jar, it is impossible to come up
with the list of classes that will be provided by that jar for a given
JVM! The only alternative would be if there was an enforced
convention that any versioned class would also version all of it's inner
classes - which may be a reasonable assumption given that they would be
compiled together, but we see nothing in the specifications that force a
jar to be assembled that way.

Is this something the other containers are assuming (ie if there is no
META-INF/versions/9/org/example/Foo$Bar.class, then the inner class is
not used by the versioned outer class and thus do not scan

Or are we going to have to engage in code analysis just to determine
which classes we need to analyse for annotations?
We (Tomcat) haven't had any reports of problems in this area yet. It is
on the list of things to worry about (and thanks for setting out the
problems so clearly) but we haven't looked at a solution yet.

The convention that any versioned class would also version all of it's
inner classes sounds like a reasonable solution to me.

Have you asked about this on jigsaw-dev@... ?



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