Support for jrt-fs.jar in JDK 7 would be very helpful in allowing FindBugs to run under Java 9

[Alan Bateman]

On 14/11/2014 16:34, Bill Pugh wrote:
> Following up on a email thread started by Dalibor Topic…
>
> In short, back porting jrt-fs.jar to work under Java 7 would be very helpful to the FindBugs project.
>
> At length:
>
> FindBugs needs access to the classifies for the standard Java libraries in order to analyze code. Right now, we access that through the file system, but that won’t be possible under jigsaw and JEP 220: Modular Run-Time Images.
>
> The JRL file system is what is recommended and needed to support FindBugs access to standard library classifies under Java 9. Having to support two different mechanisms for accessing standard library classifies would be annoying. We could move entirely over to the JRL file system. However, FindBugs is committed to running under Java 7 for a long time, at least several more years, and it would be good if FindBugs support for Java 9 were available within a year, if not sooner.
>
> So, if  jrt-fs.jar ran under Java 7, we could just rewrite that section of FindBugs to use the JRL file system, and have one version of the code running for Java 7+.

If the jrt file system provider could run on JDK 7 then it would just 
mean that a tool running on JDK 7 could access the class files and 
resources in a JDK 9 image. So I think this would mean that FindBugs 
would need to keep its existing code so that it can get to class files 
in JDK 7 and JDK 8 runtime images.

As to whether it could run on JDK 7 then it could be made to work but 
would require dropping the use of newer Java Language and APIs in the 
implementation. It would also require a few contortions in the build as 
JDK 9 uses a JDK 8 for bootstrapping (we always use a JDK N-1 as the 
boot JDK). Also just to say that a one-off back-port for JDK 7 wouldn't 
work as the JDK 9 image format is intended to change and evolve, hence a 
build of jrt-fs.jar in JDK 9.0.0 may not work with a JDK 9.0.1. This 
means that for now at least, tools have to use the jrt-fs.jar in the JDK 
9 image that they want to access.

The only way that I think would be possible to avoid two mechanisms is 
for future JDK 7 and JDK 8 updates to include a jrt-fs.jar that works in 
the same way but makes it transparent that the classes and resources are 
coming from rt.jar, resources.jar, jsse.jar etc. Even then, I would 
assume that FindBugs and other important tools wanting to keep the 
existing mechanism so they can work on existing JDK 7 and JDK 8 versions.

-Alan.