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

[Remi Forax]

On 11/15/2014 11:18 AM, Alan Bateman wrote:
> 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.

First, I think we should provide a good answer to that question,
anything that can smoother the integration of jigsaw will increase its 
adoption rate.

I think it's not a good idea to try to embed jrt-fs.jar in older 
releases. Instead, we can provide
a backport of jrt-fs.jar lets name it jrt-fs-backport.jar, publish it on 
maven central
(with the same licence as the OpenJDK) and lets tools that need it to 
use it.

I have taken a look to the source code of jdk.internal.jimagefs and 
jdk.internal.jimage and
i think such backport is possible with an automatic tool if we stick 
with the following rules
for the development of the two packages:
  - you are allowed to use lambdas and functional interfaces declared 
inside java.util.function
    so API like j.i.jimage.Archive that use things like Consumer in 
public methods is fine.
  - you are not allowed to use any default methods introduced in JDK8 
(Stream API, by example)
    nor to use ReflectiveOperationException because this requires to 
patch existing JDK classes.

Given the current source code, I don't think it's a big deal to refactor 
it a little to avoid
the use of the Stream API (BTW, there is a method forEach on Iterable, 
you don't have
to create a Stream for it, and instead of Array.asList(foo).stream(), 
there is a method
Arrays.stream() for that) and it can write the tool that does the 
retro-weaving.

cheers,
Rémi