RFR - 8132734: java.util.jar.* changes to support multi-release jar files

[Xueming Shen]

On 10/15/15 1:53 AM, Paul Sandoz wrote:
>> On 15 Oct 2015, at 05:00, Xueming Shen <xueming.shen at oracle.com> wrote:
>>
>> I'm not sure if it is a good idea, from performance perspective, to add a "versionEntry" field into the JarEntry
>> to support this feature, given most of the jar files might not be multi-release-jar aware, and the Jar input&
>> output streams dont work with a multi-release jar directly. Why should they all pay a runtime price for it. If
>> we really have to add an extra field, the JarFileEntry might be a better place, and it might be desired to
>> define a new subclass JarFileEntryMR to use when the MR is enabled, instead of adding directly into the existing
>> JarFileEntry.
>>
> According to jol there is currently space available due to alignment. If there was not it would add about 4% in direct instance size. But the actual footprint is likely to be chunkier because of the string character storage for the name so the % increase in size would be smaller e.g. perhaps on average < 2% which might be ok given that i presume such entries are unlikely to be cached.
>
> So i am not concerned about the size. If there was a way to design it to avoid modification of existing classes all the better, but i dunno if it is possible. Steve will surely know more.
>
> Paul.
>

Let's try it from another angle:-) Based on the webrev, no one need to 
and actually does create a
JarEntry with a versionedEntry, except JarFile, and JarFile only creates 
its own version of JarEntry,
the JarFileEntry.

The only non-JarFile consumer of "versioned" JarEntry is the package 
private JarVerifier.getCoderSigners,
and obviously it takes a JarFile together with the source JarEntry 
(again, if this jarEntry is not from A
JarFile, it should never have a "versionedEntry")

So why do you want to put this field into the super class JarEntry, not 
the JarFileEntry, don't see any
benefit of doing that.

While I'm writing this email, it appears to me that we might have a more 
"severe" issue with the
general/base JarEntry class to hold the link to the "versionedEntry". 
The "general" JarEntry object is
not associated with a specific JarFile (the JarFileEntry does). So there 
is no way for
JarFile.getInputStream(ZipFile ze) to verify that the JarEntry passed in 
and its "versionedEntry" is
actually belong to "this" JarFile in the following implementation, if 
the "ze" is just a JarEntry but
NOT instanceof of a JarFileEntry

  759     public synchronized InputStream getInputStream(ZipEntry ze)
  760         throws IOException
  761     {
  762         maybeInstantiateVerifier();
763
764 if (ze instanceof JarEntry) {
765 ZipEntry vze = ((JarEntry)ze).versionedEntry;
766 if (vze != null) {
767 return getInputStream(vze);
768 }
769 }
770

I think it's a bug of the implementation if we don't check, as the 
"versioned entry" is really
associated to the jar file that creates it. Sure, as I said above, there 
is actually no way you can
create a general JarEntry or a JarFileEntry with a "versionedEntry" from 
"outside", but it appears
to be possible (have not tried, just in theory) to mess up the current 
mechanism by passing a
"jar entry" from one JarFile instance to another one, if two JarFile 
instances are open on the same
multi-release-jar, but with different "version setting" policy...

But again, I still believe it might be a wrong approach to add such a 
"versionedEntry" into any of the
JarEntry, including the JarFileEntry. As specified by the specification, 
the returned entry should be the
jar entry pointing to the versioned entity in the Jar File, not the root 
one. The question I would like to
ask is why do you even need the "root entry" at all, if there is a 
matched versioned one. It might be
desired to have a mechanism to return the "base/root name" for such an 
entry, but it probably does
not deserve a "dedicate" entry object.

-Sherman

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