RFR: 8350880: (zipfs) Add support for read-only zip file systems [v4]

[Jaikiran Pai]

On Thu, 15 May 2025 18:27:35 GMT, David Beaumont <duke at openjdk.org> wrote:

>> Adding read-only support to ZipFileSystem.
>> 
>> The new `accessMode` environment property allows for readOnly and readWrite values, and ensures that the requested mode is consistent with what's returned.
>> 
>> This involved a little refactoring to ensure that "read only" state was set initially and only unset at the end of initialization if appropriate.
>> 
>> By making 2 methods return values (rather than silently set non-final fields as a side effect) it's now clear in what order fields are initialized and which are final (sadly there are still non-final fields, but only a split of this class into two types can fix that, since determining multi-jar support requires reading the file system).
>
> David Beaumont has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Changes based on review feedback.

src/jdk.zipfs/share/classes/jdk/nio/zipfs/ZipFileSystem.java line 241:

> 239:         this.readOnly = forceReadOnly || multiReleaseVersion.isPresent() || !Files.isWritable(zfpath);
> 240:         if (readOnly && accessMode == AccessMode.READ_WRITE) {
> 241:             String reason = Files.isWritable(zfpath)

Nit - this additional call to Files.isWritable(...) can be avoided if we store the value of the previous call (a couple of lines above). I realize that the previous `Files.isWritable` is stashed at the end of the `||` conditionals to prevent it from being invoked in certain situations.

So maybe a better change would be something like:


String reason = multiReleaseVersion.isPresent()
   ? "A multi-release JAR file opened with a specified version is not writable"
   : "The underlying ZIP file is not writable";


which would then avoid any additional calls to `Files.isWritable`.

But I think this point may not be relevant for the reason I note below as a separate comment.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25178#discussion_r2093180117