Passing ziptime-info=false to ZipFileSystemProvider (along with other classpath performance ideas)

[Jason Zaugg]

On Sun, 16 Mar 2025 at 19:40, Jaikiran Pai <jai.forums2013 at gmail.com> wrote:

> Hello Jason,
>
> On 02/03/25 7:13 pm, Jason Zaugg wrote:
> > I've been analysing the performance of javac in a relatively large
> > codebase and found classpath scanning to be the dominant factor. There
> > are large numbers of compilation tasks, most with long classpaths
> > including large JARs.
> >
> > I've been using the wall-time profiling mode of async-profiler to
> > analyse where time is being spent.
> >
> > I've been able improve build times (most dramatically on Windows, 750s
> > reduced to 350s) by modifying javac to:
> >
> > 1. pass ziptime-info=false to the env Map of
> > jarFsProvider.newFileSystem, which means the classpath scan in the
> > constructor of ArchiveContainer need only read the ZIP central
> > directory, rather then reading per-entry Extended Time metadata.
>
> I am not familiar with the compiler implementation code, but speaking in
> context of the ZipFileSystem, it appears that this (undocumented)
> "ziptime-info" property of the ZIP file system was indeed introduced to
> help improve the performance of the ZipFileSystem. Setting it to false
> implies that the lastAccessTime() and the creationTime() returned by the
> ZipFileAttributes may not be accurate for some entries, since settings
> this property to false skips reading the per entry creation and access
> time information for the entries. Would you happen to know if either of
> these two methods are used in the code path where you are proposing to
> add "ziptime-info=false"?
>
> -Jaikiran
>

I am not aware of any use of these attributes within javac, nor does it
seem likely that they would be used.

But the difference in the attributes could be observed by a client of
JavaFileManager that
unwraps the underlying Path from a JavaFileObject representing a JAR entry.

```
jshell> var compiler = javax.tools.ToolProvider.getSystemJavaCompiler();
compiler ==> com.sun.tools.javac.api.JavacTool at 4b9e255

jshell> var fileManager = compiler.getStandardFileManager(null, null, null)
fileManager ==> com.sun.tools.javac.file.JavacFileManager at 7f416310

jshell> var head =
fileManager.list(javax.tools.StandardLocation.CLASS_PATH, "",
Set.of(javax.tools.JavaFileObject.Kind.OTHER), true).iterator().next()
head ==> JarFileObject[/Users/jz/Library/Caches/Coursier/v ...
e/protobuf/wrappers.proto]

jshell> java.nio.file.Files.readAttributes(fileManager.asPath(head),
java.nio.file.attribute.BasicFileAttributes.class)
$11 ==>     name            : /google/protobuf/wrappers.proto
    creationTime    : Fri Feb 14 21:06:54 AEST 2020
    lastAccessTime  : Fri Feb 14 21:06:54 AEST 2020
    lastModifiedTime: Fri Feb 14 21:06:54 AEST 2020
...
```

A new option could be added to toggle between the existing and proposed
behaviour if this is a concern.

Regards,

Jason Zaugg
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