Avoid `JarURLConnection` resource leak in `AbstractFileResolvingResource.exists()`

[reckart]

The AbstractFileResolvingResource in Spring 5.3.39 has a resource leak (seems to still be present at least in 6.2.2).

The leak occurs when the exists() method is called on a UrlResource obtained from a JAR/ZIP file which is not on the classpath.

exists() will open a connection and may then call a variation of getContentLengthLong() on it to obtain the size of the resource to check if it exists by comparing the size to 0.

Leak: Calling getContentLengthLong() on a JarURLConnection opens the underlying JAR file. However, that file is then not closed anymore.

The problem can be checked e.g. by using https://github.com/jenkinsci/lib-file-leak-detector

Here a stack using Spring 5.3.39

#8 /var/folders/mr/5jj8y27x76s6zd22d78gqcfw0000gn/T/junit-4912917014845844348/zip/hash-in-file.zip by thread:ForkJoinPool-1-worker-1 on Mon Mar 03 11:41:06 CET 2025
at java.base/java.io.RandomAccessFile.<init>(RandomAccessFile.java:214)
at java.base/java.util.zip.ZipFile$Source.<init>(ZipFile.java:1312)
at java.base/java.util.zip.ZipFile$Source.get(ZipFile.java:1277)
at java.base/java.util.zip.ZipFile$CleanableResource.<init>(ZipFile.java:709)
at java.base/java.util.zip.ZipFile.<init>(ZipFile.java:243)
at java.base/java.util.zip.ZipFile.<init>(ZipFile.java:172)
at java.base/java.util.jar.JarFile.<init>(JarFile.java:347)
at java.base/sun.net.www.protocol.jar.URLJarFile.<init>(URLJarFile.java:103)
at java.base/sun.net.www.protocol.jar.URLJarFile.getJarFile(URLJarFile.java:72)
at java.base/sun.net.www.protocol.jar.JarFileFactory.get(JarFileFactory.java:168)
at java.base/sun.net.www.protocol.jar.JarFileFactory.getOrCreate(JarFileFactory.java:91)
at java.base/sun.net.www.protocol.jar.JarURLConnection.connect(JarURLConnection.java:132)
at java.base/sun.net.www.protocol.jar.JarURLConnection.getContentLengthLong(JarURLConnection.java:202)
at org.springframework.core.io.AbstractFileResolvingResource.exists(AbstractFileResolvingResource.java:69)

The issue e.g. prevents temporary folders from unit tests containing ZIP resources accessed through Spring to be deleted on Windows.

[jhoeller]

What would you expect AbstractFileResolvingResource to do at that point? We could only really open an InputStream and close() it right away, like we do for our fallback existence check already, and even that step is not guaranteed to free any actual resources (but comes at the expense of opening the content stream first). URLConnection is not an API designed for specifically managing the resource lifecycle there. To some degree, it even suggests to keep the resource handles open for faster subsequent access.

It seems that the only reliable way to avoid such leaks is to manage direct file handles, avoiding any URLConnection to begin with. Are you manually performing the UrlResource access there, or is this implicitly happening within the framework somewhere?

[reckart]

I am retrieving the UrlResource from a PatternMatchingResourcePatternResolver() which I use to access JAR/ZIP files. Is this happening within the framework - if you consider PatternMatchingResourcePatternResolver() to be part of the framework, maybe yes.

I believe it could be considered to introduce a special handling for JarURLConnection like it exists for HttpURLConnection - and in this branch directly go to the getInputStream().close() branch.

A more involved approach could also be to hoist the access to the JAR into the AbstractFileResolvingResource itself. I.e. if the file is a JAR/ZIP residing on the file system, the archive could be opened explicitly, checked for the existence of the resource and be closed again.

[jhoeller]

I've tweaked this to simply skip the getContentLengthLong() call on a JarURLConnection, always going to the getInputStream().close() branch instead. This will be available in the upcoming 6.2.4 snapshot, feel free to give it an early try!

FWIW this goes along nicely with the recent #34217 in 6.2.3.