fix: resolve double-slash path in FileResolverImpl when fileName is root

[waterWang]

Description

Fixes #6280

When FileResolverImpl.resolve() is called with an absolute path (e.g., /application.yml), the parentFile loop in resolveFile2() reaches the root path / and calls unpackUrlResource(url, "/", cl, true). In unpackFromFileURL, when iterating over the directory listing, the subResource construction concatenates fileName + "/" + file, producing //application.yml (double-slash prefix) instead of /application.yml.

This causes the subsequent getValidClassLoaderResource(cl, subResource) call to fail because //application.yml is not a valid classpath resource path.

Root cause

unpackFromFileURL unconditionally adds / between fileName and file, but when fileName is already / (the root path), this produces a double-slash prefix.

Fix

Use a conditional separator: append / only when fileName does not already end with /.

String subResource = fileName + (fileName.endsWith("/") ? "" : "/") + file;

Steps to reproduce

1. Call fileResolver.resolveFile("/application.yml") when the classloader has a resource root at a temp directory
2. Observe the exception caused by //application.yml not being found

[timastered]

the file listing is done on resource (which potentially can be a new path on the classpath)
Doesn't this need to be taken into account if you want to set the name of the resource?
e.g.
String subResource = fileName + (fileName.endsWith("/") ? "" : "/") + file;
change to
String subResource = resource.getPath() + (fileName.endsWith("/") ? "" : "/") + file;

Otherwise it will try to locate the file in the root of your application/running instance?

example in code:

• URL = %TMP%/properties
• listing on this resource gives back application.properties
• subResource should be = %TMP%/properties/application.properties?
  • instead of /application.properties in your example?

[tsegismont]

Good points @timastered

@waterWang can you add tests for this change? Thanks