ClassLoader API to look up class files

[Eirik Bjørsnøs]

On Sun, Sep 22, 2024 at 6:46 PM Rafael Winterhalter <rafael.wth at gmail.com>
wrote:

Sorry, still not sure I understand the scenario..

That is only true for Class::getResource which resolves resources relative
> to the loaded class's class file location.
>

Class:getResource simply adds the package name prefix and calls
ClassLoader::getResource. It has no separate knowledge about the class
file's location.


> When loading a class, ClassLoader::findClass respects the multi-release
> characteristic. But for ClassLoader::getResource, this is not the case.
>

ClassLoader as such has no concept of multi-release JAR files, that's
entirely up to the class loader implementation. In OpenJDK, this is
supported by URLClassLoader, or rather URLClassPath.JarLoader::getResource,
which calls JarFile::getEntry which is where the version lookup happens.

If you have a class loader which respects multi-release characteristics in
ClassLoader::loadClass, but not in ClassLoader::getResource, then that is a
strange way to implement a class loader.

Is there some intricacy of class loader hierarchies that I'm missing here?

The following unit test demonstrates how URLClassLoader::getResource
resolves versions:

@Test
public void getVersionedClass() throws IOException {
    Path zip = Path.of("multi.zip");
    Manifest man = new Manifest();
    Attributes attrs = man.getMainAttributes();
    attrs.put(Attributes.Name.MANIFEST_VERSION, "1.0");
    attrs.put(Attributes.Name.MULTI_RELEASE, "true");
    try (JarOutputStream jo = new
JarOutputStream(Files.newOutputStream(zip), man)) {
        jo.putNextEntry(new
ZipEntry("META-INF/versions/21/com/example/HelloWorld.class"));
        jo.putNextEntry(new ZipEntry("com/example/HelloWorld.class"));
    }
    ClassLoader loader = new URLClassLoader(new URL[]
{zip.toUri().toURL()});
    URL resource = loader.getResource("com/example/HelloWorld.class");
    String path =
resource.getFile().substring(resource.getFile().indexOf("!/") + 2);
    assertEquals("META-INF/versions/21/com/example/HelloWorld.class", path);
}
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