On Sun, 10 Nov 2024 04:50:36 GMT, jyxzwd <[email protected]> wrote:
>> The jrt file system provider supports both the current JDK and a
>> remote/target JDK. When the JDK is not the current JDK then it loads the jrt
>> file system provider from target's JDK jrt-fs.jar. To understand this more,
>> try this example where you set targetJDK to the file path of another JDK on
>> your system.
>>
>>
>> String targetJDK = .
>> var fs = FileSystems.newFileSystem(URI.create("jrt:/"), Map.of("java.home",
>> targetJDK));
>> byte[] classBytes =
>> Files.readAllBytes(fs.getPath("/modules/java.base/java/lang/String.class"));
>>
>>
>> Run with `-Xlog:class+load` and you'll see jrtfs and support jimage class
>> files loaded from the target JDK. If you dig deeper you'll see they are
>> loaded by a custom class loader, they are not defined by the boot loader and
>> aren't in java.base. Hopefully this makes it clear why classes in
>> jdk.internal.jimage or jdk.internal.jrtfs can't access JDK internal classes.
>
> I got it.Thank you for the detailed explanation!Maybe we should consider
> another way to load the custom FileSystemProvider.
If we load the custom DefaultFileSystemProvider by SystemClassLoader,then we
will step into the method
SystemModuleFinders$SystemModuleReader#findImageLocation again, and the var
imageReader will be null.But we can not define a custom classLoader to load the
custom DefaultFileSystemProvider since it needs to maintain JDK 8 source
compatibility.So the only way that I can think of is to use the
installedProvider which has the 'file' scheme to directly get the 'modules'
path value.
In ImageReaderFactory:
private static final Path BOOT_MODULES_JIMAGE = null;
static {
// check installed providers
for (FileSystemProvider provider :
FileSystemProvider.installedProviders()) {
if ("file".equalsIgnoreCase(provider.getScheme())) {
try {
BOOT_MODULES_JIMAGE =
provider.getFileSystem(URI.create("file:/")).getPath(JAVA_HOME, "lib",
"modules");
if (BOOT_MODULES_JIMAGE != null) break;
} catch (UnsupportedOperationException uoe) {
}
}
}
}
What do you think of this?I am new to openjdk source development.So I truely
appreciate the time you dedecate to my question!
-------------
PR Review Comment: https://git.openjdk.org/jdk/pull/21997#discussion_r1835713466
