When we create Java runtime by jlink it takes all java classes/resources and places them into JRT image file: lib/modules
.
Here is basic Maven project resources structure I use:
src
main
resources
dict
xkcd_en
I'm just trying to read xkcd_en
text file. If we look into JRT file, here it is:
>> jimage list /path/to/lib/modules
...
Module: main
dict/xkcd_en
...
Also I've explicitly opened it in module-info
, just in case:
module main {
opens dict;
// ..rest code omitted
}
The only way I could read the file is obtaining it as input stream:
WORKS:
public static InputStream getResourceAsStream(String resource) {
return FileUtils.class.getResourceAsStream(resource);
}
System.out.println(new BufferedReader(
new InputStreamReader(getResourceAsStream("/dict/xkcd_en")))
.lines().collect(Collectors.joining("\n"))
);
DOESN'T WORK:
But if I'm trying to get the file URI and read it via Java NIO API, it doesn't work:
public static URL getResourceOrThrow(String resource) {
URL url = FileUtils.class.getResource(resource);
Objects.requireNonNull(url);
return url;
}
1 - Java NIO can't find the file. But it definitely does exist, otherwise getResource()
returns null
.
System.out.println(Paths.get(getResourceOrThrow("/dict/xkcd_en").toURI()));
// /main/dict/xkcd_en
Files.readAllLines(Paths.get(getResourceOrThrow("/dict/xkcd_en").toURI()));
Caused by: java.nio.file.NoSuchFileException: /main/dict/xkcd_en
at java.base/jdk.internal.jrtfs.JrtFileSystem.checkNode(JrtFileSystem.java:494)
at java.base/jdk.internal.jrtfs.JrtFileSystem.getFileContent(JrtFileSystem.java:253)
at java.base/jdk.internal.jrtfs.JrtFileSystem.newInputStream(JrtFileSystem.java:342)
at java.base/jdk.internal.jrtfs.JrtPath.newInputStream(JrtPath.java:631)
at java.base/jdk.internal.jrtfs.JrtFileSystemProvider.newInputStream(JrtFileSystemProvider.java:322)
2 - The same if you'll use FileSystem
directly:
FileSystem fs = FileSystems.getFileSystem(URI.create("jrt:/"));
System.out.println(fs.getPath("main/dict/xkcd_en"));
// main/dict/xkcd_en
Files.readAllLines(fs.getPath("main/dict/xkcd_en")));
Caused by: java.nio.file.NoSuchFileException: /main/dict/xkcd_en
at java.base/jdk.internal.jrtfs.JrtFileSystem.checkNode(JrtFileSystem.java:494)
3 - Java NIO doesn't even know what jrt:/
scheme is.
Files.readAllLines(Paths.get(getResourceOrThrow("/dict/xkcd_en").toExternalForm()));
Caused by: java.nio.file.InvalidPathException: Illegal char <:> at index 3: jrt:/main/dict/xkcd_en
at java.base/sun.nio.fs.WindowsPathParser.normalize(WindowsPathParser.java:182)
at java.base/sun.nio.fs.WindowsPathParser.parse(WindowsPathParser.java:153)
at java.base/sun.nio.fs.WindowsPathParser.parse(WindowsPathParser.java:77)
at java.base/sun.nio.fs.WindowsPath.parse(WindowsPath.java:92)
at java.base/sun.nio.fs.WindowsFileSystem.getPath(WindowsFileSystem.java:229)
at java.base/java.nio.file.Path.of(Path.java:147)
at java.base/java.nio.file.Paths.get(Paths.java:69)
Here is the specs of JRT FS.
A jrt URL is a hierarchical URI, per RFC 3986, with the syntax
jrt:/[$MODULE[/$PATH]]
where $MODULE is an optional module name and $PATH, if present, is the path to a specific class or resource file within that module. The meaning of a jrt URL depends upon its structure:
jrt:/$MODULE/$PATH refers to the specific class or resource file named $PATH within the given $MODULE. jrt:/$MODULE refers to all of the class and resource files in the module $MODULE. jrt:/ refers to the entire collection of class and resource files stored in the current run-time image.
So obtained path looks ok to me. Where am I wrong?
The part of the spec you quote deals with URLs, not NIO. The two are different mechanisms. A URL simply represents the location; how the real location is resolved depends on the URL protocol. In this case, the form is jrt:/[$MODULE]/[$PATH]
and it will search the image and return the proper resource.
However, when using NIO you are accessing the runtime image as a file system. If you look at JDK-8066492 you'll see the JRT file system was enhanced at one point to add two directories: modules
and packages
. These directories are directly under the root and their meaning is described in the issue. If you want to access the image as a file system you'll need to take the modules
directory into account.
It's not that NIO can't read files from the JRT image, it's that /main/dict/xkcd_en
really does not exist; the real location is /modules/main/dict/xkcd_en
.
According to JDK-8216553 (linked by Alan Bateman in the question comments), the fact Paths.get(/* jrt uri */)
returns a non-existent Path
is a bug.
You can see an example of this here.