On 10/03/2017 06:09, Jayaprakash Artanareeswaran wrote:
Hello,
I remember there was an open issue about not being able to use a different JDK
9 (different than the current one) for class lookup. I raised this question
some time back and the following solution was suggested:
<http://permalink.gmane.org/gmane.comp.java.openjdk.jigsaw/820>http://permalink.gmane.org/gmane.comp.java.openjdk.jigsaw/820
At that point this solution only worked when the compiler was from a JDK 8 and
not across different versions of JDK 9. This continues to be the case even
today with Eclipse.
For convenience, the code proposed was this:
URL url = Paths.get(jdkHome, "jrt-fs.jar").toUri().toURL();
URLClassLoader loader = new URLClassLoader(new URL[] { url });
FileSystem fs = FileSystems.newFileSystem(URI.create("jrt:/"),
Collections.emptyMap(), loader);
Even today when I run the compiler with ea+159 and trying load JRT from ea+153,
I can clearly see that the JrtFileSystem returned is still pointing to ea+159.
But I also noticed Javac is able to refer to different JDK 9 without any issue
with --system., which makes me wonder if we should adjust our code in some way
too.
JDK 8 does not have a `jrt` file system provider so the above code
allows it to be loaded from the target run-time image when needed. A
small nit in the above is that the path to jrt-fs.jar should be created
with Paths.get(jdkHome, "lib", "jrt-fs.jar").
JDK 9 has a built-in `jrt` file system provider. In the above code
fragment then your URLClassLoader does parent delegation and so the
built-in `jrt` file system provider will be loaded, nothing will be
loaded from jrt-fs.jar. This is why it accesses the "local" jdk-9+159
image rather the "remote" jdk-9+153 image.
In any case, things have moved on significantly since that thread in
early 2015. The `jrt` file system provider was updated to support access
to other run-time images. This is done by specifying a key of
"java.home" in the map that you use to configure the file system. The
map value is the file system location of the runtime image. Can you try
this:
FileSystem fs = FileSystems.newFileSystem(URI.create("jrt:/"),
Map.of("java.home", target));
where `target` is a string with the location of the target run-time image.
-Alan
