Referring to a different --system

[Alan Bateman]

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