On 03/03/2021 10:43, Volker Simonis wrote:
:
My question now is if this is an inherent property of the module
system or merely an implementation detail? I.e. would it be possible
to put the application module into its own layer and initialize that
only later when the custom system class loader will be available? I
think this would be relatively easy if the custom class loader can be
found on the class path. If the custom class loader is in a module
itself, that module would have to be in the boot layer to make it
accessible to the default system class loader.
The ability to set a custom class loader as the system class loader is
somewhat legacy feature. There was consideration given to deprecating
and removing it a few years ago but we ended up leaving it "as is" in
case there are still application servers using it. So it should work the
same in JDK 16 as it did in JDK 8/older releases. That is, it will be
created with the built-in application class loader as parent and the
custom system class loader will be the default parent class loader for
delegation.
You are correct that an initial module on the application module path
will be loaded by the built-in application class loader. All modules on
the application module path are mapped to the built-in application class
loader. If an initial module is specified to the java launcher (--module
or -m) then it just locates the module in the boot layer and invokes its
main class. There is no role for a custom system class loader here.
If you are looking to deploy a custom system class loader in a module on
the module path then it should work as long as you export the package
with the custom class loader to java.base. I'm not sure if you really
want to do this or whether it's a means to an end. Maybe it would be
easier to start with a summary on what you are looking it do? Are you
looking to intercept all attempts to load classes? Is there a java agent
in the picture too?
The current API documentation of the system class loader [1] mentions
that the system class loader "is typically the class loader used to
start the application". Shouldn't that be updated to mention that this
will not be the case for modular applications?
The javadoc is correct because it will be rare to deploy with
java.system.class.loader set to a custom class loader. Assuming it is
not set then you should find that the initial class will be defined by
this class loader. This goes for both the class path and module path cases.
-Alan
