Question about java.system.class.loader and the module system

[Alan Bateman]

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