Re: How to do “--add-modules” with dynamically created layer in JPMS?

[Alan Bateman]

On 15/11/2020 19:16, Alex Orlov wrote:
> Hi all,
>   
> I create JPMS layer this way:
>   
> Configuration cf = parentLayer.configuration().resolveAndBind(moduleFinder, ModuleFinder.of(), moduleNames);
> ModuleLayer layer = parentLayer.defineModulesWithOneLoader(cf, parentClassLoader);
>   
> And I have a problem with adding implementation modules. These implementation modules are ignored by JPMS because classes from these modules are not used anywhere (it is clear, that classes from API modules are used). So, JPMS doesn't add these modules to layer.
>   
> As I understand, if I spoke about implementation modules on boot layer, I could use --add-modules jvm argument. However, I couldn't find any information how to force JPMS load my module (even if its classes are not used) for dynamically created layers.
>   
> Could anyone say how to do it?
It would be useful if you could give an example of "implementation 
module". I can't tell if you mean a jdk.* module, a service provider 
module that doesn't export an API, or something else.

In any case, I think the picture you are describing is a module that is 
observable, either in the run-time image or on module path. The module 
is not resolved at startup (and so is not in the boot layer) because the 
initial module that you specify with `java -m` doesn't transitively 
require it and it doesn't provide an implementation of a service that 
any of the other resolved modules use.

The summary is a module layer is immutable and there isn't any way to 
add modules to a module layer once it has been created. For what you are 
doing then maybe `--add-modules ALL-SYSTEM` can be used as workaround. 
This will ensure that all system/platform modules are in the boot layer 
as these modules cannot be loaded into a child layer (Johannes's suggest 
to specify the module name to resolveAndBind will not work for java.* 
modules as they cannot be loaded into child layers, his suggestion may 
work for other modules).

One other point to mention is that the original set of requirement 
envisaged some means to augment the set of platform modules at run-time. 
We didn't get to that requirement except for the limited scenario that 
is tools loading the JMX agent or a java agent into a running VM. These 
limited scenario involve loading the jdk.management.agent or 
java.instrument modules and that is some with a special child layer 
rather than augmenting the boot layer. This partial solution, along with 
`--add-modules ALL-SYSTEM`as a workaround for the general cases, has 
been sufficient to date.

-Alan