On 17/01/2024 17:22, PavelTurk wrote:
Hello all.
We use JPMS for our framework where we have the following layers:
1) boot layer: framework + JRE/JDK modules (to add them we use
--add-modules ALL-DEFAULT)
2) child layers [with child layers..] where every child layer is
dynamically added component.
As I understand Layrry framework does the same. I asked several days
ago about practical use
of JPMS layers and got only one response about Layrry. So, I can
conclude that layers are used
primarily for creating dynamic substems, plugins, components,
extensions etc.
However, we couldn't find a way to add JRE and JDK modules in child
layers. Because of this we
have to add all default modules to boot layer what is a very very
serious flaw.
Why do we need it? As components are created dynamically when/if they
are required we don't
know what JRE/JDK modules will be required we create boot layer.
That's why add all of them to
boot layer.
Could anyone say if it is possible to add JRE/JDK modules to child
layers? Is there any way to do it?
Are there any plans to add such possibility?
This is the topic that was on the original requirements wish list as
"Run-time augmentation of the platform modules". There isn't any general
support for this in the JDK. One reason is that module layers, and in
this case the boot layer, are immutable. Another reason is that
integrity of the platform requires that classes in the java.* packages
be defined to the boot or platform class loaders. In other words, you
can't have child layers containing modules with java.* classes that are
mapped to custom class loaders.
That said, there are two things to be aware of:
1. ALL-DEFAULT. You've already found that. As explained in JEP 261, this
is there to support cases where a container module is in the boot layer
but it does dynamic configuration with modules that require standard
modules that might not be in the boot layer.
2. Java agents and the JMX/management agent. If an agent is started in a
running VM, and the required java.instrument or java.management modules
are not in the boot layer, then a child layer will magically comes into
being with the required standard + JDK modules. This it possible because
the starting of these agents is under the control of the VM with the
original mapping of modules to class loaders.
To your question, you can't use the API to do dynamic configuration and
create module layers that contain the standard modules. The solution for
containers is ALL-DEFAULT to ensure that all modules in the system image
that export an API are in the boot layer.
-Alan
