Artical on JPMS integration with existing module systems
tjwatson at us.ibm.com
Thu Aug 18 13:45:06 UTC 2016
Below is the plain text format from an article I wrote on using JPMS
layers with an existing module system (OSGi). Please excuse the poor
formatting. I tried to make it is pretty as I could in plain text to
allow Mailman to let it through. I have omitted the diagrams to avoid
this message getting held up by Mailman also. If the diagrams are
important I can post them some other way later:
In my previous blog post I discussed an experiment that creates a JPMS
Bundle Layer which can represent resolved OSGi bundles as JPMS modules.
This would allow child JPMS layers to be created that have modules that
require the OSGi bundles as they would any other module.
In that experiment I took a hybrid approach where the Framework
implementation and the OSGi bundles themselves did not really live in the
JPMS world. Instead I dynamically created a layer on top that attempted
to represent that OSGi world indirectly within the JPMS world. Then real
JPMS modules could be configured to work on top of this facade layer that
represented the OSGi world. This can be thought of as taking a top down
approach to migrating to JPMS. Unfortunately this approach has a major
shortcoming because all classes that are loaded in the OSGi bundle layer
will be associated with an unnamed module.
The fact that the bundle classes are associated with an unnamed module
caused me to have to do a major hack to grant access to modules
representing the OSGi bundles. This hack involved injecting code into the
jpms modules which could invoke the addReads method in order to grant the
necessary access to the unnamed module of the bundle class loaders. This
does not seem like a real viable solution for running JPMS modules on top
of and OSGi bundle layer.
I learned much about how the JMPS layer works during that experiment. The
hybrid has a major flaw because the delegation graph of class loaders
involved are not associated with named modules all the way down. A better
way would be to do a bottom up approach where each layer involved has
class loaders which are mapped to one or more named modules. This way
when the JPMS layer resolves the modules on top it will automatically
grant read access as normal from a requiring module to all of its required
modules it got resolved to. The following diagram illustrates how the
layers would look:
The boot layer contains the JPMS modules which were configured with the
JVM when it was launched. In this diagram, the framework launcher has
also been migrated to Java 9 in order to have it create a Layer for the
class loader used to load the framework implementation. This layer
configures a single module named system.bundle. This allows all the
classes for the Framework implementation to be associated with the
system.bundle module. Next is the bundle layer. This layer is configured
to map each bundle class loader to a named module representing the bundle.
Finally we have a module layer which uses all the built-in module class
loaders of Java 9 for JPMS.
Over the past few days I have been reworking my github project (OSGi-JPMS
layer) to investigate if this approach is possible. Again, I am trying to
do this without requiring any modifications to the OSGi framework
implementation itself and I am using only OSGi specified APIs. This
approach uses a bottom up strategy for JPMS modules. With that in mind
the first thing to do is to modify the OSGi Framework launcher to create
the system.bundle module.
The system.bundle Module
I did not want to modify the framework itself to make it a real JPMS
module. Instead I decided to modify the existing Equinox launcher to
create a layer itself which maps the class loader it creates to load the
OSGi Framework with a system.bundle module. While the Equinox launcher is
specific to launching the Equinox Framework a similar thing could be done
to launch any standard OSGi Framework.
The system.bundle acts as the OSGi bundle that exports for all the
non-java.* packages available in the boot layer. This allows OSGi bundles
to use Import-Package to depend on packages from the boot layer. In order
to grant the system.bundle class loader access to all packages available
from the boot layer I have to generate a ModuleDescriptor programatically
which requires all modules from the boot layer. The layer must be created
with the system.bundle module resolved which maps the module to the class
loader used to load the framework implementation before any classes are
defined in packages that we want to be exported by system.bundle module.
The ModuleDescriptor used for the system.bundle must specify that it
exports the packages from the framework implementation, otherwise JPMS
will still associate them with the unknown module. With this modified
launcher, any classes defined in the packages we declared in the
ModuleDescriptor will be associated with the system.bundle module. You
can find the changes I made to the equinox launder on github at
may notice I hard coded the list of packages to export from the
system.bundle module. This was a hack to get going quickly. Ideally
these packages would be discovered programmatically.
The Bundle Layer
One important detail to understand about JPMS layers is that the class
loaders that are mapped to by the modules within a layer MUST NOT have
defined any classes in packages for which a ModuleDescriptor declares as
exports or conceals. This implies that the bundle layer used to represent
bundle JPMS modules must be created as early as possible and ideally
before any classes are loaded using the bundle class loaders. In order to
achieve this I changed the bundle osgi.jpms.layer to a system.bundle
fragment still named osgi.jpms.layer. The OSGi R6 Framework specification
added a new feature which allows system.bundle fragments to be activated
when the Framework is initializing before the rest of the bundles get
activated. This allows for the code controlling the bundle layer to get
in place in order to intercept any class defines from bundle class
loaders. That way we can map the bundle class loaders for resolved
bundles to their respective JPMS modules before any classes are defined. I
used a WovenClassListener and WeavingHook to achieve this. Here I am not
interested in actually weaving any class bytes, but these OSGi hooks allow
for us to hook directly into the bundle class loader just before it is
about to define a class.
We can now insert the code in the correct place to create the bundle
layer. I used a similar approach as before to achieve this, but some more
information is needed now that the bundle classes will belong to a named
module. Here are the steps:
1. Discover all resolved host bundles and map their symbolic name to
their wiring. Note that we could get conflicts if multiple bundles are
installed with the same symbolic name. For this experiment I choose only
one to map into the bundle layer.
2. Create a module finder that is backed by the bundle wirings. The
finder is what creates the ModuleReference and ModuleDescriptor objects to
represent the bundles. The following information is used from the wiring:
- The bundle symbolic name is the module name.
- The bundle version is the module version.
- The the package capabilities are the exports for the module.
- Private packages must be discovered to specify the module's
concealed packages. Here the private packages are treated as exported by
the module instead of concealed. I will explain why later.
- Dependencies on other bundles for class loading must become
3. Create a configuration using the bundle finder. Default to using
the system.bundle layer configuration as the parent configuration.
4. Create a layer that maps each module name to the bundle wiring
Creating this layer exposes some issues with JPMS that make it difficult
and sometimes impossible to properly represent OSGi bundles as modules.
1. JPMS-ISSUE-001 - Reflection is used by almost any framework in Java
and the OSGi Framework is no exception. In JPMS the JVM will not allow
reflection to be used on any class that is not known to JPMS as an
exported package. Once I successfully got every class from a bundle
associated with a JPMS module I found that the framework could no longer
call Class.newInstance() for bundle activator classes contained in
concealed packages!! In order to get that to work I had to treat every
private package from a bundle as exported by the ModuleDescriptor for the
bundle. This will also be necessary for other dependency injection
containers on OSGi, for example, Declarative Services. I also imagine
this has to cause issues for other DI containers such as Spring and CDI.
2. JPMS-ISSUE-002 - Private packages must be discovered and specified
to JPMS. As pointed out already, I had to make the private packages
exported by JPMS, but first I tried to make them concealed. Either way,
all packages that are associated with a module must be known to JPMS as
either exported or concealed. If this is not done then the classes from
unknown packages will be associated with the unnamed module. This places
an extra burden on the OSGi module system because in OSGi there was no
reason for the framework to discover the private packages ahead of time.
3. JPMS-ISSUE-003 - JPMS must be aware of the OSGi bundle dependencies
for class loader access. If the module descriptors representing OSGi
bundles do not declare any module requires then JPMS will not grant the
read access required to use a class from another module. The bundle class
loaders will continue to be able to load the classes from other bundles
according to import-package and require-bundle rules, but when the class
is actually used the JVM will throw access exceptions. This forces us to
translate the OSGi dependencies into module requires. If there are
multiple bundles with the same symbolic name then there is no way to tell
JPMS which version of the bundle a module depends on.
4. JPMS-ISSUE-004 - JPMS layers do not allow cycles between modules.
OSGi bundles are allowed to have cycles. Since we must make JPMS aware of
the OSGi bundle dependencies this restricts us to only bundles that have
5. JPMS-ISSUE-005 - JPMS layers provide a static module resolution
graph. This will prevent OSGi from successfully resolving dynamic package
imports if they require read access to a new module.
6. JPMS-ISSUE-006 - JPMS layers allow for multiple versions of the
same module but it does not appear that modules within that layer or
contained child layers can influence which version of the module they get
7.JPMS-ISSUE-008 - JPMS layers do not allow for split packages. If
the OSGi bundles are resolved with split packages then the bundle layer
cannot be created.
If you can look past these issues we are left with a layer that can
represent a static set of resolved OSGi bundles as real JPMS modules and
we can use that layer to create child JPMS layers for loading other JPMS
OSGi Bundle Dynamics
The bundle layer we have now represents a static set of resolved OSGi
bundles in a Framework. But the bundles in an OSGi Framework are not
static. They can be uninstalled, updated, re-resolved, and new bundles
can be installed. How can this dynamic nature be represented in JPMS
layers? The approach I took was to create a linear graph of layers where
the youngest child layer represents the current state of the bundles. This
would look something like this:
In this scenario we started out with bundle.a and bundle.b resolved in the
bundle layer 1. Then we created a module layer 1 to resolve jpms.a and
jpms.b modules. Then bundle.b was updated and bundle.c was installed and
then bundle.b was refreshed in order to flush out its old content and
class loader. This leaves bundle layer 1 with a "dead" bundle.b module
which also makes module layer 1 stale. So we decide to discard module
layer 1 and create module layer 2 for jpms.a and jpms.b modules. To do
that we need a new bundle layer that represents the current set of
Here we cannot discard bundle layer 1 because it still has at least one
valid module bundle.a. We also cannot represent bundle.a module in a new
layer because we may have already loaded classes from packages contained
in bundle.a. Instead of throwing away bundle layer 1 a new bundle layer 2
is created that uses bundle layer 1 as its parent. Bundle layer 2 will
contain all the new versions of modules that are not already represented
in the parent layers. This allows the new bundle.b to shadow the "dead"
bundle.b module in bundle layer 1. This appears to work. The only JPMS
module that cannot be shadowed by a child layer is the java.base module.
But we are left with a pretty big issue:
- JPMS-ISSUE-007 - Discarded modules from a JPMS layer will be pinned
in memory until the complete layer is discarded. This ultimately leads to
a huge class loader leak because we cannot properly free up our stale
bundle class loaders. It also causes issues for bundles that are
uninstalled completely. The "dead" modules for these bundles will
continue to be available since nothing is shadowing them from child
layers. I suppose we could create a empty module that has the same name
but exports nothing, but that will still allow modules on top to resolve
when they shouldn't.
Currently the code for the experiment is located in github at
I did this in the tjwatson/moduleClassLoader branch.
This approach allows for a pretty accurate representation of a static set
of resolved OSGi bundles as JPMS modules. But we are left with several
issues that need to be addressed before this can be considered a truly
viable solution. Some may decide these are permanent restrictions of JPMS
that we will have to live with going forward. But I believe there are
some tweaks to JPMS that could go a long ways to making this approach
close to a complete solution. Listed below are some changes that would
help. I listed them in the order of importance, but I think 1 and 2 are a
close tie for most important.
1. Allow for code that manages a JPMS layer to have more control for
establishing read access for the modules contained in the managed layer.
The Module addReads method allows for read access to be added for a module
dynamically at runtime. But it has a restriction that it must be called
by a class defined by the module that wants new read access. It would be
a great help if we could call addReads from the management code that
created the layer. Perhaps an addReads(Module wantsRead, Module toTarget)
method on Layer that checks the caller module is the same module get
created the Layer? This could be used to solve a large set of issues
- JPMS-ISSUE-003 - We could avoid having to make JPMS aware of the
OSGI dependencies if we would be allowed to establish the read access
ourselves when the bundle layer is created.
- JPMS-ISSUE-004 - If we avoid having to make JPMS aware of the
OSGi dependencies then we no longer have worry about restricting cycles.
- JPMS-ISSUE-005 - If we can dynamically add reads then we can
enable dynamic package import to work by dynamically adding read access to
the provider of the package at runtime.
- JPMS-ISSUE-008 - If we avoid having to make JPMS aware of the
OSGi dependencies then we no longer have to worry about restricting split
2. Allow for reflection on classes from concealed packages. Many
dependency injection containers depend on being able to act upon concealed
classes in order to construct objects and inject the objects with
dependencies. Forcing implementation details to be exported so that these
classes can be acted upon by DI containers is wrong.
- JPMS-ISSUE-001 - We would no longer have to declare the bundle
private packages as exported by the JPMS module. Instead they can remain
concealed as they should be.
3. Allow for sub-graphs of modules to be discarded within a layer.
- JPMS-ISSUE-007 - This would allow us to flush out the "dead"
modules which should never be used anymore.
4. Allow a layer to map a class loader to a default named module. Any
classes from unknown packages to the JPMS would be assigned this named
module instead of the unnamed module.
- JPMS-ISSUE-002 - This would allow us to avoid having to scan for
private packages. Instead we would map the bundle classloader to a module
and that module could be used for the private packages.
5. Allow the JPMS requires statement to specify a module version.
- JPMS-ISSUE-006 - This would allow us to represent multiple
versions of a bundle within the bundle layer and give JPMS modules the
ability to specify which version they want.
My hope is that this experiment is useful in providing constructive
feedback to the JPMS expert group. I hope they consider enhancing JPMS to
make JPMS layers more usable with existing module systems like OSGi.
This was extracted from my article at:
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