Atomos

Atomos - A Java Module Framework using OSGi Connect

Atomos requires an Equinox OSGi Framework implementation which supports OSGi Connect (https://blog.osgi.org/2019/09/osgi-connect-revisited.html). Currently a snapshot of the Equinox OSGi Framework is being used that implements the proposed OSGi Connect specification for an upcoming OSGi R8 Core specification. Source for the snapshot is found in https://git.eclipse.org/c/equinox/rt.equinox.framework.git in the osgiR8 branch and tempary binaries are pushed to https://github.com/tjwatson/atomos-temp-m2repo for Atomos. Atomos is an implementation of an OSGi Connect factory that can be used to create an OSGi Framework instance. Framework instances created with Atomos add support to the OSGi Framework that enables bundles to be installed which are managed outside of the OSGi Framework module layer. Currently Atomos supports three different modes for loading bundles from outside the OSGi module layer:

1. Module Path: Using the Java Platform Module System (JPMS) Atomos will discover the modules on the module path and will make any OSGi bundles found available for installation into the Framework. This also allows for Atomos and a set of OSGi bundles to be packaged into a jlink image resulting in a small fit-for-purpose JVM
2. Class Path: When loaded from the class path Atomos will discover the JARs on the class path and will make any OSGi bundles found available for installation into the Framework.
3. Graal Substrate Native Image: When compiled into a Substrate native image Atomos will discover the bundles that were included into the image. This requires configuration to enable the necessary reflection for things like bundle activators and declarative service components.

Build

Java 11 must be used to build Atomos. Atomos build uses the latest 1.0.0.Beta2 version of the moditect plugin (https://github.com/moditect/moditect.git).
This plugin provides some cool utilities for adding module-infos to existing dependency JARs and building jlink images. You can build the Atomos with the following:

mvn clean install -Pjava8

This should create a jlink image under atomos/atomos.tests/service.image/target/atomos. Executing the following command against the jlink image should produce a gogo shell prompt:

atomos/bin/atomos

You should see the following output:

Registered Echo service from activator.
____________________________
Welcome to Apache Felix Gogo

g! 

In order to successfully build a jlink image all bundles included in the image must contain a module-info.class, they cannot be automatic modules. The atomos.tests/service.image example uses the latest 1.0.0.Beta2 version of the moditect-maven-plugin to add module-info.class as necessary to the bundles used in the image.

You can also load additional modules into atomos by using the atomos.modules option when launching atomos. For example:

atomos/bin/atomos atomos.modules=/path/to/more/modules

When doing that the additional modules will be loaded into a child layer where the Atomos OSGi Framework will control the class loaders. This will produce a class loader per module bundle installed. This has advantages because it allows the module class loader for the bundle to implement the org.osgi.framework.BundleReference interface.

Substrate

An example project of using Graal Substrate is located at atomos/atomos.tests/service.substrate. This project is not built as part of the main Atomos build because it requires an installation of GraalVM CE 19.3.0 (Java 8 or Java 11 can be used) and the native-image tools for Substrate. The Java 11 version of Graal Substrate does not currently support full introspection at image runtime of the Java Platform Module System. Atomos Module support expects to have full introspection of the Java Platform Module System when running on Java versions greater than Java 8. Therefore the example will run in basic class path mode for both Java 8 and Java 11 when running with a native substrate image. To build the Substrate example the main Atomos build must first be built using the Java 8 profile:

mvn clean install -Pjava8

Note that install target must be used so that Atomos is installed into your local m2 repository. This still requires Java 11 to be used to build but the result allows the atomos.framework JAR to be used on Java 8.

To build the native image you must to install the native image support for Graal (see https://www.graalvm.org/docs/reference-manual/native-image/). You need to run the gu command that comes with Graal VM:

gu install native-image

Next you must switch to a Java installation of Graal with the Substrate native-image tools installed and then change into the atomos/atomos.tests/service.substrate and run mvn clean package

This will create a target/atomos executable. It also creates a target/substrate_lib/. This contains all the original bundle JARs that got compiled into the native image atomos. In order to launch the native atomos you must be in the directory containing both atomos and the substrate_lib/ folder. This is currently used as a simple way for Atomos to discover the available bundles and load additional resources from them at runtime. Hopefully substrate will add full introspection to the Java Platform Module System in the future which would allow Atomos to discover the modules within the image and load them as bundles. Otherwise, more work is needed to map all of the bundle resources into the native image at build time and provide more meta-data in the image for Atomos to use to discover the bundles included.

If you launch atomos it will give you a gogo g! prompt to run gogo commands. Also included in this example is a version of the Felix web console. The web console can be access with http://localhost:8080/system/console/bundles and the id/password is admin/admin.