OSGi R7 Highlights: Java 9 Support

With the Java 9 release, the Java Platform Module System (JPMS) is a major feature that impacts many areas of the Java Platform.  The JPMS is used to modularize the class libraries provided by the Java Platform.  In addition, it has been proposed that the JPMS can be used by developers to modularize their own applications, although the OSGi Alliance cautions against its use here.

Meanwhile the OSGi Core specification has provided a stable platform for developing modular Java applications for well over 17 years.  The OSGi Core R7 specification continues this tradition by providing updates to the OSGi Core specification which are fully compatible with previous versions of the OSGi Core specification.  OSGi Core R7 Framework implementations will be able to run on top of Java 9 without requiring any changes to existing OSGi application bundles (assuming no internal packages from the Java platform are being used).

The OSGi Core R7 specification has a couple of new features that build upon some features new to Java 9.  This includes support for multi-release JARs and runtime discovery of packages provided by the JPMS modules loaded by the platform.  First I will tell you about the multi-release JAR support.

Multi-Release JAR Support

Java 9 introduces a new type of JAR called a multi-release JAR.  A multi-release JAR file allows for a single JAR to support multiple major versions of the Java platform. For example, a multi-release JAR file can depend on both the Java 8 and Java 9 major platform releases, where some class depend on APIs in Java 8 and other class depend on APIs in Java 9.

The purpose of multi-release JARs is to support alternative implementation of select classes to deal with changes in the visible APIs of the Java platform.  That is, it is not meant as a means to supply new function or new API on different Java platform versions. As a best practice new APIs should be released in an updated version of an existing bundle or released as part of a new bundle.

The OSGi Core R7 specification adds support for multi-release JAR files. An OSGi bundle file can be a multi-release JAR by adding the following manifest header to the bundle manifest:

 Multi-Release: true

Any JAR included on the bundle class path can also be independently declared as a multi-release JAR by including the same manifest header:

  Multi-Release: true

in its own JAR manifest. For example, an OSGi bundle can include third-party multi-release JARs on its own bundle classpath. In this case the third-party multi-release JARs will be loaded as multi-release JARs by the OSGi Framework even if the bundle JAR itself is not a multi-release JAR.

When a JAR on the bundle class path is a multi-release JAR, then the Framework must search the JAR's versioned directories when attempting to locate a class or resource in the JAR. For more information on how classes and resources are loaded from multi-release JARs see the multi-release JAR documentation.

Different implementations of a Java class for different versions of the Java platform can affect the requirements of the bundle.  For example, what packages need to be imported from the running Java platform. The R7 Framework supports supplemental manifest files that can be used to specify alternative values for the Import-Package and Require-Capability manifest headers for different versions of the Java platform.

When a bundle file is a multi-release JAR, then the Framework must also look for a supplemental manifest file, OSGI-INF/MANIFEST.MF, in the versioned directories. For example:

  META-INF/versions/9/OSGI-INF/MANIFEST.MF

The Framework finds the highest versioned manifest available in the multi-release JAR which can be applied to the running Java platform and uses the Import-Package and Require-Capability manifest headers as replacements for the values specified in the bundle's manifest.

The purpose of the supplemental manifest is to specify requirements on capabilities provided by different versions of the Java platform which are needed by the implementation classes for that Java platform version. As a best practice, the supplemental manifests should not contain additional requirements on capabilities which are not supplied by the Java platform for the Java version associated with the supplemental manifest.

For more information about multi-release JAR support you can refer to the following sections of the OSGi Core R7 specification:

1. Bundle File Multi-release JAR
2. Bundle class path Multi-release Container

Runtime Discovery of Java Platform Packages

Now that the Java platform is modularized in Java 9, the runtime can be configured to load only the modules which are required by the application.  This allows for a smaller custom runtime that is tailored to the needs of a specific application.  The set of java.* packages provide by the running Java platform is no longer constant for a specific version of the Java platform.

Bundles typically depend on different packages by using requirements specified with the Import-Package header, but as of the OSGi Core R4 specification, bundles have been prohibited from using Import-Package to specify requirements on java.* packages.  Instead bundles have used the osgi.ee namespace (or the deprecated Bundle-RequiredExecutionEnvironment header) to specify a dependency on specific versions of the Java platform. It has been assumed that the set of java.* packages available for a specific version of the Java platform is constant and therefore there is no need to specify additional requirements on specific java.* packages.

With Java 9 this is no longer a valid assumption.  With OSGi Core R7, the osgi.ee namespace should only be used to specify the minimal level of the Java platform required by the Java class bytecode level included in the bundle.  Dependencies on specific java.* packages should to be specified using the Import-Package header.

The OSGi Core R7 specification now allows bundles to use the Import-Package header to import java.* packages.  The OSGi Framework implementation is required to discover the all available java.* packages in the running Java platform and automatically provide the java.* package as a separate system packages (see the system packages configuration property).  The system bundle is the only bundle that is allowed to provide java.* packages. It is an error for normal bundles to attempt to export java.* packages using the Export-Package header.

When importing java.* packages only the package name should be specified, no other matching attributes are needed.  For example:

  Import-Package: java.sql

Note that bundle class loaders in R7 continue to have full visibility to all available java.* packages available in the running Java platform.  The class and resource loads for the java.* packages will continue to be boot delegated by the framework implementation even when the bundle does not specify any Import-Package header. The java.* package requirements are only necessary to prevent a bundle from resolving if the necessary java.* packages are not available at runtime.  For example, if a bundle must only resolve if the java.sql module is loaded.

For more information about java* package support you can refer to the following sections of the OSGi Core R7 specification:

1. Execution Environment
2. Parent Delegation 

Bundles Supporting R6 and R7 Frameworks

The OSGi Core R6 specification prohibits bundles from importing java.* packages and will throw a BundleException if such a bundle is attempted to be installed. Bundles that must work on OSGi R6 and earlier Frameworks but also want to run on R7 Frameworks have two options:

1. Do not import the java.* packages.  The bundle will continue to work on an R7 framework because java.* packages are still boot delegated by the bundle class loaders. This implies the bundle may resolve when running on Java 9 even when all the java.* packages required by the bundle are not available.
2. Package the bundle as a multi-release JAR to allow alternative values for Import-Package to be specified.

A bundle that must still be able to be installed on an OSGi R6 or earlier Framework must not specify java.* packages in their main bundle manifest Import-Package header. A supplemental manifest must also be included to be able to specify java.* package imports.

This supplemental manifest must contain the original Import-Package header from the main bundle manifest as well as any java.* package required by the bundle. When running on an OSGi R6 Framework, this supplemental manifest will be ignored and therefore the bundle will install successfully as the bundle did not specify an import for the java.* packages in the main manifest.

If a bundle will always require OSGi R7 or greater Framework, there is no need to use multi-release JARs for this purpose even when the bundle must support Java 8 or earlier. This is because OSGi R7 frameworks must provide the java.* package capabilities on all Java platform versions the framework implementation supports. As long as the java.* package is available on the running Java platform, the bundle with an import for the java.* package must resolve.

I advise using option 1 here because it is by far the most simple approach.  If you really have a strong need to use this new R7 feature then I recommend you do not try to produce a single bundle version that supports both R6 and R7 Frameworks.  Instead simply release a new version of your bundle that only supports OSGi R7 or greater Frameworks.

In Summary

The OSGi Core R7 release continues the tradition by providing a stable modular platform with updates to the OSGi Core specification which are fully compatible with previous versions of the OSGi Core specification.  OSGi bundle developers can continue to develop advanced modular applications and run their applications on Java 9 without having to commit to migrating to the green initial release of JPMS.

The OSGi Core R7 specification does add new functionality in support of new features available in Java 9, such as the support for multi-release JARs and importing of java.* packages.  The OSGi specifications will continue to evolve in order to provide the necessary tools for developing advanced modular applications.

OSGi R7 Highlights: Proposed Final Draft Now Available

I am pleased to announce that the OSGi Alliance has published the Proposed Final Drafts of the OSGi Core R7 and Compendium R7 specifications. We expect that the final versions of these specifications will be published in April 2018 after OSGi Alliance member approval.

The R7 release builds upon the long history of the OSGi Alliance’s leadership in Java modularity and reflects a significant amount of effort from the technical members of the OSGi Alliance expert groups over the last 2 years. Thanks go to all of the members who have contributed to this release.

R7 represents many significant new features and capabilities and provides an open standards-based approach for a number of modern valuable and simple-to-use technologies important to Java developers.

This blog post is the start of a series of blog posts from the technical experts at the OSGi Alliance to share some of the key highlights of R7. The blog posts in this series will come out over the coming weeks and cover the following topics:

• Java 9 Support – Multi-release JAR support and runtime discovery of the packages provided by the JPMS modules loaded by the platform.
• Declarative Services – Constructor injection and component property types.
• JAX-RS – A whiteboard model for building JAX-RS microservices.
• Converter – A package for object type conversion.
• Cluster Information – Support for using OSGi frameworks in clustered environments.
• Transaction Control – An OSGi model for transaction life cycle management.
• Http Whiteboard – Updates to the Http Whiteboard model.
• Push Streams and Promises – The Promises packages is updated with new methods and an improved implementation and the new Push Streams package provides a stream programming model for asynchronously arriving events.
• Configurator and Configuration Admin – Configuration Admin is updated to support the new Configurator specification for delivering configuration data in bundles.
• LogService – A new logging API is added which supports logging levels and dynamic logging administration and a new Push Stream-based means of receiving log entries is also added.
• Bundle Annotations – Annotations that allow the developer to inform tooling on how to build bundles.
• CDI – Context and Dependency Injection support for OSGi developers.

Stay tuned and I hope you find the technical information in the blog post series useful to you as developers!

OSGi Community Event Slides and Videos

I can't believe we are already into the third week of 2018 and the holidays and year-end festivities are long forgotten already.  That is of course unless you celebrate Chinese New Year, which is only a month away now.

The presentations and video recordings from the OSGi Community Event 2017 conference are now available. These can be found on the OSGi Community Event 2017 home page. 

Unfortunately, for the first time in at least 5 years, I was not able to make it to the event last year. I hear from all accounts though that it was another successful co-location with EclipseCon Europe. We had a packed tutorial on OSGi and Bndtools, some excellent presentations, and an engaging and lively Birds of a Feather session. 

It was also great to have Intel delivering the OSGi Keynote discussing their experiences and key learnings from modularizing one of their applications by migrating to OSGi (Slides / Video).

New for the 2017 event, we have created a YouTube playlist of all of the presentations from the conference. So if you have some time and just want to kick back and watch lots of videos on OSGi be sure to check this out. 

We have also created video playlists for the 2015 and 2016 OSGi Community Events too and you can find all of these on the OSGi Alliance YouTube Channel.

2018 looks set to be a very busy year for the OSGi Alliance with the R7 (Release 7) specification due and several new opportunities for you to tell us and the world about how you are using OSGi whether you are an individual, a small, medium or large end user, or a software vendor.

We are always interested in your feedback and suggestions so if you have anything you want to share or suggest you can add it is a comment to this post or send it to the OSGi Marketing team.

That just leaves me to wish everyone the best for 2018 and encourage you to stay tuned to this blog, our Twitter feed and/or join our LinkedIn Group to hear the latest news and be notified about the technical posts on modularity coming up this year.

OSGi Tooling Workshop Oct 23 - Ludwigsburg

The German OSGi Users' Forum is holding an OSGi Tooling Workshop on Monday, October 23, 2017 in Ludwigsburg starting at 10.30 am.

Its free to attend and you will hear the latest news about development tooling for OSGi. This will include IntelliJ, Eclipse, Bndtools, Gradle, Maven, Eclipse RCP and PDE.

So anyone attending the OSGi Community Event and EclipseCon Europe should be very interested in this as its being held at the same location in the Forum am Sclosspark.

The full schedule and presenters can be found on the German OSGi Users' Forum event page HERE.

To register and reserve you place please send an email to germany-info@osgiusers.org

Letter to our OSGi User Community RE: the Java Platform Module System (JPMS)

Letter to the OSGi User Community:

With the JCP Executive Committee’s acceptance of JSR 376, the Java Platform Module System (JPMS) will be included in the Java SE 9 release. The OSGi Alliance has recently completed an analysis of JPMS and what it means for our user community.

We recognize that many of you have invested in OSGi technology over a number of years and have business-critical systems that rely on OSGi, so you may question if and how Java SE 9 may impact you. Please be assured that OSGi will continue to operate with its full capabilities on top of the new Java SE 9 platform.

However, the modularity capabilities in OSGi and JPMS are not the same. Our modularity experience, earned through the evolution of OSGi, has proven there are specific requirements in modularizing an application that is assembled with code from multiple sources, which have independent release cycles. Given these requirements, JPMS is not suitable for most real-world applications and we therefore caution against the use of JPMS for the application layer.

As you know, the modularization of an application, where code and dependencies are pulled from many independently created sources, presents a different set of challenges than those faced in the modularization of a defined and closely managed platform. This difference is especially true when the entire code base is managed by a single entity and released all at once, like the Java platform.

For more than 15 years, OSGi has proven successful in modularizing user applications by using dynamic resolution and semantic versioning, as well as supporting isolation, multiple class loaders and dependency injection. With its goal of modularizing the platform, JPMS has been able to rely upon static dependency resolution and ignore versioning, deferring responsibility for versioning to others.

Clearly, both OSGi and JPMS will co-exist and effectively operate in this application/platform demarcated environment. The modularization of the Java platform with Java SE 9 is a significant step forward for Java as a whole, and OSGi technology’s role in application-level modularization is as important today as it has been for previous releases of Java.

If you have any technical questions, please post them to osgi-dev@mail.osgi.org or, if you prefer an off-list inquiry, email modularity@osgi.org.

Sincerely,

Dan Bandera

President

OSGi Alliance

Java is registered trademark of Oracle and/or its affiliates. Other names may be trademarks of their respective owners. OSGi is a trademark or a registered trademark of the OSGi Alliance in the United States, other countries, or both.

Join Us for OSGi in Ghent - Weds, Sept 20, 2017

The OSGi Alliance is holding its next technical Expert Group meetings in Ghent later this month.

On Weds, Sept 20, 2017 you can join us for an evening of OSGi. Organised in collaboration with BeJUG.

Registration is required as places are limited so be sure to sign up on the BeJUG meetup event page.

ACA-IT Solutions are kindly hosting the event and providing sandwiches while everyone assembles from 18.30 hrs.  And then the evening kicks off with a welcome from 19.00hrs.

The agenda for the rest of the evening is:

• 19.10 hrs - What's coming in the OSGi R7 Specification (David Bosschaert & Carsten Ziegeler)
• 19.50 hrs - OSGi Push Streams, scalable event processing - pushing the limits (Tim Ward)
• 20.30 hrs - OSGi; from amateur to invited contributor (Tim Verbelen)

There will also be opportunity to network and ask questions with your peers and OSGi Expert Group members.

The event is being held at ACA-IT Solutions Ghent office (Voorhavenlaan, 31/008, Ghent). ACA have been actively using OSGi for many years and support the community by committing to open source OSGi projects. Thanks also to BeJUG for their support in promoting the event.

We hope you can join us. Don't forget to REGISTER in advance.

Using OSGi to Connect Things with IoT Applications - Member Webinar

Anyone using OSGi for IoT or interested in learning more about how OSGi and specifically Gateways powered by OSGi can be used to deliver joined up IoT solutions should find the 30 minute Bosch Webinar on Aug 29, 2017 at 1pm CDT of interest.

Titled "Connecting things with IoT applications - How gateways can build the bridge", Bosch Software Innovations have lots of real world experience with many millions of OSGi based gateways out in the field supporting everyday solutions that users and customers just take for granted.

Attendees of the webinar will learn about the important role OSGi has in the delivery of these stable, evolvable and easily maintainable IoT gateway solutions. Furthermore you will gain an understanding of:

• The pros and cons of gateway- and non-gateway-based IoT solutions
• Benefits of using gateways as mediators between devices and IoT applications
• Real-world use cases for gateway-based IoT solutions
• The role of OSGi in connectivity projects

You can find out more and register to attend the live webinar or receive access to the post event recording by visiting the event website.