Monthly Archives: July 2016

2 posts

Camel, Kura and OSGi, struggling with ‘sun.misc.Unsafe’

So here comes a puzzle for you … You do have Apache Camel (2.17), which internally uses com.googlecode.concurrentlinkedhashmap, which uses sun.misc.Unsafe. Now you can argue a lot about this is necessary or not. I just is that way. So starting up Apache Camel in an OSGi container which does strict processing of classes, using Apache Camel will run into a “java.lang.NoClassDefFoundError” issue due to “sun/misc/Unsafe”.

The cause

The cause is rather simple. Apache Camel makes use of sun.misc and so it should declare that in the OSGi manifest. OSGi R6 (and version before that as well) defines in section “3.9.4” of the core specification that java.* is forwarded to he parent class loader, but the rest is not. So sun.misc will not go the parent class loader (which finally is the JVM) by default.

Solutions

As always, there are a few. There may be a few more possible than I describe here, but I don’t want to list any which require changing Apache Camel itself.

Fragments

Two Fragments
Two Fragments
OSGi fragments are a way to enhance an already existing OSGi bundle. So the kind of merge in into the bundle. So it is possible to create a fragment for Apache Camel which does Import-Package: sun.misc. This should quickly resolve the issue as long as the bundle is installed into you OSGi container at the same time Apache Camel is, so that it is available at the time Apache Camel is started. The host bundle has to be org.apache.camel.camel-core, since this is the bundle requiring sun.misc.Unsafe.

Of course this brings up the next issue, there is nobody who exports sun.misc. But there is again a way to fix this.

The actual provider of sun.misc is the JVM. However the JVM does not know about OSGi. But the OSGi container itself, the framework, can act as a proxy. So if the framework bundle (aka bundle zero) would export sun.misc it would be able to actually resolve the class by using the JVM bootclasspath. The solution therefore is another fragment, which performs an Export-Package: sun.misc. That will bring both bundles with their fragments together, correctly wiring up sun.misc.

But as we have seen before, the fragment requires a “host bundle” and this would be different when e.g. using Apache Felix instead of Eclipse Equinox.

Again, there is a solution. The system bundle is also know as system.bundle. So the fragment can specify system.bundle with the attribute extension:=framework as bundle host:

Manifest-Version: 1.0
Bundle-ManifestVersion: 2
Bundle-SymbolicName: my.sun.misc.provider
Bundle-Version: 1.0.0
Fragment-Host: system.bundle; extension:=framework
Export-Package: sun.misc

Of course you can also export other JVM internal packages using that way.

There are only two things to keep in mind. First of all, but is is true to all other solutions as well, if the JVM does not provide sun.misc then this won’t work. Since the class cannot be found. Second, and this is specific to this solution, if you start “camel-core” before those two fragments are installed, then you need to “refresh” the Apache Camel Core bundle in order for the OSGi framework to re-wire your imports/exports.

There are also some pre-made extension bundles for this setup. Just search maven central.

Equinox and Felix

Some setups of Felix and Equinox do provide an “out of the box” workaround. Equinox for example does automatically forward all failed class lookups to the boot class loader, as a last resort, in the case the framework is started by using the org.eclipse.equinox.launcher_*.jar instead of the org.eclipse.osgi_*.jar launcher.

Bootclasspath delegation for Equinox

Eclipse Equinox also allows to set a few system properties in order to allow falling back to the bootclasspath and delegating the lookup of “sun.misc” to the JVM:

Also see: https://wiki.eclipse.org/Equinox_Boot_Delegation

osgi.compatibility.bootdelegation=true
This fill fall back to the bootclassloader like using the launcher “org.eclipse.equinox.launcher”

Bootclasspath delegation for all

The OSGi core specification also allows to configure direct delegation of lookups to the boot classloader (Section 3.9.3 of the OSGi core specificion):

org.osgi.framework.bootdelegation=sun.misc.*
This will forward requests for “sun.misc.*” directly to the boot class loader.

Conclusion

Now people may complain “oh how complicates this OSGi-thingy is”. Well, “sun.misc.Unsafe” was never intended to be used outside the JVM. Java 9 will correct this with their module system. OSGi already can do that. But it also provides a way to solve this.

If you prefer to use system properties, a different launcher or the “two fragment” approach, that is up to you and your situation. For me the problem simply was to make it happen without changing either Apache Camel or the launcher configuration of Eclipse Kura. So I went with the “two fragments” approach.

Thanks

I am just writing this down in order to help others. And I got help from others to solve this myself. So thanks to some people who posted this “on the net”, it is a long time, I stumbled over you googling about a solutions some time ago. Sorry I forgot where I initially found this solution.

Also thanks to Neil Bartlett for pointing out the OSGi conform solution with “org.osgi.framework.bootdelegation”.

Bringing OPC UA to Apache Camel

My first two weeks at Red Hat have been quite awesome! There is a lot to learn and one the first things I checked out was Apache Camel and Eclipse Milo. While Camel is more known open source project, Eclipse Milo is a newcomer project at the Eclipse Foundation. And while it is officially still in the Incubation Phase, it is a tool you can really work with! Eclipse Milo is an OPC UA client and server implementation.

Overview

Although Apache Camel already has an OPC DA connector, based on OpenSCADA’s Utgard library (sorry, but I couldn’t resist ;-) ), OPC UA is a complete different thing. OPC DA remote connectivity is based on DCOM and has always been really painful. That’s also the reason for the name: Utgard. But with OPC UA that has been cleared up. It features different communication layers, the most prominent, up until now, is the custom, TCP based binary protocol.

I started by investigating the way Camel works and dug a little bit in the API of Eclipse Milo. From an API perspective, both tools couldn’t be more different. Where Camel does try to focus on simplicity and reducing the full complexity down to a very slim and simple interface, Milo simply unleashes the full complexity of OPC UA into a very nice, but complex, Java 8 style API. Now you can argue night and day what is the better approach, with a little bit of glue code, both sides work perfectly together.

To make it short, the final result is an open source project on GitHub: ctron/de.dentrassi.camel.milo, which features two Camel components providing OPC UA client and OPC UA server support. Meaning that it is possible now to consume or publish OPC UA value updates by simply configuring a Camel URI.

Examples

For example:

milo-client:tcp://foo:bar@localhost:12685?nodeId=items-MyItem&namespaceUri=urn:org:apache:camel

Can be used to configure an OPC UA Client connection to “localhost:12685”. Implementing both Camel producer and consumer, it is possible to subscribe/monitor this item or write value updates.

The following URI does create a server item named “MyItem”:

milo-server:MyItem

Which can then be accessed using an OPC UA client. For the server component the configuration options like bind address and port are located on the Camel component, not the endpoint. However it is possible with Camel to register the same component type with different configurations.

Also see the testing classes, which show a few examples.

What is next?

With help from @hekonsek, who knows a lot more about Camel than I do, we hope to contribute this extension to the Apache Camel project. So once Eclipse Milo has it’s first release, this could become an “out-of-the-box” experience when using Apache Camel, thanks to another wonderful project of Eclipse IoT of course ;-)

Also, with a little bit of luck, there will be a talk at EclipseCon Europe 2016 about this adventure. It will even go a bit further because I do want to bring this module into Eclipse Kura, so that Eclipse Kura will feature OPC UA support using Apache Camel.