[osgi logging] part 3 – slf4j + logback as osgi bundles

This is part 3 of my blogseries about ‘Logging in OSGI Enterprise Applications‘ – you’ll find all parts here.

Download SLF4J and Logback

Now its time to integrate SLF4J and Logback into an OSGI app – at first you should download the newest releases:

• download SLF4J from here: http://www.slf4j.org/download.html Version 1.5.8
• download Logback from here: http://logback.qos.ch/download.html Version 0.9.17

Search for Bundles

If we take a look at the MANIFEST.MF files from the different Jar’s we’ll see that only some include OSGI informations, so as good citizens of OSGI world we search if bundles are available from a repository:

• Eclipse Orbit: http://wiki.eclipse.org/Orbit_Bundles
• OBR – OSGI Bundle Repository: http://www.osgi.org/Repository/HomePage
• S2 – SpringSource Enterprise Bundle Repository: http://www.springsource.com/repository/app/

At SpringSource (S2) we find Bundles for SLF4J (Version 1.5.6, where 1.5.8 is the newest) and Logback (Version 0.9.15, where 0.9.17 is the newest):

Its a good idea to download the bundles from S2 even if you want to deploy your own with the newest releases. Its always good to take a look at the Dependencies and compare if you’re in trouble 😉

Which Bundles / Jar’s you need depends from your projects. For the use-cases described in the first parts of this series we’ll need:

• SLF4J API – all our bundles logging to SLF4J / Logback only need a dependency to SLF4J API
• Logback Classic + Logback Core – the Logback Implementation of SLF4J
• JUL to SLF4J – if you have to integrate bundles using JDK Logging (java.util Logging)
• Log4J over SLF4J – if you have to integrate bundles with Log4J Logging
• Jakarta Commons Logging (JCL) over SLF4J – if you have to integrate Apache Commons Logging

SLF4J + Logback Jar’s as OSGI Bundles

As I wrote above, only some of the downloaded Jar’s from SLF4J and Logback include valid OSGI information:

• You can use as OSGI Bundles without modifications:
  • slf4j-api-1.5.8.jar
  • jcl-over-slf4j-1.5.8.jar
  • logback-core-0.9.17.jar
• Jar’s with wrong OSGI Information:
  • logback-classic-0.9.17.jar Exports slf4j.impl in version 1.5.6, but slf4j-api-1.5.8 imports 1.5.8, so I re-deployed this bundle
• Jar’s without any OSGI Information: (so I deployed them new with added OSGI dependencies)
  • jul-to-slf4j-1.5.8.jar
  • log4j-over-slf4j-1.5.8.jar

In my other blogseries I already talked about HowTo use PDE to deploy a bundle (Plug-in) from an existing Jarfile, so I don’t want to repeat it here.

PDE and Cycles

Important: there are cycles between SLF4J API and Implementation (Logback):

The dependency between slf4j.api and logback.classic has arrows in both directions, this indicates that there’s a cycle.

BTW: if you’re wondering about the dependency graph – its from ‘PDE Graph Plug-in Dependencies View‘ I talked about here and here.

Because of these cycles you have to check “allow for binary cycles” if deploying the Plug-in using PDE:

PDE 2nd Dependency

Here’s another tip: if working with logging – bundles then it could happen that your IDE (or current Target Platform) contains more then one bundle providing the same packages. If you want to force PDE to use a specific bundle to resolve the dependency, simply open MANIFEST.MF and add the bundle to “Automatic Management of Dependencies“:

example: you’re using Import-Package org.apache.commons.logging. Adding the bundle ‘jcl.over.slf4j‘ (Jakarta Commons Logging over SLF4J) to ‘Automated Management of Dependencies‘ you can use your Import-Package and on the other side be sure that PDE puts the jcl.over.slf4j bundle on its Classpath. Those ‘2nd Dependencies’ are not added to the MANIFEST.MF – you’ll find them in the built.properties:

At the end of this blog series I’ll provide download links, where you can get all needed bundles, so you don’t have to do it by yourself 🙂

you’ll find the overview of this blog series here. stay tuned… part4 follows…