A Quick Start with OSGi Logging: the Pax library
The previous technical note on java logging presents the principles of code logging, as well as some of the most widespread logging libraries for java: Sun J2SE Logging API, Apache Log4J framework, Simple Log library, Jakarta Common Logging API, Simple Logging Facade for Java, X4juli. The configuration of the logging is done through a unique config file for instance log4j/log4j.properties.
In the context of an OSGi platform, each bundle (i.e. OSGi archives) have its own view on the file system, so using a centralized config file can not conduct to anything but dirty hacks. The solution is provided by the Pax logging facility, which bundelizes both the library code and the configuration file.
This technical note presents a Quick Start view on using the Pax logging library: what steps are required to let a proper logging be available on your OSGi platform ?
Following aspects will be shown:
Technical Environment
This Quick Start assumes that you are ready to install suitable code for installing OSGi and the Pax library.
We use the Felix OSGi implementation.
The Maven development environment is used for compiling both Felix and Pax.
All necessary logging libraries are provided in Pax. We provide some convenient extension.
Restriction: no other logging API should be available in the platform.
Installation
Installation
This section introduces how to install, configure and launch a OSGi platform with the Pax logging facility.
Of coure, you can just skip the items that are already available on your system.
svn co https://svn.apache.org/repos/asf/incubator/felix/trunk/tools/maven2
cd ./maven
mvn install
If you want to learn Maven, you can do it is 5 minutes, or in 10 minutes
svn co http://svn.apache.org/repos/asf/incubator/felix felix
cd ./felix/trunk
mvn install
(include the script for launching felix in your path)
If you want to start with felix, you can have a look here.
- Checkout the code for Pax
mkdir ops4j
cd ./ops4j
svn co https://scm.ops4j.org/repos/ops4j/projects/base base
svn co https://scm.ops4j.org/repos/ops4j/projects/pax pax
If you are new to Pax, you will find an introduction here
cd ./base
mvn install
cd ../pax
mvn install
Configuration
- Configure the logging properties in Pax
in logging/service/src/main/resources/log4j.properties
cd ./logging/
mvn clean install
Launch
- Launch the osgi platform, and suitable bundles
felix
(assuming that felix is available in your path)
start file:///XX/felix/trunk/org.osgi.compendium/target/org.osgi.compendium-0.8.0-SNAPSHOT.jar
start file:///XX/pax/logging/api/target/api-0.9.5-SNAPSHOT.jar
start file:///XX/pax/logging/service/target/service-0.9.5-SNAPSHOT.jar
start file:///XX/pax/logging/log4j/target/log4j-0.9.5-SNAPSHOT.jar
(with XX denoting the suitable path in your system)
You have just installed maven, felix, pax, and started the pax bundles that are required for performing logging with log4j in a OSGi platform.
Set your own logging configuration for Log4j
This section introduces how to configure, install and test the log4j facility. An example highlight the behavior of log4j with Pax.
Configure
Have a look at the content of the logging/service/src/main/resources/log4j.properties configuration file.
Two profiles are defined: demoLogger (logging level set to ERRORS and worse probblems), and develLogger (logging level set to DEBUG).
You can change this configuration.
Do not forget to reinstall (in Maven) and reload (in the OSGi platform) the serevice.jar bundle
Install
- if your OSGi platform with logging bundles is available (section Installation)
update the bundles you just compiled in your OSGi plaform
update N
with N the ID of the service.jar bundle
follow the instruction of part 'Installation'.
Test
- Start the log4j client in your OSGi platform
start file:///XX/logging/log4jclient/target/log4jclient-0.1.jar
2 loggers (demoLogger, and develLogger) are created, which each issue an error and a warning,
the configuration is set so that the demoLogger print logs that are at least as serious logging/log4jclient/target/log4jclient-0.1.jaras ERROR, and the develLogger prints logs that are at least as serious as DEBUG.
Example of Jcl, Avalon, Sl4j, Jdk logger
This section introduces how to launch the standard example provided with Pax, that uses simultaneously Jcl, Avalon, Sl4j and Jdk logger with a log4j backup.
Installation of the bundles
- install following bundles
OSGi Service, OSGi Util, Servlet
from OBR,
or manually from following URL: http://oscar-osgi.sourceforge.net/
or in the log4jlogging.doc/repository/ directory.
- install the logger you want to test
(in the example, all are taken into account)
start file:///XX/logging/avalon/target/avalon-0.9.5-SNAPSHOT.jar
start file:///XX/logging/slf4j/target/slf4j-0.9.5-SNAPSHOT.jar
start file:///XX/logging/jcl/target/jcl-0.9.5-SNAPSHOT.jar
- go the logging/example directory
cd logging/example
- Specify the activator in the pom.xml file
<osgi-BundleActivator>org.ops4j.pax.logging.example.Activator</osgi-BundleActivator>
mvn clean install
Test
- start the example bundle in your osgi platform
start file://XX/logging/example/target/example-0.9.5-SNAPSHOT.jar
This section introduces how to use the Pax facility in an existing Maven project.
Configuration
- Add following dependencies to the pom.xml file of your project
<dependency>
<groupId>org.ops4j.pax.logging</groupId>
<artifactId>log4j </artifactId>
<version>0.9.5-SNAPSHOT</version>
<scope>provided</scope>
</dependency>
<dependency>
<groupId>org.ops4j.pax.logging</groupId>
<artifactId>api</artifactId>
<version>0.9.5-SNAPSHOT</version>
<scope>provided</scope>
</dependency>
Use
- you can retrieve the singleton Logger by calling
Logger.getLogger("name");
from all over the OSGi platform.
Beware: no other version of the log4j library must be available on your platform.
Otherwise, You should consider using a Logger service.
Links
Related tutorials
External resources