Apache Felix Logback

Apache Felix Logback is a small integration of the Logback backend with OSGi.

With OSGi R7 the Log Service Specification 1.4 (Log 1.4) brings a slew of new features designed to improve the developer experience with logging, the details of which can be read about here.

This project is intended to help bridge the last frontier of OSGi logging by leveraging many capabilities of Logback, the new Log 1.4 features to provide a unified backend where:

1. all logging can be configured in one place
2. all log records are appended together (or at least all appenders are setup in one place)
3. support as many logging APIs as possible

Depedencies¶

To add Logback to an OSGi framework include the following dependencies

<dependency>
    <groupId>ch.qos.logback</groupId>
    <artifactId>logback-classic</artifactId>
    <version>1.2.x</version>
</dependency>
<dependency>
    <groupId>ch.qos.logback</groupId>
    <artifactId>logback-core</artifactId>
    <version>1.2.x</version>
</dependency>
<dependency>
    <groupId>org.slf4j</groupId>
    <artifactId>slf4j-api</artifactId>
    <version>1.7.x</version>
</dependency>

These provide the slf4j logging API over the Logback backend.

Basic Configuration¶

Configuring logback is easily handled by setting the system property logback.configurationFile to point to a file on the file system.

This is an example using a bndrun file:

-runproperties: logback.configurationFile=${fileuri;${.}/logback.xml}

where the bnd macro ${.} gives the path to the directory of the bndrun file and macro ${fileuri;<path>} converts the path to a system compatible path.

Logback offers many features from it's configuration file, so make sure to look through the documentation.

Note When using the bnd export function to produce an executable jars -runproperties are copied verbatim and as defined above the property will be the literal value provided at build time. This is likely not desirable. You can instead use a path without a URI protocol to ensure logback searches for the file on the classpath of the executable jar.

A default configuration file can be embedded in the executable jar with the -includeresource directive in the bndrun file:

-includeresource: META-INF/logback.xml=${.}/logback.xml

This places the default logback config inside the executable jar at META-INF/logback.xml. Now the following directive can be used:

-runproperties: logback.configurationFile=META-INF/logback.xml

The defaults will be used when executing the jar.

java -jar executable.jar

Of course this property can be overloaded by the system property when executing the jar:

java -jar executable.jar -Dlogback.configurationFile=file:/some/path/logback.xml

Remember to use the file: protocol in this case to alert logback to the fact this path is not on the classpath.

Deployment Options¶

There are at least two possible deployment scenarios.

The Standard Approach¶

The standard approach is to install all bundles into the OSGi runtime.

pros:

• this is the most understood and expected deployment scenario
• bundles are managed through any OSGi management agent

cons:

• due to the fact that logging APIs are most commonly static there is an inevitable race condition which is exacerbated by aspects such as:
• the order in which the bundles are started
• which bundle makes the first call the static API that most-likely has the side effect of binding an implementation
• buffering is required to collect the complete set of logs, but configuring the buffer to hold that many logs can cause memory issues, or if too small to lose records. Also, some logging frameworks simply don't provide such buffering, and some actually simply fail in this scenario. In the case of the OSGi Log Service this is handled by implementations that support Log 1.4, or perhaps earlier through proprietary means.

The Immediate Approach¶

The immediate approach overcomes the cons of the standard approach and initializes the log engine as early as possible in order to avoid buffering, binding issues, or losing records.

pros:

• there's very little chance to miss any log records
• there are no temporal issues for binding or bundle start order since the log engine is setup before most other application activities take place

cons:

• bundles on the framework class path are not installed into the framework so they cannot be managed through regular means. On the other hand, logging is one of those infrastructural aspects that transcends the application and for which there are very few scenarios under which one would want to manage logging API or implementation bundles this way. Configuration? Sure!

Unified Backend¶

Of course adding Logback does not magically result in all logs being funneled into the same appenders, particularly the OSGi logs. However, it is quite common to assemble an OSGi application from a variety of bundles which use a variety of logging APIs. Many of these can already be mapped onto Logback.

Many examples of setting up the various logging APIs can be found in the integration tests of the project.

The following APIs are tested:

• JBoss Logging 3.3.x
• Commons Logging 1.2
• JUL (Java Util Logging)
• Log4j 1
• Log4j 2
• Slf4j

Mapping of OSGi Events¶

The OSGi Log specification 1.4 describes events resulting in log records. Log 1.4 defines logger names mapping to these events.

Note: In order to improve the granularity of the logging associated with these events, Apache Felix Logback allows appending a period (.) followed by any number of segments (separated by periods) of the Bundle-SymbolicName for the logger names used in configuring the levels. This greatly improves the configurability.

Consider the following logback.xml example:

<configuration>

    <!-- defined a console append -->
    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
        <encoder>
            <pattern>%d{HH:mm:ss.SSS} [%.15thread] %-5level %logger{36}:%line - %msg%n</pattern>
        </encoder>
    </appender>

    <!-- log all Bundle events -->
    <logger name="Events.Bundle" level="TRACE"/>

    <!-- log WARN Framework service events of bundles whose BSN starts with `org.eclipse.osgi` (guess who that is?) -->
    <logger name="Events.Service.org.eclipse.osgi" level="WARN"/>

    <!-- turn OFF legacy Log Service records from bundles whose BSN starts with `org.baz` -->
    <logger name="LogService.org.baz" level="OFF"/>

    <!-- log DEBUG Service events for bundles whose BSN starts with `org.fum` -->
    <logger name="Events.Service.org.fum" level="DEBUG"/>

    <!-- log DEBUG from any log API using a logger name starting with `org.my.foo` -->
    <logger name="org.my.foo" level="DEBUG"/>

    <root level="ERROR">
        <appender-ref ref="STDOUT" />
    </root>
</configuration>

Notes¶

• Apache Felix Logback supports Logback's automatic reloading upon file modification
• When using equinox framework you may want to disable it's internal appenders using the system property eclipse.log.enabled=false