OSGi Compendium R7 provides a major update to the OSGi LogService specification. A new logging API is added which supports logging levels and dynamic logging administration. A new Push Stream-based means of receiving log entries is also added. But it is quite often the case you need to use other code such as open source projects which are using slf4j for their logging API. This session will explore the new OSGi LogService changes and how you can integrate code using both slf4j logging and OSGi LogService logging.

1. The new OSGi LogService 1.4
and integrating with SLF4J
BJ Hargrave, IBM

2. Log Service
• An OSGi specification for logging
• Introduced in Release 1 in 2000 as an OSGi logging API
• Many other OSGi spec require logging to the Log Service, for example DS
• Pre-whiteboard; pre-log4j/slf4j
• Versions 1.1, 1.2 and 1.3 only made fairly minor updates
• Version 1.4 is a MAJOR update to the API inspired by SLF4J.

4. Logger
• An interface that allows a bundle to log information, including a message, a
level, an exception, and a ServiceReference object
• Named, typically the name of the class which will log to it
• Associated with the bundle which creates the Logger
• Provides log level specific methods to avoid message formatting work if the log
level is not in effect
• ‘{}’ message placeholders as well as support for java.util.Formatter
placeholders: %s, %d, …

6. Log Levels
• Six log levels are defined
Log Level Description
AUDIT Always logged
ERROR Error occurred
WARN Non-blocking failure or unwanted event
INFO Normal operation
DEBUG Detailed output useful for developers
TRACE High volume output for tracing purposes

7. Logger Factory
• OSGi service which is used to obtain Logger objects
• LogService now extends LoggerFactory
• LogService members deprecated
• Logger names form a hierarchy using Java package.Class naming style
• Special Logger named ROOT is the ancestor of all Loggers
• Logger name hierarchy is used to manage Logger configuration

8. Logger Configuration
• The Logger Admin service is used to
configure Logger Context objects
• Each bundle can have a Logger Context
object which can control the Log Levels for
the Logger objects of that bundle
• Map<String,LogLevel>
com.foo.Bar=INFO
com.foo=DEBUG
com=INFO
ROOT=ERROR

9. Logger Configuration
• There is also an unnamed “root” Logger Context from which all named Logger
Contexts inherit configuration
• Most people will just need to configure this Logger Context
• Configuration Admin can be used to configure LoggerContexts using PIDs starting
with “org.osgi.service.log.admin”
• PID “org.osgi.service.log.admin” configures the “root” Logger Context
• PID “org.osgi.service.log.admin|<bundle-symbolic-name>”
configures the Logger Context for the bundle

10. Where does logged information go?
• The OSGi Log Service specification does not say anything about writing
logged information to the console, files, etc.
• This would be done by using the Log Reader Service or the new Log Stream
Provider service
• So the Log Service specification is “broker” between bundles wanting to log
and bundles wanting to consume the logged information
• Such a consuming bundle can be thought of like Appenders in other logging
back ends such as Logback

11. Log Stream Provider
• Logged information can be thought of as an
ongoing stream of log entries that never
ends having asynchronous arrival
• This fits perfectly with the new OSGi Push
Stream specification!
• So we added a new Log Stream Provider
service which can be uses by a consuming
bundle to receive and process log entries
as a push stream
• The push stream can be primed with the
log history, if any, which is put in the stream
ahead of any new entries

12. Log Reader Service still remains
• Even though the design of the Log Reader Service predates the advent of the
whiteboard pattern, it remains supported in the specification
• Existing code may use it
• This also allows the Log Stream Provider implementation to be separate from
the Log Service implementation
• Which is handy for Equinox which implements the Log Service specification
in the framework and was not quite ready to require Java 8 (which Push
Stream requires)

13. Log Entry expanded to hold new information
• Logged information is packaged and delivered as a Log Entry object
• With the new support for named Loggers, we enhanced Log Entry to provide
more information
• name of the Logger
• a sequence number which orders log entries
• thread and stack trace information about the logging call site

14. Declarative Services support for using Loggers
• DS will support creating and injecting a Logger for the component
implementation class
• If the reference is to the Logger Factory service and the injection target is
Logger for Formatting Logger, then SCR must use the Logger Factory to
create a Logger which is then injected
@Component
public class ExampleImpl implements Example {
@Reference(service=LoggerFactory.class)
// LoggerFactory.getLogger(ExampleImpl.class)
private Logger logger;
@Activate
void activate() { logger.info("initialized"); }
}

15. But lots of projects use SLF4J?

16. Lots of projects use SLF4J as their logging API
• When you build an OSGi system, you will probably use a number of bundles
from open source projects that log using SLF4J
• And you will probably use a number of OSGi specification implementations
which log using OSGi Log Service
• So now you have a mix of bundles using different logging APIs and you need
to the logging combined into a single stream
• How to make the twain meet?

17. Choices
• We can either send the information logged to the OSGi Log Service to the
SLF4J logging backend (e.g. Logback)
• This means all the logged information is controlled by SLF4J logging
backend configuration
• Or we can send the information logged to the SLF4J API to the OSGi Log
Service
• This means all the logged information is controlled by the Logger
configuration in the Logger Admin service

18. slf4j.osgi
• SLF4J implementation which maps using the SLF4J API, which is a static API,
onto the OSGi Log Service
• CODE!

19. osgi.logback
• Component which consumes Log Entries and sends them to the Logback
backend
• CODE!

20. osgi.logback
• See the Apache Felix Logback project for a more capable implementation of
this choice
• http://felix.apache.org/documentation/subprojects/apache-felix-logback.html

21. Some notes…
• We are using Equinox framework with its in-built Log Service implementation
• So we need configure a log history so our bundles can find past logged
information when they start
• And we need to configure the Loggers so we have some logged information to
process!
• -runproperties:
equinox.log.history.max=1000,
org.osgi.service.log.admin.loglevel=DEBUG

22. Thanks!

23. Evaluate the Sessions
-1 0 +1
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