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Integrating SLF4J and the new OSGi LogService 1.4 - BJ Hargrave (IBM)


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OSGi Community Event 2018 Presentation by BJ Hargrave (IBM)

Abstract: 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.

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Integrating SLF4J and the new OSGi LogService 1.4 - BJ Hargrave (IBM)

  1. 1. The new OSGi LogService 1.4 and integrating with SLF4J BJ Hargrave, IBM
  2. 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.
  3. 3. 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, …
  4. 4. 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
  5. 5. 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
  6. 6. 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=INFO ROOT=ERROR
  7. 7. 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
  8. 8. 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
  9. 9. 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
  10. 10. 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)
  11. 11. 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
  12. 12. 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() {"initialized"); } }
  13. 13. But lots of projects use SLF4J?
  14. 14. 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?
  15. 15. 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
  16. 16. slf4j.osgi • SLF4J implementation which maps using the SLF4J API, which is a static API, onto the OSGi Log Service • CODE!
  17. 17. osgi.logback • Component which consumes Log Entries and sends them to the Logback backend • CODE!
  18. 18. osgi.logback • See the Apache Felix Logback project for a more capable implementation of this choice •
  19. 19. 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: 
  20. 20. Thanks!
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