Apache Felix Log Service

The OSGi Service Compendium specification defines a general purpose Log Service for the OSGi Platform.
It is a very simple specification that doesn't provide all the functionalities commonly available in enterprise-level logging tools, but its extensible service model can be used to build fairly sophisticated logging solutions.

The Log Service specification defines four main entities:

• org.osgi.service.log.LogService: service to log informations. It's possible to set a log level (LOG_DEBUG, LOG_INFO, LOG_WARNING or LOG_ERROR), a message, an Exception and the ServiceReference that generated the log
• org.osgi.service.log.LogReaderService: service to add and remove LogListener and to retrieve recent log entries
• org.osgi.service.log.LogEntry: interface defining a log entry
• org.osgi.service.log.LogListener: interface defining a listener for log entries. A LogListener is notified about every new log entry

Accessing the LogService

If the OSGi platform provides a Log Service, getting a reference to it is as simple as doing a normal OSGi service lookup:

public class Activator implements BundleActivator
{
	public void start(BundleContext context) throws Exception 
	{	
		ServiceReference ref = context.getServiceReference(LogService.class.getName());
		if (ref!=null)
		{
			LogService log = (LogService) context.getService(ref);
			
			//use the log
		}
	}

	//..

(Of course it's possible, and advisable, to use more sophisticated service acquisition mechanisms like a Service Tracker, Dynamic Services or iPOJO)

Using the LogService

The Log Service provides four methods to define logs:

public interface LogService
{
	//..

	// log a message specifying a log level
	public log(int level, java.lang.String message)  

	//log an exception
	public log(int level, java.lang.String message, java.lang.Throwable exception)  

	//log a message specifying the ServiceReference that generates it
	public log(ServiceReference sr, int level, java.lang.String message)  

	public log(ServiceReference sr, int level, java.lang.String message, java.lang.Throwable exception)  
}	

Log levels are defined in the same interface:

• LogService.LOG_DEBUG
• LogService.LOG_INFO
• LogService.LOG_WARNING
• LogService.LOG_ERROR

Retrieving log entries

The LogReaderService provides a getLog() method to retrieve an Enumeration of the latest log entries.

ServiceReference ref = context.getServiceReference(LogReaderService.class.getName());
if (ref!=null)
{
	LogReaderService reader = (LogReaderService) context.getService(ref);	
	Enumeration<LogEntry> latestLogs = reader.getLog();
}

Creating and registering a LogListener

The Log Service specification doesn't define any particular entity to store, display or write log entries; it's up to the developer to implement this functionality, or to choose an already available bundle capable of doing that.

To create such a bundle, the first step is to create an implementation of the LogListener interface:

public class LogWriter implements LogListener
{
	public void logged(LogEntry entry) 
	{
		//invoked by the OSGi platform every time a log entry is created
		System.out.println(entry.getMessage());
	}
}

The only method to implement is public void logged(LogEntry entry); this method will be called every time an installed bundle submits a log entry.

The second and final step is to register the newly created LogListener so it can start receving log entries:

ServiceReference ref = context.getServiceReference(LogReaderService.class.getName());
if (ref!=null)
{
	LogReaderService reader = (LogReaderService) context.getService(ref);
	reader.addLogListener(new LogWriter());
}

Setup of Apache Felix Log Service

The Apache Felix Log Service bundle doesn't have any specific dependency on Felix, so it can run on any OSGi container. For its configuration, it will use the following optional system properties: