Apache Felix Log

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 functionality 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 interface to log information, including log level, message, exception, and ServiceReference that generated the log.
• org.osgi.service.log.LogReaderService - service interface to add and remove LogListener instances 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, which is notified about new log entries.

Accessing the log service

To access a LogService instance it is necessary to look it up in the OSGi service registry as demonstrated in the following code snippet:

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...
        }
    }

    //..

It is possible, and advisable, to use more sophisticated service acquisition mechanisms like a Service Tracker, Dynamic Services or iPOJO.

Using the log service

The LogService interface provides four methods for logging:

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 generated it
    public log(ServiceReference sr, int level, java.lang.String message)  

    // Log a message specifying the ServiceReference and exception
    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. The following code snippets demonstrates how to retrieve it from the service registry and use it:

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 available implementation capable of doing that. To create such a bundle, the first step is to create an implementation of the LogListener interface. The following code shows a simple implementation that echoes the log message:

public class LogWriter implements LogListener
{
    // Invoked by the log service implementation for each log entry
    public void logged(LogEntry entry) 
    {
        System.out.println(entry.getMessage());
    }
}

The only method to implement is logged() method, which is called every time a log entry is created in the associated logging service. A LogListener implementation must be registered with the LogReaderService so it can start receiving log entries, as demonstrated in the following code snippet:

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: