The OSGi specification defines a log service (Section 101.1) in the Service Compendium. Like most logging facilities, the log service allows you to specify a message, exception, log level and service reference to be logged. The log service can be acquired using typical OSGi service acquisition mechanisms like a ServiceTracker (see snippet below). However, I highly recommend that you look at using Declarative Services (see Section 112) if you’re working with OSGi services.

public class Activator implements BundleActivator {

	private ServiceTracker logServiceTracker;
	private LogService logService;

	public void start(BundleContext context) throws Exception {
		// create a tracker and track the log service
		logServiceTracker =
			new ServiceTracker(context, LogService.class.getName(), null);
		logServiceTracker.open();

		// grab the service
		logService = (LogService) logServiceTracker.getService();

		if(logService != null)
			logService.log(LogService.LOG_INFO, "Yee ha, I'm logging!");
	}

	public void stop(BundleContext context) throws Exception {
		if(logService != null)
			logService.log(LogService.LOG_INFO, "Yee ha, I'm logging!");

		// close the service tracker
		logServiceTracker.close();
		logServiceTracker = null;
	}
}

Once acquired, you can log messages using the various utility methods LogService:

public interface LogService {
	public static final int	LOG_ERROR	= 1;
	public static final int	LOG_WARNING	= 2;
	public static final int	LOG_INFO	= 3;
	public static final int	LOG_DEBUG	= 4;
	public void log(int level, String message);
	public void log(int level, String message, Throwable exception);
	public void log(ServiceReference sr, int level, String message);
	public void log(ServiceReference sr, int level, String message,
			Throwable exception);
}

The messages are logged as LogEntry objects.

public interface LogEntry {
     public Bundle getBundle();
     public ServiceReference getServiceReference();
     public int getLevel();
     public String getMessage();
     public Throwable getException();
     public long getTime();
}

It’s that simple! The problem with this simplicity is that this type of logging may not be enough for certain projects. The Equinox project at Eclipse has recently graduated an extended log service. The extended log service is similar to the log service we discussed, but there are a few useful additions like named loggers:

public interface ExtendedLogService extends LogService, Logger {
	public Logger getLogger(String loggerName);
	public Logger getLogger(Bundle bundle, String loggerName);
}
public interface Logger {
	public void log(int level, String message);
	public void log(int level, String message, Throwable exception);
	public void log(ServiceReference sr, int level, String message);
	public void log(ServiceReference sr, int level, String message, Throwable exception);
	public void log(Object context, int level, String message);
	public void log(Object context, int level, String message, Throwable exception);
	public boolean isLoggable(int level);
	public String getName();
}

There is also an extended log entry that has information like a logger name and thread-related information:

public interface ExtendedLogEntry extends LogEntry {
	String getLoggerName();
	Object getContext();
	long getThreadId();
	String getThreadName();
	long getSequenceNumber();
}

That’s it!

For those working in RCP or OSGi projects, how are you guys handling logging? Are you using the LogService, Pax Logging or something more exotic?

Related posts:

• Tip: Custom OSGi (Equinox) Console Commands
• Using OSGi services to single-source an RCP and RAP Application
• Lightweight OSGi Applications using RAP’s Widget Toolkit
• REST, the OSGi and ECF way
• Dining philosophers using OSGi (Equinox) data service locations

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Tags: equinox, OSGi