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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 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 informations. It's possible to set a log level (LOG_DEBUG, LOG_INFO, LOG_WARNING or LOG_ERROR), a message, an Exception information, including log level, message, exception, and the ServiceReference that generated the log.
• org.osgi.service.log.LogReaderService: service interface to add and remove {{LogListener}}s 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 , which is notified about every new log entryentries.

Accessing the

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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 snippetIf 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
            // Use the log
		}
	...
        }
    }

	    //..

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

Using the

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log service

The Log Service LogService interface provides four methods to define logsfor logging:

public interface LogService
{
	    //..

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

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

	    //log Log a message specifying the ServiceReference that generatesgenerated 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 already available bundle 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) 
	{
		//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 method, which is called every time an installed bundle submits a log entry .The second and final step is to register the newly created LogListener is created in the associated logging service. A LogListener implementation must be registered with the LogReaderService so it can start receving 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());
}

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