At 03:19 PM 6/6/2004, Jacob Kjome wrote:
Ok, that brainfart was a bit embarrassing! Of course you are right and it works just fine. I've given the above advice before to others and yet I didn't think about using it myself. Ug!!!
Bah, your oversight just tells us that you are human. Besides, without it we would not be having this conversation.
The whole Monitor idea kind of came from this post: http://paulhammant.com/blog//000241.html
How is the monitor handed down to the reusable component? It looks to me like the logger initialization problem has been replaced by the monitor initialization problem. I don't see the gain... Anyone care to explain?
A single monitor is handed to, essentially, a container which then provides this component to all classes in the framework that require it. In each class that will use the monitor, I just provide a Monitor instance variable. I don't create the monitor in the class itself as is the normal Log4j usage. It it up to the user to configure a Monitor implementation (or get a default backed by either nothing or System.out) for the container and the container will provide it to any class that needs it. This way the user has full control over the components rather than the programmer defining components inside the class outside of the control of the user. It's all about IOC (Inversion of Control). You may have read about this stuff before in articles about Picocontainer ( http://www.picocontainer.org/ ) and Spring Framework ( http://www.springframework.org/ ), to name only two. Technically, we aren't using either of these right now, but the same principal applies.
While looking at picocontainer, I ran into a delightful article by Martin Fowler:
http://www.martinfowler.com/articles/injection.html
Besides the user having control over components, this completely removes any runtime dependency on Log4j (or any other logging framework) unless one explicitly configures the app to use the Log4jMonitor implementation. Unlike commons-logging where you are just trading a dependency on Log4j specifically for a dual dependency on commons-logging plus some logging implementation such as Log4j or JDK1.4 logging, there is no dependency whatsoever unless it is desired.
I still don't get it. While removing the dependency on log4j or c-l, this introduces a dependency on the monitor interface.
To make point clearer, assume for just one moment that all logging implementations were based on the following interface:
// The Universal Generic Logging Interface package org.apache.ugli;
public interface Logger {public boolean isTraceEnabled(); public void trace(Object msg); public void trace(Object msg, Throwable t);
public boolean isDebugEnabled(); public void debug(Object msg); public void debug(Object msg, Throwable t);
public boolean isInfoEnabled(); public void info(Object msg); public void info(Object msg, Throwable t);
public boolean isWarnEnabled(); public void warn(Object msg); public void warn(Object msg, Throwable t);
public boolean isErrorEnabled(); public void error(Object msg); public void error(Object msg, Throwable t);
}
Now assume that an implementation of the UGLI Logger were injected into client code. How is this different from injecting a monitor implementation into the client code? As I understand it, there is no difference, except that it puts more conceptual distance between the user and logging.
-- Ceki Gülcü
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