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From Ole Ersoy <ole.er...@gmail.com>
Subject [Logging] Tutorial Contribution
Date Wed, 25 Jun 2008 16:21:19 GMT
Hi,

Initially I was going to add to the WIKI FAQ, but all the Logging questions are so inter related
that I decided to write a tutorial instead.  I have not "Tested" this so there may be some
inaccuracies.  I'd appreciate feedback and will update the tutorial.  As soon as the dust
settles I'll post the end result on the WIKI.  Note that there's also a couple of FAQs added
at the end.

Here's the tutorial:



This document is provided "as is", without warranty of any kind, 
express or implied, including but not limited to the warranties of merchantability, 
fitness for a particular purpose and noninfringement. 
In no event shall the author be liable for any claim, 
damages or other liability, whether in an action of contract, 
tort or otherwise, arising from, out of or in connection with 
this document or the use or other dealings in this document.

PRIMER ON java.util.logging and JULI 

The goal of this primer is to demonstrate how to to log with 
java.util.logging (JULI) as a backdrop to the rest of the tutorial, in particular
the sections related to configuring Tomcat logging with the configuration file.
One may think it's wise to skip this section and proceed directly to the section
on the configuration file, but all the concepts talked about here and necessary
to understand the configuration file.  The examples used in this section
show programmatically how logging is configured.  The section on Tomcat's configuration
file discusses how to accomplish declaratively what is done programmatically in
this section.

The following concepts (Classes) are important:
- LogManager
- Loggers
- Handlers
- Levels
- Root Logger
- Root Handler

The official Tomcat logging documentation refers to 
these above concepts / classes extenstively.  

Lets start with Loggers.
What's the purpose of a Logger?  A Logger is what a developer uses to write a log statements

to the console, to a file, to the network, etc.  If you wanted to log something from
your web application's class CriticalComponent using java.util.logging you would 
first create a logger like this:

private String nameOfLogger = 'com.example.myapp.CriticalComponent';
or
private String nameOfLogger = CriticalComponent.class.getName();

private static Logger myLogger = Logger.getLogger(nameOfLogger);

Pay attention to how we defined the name of the Logger.  This is important
to the material explaining Tomcat's logging configuration.  You may also
want to think about why the myLogger is a static field (Hint myLogger is shared
among all instances of CriticalComponent).

Now you have a logger to create logging messages from you class CriticalComponent.
For example you could now try something like this:

public void wasssup() 
{
   myLogger.info("Ah Yeah Baby - that's the end of System.out.println");
}

If you ran this code with a deployed web application you would see this statement
on the console or in catalina.out out like:

INFO; 322125105255ms; 4407662;# 1; com.example.myapp.CriticalComponent; wasssup; Ah Yeah Baby
- that's the end of System.out.println

Notice that both the name of the Logger and the method that logged the message are 
mentioned in the log statement.  This is important to know when you want to alter 
the configuration of the Logger.  For example you might want to turn this logger off,
because it's not that useful.

What if you wanted the output to appear in a file and on the console?  For that you need to

define a 2 Handlers.  Create the two like this:

Handler fileHandler = new FileHandler("/var/log/tomcat/myapp.log");
Handler consoleHandler = new ConsoleHandler();
myLogger.addHandler(fileHandler);
myLogger.addHandler(consoleHandler);

Now myLogger will log to both the console and the file /var/log/tomcat/myapp.log.

So now we understand Loggers and Handlers, but we have not touched on Levels, 
Root Loggers, and Root Handlers yet.  What are those?

Lets start with a root Logger.  A root logger is a logger whose name is "".
What's the purpose of it.  Suppose you tried to do some logging with 
myLogger like this:

myLogger.finest("Sooooo Fine");

And nothing shows up in your log, but you know that this statement is being 
called.  What's going on?  The answer is that the JULI root logger's Level
is set to INFO by default (TRUE?).  The level INFO has a corresponding integer
assigned to it, which is 3 (TRUE? - Comment: In any case I'm just using it as an example
which fits with the rest of the tutorial...so the logic works out regardless).  
When myLogger attempts to log it first checks it's level.  If myLogger's level is greater

than the level intrinsic to the method doing the 
logging (finest), then the record will be logged.  In this case the logging method finest

corresponds to Level zero.  Zero is less than 3, hence the logger does not log the message.
My logger will only log messages with a level that is greater than or equal to 3.  So for

instance if myLogger.severe("Oohhh %#$@#$!!!") is called, it will get logged because the level
intrinsic to the method severe is greater than 3.  So you are following this, but wondering
how myLogger's level got set to 3 (INFO), since we never explicitly set it.  The answer
is that it comes the root Logger.  Now suppose you created logger named 'com.example.myapp'
and set it's level.  Would myLogger still get it's level from the root Logger.  It would not.
The reason is that the logger named 'com.example.myapp' is now a parent logger to myLogger,
and myLogger gets it's level from it instead.  How did the 'com.example.myapp' Logger
become a parent?
It's because of it's name 'com.example.myapp'.  If it were named 'org.charity.generous', it
would not be a parent logger.  Do you see the pattern (myLogger includes 'com.example.myapp'
in its name)?  

Now that we mulled that over you have an idea of what a level is as well.  The Level
determines what gets logged.  You can set the level directly on myLogger like this:

myLogger.setLevel(Level.WARNING);

In this case only messages with a level of WARNING or SEVERE will get logged by myLogger.

Now remember that once something gets logged by myLogger, it's handed over to myLogger's
Handlers.  But what if we never assigned any Handlers to myLogger.  How would it get 
it's Handler?

The answer is the root Handlers.  The root logger has root Handlers.  So if you don't 
explicitly define any Handlers for myLogger, it will use the Handlers attached to 
the root Logger.  Now if you did attach Handler(s) to the logger named 'com.example.myapp',
but not to myLogger, then myLogger would use the Handler(s) attached to 'com.example.myapp',
instead of the Handlers attached to the root Logger.  In other words myLogger uses the 
Handlers of the closest Logger ancestor, which in this case is either the 'com.example.myapp'

Logger or the root Logger.

So all of the above is important to understand in order to be able to configure Tomcat
logging with JULI.  Now we can talk about doing so via the configuration file.

CONFIGURING TOMCAT JULI

* Configuring Loggers

Suppose we wanted to configure myLogger via a logging.properties file, rather
than programmatically as we did earlier, and simply set it's Level to SEVERE.  
We would put the following in the configuration file:

-------------------------------------------------------
com.example.myapp.CriticalComponent.level = SEVERE
-------------------------------------------------------

That's how Loggers are configured.  We specified the name of the 
Logger followed by followed by a period followed by the name of the 
property we wanted to set, which is 'level' in this case.  If we had several
classes with corresponding loggers, and we did not want to set the 
logging level for each class individually, we could set the logging
level on the root Logger like this:
-------------------------------------------------------
.level = SEVERE
-------------------------------------------------------

This will set the level on all loggers to SEVERE, unless the Logger uses a non-default
level (Either programmatically or via configuration).

So if you wished to turn logging off all together simply configure the root 
logger like this and make sure that there are no child loggers that override this
setting:
.level = OFF

* Configuring Handlers

Suppose that we also wanted to add a Handler to the Logger named 
com.example.myapp.CriticalComponent.   

We could define our handler like this:	

handlers = org.apache.juli.FileHandler

And configure it like this:
org.apache.juli.FileHandler.level = WARNING
org.apache.juli.FileHandler.directory = /var/log/tomcat/
org.apache.juli.FileHandler.prefix = mywebapp

Then assign it to our Logger like this:

com.example.myapp.CriticalComponent.handlers = org.apache.juli.FileHandler

Now we've done the same thing declaratively in our logging.properties
file that we did programmatically earlier.

What if we wanted to make the Handler we just configured the root
Handler?  We could configure it as the root Handler like this:
.handlers = org.apache.juli.FileHandler.

Now all Tomcat Loggers that do not have a Handler assigned to them directly,
either programmatically or declaratively via the logging.properties file, 
will delegate to this Handler.

CONFIGURING CONTEXT LOGGERS
So you now had a look at the example logging.properties file
in the official documentation again, and you understand most of it.

Then you get to these lines:

org.apache.catalina.core.ContainerBase.[Catalina].[localhost].[/manager].level = INFO
org.apache.catalina.core.ContainerBase.[Catalina].[localhost].[/manager].handlers = \
   3manager.org.apache.juli.FileHandler
   
These don't follow the same syntax we've been using thus far to configure Loggers and Handlers.

These are for configuring Context loggers.  What's a context logger?  First you need to know
a little about the ServletContext.   

For a description of what a Servlet Context is see:
http://java.sun.com/j2ee/1.4/docs/api/javax/servlet/ServletContext.html

You'll notice specifically that it mentions that there is one ServletContext
per webapp, and that this ServletContext can be used to perform logging.

Looking down a little further in the ServletContext.html document
we see that the ServletContext has a log method that takes a String. 

If a developer working on the manager application were to use the ServletContext.log method,
without the corresponding configuration lines shown above, output would go
to standard out, and end up on the console or catalina.out.  However now that
the ServletContext Logger has been configured for the manager application, 
ServletContext.log("...") statements made within the /manager application will
end up in manager.dd.mm.yyy.log.



FAQ
-------------------------------------------------------------------------
How do I customize the location of the tomcat logging.properies file?
-------------------------------------------------------------------------
Set the following property when starting tomcat:

java.util.logging.config.file

Example:
-Djava.util.logging.config.file=/etc/tomcat/logging.properties

Note that this property is not set when running catalina.sh.

The reason for this is that the download provided by the Apache Tomcat project
knows where the file is already.  

Projects such as JPackage that repackage Tomcat for Linux typically
move the configuration to a directory dictated by the FHS standard
(http://www.pathname.com/fhs/), and therefore use the java.util.logging.config.file
property to set the location of the logging.properties file in the Tomcat
startup script. 

-------------------------------------------------------------------------
What role does commons-logging play in logging?
-------------------------------------------------------------------------
Tomcat wants to support multiple logging implementations, so it uses commons-logging.
In case that's unclear, think of it like this.  You are a Tomcat developer.  The car
you drive when logging is the commons-logging car.  The engine of that car is either
JULI or log4j.  Without one of these engines, the car goes no where.  However regardless
of whether you use JULI or log4j, the steering wheel, break, gas pedal, etc. are the same.


Related FAQ:
What role does JULI and log4j play in logging?


-------------------------------------------------------------------------
What role does JULI and log4j play in logging?
-------------------------------------------------------------------------
First see:
What role does commons-logging play in logging?

Note in addition that in your own applications you could log directly
with JULI or log4j.  But once you choose one, you can't easily switch
to the other later.  If you use commons-logging you can.


Ole

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