Commons Resources (Unofficial)
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The Jakarta Commons Logging (JCL) provides a Log interface that
is intended to be both light-weight and an independent abstraction of other logging toolkits.
It provides the middleware/tooling developer with a simple
logging abstraction, that allows the user (application developer) to plug in
a specific logging implementation.
JCL provides thin-wrapper Log implementations for
other logging tools, including
Log4J
,
Avalon LogKit
,
the Avalon Framework's logging infrastructure,
JDK 1.4, and an implementation of JDK 1.4 logging APIs (JSR-47) for pre-1.4
systems.
The interface maps closely to Log4J and LogKit.
Familiarity with high-level details of the relevant Logging implementations is presumed.
As far as possible, JCL tries to be as unobtrusive as possible.
In most cases, including the (full) commons-logging.jar in the classpath
should result in JCL configuring itself in a reasonable manner.
There's a good chance that it'll guess your preferred logging system and you won't
need to do any configuration at all!
There are two base abstractions used by JCL: Log
(the basic logger) and LogFactory (which knows how to create Log
instances). Using LogFactory implementations other than the default is a
subject for advanced users only, so let's concentrate on configuring the default
implementation.
The default LogFactory implementation uses the following discovery process
to determine what type of Log implementation it should use
(the process terminates when the first positive match - in order - is found):
-
Look for a configuration attribute of this factory named
org.apache.commons.logging.Log (for backwards
compatibility to pre-1.0 versions of this API, an attribute
org.apache.commons.logging.log is also consulted).
-
Look for a system property named
org.apache.commons.logging.Log (for backwards
compatibility to pre-1.0 versions of this API, a system property
org.apache.commons.logging.log is also consulted).
-
If the Log4J logging system is available in the application
class path, use the corresponding wrapper class
(
Log4JLogger
).
-
If the application is executing on a JDK 1.4 system, use
the corresponding wrapper class
(
Jdk14Logger
).
-
Fall back to the default simple logging wrapper
(
SimpleLog
).
Consult the JCL javadocs for details of the various Log
implementations that ship with the component. (The discovery process is also covered in more
detail there.)
The JCL SPI
can be configured to use different logging toolkits (see
above
).
JCL provides only a bridge for writing log messages. It does not (and will not) support any
sort of configuration API for the underlying logging system.
Configuration of the behavior of the JCL ultimately depends upon the
logging toolkit being used. Please consult the documentation for the chosen logging system.
Log4J is a very commonly used logging implementation (as well as being the JCL primary default),
so a few details are presented herein to get the developer/integrator going.
Please see the
Log4J Home
for more details
on Log4J and it's configuration.
Configure Log4J using system properties and/or a properties file:
-
log4j.configuration=log4j.properties
Use this system property to specify the name of a Log4J configuration file.
If not specified, the default configuration file is log4j.properties.
-
log4j.rootCategory=priority [, appender]*
Set the default (root) logger priority.
-
log4j.logger.logger.name=priority
Set the priority for the named logger
and all loggers hierarchically lower than, or below, the
named logger.
logger.name corresponds to the parameter of
LogFactory.getLog(logger.name) ,
used to create the logger instance. Priorities are:
DEBUG ,
INFO ,
WARN ,
ERROR ,
or FATAL .
Log4J understands hierarchical names,
enabling control by package or high-level qualifiers:
log4j.logger.org.apache.component=DEBUG
will enable debug messages for all classes in both
org.apache.component
and
org.apache.component.sub .
Likewise, setting
log4j.logger.org.apache.component=DEBUG
will enable debug message for all 'component' classes,
but not for other Jakarta projects.
-
log4j.appender.appender.Threshold=priority
Log4J appenders correspond to different output devices:
console, files, sockets, and others.
If appender's threshold
is less than or equal to the message priority then
the message is written by that appender.
This allows different levels of detail to be appear
at different log destinations.
For example: one can capture DEBUG (and higher) level information in a logfile,
while limiting console output to INFO (and higher).
To use the JCL SPI from a Java class,
include the following import statements:
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
Note that some components using JCL may
either extend Log,
or provide a component-specific LogFactory implementation.
Review the component documentation for guidelines
on how commons-logging should be used in such components.
For each class definition, declare and initialize a
log attribute as follows:
Messages are logged to a logger, such as log
by invoking a method corresponding to priority.
The org.apache.commons.logging.Log interface defines the
following methods for use
in writing log/trace messages to the log:
log.fatal(Object message);
log.fatal(Object message, Throwable t);
log.error(Object message);
log.error(Object message, Throwable t);
log.warn(Object message);
log.warn(Object message, Throwable t);
log.info(Object message);
log.info(Object message, Throwable t);
log.debug(Object message);
log.debug(Object message, Throwable t);
log.trace(Object message);
log.trace(Object message, Throwable t);
Semantics for these methods are such that it is expected
that the severity, from highest to lowest, of messages is ordered as above.
In addition to the logging methods, the following are provided for code guards:
Best practices for JCL are presented in two categories:
General and Enterprise.
The general principles are fairly clear.Enterprise practices are a bit more involved
and it is not always as clear as to why they are important.
Enterprise best-practice principles apply to middleware components
and tooling that is expected to execute in an "Enterprise" level
environment.
These issues relate to Logging as Internationalization,
and fault detection.
Enterprise requires more effort and planning, but are strongly encouraged (if not required)
in production level systems. Different corporate enterprises/environments have different
requirements, so being flexible always helps.
Code guards are typically used to guard code that
only needs to execute in support of logging,
that otherwise introduces undesirable runtime overhead
in the general case (logging disabled).
Examples are multiple parameters, or expressions (i.e. string + " more") for parameters.
Use the guard methods of the form log.is<Priority>() to verify
that logging should be performed, before incurring the overhead of the logging method call.
Yes, the logging methods will perform the same check, but only after resolving parameters.
It is important to ensure that log message are
appropriate in content and severity.
The following guidelines are suggested:
-
fatal - Severe errors that cause premature termination.
Expect these to be immediately visible on a status console.
See also
Internationalization
.
-
error - Other runtime errors or unexpected conditions.
Expect these to be immediately visible on a status console.
See also
Internationalization
.
-
warn - Use of deprecated APIs, poor use of API, 'almost' errors,
other runtime situations that are undesirable or unexpected, but not
necessarily "wrong".
Expect these to be immediately visible on a status console.
See also
Internationalization
.
-
info - Interesting runtime events (startup/shutdown).
Expect these to be immediately visible on a console,
so be conservative and keep to a minimum.
See also
Internationalization
.
-
debug - detailed information on the flow through the system.
Expect these to be written to logs only.
-
trace - more detailed information.
Expect these to be written to logs only.
By default the message priority should be no lower than info.
That is, by default debug message should not be seen in the logs.
The general rule in dealing with exceptions is to assume that
the user (developer using a tooling/middleware API) isn't going
to follow the rules.
Since any problems that result are going to be assigned to you,
it's in your best interest to be prepared with the proactive
tools necessary to demonstrate that your component works correctly,
or at worst that the problem can be analyzed from your logs.
For this discussion, we must make a distinction between different types of exceptions
based on what kind of boundaries they cross:
-
External Boundaries - Expected Exceptions.
This classification includes exceptions such as
FileNotFoundException
that cross API/SPI boundaries, and are exposed to the user of a component/toolkit.
These are listed in the 'throws' clause of a method signature.
Appropriate handling of these exceptions depends upon the type
of code you are developing.
API's for utility functions and tools should log these at the debug level,
if they are caught at all by internal code.
For higher level frameworks and middleware components,
these exceptions should be caught immediatly prior to crossing
the API/SPI interface back to user code-space,
logged with full stack trace at info level,
and rethrown.
The assures that the log contains a record of the root cause for
future analysis in the event that the exception is not caught and resolved
as expected by the user's code.
-
External Boundaries - Unexpected Exceptions.
This classification includes exceptions such as
NullPointerException
that cross API/SPI boundaries, and are exposed to the user of a component/toolkit.
These are runtime exceptions/error that are NOT
listed in the 'throws' clause of a method signature.
Appropriate handling of these exceptions depends upon the type
of code you are developing.
API's for utility functions and tools should log these at the debug level,
if they are caught at all.
For higher level frameworks and middleware components,
these exceptions should be caught immediatly prior to crossing
the API/SPI interface back to user code-space,
logged with full stack trace at info level,
and rethrown/wrapped as ComponentInternalError .
The assures that the log contains a record of the root cause for
future analysis in the event that the exception is not caught and
logged/reported as expected by the user's code.
-
Internal Boundaries.
Exceptions that occur internally and are resolved internally.
These should be logged when caught as debug or info messages,
at the programmer's discretion.
-
Significant Internal Boundaries.
This typically only applies to middleware components
that span networks or runtime processes.
Exceptions that cross over significant internal component boundaries, such as networks.
These should be logged when caught as info messages.
Do not assume that such a (process/network) boundary will deliver exceptions to the 'other side'.
You want to have exception/problem information available for
first-pass problem determination in a production level
enterprise application without turning on debug
as a default log level. There is simply too much information
in debug to be appropriate for day-to-day operations.
If more control is desired for the level of detail of these
'enterprise' exceptions, then consider creating a special
logger just for these exceptions:
This allows the 'enterprise' level information to be turned on/off explicitly
by most logger implementations.
NLS internationalization involves looking up messages from
a message file by a message key, and using that message for logging.
There are various tools in Java, and provided by other components,
for working with NLS messages.
NLS enabled components are particularly appreciated
(that's an open-source-correct term for 'required by corporate end-users' :-)
for tooling and middleware components.
NLS internationalization SHOULD be strongly considered for used for
fatal, error, warn, and info messages.
It is generally considered optional for debug and trace messages.
Perhaps more direct support for internationalizing log messages
can be introduced in a future or alternate version of the Log interface.
JCL is designed to encourage extensions to be created that add functionality.
Typically, extensions to JCL fall into two categories:
- new
Log implementations that provide new bridges to logging systems
-
new
LogFactory implementations that provide alternative discovery strategies
When creating new implementations for Log and LogFactory ,
it is important to understand the implied contract between the factory
and the log implementations:
- Life cycle
The JCL LogFactory implementation must assume responsibility for
either connecting/disconnecting to a logging toolkit,
or instantiating/initializing/destroying a logging toolkit.
- Exception handling
The JCL Log interface doesn't specify any exceptions to be handled,
the implementation must catch any exceptions.
- Multiple threads
The JCL Log and LogFactory implementations must ensure
that any synchronization required by the logging toolkit
is met.
The minimum requirement to integrate with another logger
is to provide an implementation of the
org.apache.commons.logging.Log interface.
In addition, an implementation of the
org.apache.commons.logging.LogFactory interface
can be provided to meet
specific requirements for connecting to, or instantiating, a logger.
The default LogFactory provided by JCL
can be configured to instantiate a specific implementation of the
org.apache.commons.logging.Log interface
by setting the property of the same name (org.apache.commons.logging.Log ).
This property can be specified as a system property,
or in the commons-logging.properties file,
which must exist in the CLASSPATH.
If desired, the default implementation of the
org.apache.commons.logging.LogFactory
interface can be overridden,
allowing the JDK 1.3 Service Provider discovery process
to locate and create a LogFactory specific to the needs of the application.
Review the Javadoc for the LogFactoryImpl.java
for details.
JCL doesn't (and cannot) impose any requirement on thread safety on the underlying implementation
and thus its SPI contract doesn't guarantee thread safety.
However, JCL can be safely used in a multi-threaded environment
as long as the underlying implementation is thread-safe.
It would be very unusual for a logging system to be thread unsafe.
Certainly, JCL is thread safe when used with the distributed Log implementations.
Upon application startup (especially in a container environment), an exception is thrown
with message 'xxxLogger does not implement Log'! What's the cause and how can
I fix this problem?
This almost always a classloader issue. Log has been loaded by a different
classloader from the logging implementation. Please ensure that:
-
all the logging classes (both Log and the logging implementations)
are deployed by the same classloader
-
there is only copy of the classes to be found within the classloader hierarchy.
In application container environments this means ensuring that if the classes
are found in a parent classloader, they are not also present in the leaf classloader
associated with the application.
So, if the jar is deployed within the root classloader of the container then it
should be removed from the application's library.
The configuration supported by JCL is limited to choosing the underlying logging system.
JCL does not (and will never) support changing the configuration of the wrapped logging system.
Please use the mechanisms provided by the underlying logging system.
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