Logging with Apache Commons Logging

Imports required

To use Jakarta commons logging from a Java class, please include the following import statements:
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

For each class definition, declare and initialize a log attribute as follows:
public class CLASS
{
   private Log log = LogFactory.getLog(getClass());
   ...

Using the API

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:
log.isFatalEnabled();
log.isErrorEnabled();
log.isWarnEnabled();
log.isInfoEnabled();
log.isDebugEnabled();
log.isTraceEnabled();

Code Example

Example of logging calls

/** log at appropriate level **/

// following logs at FATAL level
if (log.isFatalEnabled()) {
    log.fatal("Testing Fatal output");
}

// following logs at ERROR level
if (log.isErrorEnabled()) {
    log.error("Testing Error output");
}

// following logs at WARN level
if (log.isWarnEnabled()) {
    log.warn("Testing Warn output");
}

// following logs at INFO level
if (log.isInfoEnabled()) {
    log.info("Testing Info output");
}

// following logs at DEBUG level
if (log.isDebugEnabled()) {
    log.debug("Testing Debug output");
}

// following logs at TRACE level
if (log.isTraceEnabled()) {
    log.trace("Testing Trace output");
}

General - Message Priorities/Levels

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.
• ERROR - Other runtime errors or unexpected conditions. Expect these to be immediately visible on a status console.
• 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.
• INFO - Interesting runtime events (startup/shutdown). Expect these to be immediately visible on a console, so be conservative and keep to a minimum.
• DEBUG - Detailed information on flow of through the system. Expect these to be written to logs only.
• TRACE - More detailed information. Expect these to be written to logs only.

General - Default Message Priority/Level

By default the message priority should be no lower than info. That is, by default debug message should not be seen in the logs.

Why info level instead of debug?
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.