Users Guide : Jakarta Commons Logging Commons Documentation Team

The Jakarta Commons Logging (JCL) provides a Log interface that is intended to be both light-weight and independent of numerous 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.

Familiarity with high-level details of various Logging implementations is presumed.

The Jakarta Commons Logging provides a Log interface with thin-wrapper implementations for other logging tools, including Log4J, Avalon LogKit, and JDK 1.4. The interface maps closely to Log4J and LogKit.

As far as possible, Commons-Logging tries to be as unobtrusive as possible. In most cases, including the (full) commons-logging.jar in the classpath should result in Commons-Logging 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 Commons-Logging: 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):

  1. 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).
  2. 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).
  3. If the Log4J logging system is available in the application class path, use the corresponding wrapper class (Log4JLogger).
  4. If the application is executing on a JDK 1.4 system, use the corresponding wrapper class (Jdk14Logger).
  5. Fall back to the default simple logging wrapper (SimpleLog).

Consult the Commons-Logging javadocs for details of the various Log implementations that ship with the component. (The discovery process is also covered in more detail there.)

To use the JCL SPI from a Java class, include the following import statements:

Note that some components using commons-logging 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:

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 programming/planning 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:

    Log log = LogFactory.getLog("org.apache.component.enterprise");

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.

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.

The Jakarta Commons Logging SPI uses the implementation of the org.apache.commons.logging.Log interface specified by the system property org.apache.commons.logging.Log. If the property is not specified or the class is not available then the JCL provides access to a default logging toolkit by searching the CLASSPATH for the following toolkits, in order of preference:

  • Log4J
  • JDK 1.4
  • JCL SimpleLog

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.

The JCL LogFactory implementation must assume responsibility for either connecting/disconnecting to a logging toolkit, or instantiating/initializing/destroying a logging toolkit.

The JCL Log interface doesn't specify any exceptions to be handled, the implementation must catch any exceptions.

The JCL Log and LogFactory implementations must ensure that any synchronization required by the logging toolkit is met.

The Jakarta Commons Logging (JCL) SPI can be configured to use different logging toolkits.

Configuration of the behavior of the JCL ultimately depends upon the logging toolkit being used. The JCL SPI uses Log4J by default if it is available (in the CLASSPATH).

As Log4J is the default logger, a few details are presented herein to get the developer/integrator going.

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).

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.