Listeners are the last step in logging. Listeners receive formatted message and a LogMeta (see below) to construct the final message. The final message is then handed off to the underlying logging framework. (See integration.)

A LogMeta is a small object that contains meta-data for the final log statement. At minimum this contains the expected Date (timestamp), and a Severity. LogMeta instances are created by LogMetaProvider objects which are used by Logger implementations.

Listeners most commonly follow the adapter pattern, usually sending the log message to the underlying logger framework. Bug Listeners can take other forms, such as JLogViewer.

Listener implementations may use a MessageFormatter to simplify creating the final message.

There are also some Listener implementations included in that package. These can be used to help build a custom Listener, or to write to some non-standard storage system.

• ConsoleListener - Writes to System.out
• StreamListener - Writes to a provided OutputStream
• WriterListener - Writes to a provided Writer
• JavaLoggerListener - Adapter for the Java Logging API

One useful example would be to use a StreamListener with a CipherOutputStream to create a log that may contain confidential information.

Listeners can also be used for more than just writing directly to a file or stream. For example, Listeners can be used to control the flow of log messages. The FailSafeListener is designed to handle unreliable Listeners. The FailSafeListener takes two Listeners, a primary and a backup Listener. If the primary Listener throws an exception, the log message is logged to the backup Listener.

Below are implementations for well-know logging frameworks. These implementations are not part of the core package because they depend on external libraries -- the logging framework in question.

The code here is intended to be copied directly into your code and customized to your needs. This code is also part of the sources package if you wish to download all of them at once.

• Log4jListener for log4j
• CommonsLoggingListener for Apache Commons-Logging
• Slf4jListener for SLF4J

If there is no Listener implementation for your preferred logger framework, implementing one is very simple.

Logahawk also provides a LoggerRegistry interface that mimics the behavior of LoggerFactory (and similarly named classes) found in log4j, SLF4J, Commons Logging, and other logger frameworks. These classes provide convenient methods for finding the appropriate Logger for a given Class.

There is one significant difference between of Logahawk's LoggerRegistries and other logger framework's equivalents -- LoggerRegistry is an interface and all members are non-static. That means you have the option of creating multiple LoggerRegistry instances, and configuring & sharing them in application specific ways. This allows for the possibility of creating Loggers at run-time, or Loggers whose formatting is customized to the requesting object.

The use of a LoggerRegistry is optional -- Logahawk does not require their usage. (For simple applications they may be overkill.)

There are some LoggerRegistry implementations included with Logahawk:

• HashLoggerRegistry - Loggers registed by keys (like a Map)
• HierarchicalLoggerRegistry - Loggers accessed by package and class name

Below are LoggerRegistry implementations for well-know logging frameworks. These implementations are not part of the core package because they depend on external libraries -- the logging framework in question.

The code here is intended to be copied directly into your code and customized to your needs. This code is also part of the sources package if you wish to download all of them at once.

• Log4jLoggerRegistry for log4j
• CommonLoggerRegistry for Apache Commons-Logging
• Slf4jLoggerRegistry for SLF4J

If there is no LoggerRegistry implementation for your preferred loggerframework, please use the code above to build your own implementation.