Loggers
Loggers are used by applications and runtime components to capture message and trace events. When situations occur that are significant either due to a change in state (for example when a server has completed startup) or because a potential problem has been detected (such as a timeout waiting for a resource) a message will be written to the logs. Trace events are logged in debugging scenarios, where a developer needs a clear view of what is occurring in each component to understand what might be going wrong. Logged events are often the only events available when a problem is first detected, and are used during both problem recovery and problem resolution.
Loggers are organized hierarchically. Each logger can have zero or more child loggers.
Loggers can be associated with a resource bundle. If specified, the resource bundle is used by the logger to localize messages logged to it. If the resource bundle is not specified, a logger will use the same resource bundle as its parent.
Loggers can be configured with a level. If specified the level is compared by the logger to incoming events -- events less severe than the level set for the logger are ignored by the logger. If the level is not specified, a logger will take on the level used by its parent. The default level for loggers is Level.INFO.
Loggers can have zero or more attached handlers. If supplied, all events logged to the logger will be passed to the attached handlers (for more details see Log handlers). Handlers write events to output destinations such as log files or network sockets. When a logger has finished passing a logged event to all of the handlers attached to that logger, the logger then passes the event to the handlers attached to the parents of the logger. This process will stop if a parent logger has been configured not to use its parent handlers. Handlers in WAS are attached to the root logger. Set the useParentHandlers logger property to false to prevent the logger from writing events to handlers that are higher up in the hierarchy.
Loggers can have a Filter. If supplied, the filter is invoked for each incoming event to tell the logger whether or not to ignore it.
Applications interact directly with loggers to log events. To obtain or create a logger, a call is made to the Logger.getLogger method with a name for the logger. Typically, the logger name is either the package qualified class name or the name of the package that the logger is used by. The hierarchical logger namespace is automatically created by using the dots in the logger name. For example, the logger "com.ibm.websphere.ras" has a parent logger named "com.ibm.websphere", which has a parent named "com.ibm", and so on. The parent at the top of the hierarchy is referred to as the root logger. This root logger is created during initialization. The root logger is the parent of the logger "com".
Loggers are structured in a hierarchy. Every logger except the root logger has one parent. Each logger can also have 0 or more children. A logger inherits log handlers, resource bundle names, and event filtering settings from its parent in the hierarchy. The logger hierarchy is managed by the LogManager function.
Loggers create log records. A log record is the container object for the data of an event. This object is used by filters, handlers, and formatters in the logging infrastructure.
The logger provides several sets of methods for generating log messages. Some log methods take only a level and enough information to construct a message. Other, more complex logp (log precise) methods support the caller in passing class name and method name attributes in addition to the level and message information. The logrb (log with ResourceBundle) methods add the capability of specifying a ResourceBundle as well as the level, message information, class name and method name. Using methods such as severe, warning, fine, finer, and finest one can log a message at a particular level. For a complete list of methods, see the java.util.logging documentation at http://java.sun.com/j2se/.
See Also
Log handlers
Log levels
Log filters
Log formatters