# Package org.apache.log4j.spi

- Interface Summary
  Interface | Description
  --- | ---
  Component | A common interface shared by log4j components.
  Decoder | Allow LoggingEvents to be reconstructed from a different format (usually XML).
  LoggerEventListener | Interface used to listen for Logger related events such as add/remove appender or changing levels.
  LoggerRepositoryEventListener | Interface used to listen for LoggerRepository related events such as startup, reset, and shutdown.
  LoggerRepositoryEx | A `LoggerRepository` is used to create and retrieve `Loggers`.
  Thresholdable | An interface that defines the required methods for supporting the setting and getting of a level threshold.

- Class Summary
  Class | Description
  --- | ---
  ComponentBase | Most log4j components derive from this class.
  ErrorItem | Used to store special log4j errors which cannot be logged using internal logging.
  Log4JULogger | An implementation of ULogger on org.apache.log4j.Logger.
  NOPULogger | A no operation (NOP) implementation of `ULogger`.
  SimpleULogger | A simple implementation that logs messages of level INFO or higher on the console (`System.out`).