What is an Appender?

Logback delegates the task of writing a logging event to components called appenders. Appenders must implement the ch.qos.logback.core.Appender interface. The salient methods of this interface are summarized below:

package ch.qos.logback.core;

import ch.qos.logback.core.spi.ContextAware;
import ch.qos.logback.core.spi.FilterAttachable;
import ch.qos.logback.core.spi.LifeCycle;


public interface Appender<E> extends LifeCycle, ContextAware, FilterAttachable {

  public String getName();
  public void setName(String name);
  void doAppend(E event);

}

Most of the methods in the Appender interface are setters and getters. A notable exception is the doAppend() method taking an object instance of type E as its only parameter. The actual type of E will vary depending on the logback module. Within the logback-classic module E would be of type ILoggingEvent and within the logback-access module it would be of type AccessEvent. The doAppend() method is perhaps the most important in the logback framework. It is responsible for outputting the logging events in a suitable format to the appropriate output device.

Appenders are named entities. This ensures that they can be referenced by name, a quality confirmed to be instrumental in configuration scripts. The Appender interface extends the FilterAttachable interface. It follows that one or more filters can be attached to an appender instance. Filters are discussed in detail in a subsequent chapter.

Appenders are ultimately responsible for outputting logging events. However, they may delegate the actual formatting of the event to a Layout or to an Encoder object. Each layout/encoder is associated with one and only one appender, referred to as the owning appender. Some appenders have a built-in or fixed event format. Consequently, they do not require nor have a layout/encoder. For example, the SocketAppender simply serializes logging events before transmitting them over the wire.