public class MapAppender extends AppenderBase<ILoggingEvent> {
    private ConcurrentMap<String, ILoggingEvent> eventMap 
      = new ConcurrentHashMap<>();
    @Override
    protected void append(ILoggingEvent event) {
        eventMap.put(String.valueOf(System.currentTimeMillis()), event);
    public Map<String, ILoggingEvent> getEventMap() {
        return eventMap;
Next, to enable the MapAppender to start receiving logging events, let’s add it as an appender in our configuration file logback.xml:
<configuration>
    <appender name="map" class="com.baeldung.logback.MapAppender"/>
    <root level="info">
        <appender-ref ref="map"/>
    </root>
</configuration>
4. Setting Properties
Logback uses JavaBeans introspection to analyze properties set on the appender. Our custom appender will need getter and setter methods to allow the introspector to find and set these properties.
Let’s add a property to MapAppender that gives the eventMap a prefix for its key:
public class MapAppender extends AppenderBase<ILoggingEvent> {
    //...
    private String prefix;
    @Override
    protected void append(ILoggingEvent event) {
        eventMap.put(prefix + System.currentTimeMillis(), event);
    public String getPrefix() {
        return prefix;
    public void setPrefix(String prefix) {
        this.prefix = prefix;
    //...
Next, add a property to our configuration to set this prefix:
<configuration debug="true">
    <appender name="map" class="com.baeldung.logback.MapAppender">
        <prefix>test</prefix>
    </appender>
    //...
</configuration>
5. Error Handling
To handle errors during the creation and configuration of our custom appender, we can use methods inherited from AppenderBase.

For example, when the prefix property is a null or an empty string, the MapAppender can call addError() and return early:
public class MapAppender extends AppenderBase<ILoggingEvent> {
    //...
    @Override
    protected void append(final ILoggingEvent event) {
        if (prefix == null || "".equals(prefix)) {
            addError("Prefix is not set for MapAppender.");
            return;
        eventMap.put(prefix + System.currentTimeMillis(), event);
    //...
When the debug flag is turned on in our configuration, we’ll see an error in the console that alerts us that the prefix property has not been set:
<configuration debug="true">
    //...
</configuration>
6. Conclusion
In this quick tutorial, we focused on how to implement our custom appender for Logback.
As usual, the example can be found over on Github.
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