Creating a Custom Logback Appender

1. Introduction

In this article, we’ll explore creating a custom Logback appender. If you are looking for the introduction to logging in Java, please take a look at this article.

Logback ships with many built-in appenders that write to standard out, file system, or database. The beauty of this framework’s architecture is its modularity, which means we can easily customize it.

In this tutorial, we’ll focus on logback-classic, which requires the following Maven dependency:

<dependency>
    <groupId>ch.qos.logback</groupId>
    <artifactId>logback-classic</artifactId>
    <version>1.5.18</version>
</dependency>

The latest version of this dependency is available on Maven Central.

2. Base Logback Appenders

Logback provides base classes we can extend to create a custom appender.

Appender is the generic interface that all appenders must implement. The generic type is either ILoggingEvent or AccessEvent, depending on if we’re using logback-classic or logback-access, respectively.

Our custom appender should extend either AppenderBase or UnsynchronizedAppenderBase, which both implement Appender and handle functions such as filters and status messages.

AppenderBase is thread-safe; UnsynchronizedAppenderBase subclasses are responsible for managing their thread safety.

Just as the ConsoleAppender and the FileAppender both extend OutputStreamAppender and call the super method setOutputStream(), the custom appender should subclass OutputStreamAppender if it is writing to an OutputStream.

3. Custom Appender

For our custom example, we’ll create a toy appender named MapAppender. This appender will insert all logging events into a ConcurrentHashMap, with the timestamp for the key. To begin, we’ll subclass AppenderBase and use ILoggingEvent as the generic type:

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.