Add additional-field-mapper extension point as described in osiegmar#55

src/main/java/de/siegmar/logbackgelf/GelfAdditionalFieldMapper.java

@@ -0,0 +1,24 @@
+package de.siegmar.logbackgelf;
+
+import ch.qos.logback.classic.spi.ILoggingEvent;
+
+public interface GelfAdditionalFieldMapper {
+
+  KeyValue<String, Object> mapAdditionalField(ILoggingEvent event);
+
+  class KeyValue<K, V> {
+    final K key;
+    final V value;
+
+    public KeyValue(K key, V value) {
+      this.key = key;
+      this.value = value;
+    }
+
+    @Override
+    public String toString() {
+      return key + "=" + value;
+    }
+  }
+
+}

src/main/java/de/siegmar/logbackgelf/GelfEncoder.java

@@ -22,9 +22,11 @@
 import java.math.BigDecimal;
 import java.net.UnknownHostException;
 import java.nio.charset.StandardCharsets;
+import java.util.ArrayList;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.Iterator;
+import java.util.List;
 import java.util.Map;
 import java.util.Objects;
 import java.util.regex.Pattern;
@@ -126,6 +128,8 @@ public class GelfEncoder extends EncoderBase<ILoggingEvent> {
      */
     private Map<String, Object> staticFields = new HashMap<>();
 
+    private List<GelfAdditionalFieldMapper> additionalFieldMappers = new ArrayList<>();
+
     public String getOriginHost() {
         return originHost;
     }
@@ -283,6 +287,10 @@ private Object processValue(final String value) {
         return value;
     }
 
+    public void addAdditionalFieldMapper(GelfAdditionalFieldMapper additionalFieldMapper) {
+        additionalFieldMappers.add(additionalFieldMapper);
+    }
+
     @Override
     public void start() {
         if (originHost == null || originHost.trim().isEmpty()) {
@@ -321,6 +329,13 @@ public byte[] encode(final ILoggingEvent event) {
         final String shortMessage = shortPatternLayout.doLayout(event);
         final String fullMessage = fullPatternLayout.doLayout(event);
         final Map<String, Object> additionalFields = mapAdditionalFields(event);
+        for (GelfAdditionalFieldMapper additionalFieldMapper : additionalFieldMappers) {
+            final GelfAdditionalFieldMapper.KeyValue<String, Object> keyValue =
+                additionalFieldMapper.mapAdditionalField(event);
+            if (keyValue != null) {
+                additionalFields.put(keyValue.key, keyValue.value);
+            }
+        }
 
         final GelfMessage gelfMessage = new GelfMessage(originHost, shortMessage, fullMessage,
             event.getTimeStamp(), LevelToSyslogSeverity.convert(event), additionalFields);