--- GelfLayout.java.orig	2016-12-07 11:19:27.548000000 +0200
+++ GelfLayout.java	2016-12-07 11:39:10.984000000 +0200
@@ -1,4 +1,4 @@
-/*
+/*
  * Licensed to the Apache Software Foundation (ASF) under one or more
  * contributor license agreements. See the NOTICE file distributed with
  * this work for additional information regarding copyright ownership.
@@ -21,6 +21,8 @@
 import java.io.OutputStream;
 import java.io.PrintWriter;
 import java.io.StringWriter;
+import java.net.InetAddress;
+import java.net.UnknownHostException;
 import java.nio.charset.StandardCharsets;
 import java.util.Collections;
 import java.util.Map;
@@ -28,13 +30,17 @@
 import java.util.zip.GZIPOutputStream;
 
 import org.apache.logging.log4j.Level;
+import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.core.Layout;
 import org.apache.logging.log4j.core.LogEvent;
+import org.apache.logging.log4j.core.LoggerContext;
+import org.apache.logging.log4j.core.config.Configuration;
 import org.apache.logging.log4j.core.config.Node;
 import org.apache.logging.log4j.core.config.plugins.Plugin;
 import org.apache.logging.log4j.core.config.plugins.PluginAttribute;
 import org.apache.logging.log4j.core.config.plugins.PluginElement;
 import org.apache.logging.log4j.core.config.plugins.PluginFactory;
+import org.apache.logging.log4j.core.lookup.StrSubstitutor;
 import org.apache.logging.log4j.core.net.Severity;
 import org.apache.logging.log4j.core.util.JsonUtils;
 import org.apache.logging.log4j.core.util.KeyValuePair;
@@ -106,15 +112,17 @@
     private final CompressionType compressionType;
     private final String host;
     private final boolean includeStacktrace;
+    private final boolean includeThreadContext;
 
     public GelfLayout(final String host, final KeyValuePair[] additionalFields, final CompressionType compressionType,
-                      final int compressionThreshold, final boolean includeStacktrace) {
+                      final int compressionThreshold, final boolean includeStacktrace, final boolean includeThreadContext) {
         super(StandardCharsets.UTF_8);
-        this.host = host;
+        this.host = getHostName(host);
         this.additionalFields = additionalFields;
         this.compressionType = compressionType;
         this.compressionThreshold = compressionThreshold;
         this.includeStacktrace = includeStacktrace;
+        this.includeThreadContext = includeThreadContext;
     }
 
     @PluginFactory
@@ -127,9 +135,10 @@
             @PluginAttribute(value = "compressionThreshold",
                 defaultInt = COMPRESSION_THRESHOLD) final int compressionThreshold,
             @PluginAttribute(value = "includeStacktrace",
-                defaultBoolean = true) final boolean includeStacktrace) {
+                defaultBoolean = true) final boolean includeStacktrace,
+            @PluginAttribute(value = "includeThreadContext", defaultBoolean = true) boolean includeThreadContext) {
             // @formatter:on
-        return new GelfLayout(host, additionalFields, compressionType, compressionThreshold, includeStacktrace);
+        return new GelfLayout(host, additionalFields, compressionType, compressionThreshold, includeStacktrace, includeThreadContext);
     }
 
     @Override
@@ -184,9 +193,7 @@
     }
 
     private StringBuilder toText(final LogEvent event, final StringBuilder builder, final boolean gcFree) {
-        builder.append('{');
-        builder.append("\"version\":\"1.1\",");
-        builder.append("\"host\":\"");
+        builder.append("{\"version\":\"1.1\",\"host\":\"");
         JsonUtils.quoteAsString(toNullSafeString(host), builder);
         builder.append(QC);
         builder.append("\"timestamp\":").append(formatTimestamp(event.getTimeMillis())).append(C);
@@ -202,14 +209,12 @@
             builder.append(QC);
         }
 
-        for (final KeyValuePair additionalField : additionalFields) {
-            builder.append(QU);
-            JsonUtils.quoteAsString(additionalField.getKey(), builder);
-            builder.append("\":\"");
-            JsonUtils.quoteAsString(toNullSafeString(additionalField.getValue()), builder);
-            builder.append(QC);
+        if (additionalFields.length > 0) {
+            additionalFieldsToText(event, builder);
+        }
+        if (includeThreadContext) {
+            event.getContextData().forEach(WRITE_KEY_VALUES_INTO, builder);
         }
-        event.getContextData().forEach(WRITE_KEY_VALUES_INTO, builder);
         if (event.getThrown() != null) {
             builder.append("\"full_message\":\"");
             if (includeStacktrace) {
@@ -240,6 +245,23 @@
         return builder;
     }
 
+    private void additionalFieldsToText(final LogEvent event, final StringBuilder builder) {
+        // somehow the local configuration member on the layout is always null
+        Configuration config = configuration;
+        if (config == null) {
+            config = ((LoggerContext) LogManager.getContext(false)).getConfiguration();
+        }
+        final StrSubstitutor strSubstitutor = config.getStrSubstitutor();
+        for (final KeyValuePair additionalField : additionalFields) {
+            builder.append(QU);
+            JsonUtils.quoteAsString(additionalField.getKey(), builder);
+            builder.append("\":\"");
+            final String value = strSubstitutor.replace(event, additionalField.getValue());
+            JsonUtils.quoteAsString(toNullSafeString(value), builder);
+            builder.append(QC);
+        }
+    }
+
     private static final TriConsumer<String, Object, StringBuilder> WRITE_KEY_VALUES_INTO = new TriConsumer<String, Object, StringBuilder>() {
         @Override
         public void accept(final String key, final Object value, final StringBuilder stringBuilder) {
@@ -295,7 +317,7 @@
     /**
      * http://en.wikipedia.org/wiki/Syslog#Severity_levels
      */
-    private int formatLevel(final Level level) {
+    private static int formatLevel(final Level level) {
         return Severity.getSeverity(level).getCode();
     }
 
@@ -310,4 +332,24 @@
         pw.flush();
         return sw.getBuffer();
     }
-}
+
+    /**
+     * Resolves the HOST attribute in case it is not provided to the local host name.
+     *
+     * @param host
+     * @return
+     */
+    private static String getHostName(final String host) {
+        if (host != null) {
+            return host;
+        }
+
+        try {
+            return InetAddress.getLocalHost().getHostName();
+        }
+        catch (final UnknownHostException e) {
+            StatusLogger.getLogger().warn("Could not detect localhost name, falling back to \"localhost\"");
+            return "localhost";
+        }
+    }
+}
\ No newline at end of file