Adding extension point allowing subclasses to add more data to the json log

src/main/java/net/logstash/log4j/JSONEventLayoutV1.java

@@ -77,6 +77,15 @@ public String format(LoggingEvent loggingEvent) {
         logstashEvent.put("@version", version);
         logstashEvent.put("@timestamp", dateFormat(timestamp));
 
+        // allow sub-classes to add extra-data to log
+        Map extraFields = getExtraFields();
+        if (extraFields != null) {
+            for (Object entryObj : extraFields.entrySet()) {
+                Map.Entry entry = (Map.Entry) entryObj;
+                addEventData(entry.getKey().toString(), entry.getValue() != null ? entry.getValue().toString() : "null");
+            }
+        }
+
         /**
          * Extract and add fields from log4j config, if defined
          */
@@ -166,6 +175,11 @@ public void activateOptions() {
         activeIgnoreThrowable = ignoreThrowable;
     }
 
+    /** Extension point allowing subclasses to add more data to the json log */
+    protected Map getExtraFields() {
+        return null;
+    }
+
     private void addUserFields(String data) {
         if (null != data) {
             String[] pairs = data.split(",");

src/test/java/net/logstash/log4j/JSONEventLayoutV1Test.java

@@ -1,26 +1,21 @@
 package net.logstash.log4j;
 
+import java.util.HashMap;
+import java.util.Map;
+
 import junit.framework.Assert;
 import net.minidev.json.JSONObject;
 import net.minidev.json.JSONValue;
-import org.apache.log4j.Layout;
+
 import org.apache.log4j.Level;
 import org.apache.log4j.Logger;
 import org.apache.log4j.NDC;
 import org.junit.After;
 import org.junit.Before;
-import org.junit.AfterClass;
 import org.junit.BeforeClass;
 import org.junit.Ignore;
 import org.junit.Test;
 
-/**
- * Created with IntelliJ IDEA.
- * User: jvincent
- * Date: 12/5/12
- * Time: 12:07 AM
- * To change this template use File | Settings | File Templates.
- */
 public class JSONEventLayoutV1Test {
     static Logger logger;
     static MockAppenderV1 appender;
@@ -37,13 +32,14 @@ public class JSONEventLayoutV1Test {
             "@version"
     };
 
-    @BeforeClass
-    public static void setupTestAppender() {
+    @Before
+    public void init() {
         appender = new MockAppenderV1(new JSONEventLayoutV1());
-        logger = Logger.getRootLogger();
         appender.setThreshold(Level.TRACE);
         appender.setName("mockappenderv1");
         appender.activateOptions();
+        logger = Logger.getRootLogger();
+        logger.removeAllAppenders();
         logger.addAppender(appender);
     }
 
@@ -133,6 +129,25 @@ public void testJSONEventLayoutUserFieldsPropOverride() {
 
     }
 
+    @Test
+    public void testJSONEventLayoutExtraFields() {
+        MockAppenderV1 mockAppenderWithExtra = new MockAppenderV1(new JSONEventLayoutV1WithExtraField());
+        Logger loggerWithExtra = Logger.getRootLogger();
+        mockAppenderWithExtra.setThreshold(Level.TRACE);
+        mockAppenderWithExtra.setName("mockappenderv1-with-extra");
+        mockAppenderWithExtra.activateOptions();
+        loggerWithExtra.removeAllAppenders();
+        loggerWithExtra.addAppender(mockAppenderWithExtra);
+
+        loggerWithExtra.info("this is an info message with user fields");
+        String message = mockAppenderWithExtra.getMessages()[0];
+        Assert.assertTrue("Event is not valid JSON", JSONValue.isValidJsonStrict(message));
+        Object obj = JSONValue.parse(message);
+        JSONObject jsonObject = (JSONObject) obj;
+        Assert.assertTrue("Event does not contain field 'extra-field'" , jsonObject.containsKey("extra-field"));
+        Assert.assertEquals("Event does not contain value 'extra-value'", "extra-value", jsonObject.get("extra-field"));
+    }
+
     @Test
     public void testJSONEventLayoutHasKeys() {
         logger.info("this is a test message");
@@ -263,4 +278,14 @@ public void testDateFormat() {
         long timestamp = 1364844991207L;
         Assert.assertEquals("format does not produce expected output", "2013-04-01T19:36:31.207Z", JSONEventLayoutV1.dateFormat(timestamp));
     }
+
+    private static class JSONEventLayoutV1WithExtraField extends JSONEventLayoutV1 {
+
+        @Override
+        protected Map getExtraFields() {
+            Map extra = new HashMap(1);
+            extra.put("extra-field", "extra-value");
+            return extra;
+        }
+    }
 }