Index: log4j-core/src/test/java/org/apache/logging/log4j/core/pattern/MdcPatternConverterTest.java
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- log4j-core/src/test/java/org/apache/logging/log4j/core/pattern/MdcPatternConverterTest.java	(date 1475431991000)
+++ log4j-core/src/test/java/org/apache/logging/log4j/core/pattern/MdcPatternConverterTest.java	(revision )
@@ -78,6 +78,23 @@
         assertEquals(expected, str);
     }
 
+    @Test
+    public void testConverterWithKeyAndDefault() {
+        final Message msg = new SimpleMessage("Hello");
+        final String [] options = new String[] {"not_exist", "default_value"};
+        final MdcPatternConverter converter = MdcPatternConverter.newInstance(options);
+        final LogEvent event = Log4jLogEvent.newBuilder() //
+                .setLoggerName("MyLogger") //
+                .setLevel(Level.DEBUG) //
+                .setMessage(msg) //
+                .build();
+        final StringBuilder sb = new StringBuilder();
+        converter.format(event, sb);
+        final String str = sb.toString();
+        final String expected = "default_value";
+        assertEquals(expected, str);
+    }
+
     @Test
     public void testConverterWithKeys() {
         final Message msg = new SimpleMessage("Hello");
@@ -94,6 +111,23 @@
         final String expected = "{object=Log4j, subject=I}";
         assertEquals(expected, str);
     }
+
+    @Test
+    public void testConverterWithKeysAndDefaults() {
+        final Message msg = new SimpleMessage("Hello");
+        final String [] options = new String[] {"object_not_exist, subject_not_exist", "obj, sub"};
+        final MdcPatternConverter converter = MdcPatternConverter.newInstance(options);
+        final LogEvent event = Log4jLogEvent.newBuilder() //
+                .setLoggerName("MyLogger") //
+                .setLevel(Level.DEBUG) //
+                .setMessage(msg) //
+                .build();
+        final StringBuilder sb = new StringBuilder();
+        converter.format(event, sb);
+        final String str = sb.toString();
+        final String expected = "{object_not_exist=obj, subject_not_exist=sub}";
+        assertEquals(expected, str);
+    }
 
 }
 
Index: log4j-core/src/main/java/org/apache/logging/log4j/core/pattern/MdcPatternConverter.java
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- log4j-core/src/main/java/org/apache/logging/log4j/core/pattern/MdcPatternConverter.java	(date 1475431991000)
+++ log4j-core/src/main/java/org/apache/logging/log4j/core/pattern/MdcPatternConverter.java	(revision )
@@ -16,12 +16,12 @@
  */
 package org.apache.logging.log4j.core.pattern;
 
-import org.apache.logging.log4j.util.ReadOnlyStringMap;
 import org.apache.logging.log4j.core.LogEvent;
 import org.apache.logging.log4j.core.config.plugins.Plugin;
 import org.apache.logging.log4j.core.util.Constants;
-import org.apache.logging.log4j.util.TriConsumer;
+import org.apache.logging.log4j.util.ReadOnlyStringMap;
 import org.apache.logging.log4j.util.StringBuilders;
+import org.apache.logging.log4j.util.TriConsumer;
 
 /**
  * Able to handle the contents of the LogEvent's MDC and either
@@ -31,7 +31,7 @@
  * when this pattern converter has the option set.
  */
 @Plugin(name = "MdcPatternConverter", category = PatternConverter.CATEGORY)
-@ConverterKeys({ "X", "mdc", "MDC" })
+@ConverterKeys({"X", "mdc", "MDC"})
 public final class MdcPatternConverter extends LogEventPatternConverter {
 
     private static final ThreadLocal<StringBuilder> threadLocal = new ThreadLocal<>();
@@ -44,6 +44,7 @@
     private final String key;
     private final String[] keys;
     private final boolean full;
+    private final String[] defaultValues;
 
     /**
      * Private constructor.
@@ -64,10 +65,29 @@
                 keys = null;
                 key = options[0];
             }
+            if (options.length > 1) {
+                if (options[1].indexOf(',') > 0) {
+                    defaultValues = options[1].split(",");
+                    if (keys == null || keys.length != defaultValues.length){
+                        throw new IllegalArgumentException("number of default values is not EQUALS to number of keys.");
+                    }
+
+                    for (int i = 0; i < defaultValues.length; i++) {
+                        defaultValues[i] = defaultValues[i].trim();
+                    }
+                } else {
+                    defaultValues = new String[1];
+                    defaultValues[0] = options[1];
+                }
+            } else {
+                defaultValues = null;
+            }
+
         } else {
             full = true;
             key = null;
             keys = null;
+            defaultValues = null;
         }
     }
 
@@ -112,10 +132,15 @@
                     toAppendTo.append("{}");
                     return;
                 }
-                appendSelectedKeys(keys, contextData, toAppendTo);
-            } else if (contextData != null){
+                appendSelectedKeys(keys, defaultValues, contextData, toAppendTo);
+            } else if (contextData != null) {
                 // otherwise they just want a single key output
-                final Object value = contextData.getValue(key);
+                Object value = contextData.getValue(key);
+
+                if (value == null && defaultValues != null){
+                    value = defaultValues[0];
+                }
+
                 if (value != null) {
                     StringBuilders.appendValue(toAppendTo, value);
                 }
@@ -132,13 +157,18 @@
         trimToMaxSize(sb);
     }
 
-    private static void appendSelectedKeys(final String[] keys, final ReadOnlyStringMap contextData, final StringBuilder toAppendTo) {
+    private static void appendSelectedKeys(final String[] keys, final String[] defaultValues, final ReadOnlyStringMap contextData, final StringBuilder toAppendTo) {
         // Print all the keys in the array that have a value.
         final StringBuilder sb = getStringBuilder();
         sb.append("{");
         for (int i = 0; i < keys.length; i++) {
             final String theKey = keys[i];
-            final Object value = contextData.getValue(theKey);
+            Object value = contextData.getValue(theKey);
+
+            if (value == null && defaultValues != null){
+                value = defaultValues[i];
+            }
+
             if (value != null) { // !contextData.containskey(theKey)
                 if (sb.length() > 1) {
                     sb.append(", ");