Support unset MDCFilter value

logback-classic/src/main/java/ch/qos/logback/classic/turbo/MDCFilter.java

@@ -55,7 +55,7 @@ public FilterReply decide(Marker marker, Logger logger, Level level, String form
         }
 
         String value = MDC.get(MDCKey);
-        if (this.value.equals(value)) {
+        if ((this.value == value) || (this.value != null && this.value.equals(value))) {
             return onMatch;
         }
         return onMismatch;

logback-classic/src/test/java/ch/qos/logback/classic/turbo/MDCFilterTest.java

@@ -0,0 +1,74 @@
+/**
+ * Logback: the reliable, generic, fast and flexible logging framework.
+ * Copyright (C) 1999-2015, QOS.ch. All rights reserved.
+ *
+ * This program and the accompanying materials are dual-licensed under
+ * either the terms of the Eclipse Public License v1.0 as published by
+ * the Eclipse Foundation
+ *
+ *   or (per the licensee's choosing)
+ *
+ * under the terms of the GNU Lesser General Public License version 2.1
+ * as published by the Free Software Foundation.
+ */
+package ch.qos.logback.classic.turbo;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
+import org.junit.Before;
+import org.junit.Test;
+import org.slf4j.Marker;
+import org.slf4j.MarkerFactory;
+import org.slf4j.MDC;
+
+import ch.qos.logback.core.spi.FilterReply;
+
+public class MDCFilterTest {
+
+    private static String KEY = "myKey";
+
+    private MDCFilter filter;
+
+    @Before
+    public void init() {
+        filter = new MDCFilter();
+        filter.setOnMatch("ACCEPT");
+        filter.setOnMismatch("DENY");
+        filter.setMDCKey(KEY);
+        filter.start();
+        MDC.clear();
+    }
+
+    @Test
+    public void testNoValue() {
+        filter.setMDCKey(null);
+        MDC.remove(KEY);
+        assertEquals(FilterReply.NEUTRAL, filter.decide(null, null, null, null, null, null));
+        MDC.put(KEY, "key");
+        assertEquals(FilterReply.NEUTRAL, filter.decide(null, null, null, null, null, null));
+    }
+
+    @Test
+    public void testNoMdcValue() {
+        MDC.remove(KEY);
+        assertEquals(FilterReply.ACCEPT, filter.decide(null, null, null, null, null, null));
+    }
+
+    @Test
+    public void testMdcValue() {
+        MDC.put(KEY, "myKey");
+        assertEquals(FilterReply.DENY, filter.decide(null, null, null, null, null, null));
+    }
+
+    @Test
+    public void testMdcAndValue() {
+        filter.setValue("correct");
+        MDC.put(KEY, "wrong");
+        assertEquals(FilterReply.DENY, filter.decide(null, null, null, null, null, null));
+        MDC.put(KEY, "correct");
+        assertEquals(FilterReply.ACCEPT, filter.decide(null, null, null, null, null, null));
+    }
+}
+