add support for peekByKey in MDCAdapter

slf4j-api/src/main/java/org/slf4j/helpers/BasicMDCAdapter.java

@@ -38,7 +38,7 @@
  * @author Ceki Gulcu
  * @author Maarten Bosteels
  * @author Lukasz Cwik
- * 
+ *
  * @since 1.5.0
  */
 public class BasicMDCAdapter implements MDCAdapter {
@@ -156,9 +156,14 @@ public void pushByKey(String key, String value) {
 
     @Override
     public String popByKey(String key) {
-        return threadLocalMapOfDeques.popByKey(key);    
+        return threadLocalMapOfDeques.popByKey(key);
      }
 
+    @Override
+    public String peekByKey(String key) {
+        return threadLocalMapOfDeques.peekByKey(key);
+    }
+
     @Override
     public Deque<String> getCopyOfDequeByKey(String key) {
         return threadLocalMapOfDeques.getCopyOfDequeByKey(key);

slf4j-api/src/main/java/org/slf4j/helpers/ThreadLocalMapOfStacks.java

@@ -9,7 +9,7 @@
 /**
  * A simple implementation of ThreadLocal backed Map containing values of type 
  * Deque<String>.
- * 
+ *
  * @author Ceki Guuml;c&uuml;
  * @since 2.0.0
  */
@@ -40,50 +40,53 @@ public void pushByKey(String key, String value) {
     }
 
     public String popByKey(String key) {
-        if (key == null)
+        Deque<String> deque = getDeque(key);
+        if (deque == null)
             return null;
 
-        Map<String, Deque<String>> map = tlMapOfStacks.get();
-        if (map == null)
-            return null;
-        Deque<String> deque = map.get(key);
+        return deque.pop();
+    }
+
+    public String peekByKey(String key) {
+        Deque<String> deque = getDeque(key);
         if (deque == null)
             return null;
-        return deque.pop();
+
+        return deque.peek();
     }
 
     public Deque<String> getCopyOfDequeByKey(String key) {
+        Deque<String> deque = getDeque(key);
+        if (deque == null)
+            return null;
+
+        return new ArrayDeque<String>(deque);
+    }
+
+    private Deque<String> getDeque(String key) {
         if (key == null)
             return null;
 
         Map<String, Deque<String>> map = tlMapOfStacks.get();
         if (map == null)
             return null;
-        Deque<String> deque = map.get(key);
-        if (deque == null)
-            return null;
 
-        return new ArrayDeque<String>(deque);
+        return map.get(key);
     }
-    
+
     /**
-     * Clear the deque(stack) referenced by 'key'. 
-     * 
+     * Clear the deque(stack) referenced by 'key'.
+     *
      * @param key identifies the  stack
-     * 
+     *
      * @since 2.0.0
      */
     public void clearDequeByKey(String key) {
-        if (key == null)
-            return;
-
-        Map<String, Deque<String>> map = tlMapOfStacks.get();
-        if (map == null)
-            return;
-        Deque<String> deque = map.get(key);
+        Deque<String> deque = getDeque(key);
         if (deque == null)
             return;
+
         deque.clear();
     }
 
-}
+}

slf4j-api/src/main/java/org/slf4j/spi/MDCAdapter.java

@@ -110,6 +110,19 @@ public interface MDCAdapter {
      */
     public String popByKey(String key);
 
+    /**
+     * peek the stack referenced by 'key' and return the value possibly null.
+     *
+     * @param key identifies the deque(stack)
+     * @return the value just peeked. May be null/
+     * @since 2.0.18
+     */
+    default  public String peekByKey(String key){
+        Deque<String>  deque = getCopyOfDequeByKey(key);
+        if(deque == null)
+            return  null;
+        return deque.peek();
+    }
     /**
      * Returns a copy of the deque(stack) referenced by 'key'. May be null.
      * 

slf4j-api/src/test/java/org/slf4j/helpers/MDCAdapterTestBase.java

@@ -24,18 +24,16 @@
 
 package org.slf4j.helpers;
 
-import static org.junit.Assert.assertEquals;
-import static org.junit.Assert.assertFalse;
-import static org.junit.Assert.assertNull;
-import static org.junit.Assert.fail;
-
 import java.lang.Thread.UncaughtExceptionHandler;
 import java.util.Map;
+import java.util.NoSuchElementException;
 
 import org.junit.After;
 import org.junit.Test;
 import org.slf4j.spi.MDCAdapter;
 
+import static org.junit.Assert.*;
+
 /**
  * Tests for {@link BasicMDCAdapter}
  * 
@@ -64,6 +62,25 @@ public void testSettingAndGettingWithMDC() {
         assertEquals(mdc.get("testKey"), "testValue");
     }
 
+    @Test
+    public void testPushingAndPoppingWithMDC() {
+        assertNull(mdc.popByKey("testKey"));
+        mdc.pushByKey("testKey", "testValue");
+        mdc.pushByKey("testKey", "differentTestValue");
+        assertEquals(mdc.popByKey("testKey"), "differentTestValue");
+        assertEquals(mdc.popByKey("testKey"), "testValue");
+        assertThrows(NoSuchElementException.class, () -> mdc.popByKey("testKey"));
+    }
+
+    @Test
+    public void testPushingAndPeekingWithMDC() {
+        assertNull(mdc.peekByKey("testKey"));
+        mdc.pushByKey("testKey", "testValue");
+        mdc.pushByKey("testKey", "differentTestValue");
+        assertEquals(mdc.peekByKey("testKey"), "differentTestValue");
+        assertEquals(mdc.peekByKey("testKey"), "differentTestValue");
+    }
+
     @Test
     public void testOverwritingAKeyInMDC() {
         assertNull(mdc.get("testKey"));

slf4j-reload4j/src/main/java/org/slf4j/reload4j/Reload4jMDCAdapter.java

@@ -114,6 +114,11 @@ public String popByKey(String key) {
         return threadLocalMapOfDeques.popByKey(key);    
      }
 
+    @Override
+    public String peekByKey(String key) {
+        return threadLocalMapOfDeques.peekByKey(key);
+    }
+
     @Override
     public Deque<String> getCopyOfDequeByKey(String key) {
         return threadLocalMapOfDeques.getCopyOfDequeByKey(key);