Opportunity for lazy initialization in Headers interface

Motivation:
The Headers interface had two member variables (addAllVisitor, setAllVisitor) which are not necessarily always needed but are always instantiated.  This may result in excess memory being used.

Modifications:
 - addAllVisitor will be accessed via a method addAllVisitor() which will use lazy initialization.
 - setAllVisitor will be accessed via a method addAllVisitor() which will use lazy initialization.

Result:
Potential memory savings by using lazy initialization.

codec/src/main/java/io/netty/handler/codec/DefaultHeaders.java

@@ -79,25 +79,8 @@ public class DefaultHeaders<T> implements Headers<T> {
     private static final int HASH_CODE_PRIME = 31;
     private static final int DEFAULT_BUCKET_SIZE = 17;
     private static final int DEFAULT_MAP_SIZE = 4;
-
     private static final NameConverter<Object> DEFAULT_NAME_CONVERTER = new IdentityNameConverter<Object>();
 
-    private final EntryVisitor<T> setAllVisitor = new EntryVisitor<T>() {
-        @Override
-        public boolean visit(Entry<T, T> entry) {
-            set(entry.getKey(), entry.getValue());
-            return true;
-        }
-    };
-
-    private final EntryVisitor<T> addAllVisitor = new EntryVisitor<T>() {
-        @Override
-        public boolean visit(Entry<T, T> entry) {
-            add(entry.getKey(), entry.getValue());
-            return true;
-        }
-    };
-
     private final IntObjectMap<HeaderEntry> entries;
     private final IntObjectMap<HeaderEntry> tailEntries;
     private final HeaderEntry head;
@@ -107,6 +90,8 @@ public class DefaultHeaders<T> implements Headers<T> {
     private final ValueConverter<T> valueConverter;
     private final NameConverter<T> nameConverter;
     private final int bucketSize;
+    private EntryVisitor<T> setAllVisitor;
+    private EntryVisitor<T> addAllVisitor;
     int size;
 
     @SuppressWarnings("unchecked")
@@ -752,7 +737,7 @@ public class DefaultHeaders<T> implements Headers<T> {
             }
         } else {
             try {
-                headers.forEachEntry(setAllVisitor);
+                headers.forEachEntry(setAllVisitor());
             } catch (Exception ex) {
                 PlatformDependent.throwException(ex);
             }
@@ -1255,7 +1240,7 @@ public class DefaultHeaders<T> implements Headers<T> {
             }
         } else {
             try {
-                headers.forEachEntry(addAllVisitor);
+                headers.forEachEntry(addAllVisitor());
             } catch (Exception ex) {
                 PlatformDependent.throwException(ex);
             }
@@ -1322,6 +1307,38 @@ public class DefaultHeaders<T> implements Headers<T> {
         return removed;
     }
 
+    /**
+     * Lazy initialization of the visitor which will set all headers.
+     */
+    private EntryVisitor<T> setAllVisitor() {
+        if (setAllVisitor == null) {
+            setAllVisitor = new EntryVisitor<T>() {
+                @Override
+                public boolean visit(Entry<T, T> entry) {
+                    set(entry.getKey(), entry.getValue());
+                    return true;
+                }
+            };
+        }
+        return setAllVisitor;
+    }
+
+    /**
+     * Lazy initialization of the visitor which will add all headers.
+     */
+    private EntryVisitor<T> addAllVisitor() {
+        if (addAllVisitor == null) {
+            addAllVisitor = new EntryVisitor<T>() {
+                @Override
+                public boolean visit(Entry<T, T> entry) {
+                    add(entry.getKey(), entry.getValue());
+                    return true;
+                }
+            };
+        }
+        return addAllVisitor;
+    }
+
     private final class HeaderEntry implements Map.Entry<T, T> {
         final int hash;
         final T name;