Add DefaultAttributeMap

common/src/main/java/io/netty/util/DefaultAttributeMap.java

@@ -0,0 +1,46 @@
+package io.netty.util;
+
+import java.util.IdentityHashMap;
+import java.util.Map;
+import java.util.concurrent.atomic.AtomicReference;
+
+public class DefaultAttributeMap implements AttributeMap {
+
+    // Initialize lazily to reduce memory consumption.
+    private Map<AttributeKey<?>, Attribute<?>> map;
+
+    @Override
+    public synchronized <T> Attribute<T> attr(AttributeKey<T> key) {
+        Map<AttributeKey<?>, Attribute<?>> map = this.map;
+        if (map == null) {
+            // Not using ConcurrentHashMap due to high memory consumption.
+            map = this.map = new IdentityHashMap<AttributeKey<?>, Attribute<?>>(2);
+        }
+
+        Attribute<T> attr = (Attribute<T>) map.get(key);
+        if (attr == null) {
+            attr = new DefaultAttribute<T>();
+            map.put(key, attr);
+        }
+        return attr;
+    }
+
+    private class DefaultAttribute<T> extends AtomicReference<T> implements Attribute<T> {
+
+        private static final long serialVersionUID = -2661411462200283011L;
+
+        @Override
+        public T setIfAbsent(T value) {
+            if (compareAndSet(null, value)) {
+                return null;
+            } else {
+                return get();
+            }
+        }
+
+        @Override
+        public void remove() {
+            set(null);
+        }
+    }
+}