Move duplicated code into CodecUtil

Motivation:

We have some duplicated code that can be reused.

Modifications:

Create package private class called CodecUtil that now contains the shared code / helper method.

Result:

Less code-duplication

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

@@ -70,7 +70,7 @@ public abstract class ByteToMessageCodec<I> extends ChannelDuplexHandler {
      *                              {@link ByteBuf}, which is backed by an byte array.
      */
     protected ByteToMessageCodec(boolean preferDirect) {
-        checkForSharableAnnotation();
+        CodecUtil.ensureNotSharable(this);
         outboundMsgMatcher = TypeParameterMatcher.find(this, ByteToMessageCodec.class, "I");
         encoder = new Encoder(preferDirect);
     }
@@ -84,17 +84,11 @@ public abstract class ByteToMessageCodec<I> extends ChannelDuplexHandler {
      *                              {@link ByteBuf}, which is backed by an byte array.
      */
     protected ByteToMessageCodec(Class<? extends I> outboundMessageType, boolean preferDirect) {
-        checkForSharableAnnotation();
+        CodecUtil.ensureNotSharable(this);
         outboundMsgMatcher = TypeParameterMatcher.get(outboundMessageType);
         encoder = new Encoder(preferDirect);
     }
 
-    private void checkForSharableAnnotation() {
-        if (isSharable()) {
-            throw new IllegalStateException("@Sharable annotation is not allowed");
-        }
-    }
-
     /**
      * Returns {@code true} if and only if the specified message can be encoded by this codec.
      *

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

@@ -52,9 +52,7 @@ public abstract class ByteToMessageDecoder extends ChannelInboundHandlerAdapter
     private boolean first;
 
     protected ByteToMessageDecoder() {
-        if (isSharable()) {
+        CodecUtil.ensureNotSharable(this);
-            throw new IllegalStateException("@Sharable annotation is not allowed");
-        }
     }
 
     /**

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

@@ -0,0 +1,32 @@
+/*
+ * Copyright 2014 The Netty Project
+ *
+ * The Netty Project licenses this file to you under the Apache License,
+ * version 2.0 (the "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at:
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ */
+package io.netty.handler.codec;
+
+import io.netty.channel.ChannelHandlerAdapter;
+
+final class CodecUtil {
+
+    /**
+     * Throws {@link IllegalStateException} if {@link ChannelHandlerAdapter#isSharable()} returns {@code true}
+     */
+    static void ensureNotSharable(ChannelHandlerAdapter handler) {
+        if (handler.isSharable()) {
+            throw new IllegalStateException("@Sharable annotation is not allowed");
+        }
+    }
+
+    private CodecUtil() { }
+}