Add LengthFieldPrepender.lengthAdjustment parameter

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

@@ -51,6 +51,7 @@ public class LengthFieldPrepender extends MessageToByteEncoder<ByteBuf> {
 
     private final int lengthFieldLength;
     private final boolean lengthIncludesLengthFieldLength;
+    private final int lengthAdjustment;
 
     /**
      * Creates a new instance.
@@ -79,6 +80,40 @@ public class LengthFieldPrepender extends MessageToByteEncoder<ByteBuf> {
      *         if {@code lengthFieldLength} is not 1, 2, 3, 4, or 8
      */
     public LengthFieldPrepender(int lengthFieldLength, boolean lengthIncludesLengthFieldLength) {
+        this(lengthFieldLength, 0, lengthIncludesLengthFieldLength);
+    }
+
+    /**
+     * Creates a new instance.
+     *
+     * @param lengthFieldLength the length of the prepended length field.
+     *                          Only 1, 2, 3, 4, and 8 are allowed.
+     * @param lengthAdjustment  the compensation value to add to the value
+     *                          of the length field
+     *
+     * @throws IllegalArgumentException
+     *         if {@code lengthFieldLength} is not 1, 2, 3, 4, or 8
+     */
+    public LengthFieldPrepender(int lengthFieldLength, int lengthAdjustment) {
+        this(lengthFieldLength, lengthAdjustment, false);
+    }
+
+    /**
+     * Creates a new instance.
+     *
+     * @param lengthFieldLength the length of the prepended length field.
+     *                          Only 1, 2, 3, 4, and 8 are allowed.
+     * @param lengthAdjustment  the compensation value to add to the value
+     *                          of the length field
+     * @param lengthIncludesLengthFieldLength
+     *                          if {@code true}, the length of the prepended
+     *                          length field is added to the value of the
+     *                          prepended length field.
+     *
+     * @throws IllegalArgumentException
+     *         if {@code lengthFieldLength} is not 1, 2, 3, 4, or 8
+     */
+    public LengthFieldPrepender(int lengthFieldLength, int lengthAdjustment, boolean lengthIncludesLengthFieldLength) {
         if (lengthFieldLength != 1 && lengthFieldLength != 2 &&
             lengthFieldLength != 3 && lengthFieldLength != 4 &&
             lengthFieldLength != 8) {
@@ -89,6 +124,7 @@ public class LengthFieldPrepender extends MessageToByteEncoder<ByteBuf> {
 
         this.lengthFieldLength = lengthFieldLength;
         this.lengthIncludesLengthFieldLength = lengthIncludesLengthFieldLength;
+        this.lengthAdjustment = lengthAdjustment;
     }
 
     @Override
@@ -96,8 +132,15 @@ public class LengthFieldPrepender extends MessageToByteEncoder<ByteBuf> {
             ChannelHandlerContext ctx,
             ByteBuf msg, ByteBuf out) throws Exception {
 
-        int length = lengthIncludesLengthFieldLength?
+        int length = (lengthIncludesLengthFieldLength?
-                msg.readableBytes() + lengthFieldLength : msg.readableBytes();
+                msg.readableBytes() + lengthFieldLength : msg.readableBytes()) +
+                lengthAdjustment;
+
+        if (length < 0) {
+            throw new IllegalArgumentException(
+                    "Adjusted frame length (" + length + ") is less than zero");
+        }
+
         switch (lengthFieldLength) {
         case 1:
             if (length >= 256) {

codec/src/test/java/io/netty/handler/codec/frame/LengthFieldPrependerTest.java

@@ -0,0 +1,72 @@
+/*
+ * Copyright 2012 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.frame;
+
+import io.netty.buffer.ByteBuf;
+import io.netty.channel.IncompleteFlushException;
+import io.netty.channel.embedded.EmbeddedByteChannel;
+import io.netty.handler.codec.LengthFieldPrepender;
+import io.netty.util.CharsetUtil;
+import org.junit.Before;
+import org.junit.Test;
+
+import static io.netty.buffer.Unpooled.copiedBuffer;
+import static io.netty.buffer.Unpooled.wrappedBuffer;
+import static org.hamcrest.core.Is.is;
+import static org.junit.Assert.assertThat;
+import static org.junit.Assert.fail;
+
+public class LengthFieldPrependerTest {
+
+    private ByteBuf msg;
+
+    @Before
+    public void setUp() throws Exception {
+        msg = copiedBuffer("A", CharsetUtil.ISO_8859_1);
+    }
+
+    @Test
+    public void testPrependLength() throws Exception {
+        final EmbeddedByteChannel ch = new EmbeddedByteChannel(new LengthFieldPrepender(4));
+        ch.writeOutbound(msg);
+        assertThat(ch.readOutbound(), is(wrappedBuffer(new byte[]{0, 0, 0, 1, 'A'})));
+    }
+
+    @Test
+    public void testPrependLengthIncludesLengthFieldLength() throws Exception {
+        final EmbeddedByteChannel ch = new EmbeddedByteChannel(new LengthFieldPrepender(4, true));
+        ch.writeOutbound(msg);
+        assertThat(ch.readOutbound(), is(wrappedBuffer(new byte[]{0, 0, 0, 5, 'A'})));
+    }
+
+    @Test
+    public void testPrependAdjustedLength() throws Exception {
+        final EmbeddedByteChannel ch = new EmbeddedByteChannel(new LengthFieldPrepender(4, -1));
+        ch.writeOutbound(msg);
+        assertThat(ch.readOutbound(), is(wrappedBuffer(new byte[]{0, 0, 0, 0, 'A'})));
+    }
+
+    @Test
+    public void testAdjustedLengthLessThanZero() throws Exception {
+        final EmbeddedByteChannel ch = new EmbeddedByteChannel(new LengthFieldPrepender(4, -2));
+        try {
+            ch.writeOutbound(msg);
+            fail(IncompleteFlushException.class.getSimpleName() + " must be raised.");
+        } catch (IncompleteFlushException e) {
+            // Expected
+        }
+    }
+}