Fixed in 3f3e66c31ae3da70c36cc125ca9bcac8215390e4

This line should not be changed.
 If this is not public, a user who extends `AbstractChannel` (and consequently `AbstractUnsafe`) will not be able to access the return value of `AbstractUnsafe.outboundBuffer()`.
 Any reason this is not part of the interface?

ChannelOutboundBuffer only be usable via AbstractChannel
We expose `ChannelOuboundBuffer` in `Channel.Unsafe` but make it impossible to create one instance without the usage of `AbstractChannel`. This makes it impossible to write a `Channel` implementation that not extends `AbstractChannel`.

transport/src/main/java/io/netty/channel/AbstractChannel.java transport/src/main/java/io/netty/channel/ChannelOutboundBuffer.java
transport/src/main/java/io/netty/channel/AbstractChannel.java transport/src/main/java/io/netty/channel/ChannelOutboundBuffer.java
transport/src/test/java/io/netty/channel/ChannelOutboundBufferTest.java
diff --git a/transport/src/main/java/io/netty/channel/AbstractChannel.java b/transport/src/main/java/io/netty/channel/AbstractChannel.java
index 5642f226554..e270a98acb7 100644
--- a/transport/src/main/java/io/netty/channel/AbstractChannel.java
+++ b/transport/src/main/java/io/netty/channel/AbstractChannel.java
@@ -83,7 +83,7 @@ protected AbstractChannel(Channel parent) {
     @Override
     public boolean isWritable() {
         ChannelOutboundBuffer buf = unsafe.outboundBuffer();
-        return buf != null && buf.getWritable();
+        return buf != null && buf.isWritable();
     }
 
     @Override
@@ -649,7 +649,11 @@ public void write(Object msg, ChannelPromise promise) {
                 ReferenceCountUtil.release(msg);
                 return;
             }
-            outboundBuffer.addMessage(msg, promise);
+            int size = estimatorHandle().size(msg);
+            if (size < 0) {
+                size = 0;
+            }
+            outboundBuffer.addMessage(msg, size, promise);
         }
 
         @Override
diff --git a/transport/src/main/java/io/netty/channel/ChannelOutboundBuffer.java b/transport/src/main/java/io/netty/channel/ChannelOutboundBuffer.java
index 899ca3b289c..e0508bdb5d7 100644
--- a/transport/src/main/java/io/netty/channel/ChannelOutboundBuffer.java
+++ b/transport/src/main/java/io/netty/channel/ChannelOutboundBuffer.java
@@ -40,6 +40,8 @@
 /**
  * (Transport implementors only) an internal data structure used by {@link AbstractChannel} to store its pending
  * outbound write requests.
+ *
+ * All the methods should only be called by the {@link EventLoop} of the {@link Channel}.
  */
 public final class ChannelOutboundBuffer {
 
@@ -61,7 +63,7 @@ protected ByteBuffer[] initialValue() throws Exception {
         }
     };
 
-    private final AbstractChannel channel;
+    private final Channel channel;
 
     // Entry(flushedEntry) --> ... Entry(unflushedEntry) --> ... Entry(tailEntry)
     //
@@ -109,12 +111,11 @@ protected ByteBuffer[] initialValue() throws Exception {
         this.channel = channel;
     }
 
-    void addMessage(Object msg, ChannelPromise promise) {
-        int size = channel.estimatorHandle().size(msg);
-        if (size < 0) {
-            size = 0;
-        }
-
+    /**
+     * Add given message to this {@link ChannelOutboundBuffer}. The given {@link ChannelPromise} will be notified once
+     * the message was written.
+     */
+    public void addMessage(Object msg, int size, ChannelPromise promise) {
         Entry entry = Entry.newInstance(msg, size, total(msg), promise);
         if (tailEntry == null) {
             flushedEntry = null;
@@ -133,7 +134,11 @@ void addMessage(Object msg, ChannelPromise promise) {
         incrementPendingOutboundBytes(size);
     }
 
-    void addFlush() {
+    /**
+     * Add a flush to this {@link ChannelOutboundBuffer}. This means all previous added messages are marked as flushed
+     * and so you will be able to handle them.
+     */
+    public void addFlush() {
         // There is no need to process all entries if there was already a flush before and no new messages
         // where added in the meantime.
         //
@@ -206,10 +211,18 @@ private static long total(Object msg) {
         return -1;
     }
 
+    /**
+     * Return the current message to write or {@code null} if nothing was flushed before and so is ready to be written.
+     */
     public Object current() {
         return current(true);
     }
 
+    /**
+     * Return the current message to write or {@code null} if nothing was flushed before and so is ready to be written.
+     * If {@code true} is specified a direct {@link ByteBuf} or {@link ByteBufHolder} is prefered and
+     * so the current message may be copied into a direct buffer.
+     */
     public Object current(boolean preferDirect) {
         // TODO: Think of a smart way to handle ByteBufHolder messages
         Entry entry = flushedEntry;
@@ -250,7 +263,7 @@ public Object current(boolean preferDirect) {
 
     /**
      * Replace the current msg with the given one.
-     * The replaced msg will automatically be released
+     * {@link ReferenceCountUtil#release(Object)} will automatically be called on the replaced message.
      */
     public void current(Object msg) {
         Entry entry = flushedEntry;
@@ -259,6 +272,9 @@ public void current(Object msg) {
         entry.msg = msg;
     }
 
+    /**
+     * Notify the {@link ChannelPromise} of the current message about writing progress.
+     */
     public void progress(long amount) {
         Entry e = flushedEntry;
         assert e != null;
@@ -270,6 +286,11 @@ public void progress(long amount) {
         }
     }
 
+    /**
+     * Will remove the current message, mark its {@link ChannelPromise} as success and return {@code true}. If no
+     * flushed message exists at the time this method is called it will return {@code false} to signal that no more
+     * messages are ready to be handled.
+     */
     public boolean remove() {
         Entry e = flushedEntry;
         if (e == null) {
@@ -295,6 +316,11 @@ public boolean remove() {
         return true;
     }
 
+    /**
+     * Will remove the current message, mark its {@link ChannelPromise} as failure using the given {@link Throwable}
+     * and return {@code true}. If no   flushed message exists at the time this method is called it will return
+     * {@code false} to signal that no more messages are ready to be handled.
+     */
     public boolean remove(Throwable cause) {
         Entry e = flushedEntry;
         if (e == null) {
@@ -366,9 +392,8 @@ public void removeBytes(long writtenBytes) {
 
     /**
      * Returns an array of direct NIO buffers if the currently pending messages are made of {@link ByteBuf} only.
-     * {@code null} is returned otherwise.  If this method returns a non-null array, {@link #nioBufferCount()} and
-     * {@link #nioBufferSize()} will return the number of NIO buffers in the returned array and the total number
-     * of readable bytes of the NIO buffers respectively.
+     * {@link #nioBufferCount()} and {@link #nioBufferSize()} will return the number of NIO buffers in the returned
+     * array and the total number of readable bytes of the NIO buffers respectively.
      * <p>
      * Note that the returned array is reused and thus should not escape
      * {@link AbstractChannel#doWrite(ChannelOutboundBuffer)}.
@@ -479,22 +504,39 @@ private static ByteBuffer[] expandNioBufferArray(ByteBuffer[] array, int neededS
         return newArray;
     }
 
+    /**
+     * Returns the number of {@link ByteBuffer} that can be written out of the {@link ByteBuffer} array that was
+     * obtained via {@link #nioBuffers()}. This method <strong>MUST</strong> be called after {@link #nioBuffers()}
+     * was called.
+     */
     public int nioBufferCount() {
         return nioBufferCount;
     }
 
+    /**
+     * Returns the number of bytes that can be written out of the {@link ByteBuffer} array that was
+     * obtained via {@link #nioBuffers()}. This method <strong>MUST</strong> be called after {@link #nioBuffers()}
+     * was called.
+     */
     public long nioBufferSize() {
         return nioBufferSize;
     }
 
-    boolean getWritable() {
+    boolean isWritable() {
         return writable != 0;
     }
 
+    /**
+     * Returns the number of flushed messages in this {@link ChannelOutboundBuffer}.
+     */
     public int size() {
         return flushed;
     }
 
+    /**
+     * Returns {@code true} if there are flushed messages in this {@link ChannelOutboundBuffer} or {@code false}
+     * otherwise.
+     */
     public boolean isEmpty() {
         return flushed == 0;
     }

diff --git a/transport/src/test/java/io/netty/channel/ChannelOutboundBufferTest.java b/transport/src/test/java/io/netty/channel/ChannelOutboundBufferTest.java
index 11c0f6a1415..296a0e7d131 100644
--- a/transport/src/test/java/io/netty/channel/ChannelOutboundBufferTest.java
+++ b/transport/src/test/java/io/netty/channel/ChannelOutboundBufferTest.java
@@ -51,7 +51,7 @@ public void testNioBuffersSingleBacked() {
 
         ByteBuf buf = copiedBuffer("buf1", CharsetUtil.US_ASCII);
         ByteBuffer nioBuf = buf.internalNioBuffer(0, buf.readableBytes());
-        buffer.addMessage(buf, channel.voidPromise());
+        buffer.addMessage(buf, buf.readableBytes(), channel.voidPromise());
         assertEquals("Should still be 0 as not flushed yet", 0, buffer.nioBufferCount());
         buffer.addFlush();
         ByteBuffer[] buffers = buffer.nioBuffers();
@@ -75,7 +75,7 @@ public void testNioBuffersExpand() {
 
         ByteBuf buf = directBuffer().writeBytes("buf1".getBytes(CharsetUtil.US_ASCII));
         for (int i = 0; i < 64; i++) {
-            buffer.addMessage(buf.copy(), channel.voidPromise());
+            buffer.addMessage(buf.copy(), buf.readableBytes(), channel.voidPromise());
         }
         assertEquals("Should still be 0 as not flushed yet", 0, buffer.nioBufferCount());
         buffer.addFlush();
@@ -99,7 +99,7 @@ public void testNioBuffersExpand2() {
         for (int i = 0; i < 65; i++) {
             comp.addComponent(buf.copy()).writerIndex(comp.writerIndex() + buf.readableBytes());
         }
-        buffer.addMessage(comp, channel.voidPromise());
+        buffer.addMessage(comp, comp.readableBytes(), channel.voidPromise());
 
         assertEquals("Should still be 0 as not flushed yet", 0, buffer.nioBufferCount());
         buffer.addFlush();
