MessageToByteEncoder always starts with ByteBuf that use initalCapacity == 0

Motivation:

MessageToByteEncoder always starts with ByteBuf that use initalCapacity == 0 when preferDirect is used. This is really wasteful in terms of performance as every first write into the buffer will cause an expand of the buffer itself.

Modifications:

 - Change ByteBufAllocator.ioBuffer() use the same default initialCapacity as heapBuffer() and directBuffer()
 - Add new allocateBuffer method to MessageToByteEncoder that allow the user to do some smarter allocation based on the message that will be encoded.

Result:

Less expanding of buffer and more flexibilty when allocate the buffer for encoding.

buffer/src/main/java/io/netty/buffer/AbstractByteBufAllocator.java

@@ -96,9 +96,9 @@ public abstract class AbstractByteBufAllocator implements ByteBufAllocator {
     @Override
     public ByteBuf ioBuffer() {
         if (PlatformDependent.hasUnsafe()) {
-            return directBuffer(0);
+            return directBuffer(DEFAULT_INITIAL_CAPACITY);
         }
-        return heapBuffer(0);
+        return heapBuffer(DEFAULT_INITIAL_CAPACITY);
     }
 
     @Override

buffer/src/main/java/io/netty/buffer/ByteBufAllocator.java

@@ -43,7 +43,7 @@ public interface ByteBufAllocator {
     ByteBuf buffer(int initialCapacity, int maxCapacity);
 
     /**
-     * Allocate a {@link ByteBuf} whose initial capacity is 0, preferably a direct buffer which is suitable for I/O.
+     * Allocate a {@link ByteBuf}, preferably a direct buffer which is suitable for I/O.
      */
     ByteBuf ioBuffer();
 

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

@@ -102,11 +102,7 @@ public abstract class MessageToByteEncoder<I> extends ChannelOutboundHandlerAdap
             if (acceptOutboundMessage(msg)) {
                 @SuppressWarnings("unchecked")
                 I cast = (I) msg;
-                if (preferDirect) {
-                    buf = ctx.alloc().ioBuffer();
-                } else {
-                    buf = ctx.alloc().heapBuffer();
-                }
+                buf = allocateBuffer(ctx, cast, preferDirect);
                 try {
                     encode(ctx, cast, buf);
                 } finally {
@@ -134,6 +130,19 @@ public abstract class MessageToByteEncoder<I> extends ChannelOutboundHandlerAdap
         }
     }
 
+    /**
+     * Allocate a {@link ByteBuf} which will be used as argument of {@link #encode(ChannelHandlerContext, I, ByteBuf)}.
+     * Sub-classes may override this method to returna {@link ByteBuf} with a perfect matching {@code initialCapacity}.
+     */
+    protected ByteBuf allocateBuffer(ChannelHandlerContext ctx, @SuppressWarnings("unused") I msg,
+                               boolean preferDirect) throws Exception {
+        if (preferDirect) {
+            return ctx.alloc().ioBuffer();
+        } else {
+            return ctx.alloc().heapBuffer();
+        }
+    }
+
     /**
      * Encode a message into a {@link ByteBuf}. This method will be called for each written message that can be handled
      * by this encoder.