Netty分布式抽象编码器MessageToByteEncoder逻辑分析

public abstract class MessageToByteEncoder<I> extends ChannelOutboundHandlerAdapter{
    //省略类体
}

public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
    ByteBuf buf = null;
    try {
        if (acceptOutboundMessage(msg)) {
            @SuppressWarnings("unchecked")
            I cast = (I) msg;
            buf = allocateBuffer(ctx, cast, preferDirect);
            try {
                encode(ctx, cast, buf);
            } finally {
                ReferenceCountUtil.release(cast);
            }

            if (buf.isReadable()) {
                ctx.write(buf, promise);
            } else {
                buf.release();
                ctx.write(Unpooled.EMPTY_BUFFER, promise);
            }
            buf = null;
        } else {
            ctx.write(msg, promise);
        }
    } catch (EncoderException e) {
        throw e;
    } catch (Throwable e) {
        throw new EncoderException(e);
    } finally {
        if (buf != null) {
            buf.release();
        }
    }
}

protected ByteBuf allocateBuffer(ChannelHandlerContext ctx, @SuppressWarnings("unused") I msg, 
                           boolean preferDirect) throws Exception {
    if (preferDirect) {
        return ctx.alloc().ioBuffer();
    } else {
        return ctx.alloc().heapBuffer();
    }
}