разделил encoder на две части

proto_1.12.2_netty/src/main/java/mc/core/network/proto_1_12_2/netty/PacketEncoder.java

@@ -10,7 +10,6 @@ import io.netty.handler.codec.MessageToByteEncoder;
 import lombok.extern.slf4j.Slf4j;
 import mc.core.network.NetOutputStream;
 import mc.core.network.SCPacket;
-import mc.core.network.proto_1_12_2.ByteArrayOutputNetStream;
 import mc.core.network.proto_1_12_2.State;
 import mc.core.network.proto_1_12_2.netty.wrappers.WrapperNetOutputStream;
 
@@ -18,17 +17,6 @@ import static mc.core.network.proto_1_12_2.netty.NettyServer.ATTR_STATE;
 
 @Slf4j
 public class PacketEncoder extends MessageToByteEncoder<SCPacket> {
-    private static int sizeVarInt(final int value) {
-        byte size = 0;
-        int v = value;
-
-        do {
-            v >>>= 7;
-            size++;
-        } while (v != 0);
-
-        return size;
-    }
 
     @Override
     protected void encode(ChannelHandlerContext ctx, SCPacket packet, ByteBuf out) {
@@ -41,13 +29,8 @@ public class PacketEncoder extends MessageToByteEncoder<SCPacket> {
 
         log.debug("Send {}:{}", state, packet);
 
-        NetOutputStream netStream = new ByteArrayOutputNetStream();
+        NetOutputStream netStream = new WrapperNetOutputStream(out);
-        packet.writeSelf(netStream);
-        byte[] bytes = ((ByteArrayOutputNetStream) netStream).toByteArray();
-        netStream = new WrapperNetOutputStream(out);
-
-        netStream.writeVarInt(bytes.length + sizeVarInt(id));
         netStream.writeVarInt(id);
-        netStream.writeBytes(bytes);
+        packet.writeSelf(netStream);
     }
 }

proto_1.12.2_netty/src/main/java/mc/core/network/proto_1_12_2/netty/PacketPostEncoder.java

@@ -0,0 +1,38 @@
+package mc.core.network.proto_1_12_2.netty;
+
+import io.netty.buffer.ByteBuf;
+import io.netty.channel.ChannelHandlerContext;
+import io.netty.handler.codec.MessageToByteEncoder;
+import lombok.extern.slf4j.Slf4j;
+import mc.core.network.proto_1_12_2.netty.wrappers.WrapperNetOutputStream;
+
+/**
+ * Подсчет размера отправляемого пакета
+ */
+@Slf4j
+public class PacketPostEncoder extends MessageToByteEncoder<ByteBuf> {
+
+    private static int getVarIntSize(int input) {
+        for (int i = 1; i < 5; ++i) {
+            if ((input & -1 << i * 7) == 0) {
+                return i;
+            }
+        }
+
+        return 5;
+    }
+
+    @Override
+    protected void encode(ChannelHandlerContext ctx, ByteBuf msg, ByteBuf out) {
+        final int pktSize = msg.readableBytes();
+        final int sizeOfPktSize = getVarIntSize(pktSize);
+
+        if (sizeOfPktSize > 3) {
+            log.warn("unable to fit {} into {}", pktSize, 3);
+        }
+
+        out.ensureWritable(sizeOfPktSize + pktSize);
+        (new WrapperNetOutputStream(out)).writeVarInt(pktSize);
+        out.writeBytes(msg);
+    }
+}