Let me fix it
 Fixes by https://github.com/netty/netty/pull/5876

Http2EventAdapter's onUnknownFrame() is missing throws Http2Exception
`Http2EventAdapter` implements the `Http2FrameListener` interface but implements the `#onUnknownFrame(...)` method without the interface's `throws Http2Exception`.

... causing compiler errors down the road if you're trying to implement a delegate or something like that.

codec-http2/src/main/java/io/netty/handler/codec/http2/Http2EventAdapter.java
codec-http2/src/main/java/io/netty/handler/codec/http2/Http2EventAdapter.java
diff --git a/codec-http2/src/main/java/io/netty/handler/codec/http2/Http2EventAdapter.java b/codec-http2/src/main/java/io/netty/handler/codec/http2/Http2EventAdapter.java
index 3f5665a40eb..b51e083f150 100644
--- a/codec-http2/src/main/java/io/netty/handler/codec/http2/Http2EventAdapter.java
+++ b/codec-http2/src/main/java/io/netty/handler/codec/http2/Http2EventAdapter.java
@@ -82,7 +82,7 @@ public void onWindowUpdateRead(ChannelHandlerContext ctx, int streamId, int wind
 
     @Override
     public void onUnknownFrame(ChannelHandlerContext ctx, byte frameType, int streamId, Http2Flags flags,
-            ByteBuf payload) {
+            ByteBuf payload) throws Http2Exception {
     }
 
     @Override
