New and noteworthy in 5.0
This document walks you through the list of notable changes and new features in the major Netty release (since 4.1) to give you an idea to port your application to the new version.
Unlike the changes between 3.x and 4.0, 5.0 did not change a lot although it made quite a bit of breakthrough in its design simplicity. We tried to make the transition from 4.x to 5.0 as smooth as possible, but please let us know if you encounter any issues during migration.
New Buffer API replaces ByteBuf
Netty 5 introduces a new Buffer API, that is simpler and safer to use than ByteBuf. For details, see the pull request #11347. In summary, the new API has the following headline differences:
- Aliasing is no longer allowed. In other words, you can no longer have multiple buffers referring to the same memory.
- This means
duplicate, and their retaining variants are gone.
- New APIs are introduced as replacements:
send. The contracts of these methods prevent aliasing.
- This means
- Reference counting is effectively gone.
releasemethods are gone. Buffers instead have a
closemethod, which will be called at the end of the lifetime for the buffer.
- APIs and integrations should be designed such that buffers always have a unique and clear ownership. "Borrowing" of buffers should be minimised and discouraged in APIs, since reference counting can no longer be used to keep track of borrows or references at runtime.
- Most places where
retainwas used were effectively just trying to cancel the effect of an unconditional
releasein a super-class. In these cases, you can use
split, since you most likely only want to pass down the readable part of the buffer anyway.
sendmethod and Send interface can be used to encode the transfer of ownership in the type system.
- Buffers are always big-endian, and the
*LEmethods are gone.
- To perform little-endian reads or writes, use the
reverseBytesfamily of methods on
Long, etc. in conjunction with the big-endian read and write methods on the buffer.
BufferUtilhas a method for reversing "medium" (3 byte) integers.
- To perform little-endian reads or writes, use the
- Buffer implementations have higher test coverage, and more consistent behaviours.
Simplified handler type hierarchy
ChannelOutboundHandler have been merged into [
ChannelHandler] now has both inbound and outbound handler methods.
ChannelDuplexHandlerAdapter have been removed and replaced by
Because it is now impossible to tell if a handler is an inbound handler or an outbound handler,
CombinedChannelDuplexHandler has been replaced by
For more information about this change, please refer to the pull request #1999.
If you are using
SimpleChannelInboundHandler, you have to rename
ChannelHandler method signature changes.
ChannelHandler outbound methods (except
read) now return a
Future<Void>. This ensures proper propagation and chaining.
Beside this we also renamed
channelExceptionCaught(...) to make it clear that this handles inbound exceptions.
It's now possible to fire user / custom events in both directions through the pipeline. For inbound events you would use
fireChannelInboundEvent(...) (replacement of
fireUserEventTriggered(...) and for outbound events
Both of these can be intercepted by methods defined in
ChannelHandler as usual.
ChannelHandler.pendingOutboundBytes(...) method added
It's now easily possible to influence the writability of the
Channel by the contained
ChannelHandler in the
ChannelPipeline. This makes it possible to influence back pressure when the
ChannelHandler itself buffers outbound data.
Netty5 has support for half-closure build in its core now. For this
ChannelHandler.channelShutdown were introduced. Beside this also
ChannelOutboundInvoker.shutdown(...) where added. This replace the old
DuplexChannel abstraction, which was completely removed. For more details see the pull request #12468.
ChannelHandlerContext doesn't extend
We Changed ChannelHandlerContext to not extend AttributeMap anymore. In the case of you using attributes you should directly use
Channel which still extends
In netty 4.x we added the ability to add
ChannelHandlers to a
ChannelPipeline with an explicit
EventExecutorGroup. While this seemed like a good idea it turned out that it has quite some problems when it comes to life-cycles:
handlerAdded(...)may be called at the "wrong time". This can result in a lot of problems. At worse it could mean that
handlerRemoved(...)is called and the handler frees some native memory as it expect that the handler will never used anymore. What could happen here is that after it is called
channelRead(...)may be called which then try to access the previous freed memory and so crash the JVM.
- correctly implement "visibility" in terms of concurrent access / modifications of the pipeline is quite problematic as well.
With this in mind we realised that really what the user mostly want is to have the incoming messages handled by another thread to process the business-logic. This is better be done in a custom implementation provided by the user as the user has a better handle on when things can be destroyed or not.
Channel.eventLoop() renamed to
In netty 5.x we added the
executor() method to
ChannelOutboundInvoker and as this method returns
EventExecutor we did decide to remove the
eventLoop() method from
Channel and just override
executor() to return
EventLoopGroup.isCompatible(...) method added
It's now possible to check if a
Channel sub-type is compatible with the
EventLoop before trying to use it. This can help to select the correct
New methods were added to the
EventLoop interface to allow registration and deregistration of
Channels. These should not be used by the user directly but by
Channel implementations itself.
The Channel.Unsafe interface was completely removed so its not possible for the end-user to mess-up internals.
ChannelOutboundBuffer not part of the
Channel API anymore
ChannelOutboundBuffer is an implementation details of our
AbstractChannel implementation and so was removed for the
Channel itself completely.
Channel.beforeBeforeWritable() removed and
Channel.bytesBeforeUnwritable() renamed to
We removed the
Channel.beforeBeforeWritable() method as it was not used at all and renamed
Future / Promise
The support for ProgressiveFuture / ProgressivePromise was completely removed in netty 5. The reason for this was that while it may have been useful sometimes it also did require that all handlers in the pipeline take special action here if they did chain up promises. This was not really the case and is quite cumbersome to do in reality.
Due the stated problems we decided that it would be best to just remove this feature all together as there was not much usage of this feature anyway. It is better to not support something at all than have something only work "sometimes". This also means there is less code to maintain.
In netty 4.1.x it was possible to use the
voidPromise() method to obtain a special
ChannelPromise implementation that could be used for various IO operations (like
write) to reduce the number of objects created. While the motivation of this feature was good it turned out that this special case of a
ChannelPromise did actually bring a lot of problems:
ChannelHandlerthat did add a
ChannelFutureListenerto the promise did have to call
unvoid()first to ensure that it is safe to add a listener. Missing to do so would lead to a
- wait() / sync() operations were not supported at all.
- Some operations would allow to use the
VoidChannelPromiseand some not.
API changes to
Future.removeListener()have been removed. We removed the ability to remove previous added listeners. This feature was not really used and so it allowed us to remove some complexity and remove some API surface.
- The uninterruptible variants of the
awaitmethods have been removed.
Future.isFailed()method has been added, that checks that the future is both completed and failed. This is similar to the existing
Future.isSuccess()which checks that a future is both completed and successfully so.
Promise.setUncancellablenow only returns
trueif the promise transitions from "incomplete" to "uncancellable". Specifically, this method now returns
falseif the promise has already been completed, where in 4.1 it would return
truein such cases.
Future.flatMap()methods have been added, which makes it easy to compose and create new futures based on existing ones. These methods handle failures and cancellation properly through propagation.
- All blocking methods were removed from the
Futureinterface as it was easy for people to miss-use these and so block the
EventLoop. If you still need to block from outside the
EventLoopyou will need to convert the
Future.asStage(). The returned
FutureCompletionStageprovides blocking methods.
All our compression implementations were changed to make use of the new Compression API to make it easier to re-use in different codecs without the need to create an extra
- Dropped support for multi-part from netty-core. This is to be migrated to netty 5 as a contributor repository in the future. (https://github.com/netty/netty/pull/11830)
- Dropped older websocket drafts (https://github.com/netty/netty/pull/11831)
- HTTP/2 Header validation is now enabled by default. This will cause malformed requests to be rejected by default.
Rarely used codecs moved to Netty Contrib
To slim the code base down and ease the maintenance burden, the following codecs and handlers have been moved to Netty Contrib:
GraalVM and Native Image
Netty is now automatically initialised at runtime, with minimum size overhead in the native image. The minimum supported Graal version is now 22.1, Java 17.