netty4.0

http://netty.io/wiki/new-and-noteworthy.html

New and noteworthy

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• TABLE OF CONTENTS
• Project structure changes
• General API changes
• Buffer API changes
  • ChannelBuffer → ByteBuf
  • Most buffers are dynamic with configurable maximum capacity
  • New interface: CompositeByteBuf
  • Predictable NIO buffer conversion
  • Little endian support changes
  • Pooled ByteBuf
• Channel API changes
  • Revamped ChannelHandler interface
    • Upstream → Inbound, Downstream → Outbound
    • New ChannelHandler hierarchy
    • ChannelHandler with no event object
    • Simplified channel state model
    • Per-handler buffer
  • Sensible and less error-prone inbound traffic suspension
    • Suspension of accepting incoming connections
  • Half-closed sockets
  • Flexible I/O thread allocation
  • Ability to create a Channel from an existing JDK socket
  • Deregistration and re-registration of a Channel from·to an I/O thread
  • Scheduling an arbitrary task to be run by an I/O thread
  • Simplified shutdown
  • Type-safe ChannelOptions
  • AttributeMap
  • New bootstrap API
    • ChannelPipelineFactory → ChannelInitializer
  • ChannelFuture split to ChannelFuture and ChannelPromise
• Well-defined thread model
  • No more ExecutionHandler - it's in the core.
• Codec framework changes
  • Codec embedder → EmbeddedChannel
  • HTTP codec
  • Changes in transport implementations
• Case study: porting the Factorial example
  • Porting the server
  • Porting the client

This document walks you through the list of notable changes and new features in the major Netty release to give you an idea to port your application to the new version.

Project structure changes

The package name of Netty has been changed from org.jboss.netty to io.netty since we are not part of JBoss.org anymore.

The binary JAR has been split into multiple submodules so that a user can exclude unnecessary features from the class path. The current structure looks like this following:

Module	Description
netty	project parent
common	utility and logging
buffer	buffer API
transport	channel API and its core implementations
transport-rtrx	RTRX transport implementation
transport-sctp	SCTP transport implementation
transport-udt	UDT transport implementation
handler	channel handlers
codec	codec framework
codec-http	HTTP, Web Sockets, SPDY, and RTSP codec
codec-socks	Socks codec
example	examples
all	generates an all-in-one JAR
tarball	generates a tarball distribution

All Netty jars (except of the netty-all) are now OSGI bundles and can be used in your favorite OSGI-Container.

General API changes

• Most operations in Netty now supports method chaining for brevity.
• Non-configuration getters have no get/is prefix anymore (e.g. Channel.getRemoteAddress() →Channel.remoteAddress())

Buffer API changes

ChannelBuffer → ByteBuf

Thanks to the structural changes mentioned above, the buffer API can be used as a separate package. Therefore, the type name ChannelBuffer does not make sense anymore, and has been renamed to ByteBuf.

The utility class ChannelBuffers, which creates a new buffer, has been split into two utility classes, Unpooledand BufUtil. As can be guessed from its name, 4.0 introduced pooled ByteBufs which can be allocated via the ByteBufAllocator implementations.

Most buffers are dynamic with configurable maximum capacity

In 3.x, buffers were fixed or dynamic. The capacity of a fixed buffer does not change once it is created while the capacity of a dynamic buffer changes whenever write*(...) method requires more space.

Since 4.0, all buffers are dynamic. However, it's better than the old dynamic. You can decrease or increase the capacity of a buffer more easily and more safely. It's easy because there is a new methodByteBuf.capacity(int newCapacity). It's safe because you can set the maximum capacity of a buffer so that it does not grow boundlessly.

// No more dynamicBuffer() - use buffer(). ByteBuf buf = ByteBuf.buffer(); // Increase the capacity of the buffer. buf.capacity(1024); ... // Decrease the capacity of the buffer (the last 512 bytes are deleted.) buf.capacity(512);

The only exception is the buffer which wraps a single buffer or a single byte array, created by wrappedBuffer(). You cannot increase its capacity because it invalidates the whole point of wrapping an existing buffer - saving memory copies. If you want to change the capacity after you wrap a buffer, you should just create a new buffer with enough capacity and copy the buffer you wanted to wrap.

New interface: CompositeByteBuf

A new interface named CompositeByteBuf defines various advanced operations for composite buffer implementations. A user can save bulk memory copy operations using a composite buffer at the cost of relatively expensive random access. To crease a new composite buffer, use eitherUnpooled.wrappedBuffer(...) like before or Unpooled.compositeBuffer(...).

Predictable NIO buffer conversion

The contract of ChannelBuffer.toByteBuffer() and its variants were not deterministic enough in 3.x. It was impossible for a user to know if they will return a view buffer with shared data or a copied buffer with separate data. 4.0 replaces toByteBuffer() withByteBuf.nioBufferCount(), nioBuffer(), and nioBuffers(). If nioBufferCount() returns 0, a user can always get a copied buffer by calling copy().nioBuffer().

Little endian support changes

Little endian support has been changed significantly. Previously, a user was supposed to specify a LittleEndianHeapChannelBufferFactoryor wrap an existing buffer with the desired byte order to get a little endian buffer. 4.0 adds a new method: ByteBuf.order(ByteOrder). It returns a view of the callee with the desired byte order:

import io.netty.buffer.ByteBuf; import io.netty.buffer.Unpooled; import java.nio.ByteOrder; ByteBuf buf = Unpooled.buffer(4); buf.setInt(0, 1); // Prints '00000001' System.out.format("%08x%n", buf.getInt(0)); ByteBuf leBuf = buf.order(ByteOrder.LITTLE_ENDIAN); // Prints '01000000' System.out.format("%08x%n", leBuf.getInt(0)); assert buf != leBuf; assert buf == buf.order(ByteOrder.BIG_ENDIAN);

Pooled ByteBuf

As mentioned before Netty 4 introduced pooled ByteBuf instances. This can be useful for many reasons, to list a few:

• Limit GC pressure because of heavy allocation/deallocation when using unpooled ByteBufs
• Better handling of direct (native) ByteBuf
• A ByteBuf can be obtained via a ByteBufAllocator.

public interface ByteBufAllocator { ByteBuf buffer(); ByteBuf buffer(int initialCapacity); ByteBuf buffer(int initialCapacity, int maxCapacity); ByteBuf heapBuffer(); ByteBuf heapBuffer(int initialCapacity); ByteBuf heapBuffer(int initialCapacity, int maxCapacity); ByteBuf directBuffer(); ByteBuf directBuffer(int initialCapacity); ByteBuf directBuffer(int initialCapacity, int maxCapacity); ByteBuf ioBuffer(); CompositeByteBuf compositeBuffer(); CompositeByteBuf compositeBuffer(int maxNumComponents); CompositeByteBuf compositeHeapBuffer(); CompositeByteBuf compositeHeapBuffer(int maxNumComponents); CompositeByteBuf compositeDirectBuffer(); CompositeByteBuf compositeDirectBuffer(int maxNumComponents); }

To get the current ByteBufAllocator from a handler, use the ChannelHandlerContext.alloc() or Channel.alloc() method:

Channel channel = ...; ByteBuf buf = channel.alloc().buffer(512); .... channel.write(buf); ChannelHandlerContext ctx = ... ByteBuf buf2 = ctx.alloc().buffer(512); .... channel.write(buf2)

Once a ByteBuf was written to the remote peer it will automatically get released to the pool again.

The default ByteBufAllocator is PooledByteBufAllocator. If you do not wish to use buffer pooling or use your own allocator, you can useChannel.config().setAllocator(..) with an alternative allocator such as UnpooledByteBufAllocator.

Channel API changes

In 4.0, many classes under the io.netty.channel package has gone through a major overhaul, and thus simple text search-and-replace will not make your 3.x application work with 4.0. This section will try to show the thought process behind such a big change, rather than being an exhaustive resource for all the changes.

Revamped ChannelHandler interface

Upstream → Inbound, Downstream → Outbound

The terms 'upstream' and 'downstream' was pretty confusing to beginners. 4.0 use 'inbound' and 'outbound' wherever possible.

New ChannelHandler hierarchy

In 3.x, ChannelHandler was just a tag interface, and ChannelUpstreamHandler, ChannelDownstreamHandler, andLifeCycleAwareChannelHandler defined the actual handler methods. In Netty 4, ChannelHandler mergesLifeCycleAwareChannelHandler along with a couple more methods which is going to be useful to any handler implementations:

public interface ChannelHandler { void beforeAdd(ChannelHandlerContext ctx) throws Exception; void afterAdd(ChannelHandlerContext ctx) throws Exception; void beforeRemove(ChannelHandlerContext ctx) throws Exception; void afterRemove(ChannelHandlerContext ctx) throws Exception; void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception; void userEventTriggered(ChannelHandlerContext ctx, Object evt) throws Exception; ... }

The following diagram depicts the new type hierarchy:

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ChannelHandler with no event object

In 3.x, every I/O operation created a ChannelEvent object. For each read / write, it additionally created a new ChannelBuffer. It simplified the internals of Netty quite a lot because it delegates resource management and buffer pooling to JVM. However, it often was the root cause of the GC pressure and uncertainty which are sometimes observed in a Netty-based application under high load.

4.0 removes event object creation almost completely by replacing the event objects with strongly typed method invocations. 3.x has catch-all event handler methods such as handleUpstream() and handleDownstream(), but it's not the case anymore. Every event type has its own handler method now:

// Before: void handleUpstream(ChannelHandlerContext ctx, ChannelEvent e) throws Exception; void handleDownstream(ChannelHandlerContext ctx, ChannelEvent e) throws Exception; // After: void channelRegistered(ChannelHandlerContext ctx) throws Exception; void channelUnregistered(ChannelHandlerContext ctx) throws Exception; void channelActive(ChannelHandlerContext ctx) throws Exception; void channelInactive(ChannelHandlerContext ctx) throws Exception; void inboundBufferUpdated(ChannelHandlerContext ctx) throws Exception; void bind(ChannelHandlerContext ctx, SocketAddress localAddress, ChannelFuture future) throws Exception; void connect( ChannelHandlerContext ctx, SocketAddress remoteAddress, SocketAddress localAddress, ChannelFuture future) throws Exception; void disconnect(ChannelHandlerContext ctx, ChannelFuture future) throws Exception; void close(ChannelHandlerContext ctx, ChannelFuture future) throws Exception; void deregister(ChannelHandlerContext ctx, ChannelFuture future) throws Exception; void flush(ChannelHandlerContext ctx, ChannelFuture future) throws Exception; void read(ChannelHandlerContext ctx); void sendFile(ChannelHandlerContext ctx, FileRegion region, ChannelPromise promise) throws Exception;

ChannelHandlerContext also has been changed to reflect the changes mentioned above:

// Before: ctx.sendUpstream(evt); // After: ctx.fireInboundBufferUpdated();

All these changes mean a user cannot extend the non-existing ChannelEvent interface anymore. How do the user define his or her own event type such as IdleStateEvent? ChannelHandler in 4.0 has a handler method called userEventTriggered() which is dedicated to this specific user case.

Simplified channel state model

When a new connected Channel is created in 3.x, at least three ChannelStateEvent is triggered: channelOpen, channelBound, andchannelConnected. When a Channel is closed, it's channelDisconnected, channelUnbound, and channelClosed.

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However, it's of dubious value to trigger that many events. It is more useful for a user to get notified when a Channel enters the state where it can perform reads and writes.

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channelOpen, channelBound, and channelConnected are merged to channelActive. channelDisconnected, channelUnbound, andchannelClosed are merged to channelInactive. Likewise, Channel.isBound() and isConnected() are merged to isActive().

Note that channelRegistered and channelUnregistered are not equivalent to channelOpen and channelClosed. They are new states introduced to support dynamic registration, deregistration, and re-registration of a Channel, as illustrated below:

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Per-handler buffer

Unlike 3.x, which creates a new heap buffer on every read operation to trigger a upstream MessageEvent, 4.0 does not create a new buffer every time. It reads data from a socket directly into the inbound buffer created by user's ChannelInboundByteHandler andChannelInboundMessageHandler implementation.

Because the inbound buffer created by the handler is reused until the associated channel is closed, the GC overhead and the memory bandwidth consumption stays minimal. Also, a user has complete control over when the received data is discarded, the codec implementation becomes much simpler and efficient.

Similarly, it is not Netty that creates an outbound buffer. User's ChannelOutboundByteHandler and ChannelOutboundMessageHandler do.

Not necessarily one event per message

4.0 does not have messageReceived or writeRequested handler method anymore. They are replaced with inboundBufferUpdated andflush. A user enqueues one or more messages to an inbound (or outbound) buffer and triggers an inboundBufferUpdated (or flush) event.

public void inboundBufferUpdated(ChannelHandlerContext ctx) { Queue<MyMessage> in = ctx.inboundMessageBuffer(); Queue<MyNewMessage> out = ctx.nextInboundMessageBuffer(); for (;;) { MyMessage m = in.poll(); if (m == null) { break; } MyNewMessage decoded = decode(m); out.add(decoded); } ctx.fireInboundBufferUpdated(); } public void flush(ChannelHandlerContext ctx, ChannelFuture future) { Queue<MyNewMessage> in = ctx.outboundMessageBuffer(); Queue<MyMessage> out = ctx.nextOutboundMessageBuffer(); for (;;) { MyNewMessage m = in.poll(); if (m == null) { break; } MyMessage encoded = encode(m); out.add(encoded); } ctx.flush(future); }

Alternatively, a user can trigger such event for every single inbound (or outbound) message to emulate the old behavior although it might be less efficient than the new way.

Message handler vs. Byte handler

In 3.x, a MessageEvent holds an arbitrary object. It can be either a ChannelBuffer or a user-defined object and they are treated same:

@Override public void messageReceived(ChannelHandlerContext ctx, MessageEvent evt) { Object msg = evt.getMessage(); if (msg instanceof ChannelBuffer) { ChannelBuffer buf = (ChannelBuffer) msg; ... } else { MyMessage myMsg = (MyMessage) msg; ... } }

In 4.0, they are treated differently since a handler does not handle an individual message but handles multiple messages:

public void inboundBufferUpdated(ChannelHandlerContext ctx) { if (ctx.hasInboundByteBuffer()) { ByteBuf buf = ctx.inboundByteBuffer(); ... } else { Queue<MyMessage> buf = ctx.inboundMessageBuffer(); for (;;) { MyMessage msg = buf.poll(); if (buf == null) { break; } ... } } }

You might find it interesting that a ServerChannel's handler is an inbound handler whose inbound buffer is Queue<Channel>.

Handler adapters

Most users will find creating and managing its life cycle tedious, and therefore a user is supposed to extend the predefined adapter classes to make their life easier:

• ChannelHandlerAdapter
• ChannelStateHandlerAdapter
• ChannelOperationHandlerAdapter
• ChannelInboundMessageHandlerAdapter
• ChannelInboundByteHandlerAdapter
• ChannelOutboundMessageHandlerAdapter
• ChannelOutboundByteHandlerAdapter

Sensible and less error-prone inbound traffic suspension

3.x had an unintuitive inbound traffic suspension mechanism provided by Channel.setReadable(boolean). It introduced complicated interactions between ChannelHandlers and the handlers were easy to interfere with each other if implemented incorrectly.

In 4.0, a new outbound operation called read() has been added. If you turn off the default auto-read flag withChannel.config().setAutoRead(false), Netty will not read anything until you explicitly invokes the read() operation. Once the read()operation you issued is complete and the channel again stopped reading, an inbound event called channelReadSuspended() will be triggered so that you can re-issue another read() operation. You can also intercept a read() operation to perform more advanced traffic control.

Suspension of accepting incoming connections

There was no way for a user to tell Netty 3.x to stop accepting incoming connections except for blocking the I/O thread or closing the server socket. 4.0 respects the read() operation when the auto-read flag is not set, just like an ordinary channel.

Half-closed sockets

TCP and SCTP allow a user to shut down the outbound traffic of a socket without closing it completely. Such a socket is called 'a half-closed socket', and a user can make a half-closed socket by calling SocketChannel.shutdownOutput() method. If a remote peer shuts down the outbound traffic, SocketChannel.read(..) will return -1, which was seemingly indistinguishable from a closed connection.

3.x did not have shutdownOutput() operation. Also, it always closed the connection when SocketChannel.read(..) returns -1.

To support a half-closed socket, 4.0 adds SocketChannel.shutdownOutput() method, and a user can set the 'ALLOW_HALF_CLOSURE'ChannelOption to prevent Netty from closing the connection automatically even if SocketChannel.read(..) returns -1.

Flexible I/O thread allocation

In 3.x, a Channel is created by a ChannelFactory and the newly created Channel is automatically registered to a hidden I/O thread. 4.0 replaces ChannelFactory with a new interface called EventLoopGroup which consists of one or more EventLoops. Also, a new Channel is not registered to the EventLoopGroup automatically but a user has to call EventLoopGroup.register() explicitly.

Thanks to this change (i.e. separation of ChannelFactory and I/O threads), a user can register different Channel implementations to the sameEventLoopGroup, or same Channel implementations to different EventLoopGroups. For example, you can run a NIO server socket, NIO client sockets, NIO UDP sockets, and in-VM local channels in the same I/O thread. It should be very useful when writing a proxy server which requires minimal latency.

Ability to create a Channel from an existing JDK socket

3.x provided no way to create a new Channel from an existing JDK socket such as java.nio.channels.SocketChannel. You can with 4.0.

Deregistration and re-registration of a Channel from·to an I/O thread

Once a new Channel is created in 3.x, it is completely tied to a single I/O thread until its underlying socket is closed. In 4.0, a user can deregister a Channel from its I/O thread to gain the full control of its underlying JDK socket. For example, you can take advantage of high-level non-blocking I/O Netty provides to deal with complex protocols, and then later deregister the Channel and switch to blocking mode to transfer a file at possible maximum throughput. Of course, it is possible to register the deregistered Channel back again.

java.nio.channels.FileChannel myFile = ...; java.nio.channels.SocketChannel mySocket = java.nio.channels.SocketChannel.open(); // Perform some blocking operation here. ... // Netty takes over. SocketChannel ch = new NioSocketChannel(mySocket); EventLoopGroup group = ...; group.register(ch); ... // Deregister from Netty. ch.deregister().sync(); // Perform some blocking operation here. mySocket.configureBlocking(false); myFile.transferFrom(mySocket, ...); // Register back again to another event loop group. EventLoopGroup anotherGroup = ...; anotherGroup.register(ch);

Scheduling an arbitrary task to be run by an I/O thread

When a Channel is registered to an EventLoopGroup, the Channel is actually registered to one of the EventLoops which is managed by theEventLoopGroup. EventLoop implements java.util.concurrent.ScheduledExecutorService. It means a user can execute or schedule an arbitrary Runnable or Callable in an I/O thread where the user's channel belongs to. Along with the new well-defined thread model, which will be explained later, it became extremely easier to write a thread-safe handler.

public class MyHandler extends ChannelOutboundMessageHandlerAdapter { ... public void flush(ChannelHandlerContext ctx, ChannelFuture f) { ... ctx.flush(f); // Schedule a write timeout. ctx.executor().schedule(new MyWriteTimeoutTask(), 30, TimeUnit.SECONDS); ... } } public class Main { public static void main(String[] args) throws Exception { // Run an arbitrary task from an I/O thread. Channel ch = ...; ch.executor().execute(new Runnable() { ... }); } }

Simplified shutdown

There's no more releaseExternalResources(). You can close all open channels immediately and make all I/O threads stop themselves by calling EventLoopGroup.shutdown(), just like you shut down your thread pool with java.util.concurrent.ExecutorService.shutdown().

Type-safe ChannelOptions

There are two ways to configure the socket parameters of a Channel in Netty. One is to call the setters of a ChannelConfig explicitly, such asSocketChannelConfig.setTcpNoDelay(true). It is the most type-safe way. The other is to call ChannelConfig.setOption() method. Sometimes you have to determine what socket options to configure in runtime, and this method is ideal for such cases. However, it is error-prone in 3.x because a user has to specify the option as a pair of a string and an object. If a user calls with the wrong option name or value, he or she will encounter a ClassCastException or the specified option might even be ignored silently.

4.0 introduces a new type called ChannelOption, which provides type-safe access to socket options.

ChannelConfig cfg = ...; // Before: cfg.setOption("tcpNoDelay", true); cfg.setOption("tcpNoDelay", 0); // Runtime ClassCastException cfg.setOption("tcpNoDelays", true); // Typo in the option name - ignored silently // After: cfg.setOption(ChannelOption.TCP_NODELAY, true); cfg.setOption(ChannelOption.TCP_NODELAY, 0); // Compile error

AttributeMap

In response to user demand, you can attach any object to Channel and ChannelHandlerContext. A new interface called AttributeMap, which Channel and ChannelHandlerContext extend, has been added. Instead, ChannelLocal and Channel.attachment are removed. The attributes are garbage-collected when their associated Channel is garbage-collected.

public class MyHandler extends ChannelInboundMessageHandlerAdapter<MyMessage> { private static final AttributeKey<MyState> STATE = new AttributeKey<MyState>("MyHandler.state"); @Override public void channelRegistered(ChannelHandlerContext ctx) { ctx.attr(STATE).set(new MyState()); ctx.fireChannelRegistered(); } @Override public void messageReceived(ChannelHandlerContext ctx, MyMessage msg) { MyState state = ctx.attr(STATE).get(); } ... }

New bootstrap API

The bootstrap API has been rewritten from scratch although its purpose stays same; it performs the typical steps required to make a server or a client up and running, often found in boilerplate code.

The new bootstrap also employs fluent interface.

public static void main(String[] args) throws Exception { // Configure the server. ServerBootstrap b = new ServerBootstrap(); try { b.group(new NioEventLoopGroup(), new NioEventLoopGroup()) .channel(new NioServerSocketChannel()) .option(ChannelOption.SO_BACKLOG, 100) .localAddress(8080) .childOption(ChannelOption.TCP_NODELAY, true) .childHandler(new ChannelInitializer<SocketChannel>() { @Override public void initChannel(SocketChannel ch) throws Exception { ch.pipeline().addLast(handler1, handler2, ...); } }); // Start the server. ChannelFuture f = b.bind().sync(); // Wait until the server socket is closed. f.channel().closeFuture().sync(); } finally { // Shut down all event loops to terminate all threads. b.shutdown(); } }

ChannelPipelineFactory → ChannelInitializer

As you noticed in the example above, there is no ChannelPipelineFactory anymore. It has been replaced with ChannelInitializer, which gives more control over Channel and ChannelPipeline configuration.

Please note that you don't create a new ChannelPipeline by yourself. After observing many use cases reported so far, the Netty project team concluded that it has no benefit for a user to create his or her own pipeline implementation or to extend the default implementation. Therefore,ChannelPipeline is not created by a user anymore. ChannelPipeline is automatically created by a Channel.

ChannelFuture split to ChannelFuture and ChannelPromise

ChannelFuture has been split into ChannelFuture and ChannelPromise. This does not only make the contract of consumer and producer of an asynchronous operation explicit, but also make it more safe to use the returned ChannelFuture in a chain (like filtering), because the state of the ChannelFuture cannot be changed.

Due to this change, some methods now accepts ChannelPromise rather than ChannelFuture to modify its state.

Well-defined thread model

There is no well-defined thread model in 3.x although there was an attempt to fix its inconsistency in 3.5. 4.0 defines a strict thread model that helps a user write a ChannelHandler without worrying too much about thread safety.

• Netty will never call a ChannelHandler's methods concurrently, unless the ChannelHandler is annotated with @Shareable. This is regardless of the type of handler methods - inbound, outbound, or life cycle event handler methods.
  • A user does not need to synchronize either inbound or outbound event handler methods anymore.
  • 4.0 disallows adding a ChannelHandler more than once unless it's annotated with @Sharable.
• There is always happens-before relationship between each ChannelHandler method invocations made by Netty.
  • A user does not need to define a volatile field to keep the state of a handler.
• A user can specify an EventExecutor when he or she adds a handler to a ChannelPipeline.
  • If specified, the handler methods of the ChannelHandler are always invoked by the specified EventExecutor.
  • If unspecified, the handler methods are always invoked by the EventLoop that its associated Channel is registered to.
• EventExecutor and EventLoop assigned to a handler or a channel are always single-threaded.
  • The handler methods will always be invoked by the same thread.
  • If multithreaded EventExecutor or EventLoop is specified, one of the threads will be chosen first and then the chosen thread will be used until deregistration.
  • If two handlers in the same pipeline are assigned with different EventExecutors, they are invoked simultaneously. A user has to pay attention to thread safety if more than one handler access shared data even if the shared data is accessed only by the handlers in the same pipeline.
• The ChannelFutureListeners added to ChannelFuture are always invoked by the EventLoop thread assigned to the future's associatedChannel.

No more ExecutionHandler - it's in the core.

You can specify an EventExecutor when you add a ChannelHandler to a ChannelPipeline to tell the pipeline to always invoke the handler methods of the added ChannelHandler via the specified EventExecutor.

Channel ch = ...; ChannelPipeline p = ch.pipeline(); EventExecutor e1 = new DefaultEventExecutor(16); EventExecutor e2 = new DefaultEventExecutor(8); p.addLast(new MyProtocolCodec()); p.addLast(e1, new MyDatabaseAccessingHandler()); p.addLast(e2, new MyHardDiskAccessingHandler());

EventExecutor is a supertype of EventLoop, and it also extends ScheduledExecutorService.

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Codec framework changes

There were substantial internal changes in codec framework because 4.0 requires a handler to create and manage its buffer (see Per-handler buffer section in this document.) However, the changes from a user's perspective are not very big.

• Core codec classes are moved to the io.netty.handler.codec package.
• FrameDecoder has been renamed to ByteToMessageDecoder.
• OneToOneEncoder and OneToOneDecoderwere replaced with MessageToMessageEncoder and MessageToMessageDecoder.
• The method signatures of decode(), decodeLast(), encode() were changed slightly to support generics and to remove redundant parameters.

Codec embedder → EmbeddedChannel

Codec embedder has been replaced by io.netty.channel.embedded.EmbeddedByteChannel and EmbeddedMessageChannel.EmbeddedChannel allows a user unit-test any kind of pipeline including a codec.

HTTP codec

HTTP decoders now always generates multiple message objects per a single HTTP message:

1 * HttpRequest / HttpResponse 0 - n * HttpContent 1 * LastHttpContent

For more detail, please refer to the updated HttpSnoopServer example. If you wish not to deal with multiple messages for a single HTTP message, you can put an HttpObjectAggregator in the pipeline. HttpObjectAggregator will transform multiple messages into a singleFullHttpRequest or FullHttpResponse.

Changes in transport implementations

The following transports were newly added:

• AIO socket transport which uses the NIO.2 AsynchronousSocketChannel
• OIO SCTP transport
• UDT transport

Case study: porting the Factorial example

This section shows rough steps to port the Factorial example from 3.x to 4.0. The Factorial example has been ported to 4.0 already in theio.netty.example.factorial package. Please browse the source code of the example to find every bits changed.

Porting the server

1. Rewrite FactorialServer.run() method to use the new bootstrap API.
   1. No ChannelFactory anymore. Instantiate NioEventLoop (one for accepting incoming connections and the other for handling the accepted connections) by yourself.
2. Rename FactorialServerPipelineFactory to FactorialServerInitializer.
   1. Make it extends ChannelInitializer<Channel>.
   2. Instead of creating a new ChannelPipeline, get it via Channel.pipeline().
3. Make FactorialServerHandler extends ChannelInboundMessageHandlerAdapter<BigInteger>.
   1. Replace channelDisconnected() with channelInactive().
   2. handleUpstream() is not used anymore.
4. Make BigIntegerDecoder extend ByteToMessageDecoder<BigInteger>.
5. Make NumberEncoder extend MessageToByteEncoder<Number>.
   1. encode() does not return a buffer anymore. Fill the encoded data to the buffer provided by ByteToMessageDecoder.

Porting the client

Mostly same with porting the server, but you need to pay attention when you write a potentially large stream.

1. Rewrite FactorialClient.run() method to use the new bootstrap API.
2. Rename FactorialClientPipelineFactory to FactorialClientInitializer.
3. Make FactorialClientHandler extends ChannelInboundMessageHandlerAdapter<BigInteger>
   1. At this point, you find there is no Channel.isWritable() nor channelInterestChanged() in 4.0. Instead, you maintain the number of pending writes by yourself. The new sendNumbers() could look like the following:

private void sendNumbers() { // Do not send more than 4096 numbers. boolean finished = false; MessageBuf<Object> out = ctx.nextOutboundMessageBuffer(); while (out.size() < 4096) { if (i <= count) { out.add(Integer.valueOf(i)); i ++; } else { finished = true; break; } } ChannelFuture f = ctx.flush(); if (!finished) { f.addListener(numberSender); } } private final ChannelFutureListener numberSender = new ChannelFutureListener() { @Override public void operationComplete(ChannelFuture future) throws Exception { if (future.isSuccess()) { sendNumbers(); } } };