Netty深度解析：从基础到高级应用

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"Netty实战2013版 - Netty 4.x" 本书是关于Netty 4.x的实战指南，旨在深入解析Netty框架的架构和使用方法。Netty是一个高性能、异步事件驱动的网络应用框架，主要用于快速开发可维护的高性能协议服务器和客户端。它提供了对Java NIO（非阻塞I/O）API的强大支持，是构建高并发、低延迟网络服务的首选工具。在"Part 1: Getting Started"中，作者将介绍Netty与Java NIO API的关系，帮助读者理解Netty如何利用NIO来提升性能。这部分还会引导读者创建第一个Netty应用程序，从而熟悉Netty的基本工作原理和结构。此外，"Netty from the ground up"章节将深入探讨Netty的核心组件，使读者能够构建自己的网络应用基础。 "Part 2: Core Functions/Parts"涵盖了Netty的关键组件，包括Transports（传输层）、Buffers（缓冲区）、ChannelHandler（通道处理器）以及Codec（编解码器）。这些章节详细阐述了如何使用这些组件进行数据传输、内存管理、处理网络事件以及实现自定义的数据编码和解码。 "Part 3: Netty by Example"部分通过一系列示例来展示Netty在实际应用中的能力，如单元测试、WebSocket协议实现、SPDY协议支持，以及通过UDP进行事件广播。这些例子有助于读者更好地理解Netty在不同场景下的应用和配置。在"Part 4: Advanced Topics"中，读者将学习到如何实现自定义的编解码器，选择合适的线程模型，以及如何在EventLoop中注销和重新注册。此外，案例研究章节将提供实际项目中的应用示例，进一步深化对Netty的理解。附录部分介绍了Netty社区，如何参与其中，以及相关的书籍和项目，为读者提供了一个更广泛的Netty生态系统视角，鼓励他们参与到这个活跃的开发者社区中去。这本书不仅适合初学者，也适用于有一定经验的开发者，它将帮助读者掌握Netty 4.x的各个方面，从而在开发高效、可靠的网络应用时发挥其最大潜力。通过深入学习和实践，读者可以充分利用Netty提供的强大功能，解决复杂网络通信问题。

ServerSocketChannel serverChannel = ServerSocketChannel.open();

ServerSocket ss = serverChannel.socket();

InetSocketAddress address = new InetSocketAddress(port);

ss.bind(address); #1

serverChannel.configureBlocking(false);

Selector selector = Selector.open();

serverChannel.register(selector, SelectionKey.OP_ACCEPT); #2

while (true) {

try {

selector.select(); #3

} catch (IOException ex) {

ex.printStackTrace();

// handle in a proper way

break;

}

Set readyKeys = selector.selectedKeys(); #4

Iterator iterator = readyKeys.iterator();

while (iterator.hasNext()) {

SelectionKey key = (SelectionKey) iterator.next();

iterator.remove(); #5

try {

if (key.isAcceptable()) {

ServerSocketChannel server = (ServerSocketChannel)

key.channel();

SocketChannel client = server.accept(); #6

System.out.println("Accepted connection from " +

client);

client.configureBlocking(false);

client.register(selector, SelectionKey.OP_WRITE 

SelectionKey.OP_READ, ByteBuffer.allocate(100)); #7

}

if (key.isReadable()) { #8

SocketChannel client = (SocketChannel) key.channel();

ByteBuffer output = (ByteBuffer) key.attachment();

client.read(output); #9

}

if (key.isWritable()) { #10

SocketChannel client = (SocketChannel) key.channel();

ByteBuffer output = (ByteBuffer) key.attachment();

output.flip();

client.write(output); #11

output.compact();

}

} catch (IOException ex) {

key.cancel();

try {

key.channel().close();

} catch (IOException cex) {

}

©Manning Publications Co. We welcome reader comments about anything in the manuscript - other than typos and

other simple mistakes. These will be cleaned up during production of the book by copyeditors and proofreaders.

http://www.manning-sandbox.com/forum.jspa?forumID=857

#1 Bind server to port

#2 Register the channel with the selector to be interested in new Client connections that get accepted

#3 Block until something is selected

#4 Get all SelectedKey instances

#5 Remove the SelectedKey from the iterator

#6 Accept the client connection

#7 Register connection to selector and set ByteBuffer

#8 Check for SelectedKey for read

#9 Read data to ByteBuffer

#10 Check for SelectedKey for write

#11 Write data from ByteBuffer to channel

This example is more complex than the previous version of EchoServer. This complexity is a

trade off; asynchronous code is typically more complicated than its synchronous counterpart.

Semantically, the original NIO and the new NIO.2 API are similar, but their implementations

are different. Well take a look the differences next, and well implement version 3 of

EchoServer.

1.3.4 EchoServer based on NIO.2

Unlike the original NIO implementation, NIO.2 allows you to issue IO operations and provide

what is called a completion handler (

CompletionHandler class). This completion handler

gets executed after the operation completes. Therefore, execution of the completion handler is

driven by the underlying system and the implementation is hidden from the developer. It also

guarantees that only one

CompletionHandler is executed for channel at the same time. This

approach helps to simplify the code because it removes the complexity that comes with

multithreaded execution.

The major difference between the original NIO and NIO.2 is that you dont have to check

whether an event accours on the Channel and then trigger some action. In NIO.2 you can just

trigger an IO operation and register a completion handler with it, this handler will then get

notified once the operation complates. This removes the need to create your own application

logic to check for completion, which itself results in unnecessary processing.

Now lets see how the same asynchronous EchoServer would be implemented with

NIO2,implementation as shown in the next listing.

Listing 1.7 EchoServer v3: NIO.2

public class PlainNio2EchoServer {

public void serve(int port) throws IOException {

System.out.println("Listening for connections on port " + port);

final AsynchronousServerSocketChannel serverChannel =

AsynchronousServerSocketChannel.open();

InetSocketAddress address = new InetSocketAddress(port);

serverChannel.bind(address); #1

final CountDownLatch latch = new CountDownLatch(1);

serverChannel.accept(null, new

CompletionHandler<AsynchronousSocketChannel, Object>() { #2

@Override

public void completed(final AsynchronousSocketChannel channel,

Object attachment) {

©Manning Publications Co. We welcome reader comments about anything in the manuscript - other than typos and

other simple mistakes. These will be cleaned up during production of the book by copyeditors and proofreaders.

http://www.manning-sandbox.com/forum.jspa?forumID=857

serverChannel.accept(null, this); #3

ByteBuffer buffer = ByteBuffer.allocate(100);

channel.read(buffer, buffer,

new EchoCompletionHandler(channel)); #4

}

@Override

public void failed(Throwable throwable, Object attachment) {

try {

serverChannel.close(); #5

} catch (IOException e) {

// ingnore on close

} finally {

latch.countDown();

}

});

try {

latch.await();

} catch (InterruptedException e) {

Thread.currentThread().interrupt();

}

private final class EchoCompletionHandler implements

CompletionHandler<Integer, ByteBuffer> {

private final AsynchronousSocketChannel channel;

EchoCompletionHandler(AsynchronousSocketChannel channel) {

this.channel = channel;

}

@Override

public void completed(Integer result, ByteBuffer buffer) {

buffer.flip();

channel.write(buffer, buffer, new CompletionHandler<Integer,

ByteBuffer>() { #6

@Override

public void completed(Integer result, ByteBuffer buffer) {

if (buffer.hasRemaining()) {

channel.write(buffer, buffer, this); #7

} else {

buffer.compact();

channel.read(buffer, buffer,

EchoCompletionHandler.this); #8

}

@Override

public void failed(Throwable exc, ByteBuffer attachment) {

try {

channel.close();

} catch (IOException e) {

// ingnore on close

}

});

}

©Manning Publications Co. We welcome reader comments about anything in the manuscript - other than typos and

other simple mistakes. These will be cleaned up during production of the book by copyeditors and proofreaders.

http://www.manning-sandbox.com/forum.jspa?forumID=857

@Override

public void failed(Throwable exc, ByteBuffer attachment) {

try {

channel.close();

} catch (IOException e) {

// ingnore on close

}

#1Bind Server to port

#2 Start to accept new Client connections. Once one is accepted the CompletionHandler will get called.

#3 Again accept new Client connections

#4 Trigger a read operation on the Channel, the given CompletionHandler will be notified once

something was read

#5 Close the socket on error

#6 Trigger a write operation on the Channel, the given CompletionHandler will be notified once

something was written

#7 Trigger again a write operation if something is left in the ByteBuffer

#8 Trigger a read operation on the Channel, the given CompletionHandler will be notified once

something was read

At first glance, it may look like more code than what you had when using the previous NIO

implementation. But notice that NIO.2 handles threading and the creation of the so-called

event loop for you. This approach simplifies the code needed to build a multithreaded NIO

application, even though it may not seem to be the case in this example. As the complexity of

the application increases, the gains become more evident because youll be producing cleaner

code.

In the next section, well look at some known problems that exist in the JDKs NIO

implementation.

1.4 NIO problems and how Netty comes to the rescue

In this section, well look at some problems and limitations of the Java NIO APIs and how Netty

addresses these issues. Having the NIO packages of the JDK is a great step in the right

direction, but users are limited in how they can use them. These issues are often the result of

design choices that were made in the past and arent easy to change now, others are defects.

1.4.1 Cross-platform and compatibility issues

NIO is low level and depends on how the operating system (OS) handles IO. So fulfilling the

requirement of having a unified API exposed in Java, which works and behaves the same on all

OSs, isnt trivial.

When using NIO you often find that your code works fine on Linux, for example, but has

problems on Windows. My general advice, even if youre not using NIO, but its more important

if you are, is to test on all the OSs you want to support or use. Even if all the tests pass on

your Linux workstation, be sure to verify it on the other OSs. If you dont verify, prepare to

have fun later.

other simple mistakes. These will be cleaned up during production of the book by copyeditors and proofreaders.

http://www.manning-sandbox.com/forum.jspa?forumID=857

Ideal as NIO.2 may seem, its only supported in Java 7, and if your application runs on Java

6, you may not be able to use it. Also, at the time of writing, there is no NIO.2 API for

datagram channels (for UDP applications), so its usage is limited to TCP applications only.

Netty addresses this problem by providing a unified API, which allows the same semantics

to work seamlessly on either Java 6 or 7. You dont have to worry about the underlying

version, and you benefit from a simple and consistent API.

1.4.2 Extending ByteBuffer ... or not

As you saw previously, ByteBuffer is used as data container. Unfortunately, the JDK doesnt

contain a

ByteBuffer implementation that allows wrapping an array of ByteBuffer

instances. This functionality is useful if you want to minimize memory copies. If you rethinking

Ill implement it myself, dont waste your time;

ByteBuffer has a private constructor, so it

isnt possible to extend it.

Netty provides its own

ByteBuffer implementation, which gets around this limitation and

goes further by providing several other means of constructing, using, and manipulating a

ByteBuffer all with a simpler API.

1.4.3 Scattering and gathering may leak

Many channel implementations support scattering and gathering. This feature allows writing to

or reading from many

ByteBuffer instances at the same time with better performance. Here

the kernel/OS handles how these are written/read, which often gives you the best performance

because the kernel/OS, being closer to the hardware, knows how to do it in the most efficient

way.

Scattering/gathering is often used if you want to split data in different

ByteBuffer

instances to handle each buffer separately. For example, you may want to have the header in

one

ByteBuffer and the body in another.

Figure 1.4 shows how a scattering read is performed. You pass an array of

ByteBuffer

instances to the

ScatteringByteChannel and the data gets scattered from the channel to

the buffers.

Figure 1.4 A scattering read from a channel

other simple mistakes. These will be cleaned up during production of the book by copyeditors and proofreaders.

http://www.manning-sandbox.com/forum.jspa?forumID=857

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