Netty实战第五版：从入门到高级

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《Netty in Action》第五版是Manning Early Access Program（MEAP）系列的一部分，该书专注于介绍和实践高性能网络通信框架Netty。这本书是针对Java NIO（New I/O）API的深入指南，适合网络开发者和系统架构师，特别是那些希望提升应用程序性能和可扩展性的人员。本书分为四个主要部分： 1. **入门篇** (PART1)：首先引导读者理解Netty与Java NIO的关系，包括如何编写第一个Netty应用，逐步构建对Netty核心概念的理解。这部分旨在帮助新手快速上手。 2. **核心功能与模块** (PART2)： - **传输** (Transports)：讲解Netty支持的各种网络传输协议，如TCP、UDP等。 - **缓冲区** (Buffers)：介绍数据在传输过程中的存储和处理机制。 - **Channel Handler**：剖析Netty的事件驱动模型，以及如何通过自定义处理器处理数据流。 - **Codec**：编码解码器的作用，以及如何设计和实现自己的协议转换。 - **应用启动与管理** (Bootstrapping Netty applications)：演示如何有效地初始化和配置Netty服务。 3. **实战篇** (PART3)：通过实例演示Netty的高级功能，如单元测试代码编写、WebSocket支持、SPDY（Google提出的高性能HTTP/2前身）应用，以及使用UDP进行广播消息传递。 4. **进阶主题** (PART4)：深入探讨技术细节，如自定义编码器、选择合适的线程模型、动态注册和注销事件循环，以及案例研究，帮助读者应对复杂场景。附录部分提供了丰富的资源和支持： - **社区参与** (Appendix A)：介绍如何加入Netty社区，与其他开发者交流学习。 - **相关书籍** (Appendix B)：列出其他与Netty相关的图书，便于读者进一步扩展知识面。 - **相关项目** (Appendix C)：推荐其他值得探索的Netty项目，有助于实践和项目参考。《Netty in Action》第五版不仅提供理论知识，还注重实践操作，是Netty开发者和网络编程爱好者的必备参考资料。通过阅读本书，读者将能够熟练掌握Netty的核心原理和高级技巧，从而优化网络应用程序的性能和稳定性。

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.

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