Netty in Action 第五版：大数据学习指南

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"《Netty in Action》第五版是一本专为大数据领域学习者编写的全英文技术书籍，通过Manning出版社的MEAP（Manning Early Access Program）发布。这本书深入介绍了Netty框架及其与Java NIO API的结合使用，涵盖了从基础到高级的各种主题，旨在帮助读者构建高性能、可扩展的网络应用。" 在本书中，作者共分为四个部分来详细讲解Netty：第一部分：入门 1. Netty与Java NIO APIs：这部分介绍了Netty如何与Java的非阻塞I/O（Non-blocking I/O）APIs协同工作，包括选择Netty而非标准Java NIO的原因以及它们之间的关系。 2. 你的第一个Netty应用：引导读者创建并理解一个简单的Netty应用程序，从而熟悉Netty的基本架构和编程模式。 3. Netty从零开始：深入探讨Netty的核心概念，如事件驱动模型、线程模型和内存管理。第二部分：核心功能与组件 4. 传输层：详细讲解Netty中的各种传输机制，如TCP、UDP以及本地传输等，以及如何选择合适的传输方式。 5. 缓冲区：介绍Netty的ByteBuf实现，它是高效数据处理的关键，包括读写操作、缓冲区复制和内存管理。 6. ChannelHandler：解释ChannelHandler的作用，它是Netty事件处理链中的基本组件，用于处理网络事件和数据转换。 7. 编码解码：讨论如何使用Netty的编码器和解码器来处理不同协议的数据，如HTTP、WebSocket等。 8. 预置的ChannelHandlers和Codecs：列出Netty提供的现成组件，用于快速搭建常见的网络应用。 9. Netty应用的启动：指导如何配置和启动Netty服务器，包括EventLoopGroup、Bootstrap和ChannelFuture的使用。第三部分：Netty实例 10. 单元测试你的代码：展示如何编写和集成测试，确保Netty应用的稳定性和正确性。 11. WebSocket支持：详细讲解如何利用Netty实现WebSocket服务器和客户端，支持实时双向通信。 12. SPDY协议：介绍SPDY（已弃用，但当时是重要的优化网络性能的协议），以及Netty如何支持它。 13. 通过UDP广播事件：探讨使用Netty处理多播和广播的场景，以提高网络通信效率。第四部分：进阶话题 14. 实现自定义编码器和解码器：指导读者根据需求定制自己的编码和解码逻辑，以处理特定协议或数据格式。 15. 选择合适的线程模型：分析不同的线程模型，如单线程、多线程和EventLoop，以及如何根据应用场景选择最佳方案。 16. EventLoop的注销与重新注册：讨论在动态调整系统资源时，如何优雅地注销和重新注册EventLoop。 17. 案例研究：通过实际案例来进一步理解Netty在复杂场景下的应用和优化。书末还附有社区参与指南、相关书籍推荐和相关项目的列表，为读者提供了丰富的学习和交流资源。

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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Netty in Action 第五版：大数据学习指南

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