第十四章：实现自定义的编码解码器
文章目录
本文转自互联网博客，在此感谢博主的总结归纳，本文只提供于巩固复习使用
本章讲述Netty中如何轻松实现定制的编解码器，由于Netty架构的灵活性，这些编解码器易于重用和测试。为了更容易实现，使用Memcached作为协议例子是因为它更方便我们实现。
Memcached是免费开源、高性能、分布式的内存对象缓存系统，其目的是加速动态Web应用程序的响应，减轻数据库负载；Memcache实际上是一个以key-value存储任意数据的内存小块。可能有人会问“为什么使用Memcached?因为Memcached协议非常简单，便于讲解。
14.1 编解码器的范围
我们将只实现Memcached协议的一个子集，这足够我们进行添加、检索、删除对象；在Memcached中是通过执行SET,GET,DELETE命令来实现的。Memcached支持很多其他的命令，但我们只使用其中三个命令，简单的东西，我们才会理解的更清楚。
Memcached有一个二进制和纯文本协议，它们都可以用来与Memcached服务器通信，使用什么类型的协议取决于服务器支持哪些协议。本章主要关注实现二进制协议，因为二进制在网络编程中最常用。
14.2 实现Memcached的编解码器
当想要实现一个给定协议的编解码器，我们应该花一些事件来了解它的运作原理。通常情况下，协议本身都有一些详细的记录。在这里你会发现多少细节？幸运的是Memcached的二进制协议可以很好的扩展。
在RFC中有相应的规范，并提供了Memcached二进制协议下载地址：http://code.google.com/p/memcached/wiki/BinaryProtocolRevamped。我们不会执行Memcached的所有命令，只会执行三种操作：SET,GET和DELETE。这样做事为了让事情变得简单。
###14.3 了解Memcached二进制协议
可以在http://code.google.com/p/memcached/wiki/BinaryProtocolRevamped上详细了解Memcached二进制协议结构。不过这个网站如果不FQ的话好像访问不了。
14.4 Netty编码器和解码器
14.4.1 实现Memcached编码器
先定义memcached操作码(Opcode)和响应状态码(Status)：
package netty.in.action.mem;
/**
* memcached operation codes
* @author c.king
*
*/
public class Opcode {
public static final byte GET = 0x00;
public static final byte SET = 0x01;
public static final byte DELETE = 0x04;
}
package netty.in.action.mem;
/**
* memcached response statuses
* @author c.king
*
*/
public class Status {
public static final short NO_ERROR = 0x0000;
public static final short KEY_NOT_FOUND = 0x0001;
public static final short KEY_EXISTS = 0x0002;
public static final short VALUE_TOO_LARGE = 0x0003;
public static final short INVALID_ARGUMENTS = 0x0004;
public static final short ITEM_NOT_STORED = 0x0005;
public static final short INC_DEC_NON_NUM_VAL = 0x0006;
}
继续编写memcached请求消息体：
package netty.in.action.mem;
import java.util.Random;
/**
* memcached request message object
* @author c.king
*
*/
public class MemcachedRequest {
private static final Random rand = new Random();
private int magic = 0x80;// fixed so hard coded
private byte opCode; // the operation e.g. set or get
private String key; // the key to delete, get or set
private int flags = 0xdeadbeef; // random
private int expires; // 0 = item never expires
private String body; // if opCode is set, the value
private int id = rand.nextInt(); // Opaque
private long cas; // data version check...not used
private boolean hasExtras; // not all ops have extras
public MemcachedRequest(byte opcode, String key, String value) {
this.opCode = opcode;
this.key = key;
this.body = value == null ? "" : value;
// only set command has extras in our example
hasExtras = opcode == Opcode.SET;
}
public MemcachedRequest(byte opCode, String key) {
this(opCode, key, null);
}
public int getMagic() {
return magic;
}
public byte getOpCode() {
return opCode;
}
public String getKey() {
return key;
}
public int getFlags() {
return flags;
}
public int getExpires() {
return expires;
}
public String getBody() {
return body;
}
public int getId() {
return id;
}
public long getCas() {
return cas;
}
public boolean isHasExtras() {
return hasExtras;
}
}
最后编写memcached请求编码器：
package netty.in.action.mem;
import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelHandlerContext;
import io.netty.handler.codec.MessageToByteEncoder;
import io.netty.util.CharsetUtil;
/**
* memcached request encoder
* @author c.king
*
*/
public class MemcachedRequestEncoder extends MessageToByteEncoder<MemcachedRequest> {
@Override
protected void encode(ChannelHandlerContext ctx, MemcachedRequest msg, ByteBuf out)
throws Exception {
// convert key and body to bytes array
byte[] key = msg.getKey().getBytes(CharsetUtil.UTF_8);
byte[] body = msg.getBody().getBytes(CharsetUtil.UTF_8);
// total size of body = key size + body size + extras size
int bodySize = key.length + body.length + (msg.isHasExtras() ? 8 : 0);
// write magic int
out.writeInt(msg.getMagic());
// write opcode byte
out.writeByte(msg.getOpCode());
// write key length (2 byte) i.e a Java short
out.writeShort(key.length);
// write extras length (1 byte)
int extraSize = msg.isHasExtras() ? 0x08 : 0x0;
out.writeByte(extraSize);
// byte is the data type, not currently implemented in Memcached
// but required
out.writeByte(0);
// next two bytes are reserved, not currently implemented
// but are required
out.writeShort(0);
// write total body length ( 4 bytes - 32 bit int)
out.writeInt(bodySize);
// write opaque ( 4 bytes) - a 32 bit int that is returned
// in the response
out.writeInt(msg.getId());
// write CAS ( 8 bytes)
// 24 byte header finishes with the CAS
out.writeLong(msg.getCas());
if(msg.isHasExtras()){
// write extras
// (flags and expiry, 4 bytes each), 8 bytes total
out.writeInt(msg.getFlags());
out.writeInt(msg.getExpires());
}
//write key
out.writeBytes(key);
//write value
out.writeBytes(body);
}
}
14.4.2 实现Memcached解码器
编写memcached响应消息体：
package netty.in.action.mem;
/**
* memcached response message object
* @author c.king
*
*/
public class MemcachedResponse {
private byte magic;
private byte opCode;
private byte dataType;
private short status;
private int id;
private long cas;
private int flags;
private int expires;
private String key;
private String data;
public MemcachedResponse(byte magic, byte opCode, byte dataType, short status,
int id, long cas, int flags, int expires, String key, String data) {
this.magic = magic;
this.opCode = opCode;
this.dataType = dataType;
this.status = status;
this.id = id;
this.cas = cas;
this.flags = flags;
this.expires = expires;
this.key = key;
this.data = data;
}
public byte getMagic() {
return magic;
}
public byte getOpCode() {
return opCode;
}
public byte getDataType() {
return dataType;
}
public short getStatus() {
return status;
}
public int getId() {
return id;
}
public long getCas() {
return cas;
}
public int getFlags() {
return flags;
}
public int getExpires() {
return expires;
}
public String getKey() {
return key;
}
public String getData() {
return data;
}
}
编写memcached响应解码器：
package netty.in.action.mem;
import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelHandlerContext;
import io.netty.handler.codec.ByteToMessageDecoder;
import io.netty.util.CharsetUtil;
import java.util.List;
public class MemcachedResponseDecoder extends ByteToMessageDecoder {
private enum State {
Header, Body
}
private State state = State.Header;
private int totalBodySize;
private byte magic;
private byte opCode;
private short keyLength;
private byte extraLength;
private byte dataType;
private short status;
private int id;
private long cas;
@Override
protected void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out)
throws Exception {
switch (state) {
case Header:
// response header is 24 bytes
if (in.readableBytes() < 24) {
return;
}
// read header
magic = in.readByte();
opCode = in.readByte();
keyLength = in.readShort();
extraLength = in.readByte();
dataType = in.readByte();
status = in.readShort();
totalBodySize = in.readInt();
id = in.readInt();
cas = in.readLong();
state = State.Body;
break;
case Body:
if (in.readableBytes() < totalBodySize) {
return;
}
int flags = 0;
int expires = 0;
int actualBodySize = totalBodySize;
if (extraLength > 0) {
flags = in.readInt();
actualBodySize -= 4;
}
if (extraLength > 4) {
expires = in.readInt();
actualBodySize -= 4;
}
String key = "";
if (keyLength > 0) {
ByteBuf keyBytes = in.readBytes(keyLength);
key = keyBytes.toString(CharsetUtil.UTF_8);
actualBodySize -= keyLength;
}
ByteBuf body = in.readBytes(actualBodySize);
String data = body.toString(CharsetUtil.UTF_8);
out.add(new MemcachedResponse(magic, opCode, dataType, status,
id, cas, flags, expires, key, data));
state = State.Header;
break;
default:
break;
}
}
}
14.5 测试编解码器
基于netty的编解码器都写完了，下面我们来写一个测试它的类：
package netty.in.action.mem;
import io.netty.buffer.ByteBuf;
import io.netty.channel.embedded.EmbeddedChannel;
import io.netty.util.CharsetUtil;
import org.junit.Assert;
import org.junit.Test;
/**
* test memcached encoder
* @author c.king
*
*/
public class MemcachedRequestEncoderTest {
@Test
public void testMemcachedRequestEncoder() {
MemcachedRequest request = new MemcachedRequest(Opcode.SET, "k1", "v1");
EmbeddedChannel channel = new EmbeddedChannel(
new MemcachedRequestEncoder());
Assert.assertTrue(channel.writeOutbound(request));
ByteBuf encoded = (ByteBuf) channel.readOutbound();
Assert.assertNotNull(encoded);
Assert.assertEquals(request.getMagic(), encoded.readInt());
Assert.assertEquals(request.getOpCode(), encoded.readByte());
Assert.assertEquals(2, encoded.readShort());
Assert.assertEquals((byte) 0x08, encoded.readByte());
Assert.assertEquals((byte) 0, encoded.readByte());
Assert.assertEquals(0, encoded.readShort());
Assert.assertEquals(2 + 2 + 8, encoded.readInt());
Assert.assertEquals(request.getId(), encoded.readInt());
Assert.assertEquals(request.getCas(), encoded.readLong());
Assert.assertEquals(request.getFlags(), encoded.readInt());
Assert.assertEquals(request.getExpires(), encoded.readInt());
byte[] data = new byte[encoded.readableBytes()];
encoded.readBytes(data);
Assert.assertArrayEquals((request.getKey() + request.getBody())
.getBytes(CharsetUtil.UTF_8), data);
Assert.assertFalse(encoded.isReadable());
Assert.assertFalse(channel.finish());
Assert.assertNull(channel.readInbound());
}
}
14.6 Summary
本章主要是使用netty写了个模拟memcached二进制协议的处理。至于memcached二进制协议具体是个啥玩意，可以单独了解，这里也没有详细说明。
