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今天，给大家带来一篇稍有深度的文章——《RPC之——HTTP协议栈》，好了，我们进入正题吧。

 HTTP协议属于应用层协议，它构建于TCP和IP协议之上，处于TCP/IP协议架构层的顶端，所以，它不用处理下层协议间诸如丢包补发、握手及数据的分段及重新组装等繁琐的细节，使开发人员可以专注于应用业务。

协议是通信的规范，为了更好的理解HTTP协议，我们可以基于Java的Socket API接口，通过设计一个简单的应用层通信协议，来简单分析下协议实现的过程和细节。

在我们今天的示例程序中，客户端会向服务端发送一条命令，服务端在接收到命令后，会判断命令是否是“HELLO”，如果是“HELLO”， 则服务端返回给客户端的响应为“hello”，否则，服务端返回给客户端的响应为“bye bye”。

我们接下来用Java实现这个简单的应用层通信协议：

 1、协议请求的定义

协议的请求主要包括：编码、命令和命令长度三个字段。

package com.lyz.params;

/**
 * 协议请求的定义
 * @author liuyazhuang
 *
 */
public class Request {
	/**
	 * 协议编码
	 */
	private byte encode;
	
	/**
	 * 命令
	 */
	private String command;
	
	/**
	 * 命令长度
	 */
	private int commandLength;

	public Request() {
		super();
	}

	public Request(byte encode, String command, int commandLength) {
		super();
		this.encode = encode;
		this.command = command;
		this.commandLength = commandLength;
	}

	public byte getEncode() {
		return encode;
	}

	public void setEncode(byte encode) {
		this.encode = encode;
	}

	public String getCommand() {
		return command;
	}

	public void setCommand(String command) {
		this.command = command;
	}

	public int getCommandLength() {
		return commandLength;
	}

	public void setCommandLength(int commandLength) {
		this.commandLength = commandLength;
	}

	@Override
	public String toString() {
		return "Request [encode=" + encode + ", command=" + command
				+ ", commandLength=" + commandLength + "]";
	}
	
}

2、响应协议的定义

协议的响应主要包括：编码、响应内容和响应长度三个字段。

package com.lyz.params;

/**
 * 协议响应的定义
 * @author liuyazhuang
 *
 */
public class Response {
	/**
	 * 编码
	 */
	private byte encode;
	
	/**
	 * 响应内容
	 */
	private String response;
	
	/**
	 * 响应长度
	 */
	private int responseLength;

	public Response() {
		super();
	}

	public Response(byte encode, String response, int responseLength) {
		super();
		this.encode = encode;
		this.response = response;
		this.responseLength = responseLength;
	}

	public byte getEncode() {
		return encode;
	}

	public void setEncode(byte encode) {
		this.encode = encode;
	}

	public String getResponse() {
		return response;
	}

	public void setResponse(String response) {
		this.response = response;
	}

	public int getResponseLength() {
		return responseLength;
	}

	public void setResponseLength(int responseLength) {
		this.responseLength = responseLength;
	}

	@Override
	public String toString() {
		return "Response [encode=" + encode + ", response=" + response
				+ ", responseLength=" + responseLength + "]";
	}
	
}

3、编码常量定义

编码常量的定义主要包括UTF-8和GBK两种编码。

package com.lyz.constant;

/**
 * 常量类
 * @author liuyazhuang
 *
 */
public final class Encode {
	//UTF-8编码
	public static final byte UTF8 = 1;
	//GBK编码
	public static final byte GBK = 2;
}

4、客户端的实现

客户端先构造一个request请求，通过Socket接口将其发送到远端，并接收远端的响应信息，并构造成一个Response对象。

package com.lyz.protocol.client;

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.Socket;

import com.lyz.constant.Encode;
import com.lyz.params.Request;
import com.lyz.params.Response;
import com.lyz.utils.ProtocolUtils;

/**
 * 客户端代码
 * @author liuyazhuang
 *
 */
public final class Client {
	public static void main(String[] args) throws IOException{
		//请求
		Request request = new Request();
		request.setCommand("HELLO");
		request.setCommandLength(request.getCommand().length());
		request.setEncode(Encode.UTF8);
		
		Socket client = new Socket("127.0.0.1", 4567);
		OutputStream out = client.getOutputStream();
		
		//发送请求
		ProtocolUtils.writeRequest(out, request);
		
		//读取响应数据
		InputStream in = client.getInputStream();
		Response response = ProtocolUtils.readResponse(in);
		System.out.println("获取的响应结果信息为: " + response.toString());
	}
}

5、服务端的实现

服务端接收客户端的请求，根据接收命令的不同，响应不同的消息信息，如果是“HELLO”命令，则响应“hello”信息，否则响应“bye bye”信息。

package com.lyz.protocol.server;

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.ServerSocket;
import java.net.Socket;

import com.lyz.constant.Encode;
import com.lyz.params.Request;
import com.lyz.params.Response;
import com.lyz.utils.ProtocolUtils;

/**
 * Server端代码
 * @author liuyazhuang
 *
 */
public final class Server {
	public static void main(String[] args) throws IOException{
		ServerSocket server = new ServerSocket(4567);
		while (true) {
			Socket client = server.accept();
			//读取请求数据
			InputStream input = client.getInputStream();
			Request request = ProtocolUtils.readRequest(input);
			System.out.println("收到的请求参数为: " + request.toString());
			OutputStream out = client.getOutputStream();
			//组装响应数据
			Response response = new Response();
			response.setEncode(Encode.UTF8);
			if("HELLO".equals(request.getCommand())){
				response.setResponse("hello");
			}else{
				response.setResponse("bye bye");
			}
			response.setResponseLength(response.getResponse().length());
			ProtocolUtils.writeResponse(out, response);
		}
	}
}

6、ProtocolUtils工具类的实现

ProtocolUtils的readRequest方法将从传递进来的输入流中读取请求的encode、command和commandLength三个参数，进行相应的编码转化，构造成Request对象返回。而writeResponse方法则是将response对象的字段根据对应的编码写入到响应的输出流中。

有一个细节需要重点注意：OutputStream中直接写入一个int类型，会截取其低8位，丢弃其高24位，所以，在传递和接收数据时，需要进行相应的转化操作。

package com.lyz.utils;

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;

import com.lyz.constant.Encode;
import com.lyz.params.Request;
import com.lyz.params.Response;

/**
 * 协议工具类
 * @author liuyazhuang
 *
 */
public final class ProtocolUtils {
	/**
	 * 从输入流中反序列化Request对象
	 * @param input
	 * @return
	 * @throws IOException
	 */
	public static Request readRequest(InputStream input) throws IOException{
		//读取编码
		byte[] encodeByte = new byte[1];
		input.read(encodeByte);
		byte encode = encodeByte[0];
		
		//读取命令长度
		byte[] commandLengthBytes = new byte[4];
		input.read(commandLengthBytes);
		int commandLength = ByteUtils.byte2Int(commandLengthBytes);
		
		//读取命令
		byte[] commandBytes = new byte[commandLength];
		input.read(commandBytes);
		String command = "";
		if(Encode.UTF8 == encode){
			command = new String(commandBytes, "UTF-8");
		}else if(Encode.GBK == encode){
			command = new String(commandBytes, "GBK");
		}
		//组装请求返回
		Request request = new Request(encode, command, commandLength);
		return request;
	}
	/**
	 * 从输入流中反序列化Response对象
	 * @param input
	 * @return
	 * @throws IOException
	 */
	public static Response readResponse(InputStream input) throws IOException{
		//读取编码
		byte[] encodeByte = new byte[1];
		input.read(encodeByte);
		byte encode = encodeByte[0];
		
		//读取响应长度
		byte[] responseLengthBytes = new byte[4];
		input.read(responseLengthBytes);
		int responseLength = ByteUtils.byte2Int(responseLengthBytes);
		
		//读取命令
		byte[] responseBytes = new byte[responseLength];
		input.read(responseBytes);
		String response = "";
		if(Encode.UTF8 == encode){
			response = new String(responseBytes, "UTF-8");
		}else if(Encode.GBK == encode){
			response = new String(responseBytes, "GBK");
		}
		//组装请求返回
		Response resp = new Response(encode, response, responseLength);
		return resp;
	}
	
	/**
	 * 序列化请求信息
	 * @param output
	 * @param response
	 */
	public static void writeRequest(OutputStream output, Request request) throws IOException{
		//将response响应返回给客户端
		output.write(request.getEncode());
		//output.write(response.getResponseLength());直接write一个int类型会截取低8位传输丢弃高24位
		output.write(ByteUtils.int2ByteArray(request.getCommandLength()));
		if(Encode.UTF8 == request.getEncode()){
			output.write(request.getCommand().getBytes("UTF-8"));
		}else if(Encode.GBK == request.getEncode()){
			output.write(request.getCommand().getBytes("GBK"));
		}
		output.flush();
	}
	/**
	 * 序列化响应信息
	 * @param output
	 * @param response
	 */
	public static void writeResponse(OutputStream output, Response response) throws IOException{
		//将response响应返回给客户端
		output.write(response.getEncode());
		//output.write(response.getResponseLength());直接write一个int类型会截取低8位传输丢弃高24位
		output.write(ByteUtils.int2ByteArray(response.getResponseLength()));
		if(Encode.UTF8 == response.getEncode()){
			output.write(response.getResponse().getBytes("UTF-8"));
		}else if(Encode.GBK == response.getEncode()){
			output.write(response.getResponse().getBytes("GBK"));
		}
		output.flush();
	}
}

7、ByteUtils类的实现

package com.lyz.utils;

/**
 * 字节转化工具类
 * @author liuyazhuang
 *
 */
public final class ByteUtils {
	/**
	 * 将byte数组转化为int数字
	 * @param bytes
	 * @return
	 */
	public static int byte2Int(byte[] bytes){
		int num = bytes[3] & 0xFF;
		num |= ((bytes[2] << 8) & 0xFF00);
		num |= ((bytes[1] << 16) & 0xFF0000);
		num |= ((bytes[0] << 24) & 0xFF000000);
		return num;
	}
	
	/**
	 * 将int类型数字转化为byte数组
	 * @param num
	 * @return
	 */
	public static byte[] int2ByteArray(int i){
		byte[] result = new byte[4];
		result[0]  = (byte)(( i >> 24 ) & 0xFF);
		result[1]  = (byte)(( i >> 16 ) & 0xFF);
		result[2]  = (byte)(( i >> 8 ) & 0xFF);
		result[3]  = (byte)(i & 0xFF);
		return result;
	}
}

RPC之——HTTP协议栈