TCP/IP通讯(二)

本文主要关注点：

 问：TCP方式的socket通讯的接受者不知道或者说无法确定消息的长度(比如我们聊天消息 前后的消息长度可能是不一样的)，如果能识别消息是否接受完成

答：主要有2中技术使接受者能够准确的找到消息的结束位置

       1、基于鉴定符(Delimiter-base)：消息的结束由一个唯一的标记支出，即发送者在传输完数据后显式添加一个特殊字节序列。这个特殊标记不能砸传输中数据中出现

        2、显式长度(Explicit length):在边长字段货消息前附加一个固定大小的字段，用来指示该字段货消息中包含了多少个字节(这个方法简单些，不过必须知道消息的长度的上限。发送者先确定消息的长度，将长度信息存入一个整数，作为消息的前缀。消息的藏毒上限定义了编码消息藏毒所需要的字节数：如果消息的长度小于256字节，则需要1个字节；如果消息的长度消息65536字节，则需要2个字节)

追问：第一个技术中，假如消息体重出现特殊标记字符怎么办

          答：填充(Stuffing)技术能够对消息中出现的定界符进行修改，聪哥使接受者不将其识别为界定符。这个有个缺点是发送者和接受者双方度必须扫描消息

       

2、通讯要成功并且2端都能正确解析，需要通讯的双发约定协议:即命令交互的格式。

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以下文投票为例：我们需要服务端和客户端约定一套规则来识别是投票、查询还是响应

一般2中表示方式  基本文本的表示方式(投票示例，按约定规则生成和解析字符串)和二进制表示方式

package cc.leng.tcpip.step2;

import java.io.ByteArrayInputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Scanner;
/**
 * 投票编解码<br/>
 * format： Voting <"v"|"i"> [<RESPFLAG> <CANDIADTEID> [VOTECNT]
 * @author leng-home
 *
 */
public class VoteMsgTextCoder {

	public static final String MAGIC = "Voting";
	public static final String VOTESTR = "v";
	public static final String INQSTR = "i";
	public static final String RESPONSESTR = "R";
	public static final String CHARSETNAME = "US_ASCII";
	public static final String DELIMSTR = " ";
	public static final int MAX_WIRE_LENGTH = 2000;

	/**
	 * 
	 * @param msg
	 * @return
	 * @throws IOException
	 */
	public byte[] toWire(VoteMsg msg) throws IOException {
		StringBuilder sb = new StringBuilder();
		sb.append(MAGIC).append(DELIMSTR)
				.append((msg.isInquiry() ? INQSTR : VOTESTR)).append(DELIMSTR)
				.append(msg.isResponse() ? RESPONSESTR + DELIMSTR : "")
				.append(msg.getCandidateID()).append(DELIMSTR)
				.append(msg.getVoteCount());
		byte[] data = sb.toString().getBytes(CHARSETNAME);
		
		return data;

	}
	/**
	 * 
	 * @param mesage
	 * @return
	 * @throws IOException
	 */
	public VoteMsg fromWire(byte[] mesage) throws IOException{
		ByteArrayInputStream msgStream = new ByteArrayInputStream(mesage);
		Scanner s = new Scanner(new InputStreamReader(msgStream,CHARSETNAME));
		boolean isInquiry;
		boolean isResponse;
		int candidateID;
		long voteCount;
		String token;
		try{
			token = s.next();
			if(!MAGIC.equals(token)){
				throw new IOException("Bad magic string : "+ token);
			}
			token = s.next();
			if(VOTESTR.equals(token)){
				isInquiry = false;
			}else if(!INQSTR.equals(token)){
				throw new IOException("Bad vote/inq indicator :"+token);
			}else{
				isInquiry = true;
			}
			
			token = s.next();
			
			if(RESPONSESTR.equals(token)){
				isResponse = true;
				token = s.next();
			}else{
				isResponse = false;
			}
			//current token is candidateID
			candidateID = Integer.parseInt(token);
			if(isResponse){
				token = s.next();
				voteCount = Long.parseLong(token);
			}else{
				voteCount = 0;
			}
		}catch(IOException e){
			throw new IOException("parse error....");
		}
		return new VoteMsg(isResponse, isInquiry, candidateID, voteCount);
	}
}

package cc.leng.tcpip.step2;

/**
 * 投票信息类
 * @author leng-home
 *
 */
public class VoteMsg {
	
	private boolean isInquiry;
	private boolean isResponse;
	private int candidateID;
	private long voteCount;
	
	
	public static final int MAX_CANDIDATE_ID = 1000;
	
	public VoteMsg(boolean isResponse,boolean isInquiry,int candidateID,long voteCount){
		if(voteCount !=0 && !isResponse){
			//只有响应的时候voteCount才有值
			throw new IllegalArgumentException("Request vote count must be zero");
		}
		if(candidateID < 0 || candidateID > MAX_CANDIDATE_ID){
			throw new IllegalArgumentException("Bad Candidate ID: " + candidateID);
		}
		if(voteCount < 0){
			throw new IllegalArgumentException("Total must be >= zero");
		}
		this.candidateID = candidateID;
		this.isResponse = isResponse;
		this.isInquiry = isInquiry;
		this.voteCount = voteCount;
	}
	
	

	public boolean isInquiry() {
		return isInquiry;
	}

	public void setInquiry(boolean isInquiry) {
		this.isInquiry = isInquiry;
	}

	public boolean isResponse() {
		return isResponse;
	}

	public void setResponse(boolean isResponse) {
		this.isResponse = isResponse;
	}

	public int getCandidateID() {
		return candidateID;
	}

	public void setCandidateID(int candidateID) {
		this.candidateID = candidateID;
	}

	public long getVoteCount() {
		return voteCount;
	}

	public void setVoteCount(long voteCount) {
		this.voteCount = voteCount;
	}
	
	
	public String toString() {
		String res = (isInquiry ? "inquiry" : "vote") + " for candidate "+ candidateID;
		if(isResponse){
			res = " response to" + res + " who now has " + voteCount + " vote(s)";
		}
		return res;
	}
	

}