4.17-4.18学习总结

MD5

MD5:
1、压缩性

2、容易计算
3、抗修改性

4、弱抗碰撞
5、强抗碰撞

为什么需要MD5？

存储一些敏感信息的时候，如果不进行加密会出现安全问题。

例如：系统登录的密码，如果数据库中的密码采用明文，一旦数据库泄露，则用户的密码直接泄露。但是使用MD5加密算法之后即使数据泄露，也不会被轻易的破解，相对安全。

public class MD5Utils {public static String getPWD( String strs ){/** 加密需要使用JDK中提供的类*/StringBuffer sb = new StringBuffer();try{MessageDigest digest = MessageDigest.getInstance("MD5");byte[] bs = digest.digest(strs.getBytes());for (byte b : bs) {int x = b & 255;String s = Integer.toHexString(x);if( x > 0 && x < 16 ){sb.append("0");sb.append(s);}else{sb.append(s);}}}catch( Exception e){System.out.println("加密失败");}return sb.toString();}}

原理说明： 

MD5码以512位分组来处理输入的信息，且每一分组又被划分为16个32位子分组，经过了一系列的处理后，算法的输出由四个32位分组组成，将这四个32位分组级联后将生成一个128位散列值。

总体流程如下图所示，每次的运算都由前一轮的128位结果值和当前的512bit值进行运算

 序列化

 序列化其实就是：将一个对象转化为一种格式，能够更好的传输和电脑理解。

反序列化是转换过来，便于人们观看的。

public class Person implements serializable 
{private string name;private int age;public Person ( String name，int age) {this.name = name ;this.age = age;
);public class write{public static void main(String[] args) {try (//创建一个objectoutputStream输出流objectoutputStream oos = new   ObjectoutputStream(newFileOutputStream("object.txt")));
/将对象序列化到文件sPerson person = new Person ( "1",23);oos.write( person ) ;
}catch (Exception e）{e.printStackTrace();
}

正则表达式：

正则表达式是一种字符串匹配一的模式

作业：
数据验证:比如电话号码、邮箱等
替换文本:快速找到特定文本，用于替换
快速提取字符串:基于匹配原则，快速从文档中查找或提取子字符串

比如简单实现一个身份证的正则表达式：

 

 socket简单实现多个线程对一个服务器：

package login;import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintStream;
import java.net.Socket;public class Client {public static void main(String[] args) throws IOException {Socket socket = new Socket("127.0.0.1", 20000);new Thread(new ClientTread(socket)).start();PrintStream ps = new PrintStream(socket.getOutputStream());String line = null;BufferedReader br = new BufferedReader(new InputStreamReader(System.in));while ((line = br.readLine()) != null) {ps.println(line);}if (br != null) br.close();if (socket != null) socket.close();}
}
package login;import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.Socket;public class ClientTread implements Runnable {private final Socket socket;BufferedReader br = null;public ClientTread(Socket socket) throws IOException {this.socket = socket;br = new BufferedReader(new InputStreamReader(socket.getInputStream()));}public void run() {String content = null;while (true) {try {if ((content = br.readLine()) == null) break;} catch (IOException e) {e.printStackTrace();e.getCause();}System.out.println(content);}}}
package socket;import java.io.IOException;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;public class Server {public static List<Socket> socketList = Collections.synchronizedList(new ArrayList<>());public static void main(String[] args) throws IOException {List<Socket> socketList = new ArrayList<>();ServerSocket serverSocket = new ServerSocket(20000);try {while (true) {Socket socket1 = serverSocket.accept();socketList.add(socket1);new Thread(new ServerThread(socket1)).start();}} catch (Exception e) {e.printStackTrace();socketList.removeAll(socketList);}
package socket;import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintStream;
import java.net.Socket;public class ServerThread implements Runnable {Socket socket = null;BufferedReader br = null;public ServerThread(Socket socket) throws Exception {this.socket = socket;br = new BufferedReader(new InputStreamReader(socket.getInputStream()));}public void run() {String content = null;while ((content = readFormClient()) != null) {for (Socket socket : Server.socketList) {try {PrintStream ps = new PrintStream(socket.getOutputStream());} catch (IOException e) {e.printStackTrace();e.getCause();}}}}private String readFormClient() {try {return br.readLine();} catch (IOException e) {Server.socketList.remove(socket);}return null;}
}