考研比如2022年12月考完，2023年3月初试，几个月的时间，不太需要本科毕业生做这么高难度的、功能完善的事儿。
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Overview of the Java Chat Application The Java Chat application I am going to build is a console application that is launched from the command line. I use MySQL to store the client login information, and the server will participate in each function. Besides, each meassage will be processed by the server. Explanation: https://www.geeksforgeeks.org/multithreaded-servers-in-java/
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The program runs in the console in the same computer. 注意：本项目只在单主机运行调试过，没试过在局域网和不同主机之间接发消息和文件（），有需要的自行查阅资料。
For the sake of simplicity, we’ll run our client and server programs on the same computer. If we were to execute them on different networked computers, the only thing that would change is the IP address. In this case, we’ll use localhost on 127.0.0.1. https://www.baeldung.com/a-guide-to-java-sockets#:~:text=Socket%20clientSocket%20%3D%20serverSocket.accept%20%28%29%3B%20When%20the%20server,everything%20goes%20well%2C%20the%20server%20accepts%20the%20connection.
Run the server first and then the client. The client should receive the "" message from the server and display it on the console. We have two files including Server.java and Client.java
** You need to set up a database and export parameters to edit initSQL() in both Java files.
我的设计理念： 依照别人成熟的方案，进行编码执行，没有到达很高阶；
上线了，构造函数自动给client 赋值生命Connected=true; 该属性随着client的quit()消失而消失； 查询client是否在线，则server每次都去数据库里看看当前client是Online or quit；
（中式英语）： Socket connection has solved the connection of IPs and ports for the source and destination.
- Symbol means different function. 每出现一个新功能，就多一个sym标志位的数字，来检验新功能，于是有了很多if else
- We use bConnected to judge whether a thread is alive at present.
- A symbol means 标志位-num to label different functions, i.e., functions in the requirement list.
- Every client has at most 50 messages that can be stored in local database. We seem to only have one table for all clients, which is a bug. They should set up a table for themselves each.
Maybe I need to initialize something to start some other functions, anyway.
- We use the socket to locate each client;
- Client启动的客户端，依据题目需要来运行程序；Client通过多线程创建新的客户端
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We have a list to accomdate clients
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jdbc connects the local database to store the status of the client, like whether the client is online or has quitted.
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Server主循环接收Client的Socket连接， the server will use multiple threads to connect each Client.
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I use the sql to process the data of the string.
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we use the following methods to read and write for the data stream.
dis = new DataInputStream(socket.getInputStream());
dos = new DataOutputStream(socket.getOutputStream());
- 消息都是以空格隔开的，所以我这里server 用readUTF()来读一行行消息
- I did not consider the size of receiving buffet because if there is so much data. If there exists new message, then the message will be read. I just have one while(True) loop for the server.
eg. Bob hi 1 Alice, "Bob" is the message sender, "hi" is the message,"1" means to ,"Alice" means the target receiver. For the client "Alice", "hi" should appear on her console screen.
/login: If it is the first time that the client logins, then the client will register its information, otherwise the server will check whether the login information of the client is right. *login.length()>=7)//首先名字长度得有, it must be larger than 7; there is no need to input the password, because this program does not have this function. Each client only has a unique name. //如果输错，要一直循环等待输入，且quit指令还没放入login;配合mysql &emsp if logining name has existed, System.out.println("Name exist, please choose another name."); /quit: the client go offline.
Other inputs can be summarized as invalid input.
/to : meaning broadcast the message to all other clients.
/to (client N, +message): meaning two clients talk privately. For example, client A starts the message "/to B I want to date with you", then the message will send to the client B, and it will receive the message " A: I want to date with you".
/history:
who: calculate the total number of the online clients.
3.1. What Is “Connection Timed Out”? For establishing a connection to the server from the client-side, the socket constructor is invoked, which instantiates a socket object. The constructor takes the remote host address and the port number as input arguments. After that, it attempts to establish a connection to the remote host based on the given parameters.
The operation blocks all other processes until a successful connection is made. However, if the connection isn’t successful after a certain time, the program throws a ConnectionException with a “Connection timed out” message:
https://www.baeldung.com/java-socket-connection-read-timeout
time_out is made by JAVA
https://reintech.io/blog/java-network-programming-creating-managing-sockets
我更是按照视频，做了逻辑字段的处理，主要的代码，都还是借鉴尚硅谷的旧版视频的；
原理解释：
从连接上，TCP/IP established, then the client write DataStream into the buff; Server 端的accept方法一直到客户端启动并向服务器发出请求为止一直在等待。但注意，此方法不支持多个客户端同时访问 simultaneously. //据说，即使server不accpet(), TCP has established. There is data transmitting. Java这里是怎么捕获client的socket的，为啥不会捕获错误？如何识别client1和client2，咱的client port设置的不都是一样的？多线程的client怎么标记的呢？
- 函数原型 accept函数允许在套接字上进行传入连接尝试。
SOCKET WSAAPI accept( SOCKET s, sockaddr *addr, int *addrlen );
listen监听客户端来的链接，accept将客户端的信息绑定到一个socket上，也就是给客户端创建一个socket，通过返回值返回给我们客户端的socket。//这里提到的是，信息绑定
一次只能创建一个，有几个客户端链接，就要调用几次。 ———————————————— 版权声明：本文为CSDN博主「超级D洋葱」的原创文章，遵循CC 4.0 BY-SA版权协议，转载请附上原文出处链接及本声明。 原文链接：https://blog.csdn.net/u014779536/article/details/115834445
Server and client all need to connect to the database. They all need to register and connect, thus, TCP/IP has some function here, and we can easily understand why we need to write down those codes.
一：JDBC
sun：提供了一套通用性的接口：可以连接任何的数据库： 连接数据库的具体得到实例，具体的数据库厂商实现的。
连接数据的步骤（别忘了复制jar包）:( 1）注册驱动： Class.forName（）：DriverManager （2）获得链接对象：Connection （3）创建sql容器：语句： （4）执行sql语句:: stmt （5）查询操作：遍历结果集：ResultSet （6）关闭资源：
如果是软编码的话步骤如下：
1个方言文件内容： ———————————————— 版权声明：本文为CSDN博主「东方-教育技术博主」的原创文章，遵循CC 4.0 BY-SA版权协议，转载请附上原文出处链接及本声明。 原文链接：https://blog.csdn.net/qq_41517071/article/details/84615765
今天与同学争执一个话题：由于socket的accept函数在有客户端连接的时候产生了新的socket用于服务该客户端，那么，这个新的socket到底有没有占用一个新的端口？https://blog.51cto.com/ticktick/779866
First off, a "port" is just a number. All a "connection to a port" really represents is a packet which has that number specified in its "destination port" header field.
- Read Timed Out
4.1. What Is “Read Timed Out”? The read() method call in the InputStream blocks until it finishes reading data bytes from the socket. The operation waits until it reads at least one data byte from the socket. However, if the method doesn’t return anything after an unspecified time, it throws an InterrupedIOException with a “Read timed out” error message:
another link:(awesome https://github.com/eugenp/tutorials/tree/master/core-java-modules/core-java-networking-3
生产环境中C/S架构，S一定是启用进程来对client进行服务的：
服务器一般作 为守护进程始终运行， 监听网络端口， 一旦有客户请求， 就会启动一个服务进程来响应该客 户，同时自己继续监听服务端口，使后来的客户也得到响应的服务。
IP 层主要负责网络主机的定位，
数据传输的路 由，由 IP 地址可以唯一地确定 Internet 上的一台主机。 而 TCP层则提供面向应用的可靠的 或非可靠的数据传输机制，这是网络编程的主要对象，一般不需要关心 IP 层是如何处理数 据的。 目前较为流行的网络编程模型是客户机。
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