絮叨

本人学生，往前一年左右的时间用在了Java上

都说写博客、随笔是百利一害的事情-->一害是费时间

近期也是在此申请开通了博客

此篇也算是开博第一篇，所以絮叨一下

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问题发现与解决

今天在写Socket的文件传输

程序涉及到Socket、线程、文件操作、流等

目标是Client可以向Server提交文件名

然后Server进行响应-->文件存在则传输，不存在则回复文件不存在的消息

因为想要完成多次文件传输，所以把方法块放进了循环

于是希望所有打开的资源，像InputStream, OutputStream，这些可以再运行过程中一直打开，直到程序结束才关闭

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文件传输用的是 DataInputStream 中的 read(byte[] b, int off, int len) 和 DataOutputStream 中的 write(byte[] b, int off, int len)

JavaDoc 地址：

简单描述

DataInputStream 和 DataOutputStream 是字节流可以视为-->他们操作的对象是每字节的编码-->机器认识而我不认识的东西

相对应的Writer和Reader 是字符流他们操作的是将字节编码转化为字符的我能看懂的东西

所以在传输文件的时候使用字节流是更为合适的选择

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网上有关文件传输的代码大多是这个样子

通过(FileInputStream) fileInput 从文件读取，并通过(DataOutputStream) out 发送：

1         while((length = fileInput.read(bytes)) != -1) {2             out.write(bytes, 0, length);3             out.flush();4         }

通过(DataInputStream) in 读取数据，并通过(FileOutputStream) fileOutput 写入文件

1         while((length = in.read(bytes, 0, bytes.length)) != -1) {2             fileOutput.write(bytes, 0, length);3             fileOutput.flush();4         }

这两段代码没有问题，都是可以运行的

我的问题出现在接收文件数据的时候，文件数据已经发送完毕，但是程序没有跳出循环

其症结是while()中的判断条件是要 length = -1 才会跳出循环

可以通过上方贴出来的JavaDoc的截图看到-1表示的读取到流的结尾

但是想要让它读取到流的结尾是要发送方关闭流

在发送方发送完成后需要有类似这样一句 out.close()

执行out.close() 实际上它所包装的socket也随之关闭了

也就是说整条连接都断了这不是我想要的

我想要的是发送完一个程序还能够继续响应我从Client发送来的消息

于是需要引入新的判断条件让接收方知道它想要的文件已经传输完毕

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我找到的一个简单的方法是利用文件的length

传输文件之前，在Server检查文件名是否存在，向Client反馈的时候，将文件的length也发送过去

这样就可在接收文件的时候对本地文件的length和传输过来的length进行比较

从而判断文件传输是否完成

1     private static void getBytes(String fileName, long fileLength) throws IOException { 2         File file = new File(fileName); 3         FileOutputStream f = new FileOutputStream(file); 4         byte[] bytes = new byte[1024]; 5         int length = 0; 6         while(true) { 7             length = in.read(bytes, 0, bytes.length); 8             f.write(bytes, 0, length); 9             f.flush();10             /*11              * when fileLength equals to length of local file, it12              * represents the downloading is finished13              */14             if(fileLength == file.length()) break;15         }16         f.close();17     }

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代码

下面是我所写的Socket文件传输的代码

Server

在Server端循环监听绑定端口

在listenRequest()中为每个新连接进来的socket新建一个线程

以此来并发响应每个对应Client的请求

1 import java.io.DataInputStream;  2 import java.io.DataOutputStream;  3 import java.io.File;  4 import java.io.FileInputStream;  5 import java.io.IOException;  6 import java.net.ServerSocket;  7 import java.net.Socket;  8 import java.util.logging.Level;  9 import java.util.logging.Logger; 10  11 public class Server { 12  13      14     private static ServerSocket server; 15     private static Socket socket; 16     private static final int port = 123456; 17     private static Logger logger; 18      19     static{ 20         logger = Logger.getLogger("Server"); 21         logger.setLevel(Level.INFO); 22     } 23      24     public static void main(String[] args) throws IOException { 25         //create server with port 26         server = new ServerSocket(port); 27         logger.info("Server Starts"); 28         while(true) { 29             //listen server and bind socket when accept a connection 30             socket = server.accept(); 31             //record incoming connections 32             logger.info("Get connection from " + socket.getRemoteSocketAddress()); 33             listenRequest(socket); 34         } 35     } 36  37     /** 38      * create a new thread for socket.
 39      * listen input from socket
 40      * process requests and give responses to client. 41      * @param socket the specific socket that connects to the server 42      */ 43     private static void listenRequest(Socket socket) { 44         /* 45          * create a new thread for this socket so that it  46          * can be responsible for this socket only 47          */ 48         new Thread(new Runnable(){ 49             DataInputStream in; 50             DataOutputStream out; 51             boolean status; 52              53             @Override 54             public void run() { 55                 try { 56                     logger.info("Start Listening Socket Input"); 57                     /*  58                      * create DataInputStream and DataOutputStream objects to get 59                      * the stream of socket 60                      */ 61                     in = new DataInputStream(socket.getInputStream()); 62                     out = new DataOutputStream(socket.getOutputStream()); 63                     status = true; 64                     while(status) { 65                         processInput(); 66                     } 67                 } catch (IOException e) { 68                     e.printStackTrace(); 69                 } 70             } 71  72             /** 73              * process requests from client and give response respectively 74              * @throws IOException 75              */ 76             private void processInput() throws IOException { 77                 String content = in.readUTF(); 78                 String[] command = content.split(":"); 79                 String fileName = null; 80                 switch(command[0]) { 81                 case "FILE_TRANSFER_REQUEST": 82                     /*  83                      * structure of FILE_REQUEST should be: 84                      * @COMMAND:@FILENAME 85                      */ 86                     logger.info("Get File Request From " + socket.getRemoteSocketAddress()); 87                     fileName = command[1]; 88                     fileTransferResponse(fileName); 89                     break; 90                 case "FILE_BYTES_REQUEST": 91                     /* 92                      * structure of FILE_BYTES_REQUEST 93                      * @Command:@FileName 94                      */ 95                     fileName = command[1]; 96                     logger.info("Get File Bytes Request From " + socket.getRemoteSocketAddress()); 97                     fileBytesResponse(fileName); 98                     break; 99                 case "DISCONNECT":100                     logger.info("Get Disconnect From " + socket.getRemoteSocketAddress());101                     stop();102                     break;103                     default:104                         break;105                 }106                 107             }108 109             /**110              * close relevant resources and change status to 111              * false to end this thread112              * @throws IOException113              */114             private void stop() throws IOException {115                 in.close();116                 out.close();117                 socket.close();118                 status = false;119             }120 121             /**122              * send file bytes(raw data) to client 123              * @param fileName name of file124              * @throws IOException125              */126             private void fileBytesResponse(String fileName) throws IOException {127                 File file = new File(fileName);128                 FileInputStream fileInput = new FileInputStream(file);129                 byte[] bytes = new byte[1024];130                 int length = 0;131                 while((length = fileInput.read(bytes)) != -1) {132                     out.write(bytes, 0, length);133                     out.flush();134                 }135                 logger.info("File Transfer Finished");136                 fileInput.close();137             }138 139             private void fileTransferResponse(String fileName) throws IOException {140                 File file = new File(fileName);141                 StringBuffer response = new StringBuffer();142                 /* 143                  * if file does not exist, response to the client144                  * with error message145                  */146                 if(!file.exists()) {147                     //make sure there is nothing in response148                     response.setLength(0);149                     /* 150                      * structure of FILE_NOT_EXISTS should be:151                      * @COMMAND:@MESSAGE152                      */153                     response.append("FILE_NOT_EXISTS:");154                     response.append("The file \"" + fileName+ "\" your request does not exist.");155                     out.writeUTF(response.toString());156                     out.flush();157                 }else {158                     /* 159                      * structure of FILE_EXISTS should be;160                      * @Command:@FileName:@FileLength161                      */162                     response.setLength(0);163                     response.append("FILE_EXISTS:");164                     response.append(fileName+":");165                     response.append(file.length());166                     out.writeUTF(response.toString());167                     out.flush();168                 }169             }170             171         }).start();172     }173     174 175 176 }

Client

Client中不需要多线程因此一个主线程完成所有工作

1 import java.io.DataInputStream;  2 import java.io.DataOutputStream;  3 import java.io.File;  4 import java.io.FileOutputStream;  5 import java.io.IOException;  6 import java.io.PrintStream;  7 import java.net.Socket;  8 import java.util.Scanner;  9 import java.util.logging.Level; 10 import java.util.logging.Logger; 11  12 public class Client { 13  14     private static Socket socket; 15     private static final int port = 123456; 16     private static final String host = "***********"; 17     private static Logger logger; 18     private static PrintStream display; 19     private static DataOutputStream out; 20     private static DataInputStream in; 21     private static Scanner sc; 22      23     static{ 24         logger = Logger.getLogger("Client"); 25         logger.setLevel(Level.INFO); 26         display = System.out; 27         sc = new Scanner(System.in); 28     } 29     public static void main(String[] args) { 30         try { 31             /* create a new object of Socket and try to connect to 32              * the server with host and port 33              */ 34             socket = new Socket(host, port); 35              36             //logger.info("Connect to Server "+ host + " with Port "+ port); 37             out = new DataOutputStream(socket.getOutputStream()); 38             in = new DataInputStream(socket.getInputStream()); 39             while(true) { 40                 welcomeInterface(display, sc); 41             } 42         } catch (IOException e) { 43             e.printStackTrace(); 44         } 45     } 46     /** 47      * An interface that instruct users to interact with this program 48      * @param display system out stream 49      * @param sc system input stream 50      * @throws IOException 51      */ 52     private static void welcomeInterface(PrintStream display, Scanner sc) throws IOException { 53         display.println(); 54         for(int i = 0; i < 15; i++) { 55             display.append("#"); 56         } 57         display.append(" Welcome To Week6Client "); 58         for(int i = 0; i < 15; i++) { 59             display.append("#"); 60         } 61         display.println(); 62         display.println(); 63         display.printf("%5s%-30s", " ", "Please Enter The File Name: "); 64         String fileName = sc.nextLine(); 65         display.println(); 66         fileRequest(fileName); 67     } 68  69     /** 70      * send file request to server, and process responses of this request 71      * from server. 72      * @param fileName the name of file that expect to download 73      * @throws IOException 74      */ 75     private static void fileRequest(String fileName) throws IOException { 76         StringBuffer content = new StringBuffer(); 77         /* 78          * structure of FILE_REQUEST: 79          * @command:@fileName 80          */ 81         content.append("FILE_TRANSFER_REQUEST:"); 82         content.append(fileName); 83         out.writeUTF(content.toString()); 84         out.flush(); 85         //logger.info("Send File Request to Server"); 86         // after send request to server, waiting for response from server 87         String[] response = in.readUTF().split(":"); 88         //logger.info("Get Response From Server: " + response[0]); 89         switch(response[0]) { 90         case "FILE_EXISTS": 91             fileName = response[1]; 92             File file = new File(fileName); 93             /* 94              * structure of FILE_EXISTS: 95              * @Command:@FileName:@FileLength 96              */ 97             if(file.exists()) { 98                 display.printf("%5s%s", " ", "File \"" +fileName + "\" exists in current director.\n"); 99                 display.printf("%5s%s", " ", "Do you want to overrid it? \n");100                 display.printf("%5s%s", " ", "Y/y -> Override, Other -> Cancel\n");101                 display.printf("%5s", " ");102                 if(!sc.nextLine().toLowerCase().equals("y")) break;103                 else {104                     file.delete();105                     file.createNewFile();106                 }107             } 108             content.setLength(0);109             content.append("FILE_BYTES_REQUEST:");110             content.append(fileName);111             out.writeUTF(content.toString());112             display.printf("%5s%-30s%s", "",  "Downloading ", fileName+"\n");113             long fileLength = Long.parseLong(response[2]);114             getBytes(fileName, fileLength);115             display.printf("%5s%-20s", "",  "Download Done\n");116             break;117         case "FILE_NOT_EXISTS":118             display.printf("%5s%-50s", "", response[1]);119             break;120             default:121                 break;122         }123         display.println();124         display.printf("%5s%s", " ", "Do you want to continue? \n");125         display.printf("%5s%s", " ", "Y/y -> Continue, Other -> Quit \n");126         display.printf("%5s", "");127         /*128          * if user enter other character, call stop() to close relevant129          * resources and end this process130          */131         if(!sc.nextLine().toLowerCase().equals("y")) {132             display.println();133             display.println();134             stop();135         }136         display.println();137         display.println();138 139     }140 141     /**142      * close relevant resources and call System.exit(int status) to 143      * end this process144      * @throws IOException145      */146     private static void stop() throws IOException {147         out.writeUTF("DISCONNECT");148         out.close();149         in.close();150         sc.close();151         socket.close();152         System.exit(0);153     }154 155     /**156      * Receive bytes from server. Once the length of local file is same157      * as server's, transfer ends.158      * @param fileName the name of file that is going to operate159      * @param fileLength the length of file on server160      * @throws IOException161      */162     private static void getBytes(String fileName, long fileLength) throws IOException {163         File file = new File(fileName);164         FileOutputStream f = new FileOutputStream(file);165         byte[] bytes = new byte[1024];166         int length = 0;167         while(true) {168             length = in.read(bytes, 0, bytes.length);169             f.write(bytes, 0, length);170             f.flush();171             /*172              * when fileLength equals to length of local file, it173              * represents the downloading is finished174              */175             if(fileLength == file.length()) break;176         }177         f.close();178     }179 180 }