Java通过Socket传输文件以及判断文件传输完成的方法

 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： 

https://docs.oracle.com/javase/8/docs/api/java/io/DataInputStream.html

https://docs.oracle.com/javase/8/docs/api/java/io/DataOutputStream.html

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：

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

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

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

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

 

    private static void getBytes(String fileName, long fileLength) throws IOException {
        File file = new File(fileName);
        FileOutputStream f = new FileOutputStream(file);
        byte[] bytes = new byte[1024];
        int length = 0;
        while(true) {
            length = in.read(bytes, 0, bytes.length);
            f.write(bytes, 0, length);
            f.flush();
            /*
             * when fileLength equals to length of local file, it
             * represents the downloading is finished
             */
            if(fileLength == file.length()) break;
        }
        f.close();
    }

 

 

 

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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

import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.logging.Level;
import java.util.logging.Logger;

public class Server {

    
    private static ServerSocket server;
    private static Socket socket;
    private static final int port = 123456;
    private static Logger logger;
    
    static{
        logger = Logger.getLogger("Server");
        logger.setLevel(Level.INFO);
    }
    
    public static void main(String[] args) throws IOException {
        //create server with port
        server = new ServerSocket(port);
        logger.info("Server Starts");
        while(true) {
            //listen server and bind socket when accept a connection
            socket = server.accept();
            //record incoming connections
            logger.info("Get connection from " + socket.getRemoteSocketAddress());
            listenRequest(socket);
        }
    }

    /**
     * create a new thread for socket.<br>
     * listen input from socket<br>
     * process requests and give responses to client.
     * @param socket the specific socket that connects to the server
     */
    private static void listenRequest(Socket socket) {
        /*
         * create a new thread for this socket so that it 
         * can be responsible for this socket only
         */
        new Thread(new Runnable(){
            DataInputStream in;
            DataOutputStream out;
            boolean status;
            
            @Override
            public void run() {
                try {
                    logger.info("Start Listening Socket Input");
                    /* 
                     * create DataInputStream and DataOutputStream objects to get
                     * the stream of socket
                     */
                    in = new DataInputStream(socket.getInputStream());
                    out = new DataOutputStream(socket.getOutputStream());
                    status = true;
                    while(status) {
                        processInput();
                    }
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }

            /**
             * process requests from client and give response respectively
             * @throws IOException
             */
            private void processInput() throws IOException {
                String content = in.readUTF();
                String[] command = content.split(":");
                String fileName = null;
                switch(command[0]) {
                case "FILE_TRANSFER_REQUEST":
                    /* 
                     * structure of FILE_REQUEST should be:
                     * @COMMAND:@FILENAME
                     */
                    logger.info("Get File Request From " + socket.getRemoteSocketAddress());
                    fileName = command[1];
                    fileTransferResponse(fileName);
                    break;
                case "FILE_BYTES_REQUEST":
                    /*
                     * structure of FILE_BYTES_REQUEST
                     * @Command:@FileName
                     */
                    fileName = command[1];
                    logger.info("Get File Bytes Request From " + socket.getRemoteSocketAddress());
                    fileBytesResponse(fileName);
                    break;
                case "DISCONNECT":
                    logger.info("Get Disconnect From " + socket.getRemoteSocketAddress());
                    stop();
                    break;
                    default:
                        break;
                }
                
            }

            /**
             * close relevant resources and change status to 
             * false to end this thread
             * @throws IOException
             */
            private void stop() throws IOException {
                in.close();
                out.close();
                socket.close();
                status = false;
            }

            /**
             * send file bytes(raw data) to client 
             * @param fileName name of file
             * @throws IOException
             */
            private void fileBytesResponse(String fileName) throws IOException {
                File file = new File(fileName);
                FileInputStream fileInput = new FileInputStream(file);
                byte[] bytes = new byte[1024];
                int length = 0;
                while((length = fileInput.read(bytes)) != -1) {
                    out.write(bytes, 0, length);
                    out.flush();
                }
                logger.info("File Transfer Finished");
                fileInput.close();
            }

            private void fileTransferResponse(String fileName) throws IOException {
                File file = new File(fileName);
                StringBuffer response = new StringBuffer();
                /* 
                 * if file does not exist, response to the client
                 * with error message
                 */
                if(!file.exists()) {
                    //make sure there is nothing in response
                    response.setLength(0);
                    /* 
                     * structure of FILE_NOT_EXISTS should be:
                     * @COMMAND:@MESSAGE
                     */
                    response.append("FILE_NOT_EXISTS:");
                    response.append("The file \"" + fileName+ "\" your request does not exist.");
                    out.writeUTF(response.toString());
                    out.flush();
                }else {
                    /* 
                     * structure of FILE_EXISTS should be;
                     * @Command:@FileName:@FileLength
                     */
                    response.setLength(0);
                    response.append("FILE_EXISTS:");
                    response.append(fileName+":");
                    response.append(file.length());
                    out.writeUTF(response.toString());
                    out.flush();
                }
            }
            
        }).start();
    }
    


}

 

Client

Client中不需要多线程 因此一个主线程完成所有工作

import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.PrintStream;
import java.net.Socket;
import java.util.Scanner;
import java.util.logging.Level;
import java.util.logging.Logger;

public class Client {

    private static Socket socket;
    private static final int port = 123456;
    private static final String host = "***********";
    private static Logger logger;
    private static PrintStream display;
    private static DataOutputStream out;
    private static DataInputStream in;
    private static Scanner sc;
    
    static{
        logger = Logger.getLogger("Client");
        logger.setLevel(Level.INFO);
        display = System.out;
        sc = new Scanner(System.in);
    }
    public static void main(String[] args) {
        try {
            /* create a new object of Socket and try to connect to
             * the server with host and port
             */
            socket = new Socket(host, port);
            
            //logger.info("Connect to Server "+ host + " with Port "+ port);
            out = new DataOutputStream(socket.getOutputStream());
            in = new DataInputStream(socket.getInputStream());
            while(true) {
                welcomeInterface(display, sc);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    /**
     * An interface that instruct users to interact with this program
     * @param display system out stream
     * @param sc system input stream
     * @throws IOException
     */
    private static void welcomeInterface(PrintStream display, Scanner sc) throws IOException {
        display.println();
        for(int i = 0; i < 15; i++) {
            display.append("#");
        }
        display.append(" Welcome To Week6Client ");
        for(int i = 0; i < 15; i++) {
            display.append("#");
        }
        display.println();
        display.println();
        display.printf("%5s%-30s", " ", "Please Enter The File Name: ");
        String fileName = sc.nextLine();
        display.println();
        fileRequest(fileName);
    }

    /**
     * send file request to server, and process responses of this request
     * from server.
     * @param fileName the name of file that expect to download
     * @throws IOException
     */
    private static void fileRequest(String fileName) throws IOException {
        StringBuffer content = new StringBuffer();
        /*
         * structure of FILE_REQUEST:
         * @command:@fileName
         */
        content.append("FILE_TRANSFER_REQUEST:");
        content.append(fileName);
        out.writeUTF(content.toString());
        out.flush();
        //logger.info("Send File Request to Server");
        // after send request to server, waiting for response from server
        String[] response = in.readUTF().split(":");
        //logger.info("Get Response From Server: " + response[0]);
        switch(response[0]) {
        case "FILE_EXISTS":
            fileName = response[1];
            File file = new File(fileName);
            /*
             * structure of FILE_EXISTS:
             * @Command:@FileName:@FileLength
             */
            if(file.exists()) {
                display.printf("%5s%s", " ", "File \"" +fileName + "\" exists in current director.\n");
                display.printf("%5s%s", " ", "Do you want to overrid it? \n");
                display.printf("%5s%s", " ", "Y/y -> Override, Other -> Cancel\n");
                display.printf("%5s", " ");
                if(!sc.nextLine().toLowerCase().equals("y")) break;
                else {
                    file.delete();
                    file.createNewFile();
                }
            } 
            content.setLength(0);
            content.append("FILE_BYTES_REQUEST:");
            content.append(fileName);
            out.writeUTF(content.toString());
            display.printf("%5s%-30s%s", "",  "Downloading ", fileName+"\n");
            long fileLength = Long.parseLong(response[2]);
            getBytes(fileName, fileLength);
            display.printf("%5s%-20s", "",  "Download Done\n");
            break;
        case "FILE_NOT_EXISTS":
            display.printf("%5s%-50s", "", response[1]);
            break;
            default:
                break;
        }
        display.println();
        display.printf("%5s%s", " ", "Do you want to continue? \n");
        display.printf("%5s%s", " ", "Y/y -> Continue, Other -> Quit \n");
        display.printf("%5s", "");
        /*
         * if user enter other character, call stop() to close relevant
         * resources and end this process
         */
        if(!sc.nextLine().toLowerCase().equals("y")) {
            display.println();
            display.println();
            stop();
        }
        display.println();
        display.println();

    }

    /**
     * close relevant resources and call System.exit(int status) to 
     * end this process
     * @throws IOException
     */
    private static void stop() throws IOException {
        out.writeUTF("DISCONNECT");
        out.close();
        in.close();
        sc.close();
        socket.close();
        System.exit(0);
    }

    /**
     * Receive bytes from server. Once the length of local file is same
     * as server's, transfer ends.
     * @param fileName the name of file that is going to operate
     * @param fileLength the length of file on server
     * @throws IOException
     */
    private static void getBytes(String fileName, long fileLength) throws IOException {
        File file = new File(fileName);
        FileOutputStream f = new FileOutputStream(file);
        byte[] bytes = new byte[1024];
        int length = 0;
        while(true) {
            length = in.read(bytes, 0, bytes.length);
            f.write(bytes, 0, length);
            f.flush();
            /*
             * when fileLength equals to length of local file, it
             * represents the downloading is finished
             */
            if(fileLength == file.length()) break;
        }
        f.close();
    }

}

 

 

 

 第一次写东西，没什么经验，如有错误、问题请指出

如需讨论 也可发邮件给我哈

email: [e-mail]

 

 

 

 

 

 

 

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