Question

对于我的软件工程类，我的小组必须从服务器获取摄像机流并通过套接字将其发送到客户端。我们通过不断通过套接字发送图像来做到这一点。当我们尝试运行我们的程序时，我们只获得一个图像，但下一个图像变为null，因此我们得到一个nullpointer异常。

这是UserClient类。 UserClient接收一系列图像。空指针出现在run方法中，我用注释标记它。

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
package communication;

import java.awt.image.BufferedImage;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.ObjectInputStream;
import java.net.ConnectException;
import java.net.Socket;
import java.net.UnknownHostException;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.imageio.ImageIO;

import javax.swing.JOptionPane;
import userpc.RoverGUI;

/**
 * A simple Swing-based client for the capitalization server. It has a main
 * frame window with a text field for entering strings and a textarea to see the
 * results of capitalizing them.
 */
public class UserClient extends Thread{

    //ObjectInputStream input;
    Socket s = null;
    boolean done = false;
    String serverAddress;
    RoverGUI gui;

    public static void main(String[] args){
        String serverAddress = JOptionPane.showInputDialog(
                "Enter IP Address of a machine that is\n"
                + "running the date service on port 9090:");
        try {
            UserClient c = new UserClient(null, serverAddress);
        } catch (IOException ex) {
            Logger.getLogger(UserClient.class.getName()).log(Level.SEVERE, null, ex);
        }
    }

    public UserClient(RoverGUI gui, String serverAddress) throws UnknownHostException, IOException{
        this.gui=gui;
        this.serverAddress=serverAddress;
    }

    @Override
    public void run(){
        while (!done) {
            try {
                if (serverAddress.equals("")) {
                    throw new UnknownHostException();
                }
                s = new Socket(serverAddress, 9090);
                //input =new ObjectInputStream(s.getInputStream());
                done = true;
            } catch (ConnectException e) {
                gui.display("Wait for servor...");
            } catch (UnknownHostException ex) {
                gui.display("Un known Host Exception Occur...");
            } catch (IOException ex) {
                Logger.getLogger(UserClient.class.getName()).log(Level.SEVERE, null, ex);
            }
        }

        while(true){
            try {
                    gui.setIcons(ImageIO.read(ImageIO.createImageInputStream(s.getInputStream())),null);
//NULL POINTER HERE
                //System.out.println(input.readLine());
            } catch (IOException ex) {
                Logger.getLogger(UserClient.class.getName()).log(Level.SEVERE, null, ex);
            }
        }
    }

    public void close() throws IOException {
        //input.close();
        s.close();
    }
}

RoverServer发送图像流。

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
package communication;

import camera.ImageBuffer;
import java.io.IOException;
import java.io.ObjectOutputStream;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.logging.Level;
import java.util.logging.Logger;

/**
 *
 * @author junxin
 */
public class RoverServer implements Runnable {

    Socket socket = null;
    ServerSocket listener = null;
    ObjectOutputStream out;
    ImageBuffer ib;
    RoverClient client;

    public static void main(String arg[]){
        try {
            ImageBuffer ib=new ImageBuffer();
            RoverServer server=new RoverServer(ib);
            new Thread(server).start();
            RoverClient client= new RoverClient("150.250.220.214");
            client.start();
        } catch (IOException ex) {
            Logger.getLogger(RoverServer.class.getName()).log(Level.SEVERE, null, ex);
        }
    }

    public RoverServer(ImageBuffer ib) {
        this.ib=ib;
        try {
            listener = new ServerSocket(9090);
        } catch (IOException ex) {
            Logger.getLogger(RoverServer.class.getName()).log(Level.SEVERE, null, ex);
        }

    }

    @Override
    public void run() {
        try {
            socket = listener.accept();
            System.out.println("Client accept");

            out =new ObjectOutputStream(socket.getOutputStream());
            while(true){
                //out.printf("", ib.getImage(),ib.getImage2());
                out.writeObject(ib.getImage());
                Thread.sleep(100);
            }
        } catch (IOException ex) {
            System.out.println("Unable to accept client!");
        } catch (InterruptedException ex) {
            Logger.getLogger(RoverServer.class.getName()).log(Level.SEVERE, null, ex);
        }
    }
}

感谢您的帮助！

Answer 1

 gui.setIcons(ImageIO.read(ImageIO.createImageInputStream(s.getInputStream())),null);

将套接字输入流分配给新的阅读器，对吧？
每个流只允许一次。
此外，如果您将流分配给多个阅读器，则只有第一个阅读器可以从流中读取在下面的示例中，只有第一个读取器获取值（服务器发送hi \ nat \ nall）：

        BufferedReader firstReader = new BufferedReader(new InputStreamReader(
                socket.getInputStream()));
        BufferedReader secondReader = new BufferedReader(new InputStreamReader(
                socket.getInputStream()));
        System.out.println("str = " + firstReader.readLine()); //str = hi
        System.out.println("str = " + secondReader.readLine()); //str = null !!!!!
        System.out.println("str = " + firstReader.readLine()); //str = at
        System.out.println("str = " + firstReader.readLine()); //str = all

你应该为每个连接创建一个阅读器并在循环中重复使用它。

缓冲区IO上的空指针异常

1 个答案: