Question

我是套接字编程的新手，我试图让我的客户端程序从我桌面上的文档中检索网址，将此网址发送到我的服务器程序，并获取我的服务器 - 程序将此url-website的html代码作为字符串发回：当我尝试将字符串发回给我的客户时，我一直收到NPE，有人能指出我做错了什么吗？

服务器程序：

 public class Server {
 public static void main(String args[]) {
    String fromClient;
    StringBuilder htmlCode = null;
    String str = null;
    try(ServerSocket welcomeSocket = new ServerSocket(8082)){
        System.out.println("Server started, waiting for clients...");
        while(true){
            try(Socket connectionSocket = welcomeSocket.accept();
                BufferedReader inFromClient = new BufferedReader(new InputStreamReader(connectionSocket.getInputStream()));
                DataOutputStream toClient = new DataOutputStream(connectionSocket.getOutputStream())){
                fromClient = inFromClient.readLine();
                try(InputStream url = new URL("http://"+fromClient+"/index.html").openStream();
                    BufferedReader getUrl = new BufferedReader(new InputStreamReader(url))){
                        for(String line = getUrl.readLine(); line != null; line = getUrl.readLine()){
                            //THIS IS WHERE ECLIPSE SAYS NPE IS HAPPENING
                            htmlCode = htmlCode.append(line);
                        }
                    str = htmlCode.toString();
                    toClient.writeBytes(str);   
                }
            }
        }
    }
    catch(IOException io){
        io.printStackTrace();
    }
}

}

客户端：

 public class Client {
 public static void main(String[] args) {
    String url;
    String returned;
    try(Socket clientSocket = new Socket("localhost", 8082);
        BufferedReader inFromUser = new BufferedReader(new FileReader("C:\\Users\\Anthony\\Desktop\\client.txt"));
        DataOutputStream outToServer = new DataOutputStream(clientSocket.getOutputStream());
        BufferedReader inFromServer = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()))){

        url = inFromUser.readLine();
        outToServer.writeBytes(url + "\n");
        returned = inFromServer.readLine();
        System.out.println("FROM SERVER: " + returned);
    }
    catch(IOException io){
        io.printStackTrace();
    }
}

}

Answer 1

问题在于您的服务器：

您使用__declspec(noinline) bool IsEqual(Vector2D & a, Vector2D & b) { // etc.. }初始化StringBuilder：

null

然后尝试在null引用上调用方法：

StringBuilder htmlCode = null;

尝试

htmlCode = htmlCode.append(line);

代替。这应解决StringBuilder htmlCode = new StringBuilder();。

Answer 2

在您为其提供非空值之前，您正在使用htmlCode。

StringBuilder htmlCode = null;

...

htmlCode = htmlCode.append(line);

您可以预先初始化它：

StringBuilder htmlCode = new StringBuilder();
...
htmlCode = htmlCode.append(line);

客户端 - 服务器套接字编程Java

2 个答案: