客户端-服务器套接字编程 Java

Question

我是套接字编程的新手，我正在尝试让我的客户端程序从我桌面上的文档中检索 url，将此 url 发送到我的服务器程序，并让我的服务器程序将此 url 网站的 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，谁能指出我做错了什么？

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

在给它一个非空值之前，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);

Answer 2

问题出在你的服务器：

你用 null 初始化 StringBuilder:

StringBuilder htmlCode = null;

然后尝试在空引用上调用方法：

htmlCode = htmlCode.append(line);

尝试

StringBuilder htmlCode = new StringBuilder();

相反。这应该解决 NullPointerException.

客户端-服务器套接字编程 Java

Client-Server Socket Programming Java

java

io

serversocket

socket.io