客戶端-服務器套接字編程Java

Question

我是套接字編程的新手，我正嘗試讓我的客戶端程序從桌面上的文檔中檢索url，將此URL發送到我的服務器程序，並讓我的服務器程序發送回html代碼。 url網站的字符串形式：當我嘗試將字符串發送回客戶端時，我一直得到NPE，有人可以指出我做錯了什么嗎？

服務器程序：

 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();
    }
}

}

客戶端：

 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

問題出在您的服務器上：

您可以使用null初始化StringBuilder ：

StringBuilder htmlCode = null;

然后嘗試對一個為空的引用調用方法：

htmlCode = htmlCode.append(line);

嘗試

StringBuilder htmlCode = new StringBuilder();

代替。 這應該解決NullPointerException 。

Answer 2

在給它一個非null值之前，您正在使用htmlCode 。

StringBuilder htmlCode = null;

...

htmlCode = htmlCode.append(line);

您可以改為將其初始化：

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