客戶端 - 服務器套接字編程Java

Question

我是新來的套接字編程，我試圖讓我的客戶端程序從我的桌面上的文檔檢索URL，發送此URL到我的服務器程序，並讓我的服務器程序以字符串的形式發回這個url-website的html代碼： 當我嘗試將字符串發回給我的客戶端時，我總是收到NPE，任何人都可以指向我做錯了什麼？

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

問題出在你的服務器：

您初始化StringBuilder與null：

StringBuilder htmlCode = null; 

，然後嘗試調用空方法的參考方法：

htmlCode = htmlCode.append(line); 

嘗試

StringBuilder htmlCode = new StringBuilder(); 

代替。這應該解決NullPointerException。

Answer 2

您使用htmlCode你給它一個非空值之前。

StringBuilder htmlCode = null; 

... 

htmlCode = htmlCode.append(line); 

你可以代替初始化它面前：

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