Java：無限錯誤「null」

Question

我正在寫一個簡單的web服務器在java中作爲練習。當我運行程序時，它立即開始進入無限循環，輸出「Error：null」到控制檯中。我有幾個的try/catch語句做

System.out.println("Error: " + e.getMessage()); 

我使用Eclipse調試器來找出試了一下從哪裏開始，但是當我在調試器中運行它，它的正常動作。繼承人的代碼相關的兩個片段：

public void run() { 
     // Declare server socket 
     ServerSocket serversocket = null; 

     try { 
      // Initialize serversocket on the port passed to 
      // the constructor 
      serversocket = new ServerSocket(port); 
      System.out.println("Server started on port " 
        + Integer.toString(port)); 
     } catch (Exception e) { 
      System.out.println("Fatal Error: " + e.getMessage()); 
     } 

     while (true) { 
      try { 
       // Wait for connections 
       Socket connectionsocket = serversocket.accept(); 
       // Get client IP 
       InetAddress client = connectionsocket.getInetAddress(); 
       System.out.println(client.getHostName() + " connected."); 

       // Read the request into a buffer 
       BufferedReader input = new BufferedReader(
        new InputStreamReader(connectionsocket.getInputStream())); 

       // Prepare the output stream for sending headers 
       // and requested file to the client 
       DataOutputStream output = 
        new DataOutputStream(connectionsocket.getOutputStream()); 

       http_handler(input, output); 
      } catch (Exception e) { 
       System.out.println("Error: " + e.getMessage()); 
      } 
     } 
    } 

和

private void http_handler(BufferedReader input, DataOutputStream output) { 
    int method = 0; // 1 get, 2 head, 0 not supported 
    String http = new String(); 
    String path = new String(); 
    String file = new String(); 
    String user_agent = new String(); 
    try { 
     // Two types of requests we can handle: 
     // Get /index.html HTTP/1.0 
     // HEAD /index.html HTTP/1.0 
     String tmp = input.readLine(); // Read from the stream 
     String tmp2 = new String(tmp); 
     tmp.toUpperCase(); 
     if (tmp.startsWith("GET")) { 
      method = 1; 
     } 
     if (tmp.startsWith("HEAD")) { 
      method = 2; 
     } 

     if (method == 0) { 
      try { 
       // If the request is unsupported, send the error 
       output.writeBytes(construct_http_header(501, 0)); 
       output.close(); 
       return; 
      } catch (Exception e2) { 
       System.out.println("Error: " + e2.getMessage()); 
      } 
     } 

     // Get whats between the spaces in the request 
     // without the beginning slash 
     int start = 0; 
     int end = 0; 

     for (int a = 0; a < tmp2.length(); a++) { 
      if (tmp2.charAt(a) == ' ' && start != 0) { 
       end = a; 
       break; 
      } 
      if (tmp2.charAt(a) == ' ' && start == 0) { 
       start = a; 
      } 
     } 
     path = tmp2.substring(start + 2, end); 

    } catch(Exception e) { 
     System.out.println("Error: " + e.getMessage()); 
    } 
    FileInputStream requestedfile = null; 

    try { 
     // Try to open the file 
     requestedfile = new FileInputStream(path); 
    } catch (Exception e) { 
     try { 
      output.writeBytes(construct_http_header(404, 0)); 
      output.close(); 
     } catch (Exception e2) {} 
     ; 
     System.out.println("Error: " + e.getMessage()); 
    } 

    try { 
     int type_is = 0; 
     // Find out what the filename ends with so we can 
     // put the right MIME type in the header 

     if (path.endsWith(".zip") || path.endsWith(".exe") 
       || path.endsWith(".tar")) { 
      type_is = 3; 
     } else if (path.endsWith(".jpg") || path.endsWith(".jpeg")) { 
      type_is = 1; 
     } else if (path.endsWith(".gif")) { 
      type_is = 2; 
     } else { 
      type_is = 5; 
     } 
     // write out the header, 200 -> OK 
     output.writeBytes(construct_http_header(200, type_is)); 

     // if the request was HEAD, we don't print the body 
     if (method == 1) { 
      while (true) { 
       // read the file from the filestream and ouput through 
       // the client outpustream 
       int b = requestedfile.read(); 
       if (b == -1) { 
        break; // end of file 
       } 
       output.write(b); 
      } 
     } 
     // Clean up 
     output.close(); 
     requestedfile.close(); 
    } catch (Exception e) {} 
} 

**編輯** 我打印的堆棧跟蹤，它是給在

Socket connectionsocket = serversocket.accept(); 

我一個NullPointerException認爲這種方法應該等待連接。我如何阻止它做它正在做的事情？

Answer 1

如果ServerSocket初始化失敗，最終會出現無限空指針異常，因爲沒有任何東西可以接受連接。

Answer 2

我的第一個猜測是你在第一部分代碼中有while (true)，但在粗略閱讀中，我看不到任何突出的東西。您是否嘗試過打印調試語句，而不是使用分步調試器？我知道你在調試模式下的意思，我以前曾經遇到過這種情況。

不過了好幾個步驟來追查：

1. 改變每Error:語句類似Error 1:，Error 2:和使用的輸出來縮小它的代碼塊將引發異常。
2. 打印堆棧軌跡，這些軌跡將使您的操作變得非常容易
3. 打印出每行（甚至每隔幾行，根據需要）的內容，以便直觀地瀏覽輸出中的代碼窗口

這可以讓你縮小出現問題的區域，並使用更精細的輸出語句，確切地說明哪條線和原因。