Java：无限错误“null”

Question

我正在用 java 编写一个简单的 Web 服务器作为练习。当我运行该程序时，它立即开始进入无限循环，将“错误：null”输出到控制台。我有一些 try/catch 语句，

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

我尝试使用 eclipse 调试器来找出它的启动位置，但是当我在调试器中运行它时，它会正常运行。这是两个相关的代码片段：

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) {}
}

** 编辑 ** 我打印了堆栈跟踪，它给出了一个 NullPointerException

Socket connectionsocket = serversocket.accept();

我认为这个方法应该等待连接。我如何阻止它做它正在做的事情？

Answer 1

我的第一个猜测是第一段代码中的 while (true) ，但是粗略地读了一下，我看不到任何突出的东西。您是否尝试过打印调试语句，而不是使用逐步调试器？我知道你所说的在调试模式下工作是什么意思，我自己以前也遇到过这种情况。

但需要采取几个步骤来追踪它：

1. 将每个 Error: 语句更改为 Error 1:、Error 2: 并使用的输出来缩小哪些代码块将引发异常。
2. 打印堆栈跟踪，这将比您轻松地缩小范围缩小很多范围，
3. 为每一行（甚至根据需要甚至每隔几行）打印一些内容，以直观地浏览输出窗口中的代码

这将使您缩小范围找到给您带来问题的区域，并使用更细粒度的输出语句，准确说明是哪一行以及原因。

Answer 2

如果 ServerSocket 初始化失败，您最终会遇到无限空指针异常，因为不存在任何东西可以接受连接。