Question

我尝试研究HttpURLConnection Java源代码，以了解它如何基于套接字编程实现Http连接，但我遇到了如下问题：

/**
 * Returns the value for the <code>n</code><sup>th</sup> header field.
 * Some implementations may treat the <code>0</code><sup>th</sup>
 * header field as special, i.e. as the status line returned by the HTTP
 * server.
 * <p>
 * This method can be used in conjunction with the
 * {@link #getHeaderFieldKey getHeaderFieldKey} method to iterate through all
 * the headers in the message.
 *
 * @param   n   an index, where n>=0.
 * @return  the value of the <code>n</code><sup>th</sup> header field,
 *          or <code>null</code> if the value does not exist.
 * @see     java.net.HttpURLConnection#getHeaderFieldKey(int)
 */
public String getHeaderField(int n) {
    return null;
}

在getResponseCode()方法中，有一段代码如下：

    /*
     * If we can't a status-line then re-throw any exception
     * that getInputStream threw.
     */
    String statusLine = getHeaderField(0);
    if (statusLine == null) {
        if (exc != null) {
            if (exc instanceof RuntimeException)
                throw (RuntimeException)exc;
            else
                throw (IOException)exc;
        }
        return -1;
    }

     /*
     * Examine the status-line - should be formatted as per
     * section 6.1 of RFC 2616 :-
     *
     * Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase
     *
     * If status line can't be parsed return -1.
     */
    if (statusLine.startsWith("HTTP/1.")) {
        int codePos = statusLine.indexOf(' ');
        if (codePos > 0) {

            int phrasePos = statusLine.indexOf(' ', codePos+1);
            if (phrasePos > 0 && phrasePos < statusLine.length()) {
                responseMessage = statusLine.substring(phrasePos+1);
            }

            // deviation from RFC 2616 - don't reject status line
            // if SP Reason-Phrase is not included.
            if (phrasePos < 0)
                phrasePos = statusLine.length();

            try {
                responseCode = Integer.parseInt
                        (statusLine.substring(codePos+1, phrasePos));
                return responseCode;
            } catch (NumberFormatException e) { }
        }
    }
    return -1;

为什么getHeaderField(int n)总是返回null？有什么分？我该如何解释这两种方法？

Answer 1

虽然它没有说出来，但HttpURLConnection被视为抽象基类（实际上并不是抽象的）。当你调用URL#openConnection()时，你实际上得到了一个特定于编译器的类的实例;对于Sun / Oracle编译器，它是sun.net.www.protocol.http.HttpURLConnection。你会发现getHeaderField的实现更有意义。

HttpURLConnection源代码getHeaderField总是返回null？

1 个答案: