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service()詳解

*從public<code>service</code>方法接收標準HTTP請求,並將它們分派到該類中定義的<code>do</code><i>.</i>方法。此方法是{@link javax.servlet.Servlet#service}方法的HTTP特定版本。不需要重寫此方法。

 原始碼:

 /**
     * Receives standard HTTP requests from the public
     * <code>service</code> method and dispatches
     * them to the <code>do</code><i>Method</i> methods defined in
     * this class. This method is an HTTP-specific version of the
     * {@link javax.servlet.Servlet#service} method. There's no
     * need to override this method.
     *
     * @param req   the {@link HttpServletRequest} object that
     *                  contains the request the client made of
     *                  the servlet
     *
     * @param resp  the {@link HttpServletResponse} object that
     *                  contains the response the servlet returns
     *                  to the client
     *
     * @exception IOException   if an input or output error occurs
     *                              while the servlet is handling the
     *                              HTTP request
     *
     * @exception ServletException  if the HTTP request
     *                                  cannot be handled
     *
     * @see javax.servlet.Servlet#service
     */
    protected void service(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException {

        String method = req.getMethod();

        if (method.equals(METHOD_GET)) {
            long lastModified = getLastModified(req);
            if (lastModified == -1) {
                // servlet doesn't support if-modified-since, no reason
                // to go through further expensive logic
                //servlet不支援if-modified-since因為,沒有理由進行更昂貴的邏輯
                doGet(req, resp);
            } else {
                long ifModifiedSince;
                try {
                    ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
                } catch (IllegalArgumentException iae) {
                    // Invalid date header - proceed as if none was set
                    ifModifiedSince = -1;
                }
                if (ifModifiedSince < (lastModified / 1000 * 1000)) {
                    // If the servlet mod time is later, call doGet()
                    // Round down to the nearest second for a proper compare
                    // A ifModifiedSince of -1 will always be less
                    /*
                      如果尚未設定Last-Modified實體標頭欄位,
                      如果它還沒有被設定並且這個值是有用的,則設定該欄位。在doGet之前呼叫,
                      以確保在寫入響應資料之前設定頭。子類可能已經設定了這個標頭,
                      所以我們進行檢查。
                    */
                    maybeSetLastModified(resp, lastModified);
                    doGet(req, resp);
                } else {
                    resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
                }
            }

        } else if (method.equals(METHOD_HEAD)) {
            long lastModified = getLastModified(req);
            maybeSetLastModified(resp, lastModified);
            doHead(req, resp);

        } else if (method.equals(METHOD_POST)) {
            doPost(req, resp);

        } else if (method.equals(METHOD_PUT)) {
            doPut(req, resp);

        } else if (method.equals(METHOD_DELETE)) {
            doDelete(req, resp);

        } else if (method.equals(METHOD_OPTIONS)) {
            doOptions(req,resp);

        } else if (method.equals(METHOD_TRACE)) {
            doTrace(req,resp);

        } else {
            //
            // Note that this means NO servlet supports whatever
            // method was requested, anywhere on this server.
            //
/*注意,這意味著在這個伺服器上的任何地方,任何servlet都不支援請求的任何方法。*/

            String errMsg = lStrings.getString("http.method_not_implemented");
            Object[] errArgs = new Object[1];
            errArgs[0] = method;
            errMsg = MessageFormat.format(errMsg, errArgs);

            resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
        }
    }

自己理解是,service方法負責根據客戶端請求的方式,來呼叫不同的方法執行請求和響應。

service有個過載的方法,public void service(ServletRequest req, ServletResponse res)

它的主要作用是把請求和響應的型別從

ServletRequest req   =>  HttpServletRequest  request;

ServletResponse res  => HttpServletResponse response;

看原始碼:

作用就是強轉,轉不了報錯。

    /**
     * Dispatches client requests to the protected
     * <code>service</code> method. There's no need to
     * override this method.
     *
     * @param req   the {@link HttpServletRequest} object that
     *                  contains the request the client made of
     *                  the servlet
     *
     * @param res   the {@link HttpServletResponse} object that
     *                  contains the response the servlet returns
     *                  to the client
     *
     * @exception IOException   if an input or output error occurs
     *                              while the servlet is handling the
     *                              HTTP request
     *
     * @exception ServletException  if the HTTP request cannot
     *                                  be handled
     *
     * @see javax.servlet.Servlet#service
     */
    @Override
    public void service(ServletRequest req, ServletResponse res)
        throws ServletException, IOException {

        HttpServletRequest  request;
        HttpServletResponse response;

        try {
            request = (HttpServletRequest) req;
            response = (HttpServletResponse) res;
        } catch (ClassCastException e) {
            throw new ServletException("non-HTTP request or response");
        }
        service(request, response);
    }