The Java Developers Almanac 1.4


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Home > List of Packages > javax.servlet  [11 examples] > Requests  [6 examples]

e1040. Getting a Request Parameter in a Servlet

In a GET request, the request parameters are taken from the query string (the data following the question mark on the URL). For example, the URL http://hostname.com?p1=v1&p2=v2 contains two request parameters - - p1 and p2. In a POST request, the request parameters are taken from both query string and the posted data which is encoded in the body of the request. This example demonstrates how to get the value of a request parameter in either a GET or POST request.
    // See also e1035 The Quintessential Servlet
    
    // This method is called by the servlet container to process a GET request.
    public void doGet(HttpServletRequest req, HttpServletResponse resp) throws IOException {
        doGetOrPost(req, resp);
    }
    
    // This method is called by the servlet container to process a POST request.
    public void doPost(HttpServletRequest req, HttpServletResponse resp) throws IOException {
        doGetOrPost(req, resp);
    }
    
    // This method handles both GET and POST requests.
    private void doGetOrPost(HttpServletRequest req, HttpServletResponse resp) throws IOException {
        // Get the value of a request parameter; the name is case-sensitive
        String name = "param";
        String value = req.getParameter(name);
        if (value == null) {
            // The request parameter 'param' was not present in the query string
            // e.g. http://hostname.com?a=b
        } else if ("".equals(value)) {
            // The request parameter 'param' was present in the query string but has no value
            // e.g. http://hostname.com?param=&a=b
        }
    
        // The following generates a page showing all the request parameters
        PrintWriter out = resp.getWriter();
        resp.setContentType("text/plain");
    
        // Get the values of all request parameters
        Enumeration enum = req.getParameterNames();
        for (; enum.hasMoreElements(); ) {
            // Get the name of the request parameter
            name = (String)enum.nextElement();
            out.println(name);
    
            // Get the value of the request parameter
            value = req.getParameter(name);
    
            // If the request parameter can appear more than once in the query string, get all values
            String[] values = req.getParameterValues(name);
    
            for (int i=0; i<values.length; i++) {
                out.println("    "+values[i]);
            }
        }
        out.close();
    }

 Related Examples
e1041. Preventing Concurrent Requests to a Servlet
e1042. Getting the Requesting URL in a Servlet
e1043. Getting a Request Header in a Servlet
e1044. Processing a HEAD Request in a Servlet
e1045. Getting the Client's Address in a Servlet


© 2002 Addison-Wesley.