May 12, 2021 JSP
When a browser requests a Web page, it sends a series of information to the web server that cannot be read directly because it is transmitted as part of the HTTP header. You can check the HTTP protocol for more information.
The following table lists some important elements of the browser-side header, which will be frequently seen in future network programming:
| Information | Describe |
|---|---|
| Accept | Specify the type of MIME that a browser or other client can handle. Its value is usually image/png or image/jpeg |
| Accept-Charset | Specifies the set of characters that the browser wants to use. For example, ISO-8859-1 |
| Accept-Encoding | Specify the type of encoding. Its value is usually gzip or compress |
| Accept-Language | Specifying the client's preferred language, the servlet returns the result set in the current language first, if the servlet supports that language. For example, en, en-us, ru, etc |
| Authorization | Identify different users when visiting password-protected web pages |
| Connection | Indicates whether the client can handle HTTP persistent connections. P ersistent connections allow clients or browsers to get multiple files in a single request. Keep-Alive means that persistent connections are enabled |
| Content-Length | Available only for POST requests, representing the number of bytes of POST data |
| Cookie | Return cookies previously sent to the browser to the server |
| Host | Indicates the host name and port number in the original URL |
| If-Modified-Since | Indicates that the client will only need the page if it is modified on the specified date. The server sends 304 yards to the client, indicating that there are no updated resources |
| If-Unmodified-Since | In contrast to If-Modifyed-Since, the operation will only succeed if the document has not been modified after the specified date |
| Referer | Marks the URL of the referenced page. For example, if you are on page 1 and then click a link to page 2, the URL of page 1 will be included in the header of the browser request page 2 |
| User-Agent | Used to distinguish between requests sent by different browsers or clients and to return different content to different types of browsers |
The request object is an example of the Javax.servlet.http://httpservletRequest class. Each time a client requests a page, the JSP engine produces a new object to represent the request.
The request object provides a range of methods for obtaining HTTP headers, including form data, cookies, HTTP methods, and so on.
Next, we'll introduce some of the methods commonly used in JSP programming to get HTTP headers. See the table below for details:
| Serial number | Method . . . |
|---|---|
| 1 |
Cookie[] getCookies()
Returns an array of all cookies owned by the client |
| 2 |
Enumeration getAttributeNames()
Returns a collection of all property names for the request object |
| 3 |
Enumeration getHeaderNames()
Returns a collection of names for all HTTP headers |
| 4 |
Enumeration getParameterNames()
Returns a collection of all parameters in the request |
| 5 |
HttpSession getSession()
Returns the session object for the request, and if not, creates one |
| 6 |
HttpSession getSession(boolean create)
Returns the session object for the request, and if there is no and the parameter create is true, a new session object is returned |
| 7 |
Locale getLocale()
Returns the Locale object of the current page, which can be set in response |
| 8 |
Object getAttribute(String name)
Returns a property value with the name name name, null if it does not exist. |
| 9 |
ServletInputStream getInputStream()
Returns the requested input stream |
| 10 |
String getAuthType()
Returns the name of the authentication scheme to protect the servlet, such as "BASIC" or "SSL" or null if the JSP does not have protection in place |
| 11 |
String getCharacterEncoding()
Returns the character encoding set name of the request |
| 12 |
String getContentType()
Returns the MIME type of the request body and, if unknown, null |
| 13 |
String getContextPath()
Returns the context path indicated in the request URI |
| 14 |
String getHeader(String name)
Returns the header specified by name |
| 15 |
String getMethod()
Return http methods such as GET, POST, or PUT in this request |
| 16 |
String getParameter(String name)
Returns the parameters specified by name in this request and null if they do not exist |
| 17 |
String getPathInfo()
Return any additional paths associated with this request URL |
| 18 |
String getProtocol()
Returns the protocol name and version used for this request |
| 19 |
String getQueryString()
Returns the query string contained in this request URL |
| 20 |
String getRemoteAddr()
Returns the client's IP address |
| 21 |
String getRemoteHost()
Returns the full name of the client |
| 22 |
String getRemoteUser()
Returns the user who the client has authenticated by logging on to, and null if the user is not authenticate |
| 23 |
String getRequestURI()
Returns the URI of the request |
| 24 |
String getRequestedSessionId()
Returns the session ID specified by request |
| 25 |
String getServletPath()
Returns the requested servlet path |
| 26 |
String[] getParameterValues(String name)
Returns all values of the parameters of the specified name, or null if it does not exist |
| 27 |
boolean isSecure()
Returns whether request is using an encrypted channel, such as HTTPS |
| 28 |
int getContentLength()
Returns the number of bytes contained in the request body, if unknown return -1 |
| 29 |
int getIntHeader(String name)
Returns the value of the request header with the specified name |
| 30 |
int getServerPort()
Returns the server port number |
In this example, we will use the getHeaderNames() method of the HttpServletRequest class to read the HTTP header. This method returns the header information of the current HTTP request as enumeration.
After you get the Enumeration object, you traverse the Enumeration object in a standard way, using the HasMoreElements() method to determine when to stop, and the nextElement() method to get the name of each parameter.
<%@ page import="java.io.*,java.util.*" %>
<html>
<head>
<title>HTTP Header Request Example</title>
</head>
<body>
<center>
<h2>HTTP Header Request Example</h2>
<table width="100%" border="1" align="center">
<tr bgcolor="#949494">
<th>Header Name</th><th>Header Value(s)</th>
</tr>
<% Enumeration headerNames = request.getHeaderNames(); while(headerNames.hasMoreElements()) { String paramName = (String)headerNames.nextElement(); out.print("<tr><td>" + paramName + "</td>\n");
String paramValue = request.getHeader(paramName);
out.println("<td> " + paramValue + "</td></tr>\n");
}
%>
</table>
</center>
</body>
</html>
Access main.jsp will get the following results:
| Header Name | Header Value(s) |
|---|---|
| accept | */* |
| accept-language | en-us |
| user-agent | Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; Trident/4.0; InfoPath.2; MS-RTC LM 8) |
| accept-encoding | gzip, deflate |
| host | localhost:8080 |
| connection | Keep-Alive |
| cache-control | no-cache |
You can try other methods of the HttpServletRequest class in the code above.