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Saturday, January 21, 2017

JSP - Server Response

When a Web server responds to a HTTP request to the browser, the response typically consists of a status line, some response headers, a blank line, and the document. A typical response looks like this:
HTTP/1.1 200 OK
Content-Type: text/html
Header2: ...
...
HeaderN: ...
  (Blank Line)
<!doctype ...>
<html>
<head>...</head>
<body>
...
</body>
</html>
The status line consists of the HTTP version (HTTP/1.1 in the example), a status code (200 in the example), and a very short message corresponding to the status code (OK in the example).
Following is a summary of the most useful HTTP 1.1 response headers which go back to the browser from web server side and you would use them very frequently in web programming:
HeaderDescription
AllowThis header specifies the request methods (GET, POST, etc.) that the server supports.
Cache-ControlThis header specifies the circumstances in which the response document can safely be cached. It can have values public, private or no-cache etc. Public means document is cacheable, Private means document is for a single user and can only be stored in private (nonshared) caches and no-cache means document should never be cached.
ConnectionThis header instructs the browser whether to use persistent in HTTP connections or not. A value of close instructs the browser not to use persistent HTTP connections and keep-alive means using persistent connections.
Content-DispositionThis header lets you request that the browser ask the user to save the response to disk in a file of the given name.
Content-EncodingThis header specifies the way in which the page was encoded during transmission.
Content-LanguageThis header signifies the language in which the document is written. For example en, en-us, ru, etc.
Content-LengthThis header indicates the number of bytes in the response. This information is needed only if the browser is using a persistent (keep-alive) HTTP connection.
Content-TypeThis header gives the MIME (Multipurpose Internet Mail Extension) type of the response document.
ExpiresThis header specifies the time at which the content should be considered out-of-date and thus no longer be cached.
Last-ModifiedThis header indicates when the document was last changed. The client can then cache the document and supply a date by an If-Modified-Since request header in later requests.
LocationThis header should be included with all responses that have a status code in the 300s. This notifies the browser of the document address. The browser automatically reconnects to this location and retrieves the new document.
RefreshThis header specifies how soon the browser should ask for an updated page. You can specify time in number of seconds after which a page would be refreshed.
Retry-AfterThis header can be used in conjunction with a 503 (Service Unavailable) response to tell the client how soon it can repeat its request.
Set-CookieThis header specifies a cookie associated with the page.

The HttpServletResponse Object:

The response object is an instance of a javax.servlet.http.HttpServletResponse object. Just as the server creates the request object, it also creates an object to represent the response to the client.
The response object also defines the interfaces that deal with creating new HTTP headers. Through this object the JSP programmer can add new cookies or date stamps, HTTP status codes etc.
There are following methods which can be used to set HTTP response header in your servlet program. These method are available with HttpServletResponse object which represents server response.
S.N.Method & Description
1String encodeRedirectURL(String url)
Encodes the specified URL for use in the sendRedirect method or, if encoding is not needed, returns the URL unchanged.
2String encodeURL(String url)
Encodes the specified URL by including the session ID in it, or, if encoding is not needed, returns the URL unchanged.
3boolean containsHeader(String name)
Returns a boolean indicating whether the named response header has already been set.
4boolean isCommitted()
Returns a boolean indicating if the response has been committed.
5void addCookie(Cookie cookie)
Adds the specified cookie to the response.
6void addDateHeader(String name, long date)
Adds a response header with the given name and date-value.
7void addHeader(String name, String value)
Adds a response header with the given name and value.
8void addIntHeader(String name, int value)
Adds a response header with the given name and integer value.
9void flushBuffer()
Forces any content in the buffer to be written to the client.
10void reset()
Clears any data that exists in the buffer as well as the status code and headers.
11void resetBuffer()
Clears the content of the underlying buffer in the response without clearing headers or status code.
12void sendError(int sc)
Sends an error response to the client using the specified status code and clearing the buffer.
13void sendError(int sc, String msg)
Sends an error response to the client using the specified status.
14void sendRedirect(String location)
Sends a temporary redirect response to the client using the specified redirect location URL.
15void setBufferSize(int size)
Sets the preferred buffer size for the body of the response.
16void setCharacterEncoding(String charset)
Sets the character encoding (MIME charset) of the response being sent to the client, for example, to UTF-8.
17void setContentLength(int len)
Sets the length of the content body in the response In HTTP servlets, this method sets the HTTP Content-Length header.
18void setContentType(String type)
Sets the content type of the response being sent to the client, if the response has not been committed yet.
19void setDateHeader(String name, long date)
Sets a response header with the given name and date-value.
20void setHeader(String name, String value)
Sets a response header with the given name and value.
21void setIntHeader(String name, int value)
Sets a response header with the given name and integer value.
22void setLocale(Locale loc)
Sets the locale of the response, if the response has not been committed yet.
23void setStatus(int sc)
Sets the status code for this response.

HTTP Header Response Example:

Following example would use setIntHeader() method to set Refresh header to simulate a digital clock:
<%@ page import="java.io.*,java.util.*" %>
<html>
<head>
<title>Auto Refresh Header Example</title>
</head>
<body>
<center>
<h2>Auto Refresh Header Example</h2>
<%
   // Set refresh, autoload time as 5 seconds
   response.setIntHeader("Refresh", 5);
   // Get current time
   Calendar calendar = new GregorianCalendar();
   String am_pm;
   int hour = calendar.get(Calendar.HOUR);
   int minute = calendar.get(Calendar.MINUTE);
   int second = calendar.get(Calendar.SECOND);
   if(calendar.get(Calendar.AM_PM) == 0)
      am_pm = "AM";
   else
      am_pm = "PM";
   String CT = hour+":"+ minute +":"+ second +" "+ am_pm;
   out.println("Current Time is: " + CT + "\n");
%>
</center>
</body>
</html>
Now put the above code in main.jsp and try to access it. This would display current system time after every 5 seconds as follows. Just run the JSP and wait to see the result:



Auto Refresh Header Example

Current Time is: 9:44:50 PM
To become more comfortable with other methods you can try few more above listed methods in the same fashion.
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