SERVLETS SERVER HTTP RESPONSE
As discussed in previous chapter, 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:
| Header | Description |
|---|---|
| Allow | This header specifies the request methods (GET, POST, etc.) that the server supports. |
| Cache-Control | This header specifies the circumstances in which the response document can safely be cached. It can have values public, private orno-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. |
| Connection | This 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-Disposition | This header lets you request that the browser ask the user to save the response to disk in a file of the given name. |
| Content-Encoding | This header specifies the way in which the page was encoded during transmission. |
| Content-Language | This header signifies the language in which the document is written. For example en, en-us, ru, etc. |
| Content-Length | This 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-Type | This header gives the MIME (Multipurpose Internet Mail Extension) type of the response document. |
| Expires | This header specifies the time at which the content should be considered out-of-date and thus no longer be cached. |
| Last-Modified | This 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. |
| Location | This 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. |
| Refresh | This 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-After | This header can be used in conjunction with a 503 (Service Unavailable) response to tell the client how soon it can repeat its request. |
| Set-Cookie | This header specifies a cookie associated with the page. |
Methods to Set HTTP Response Header:
There are following methods which can be used to set HTTP response header in your servlet program. These methods are available with HttpServletResponse object.
| S.N. | Method & Description |
|---|---|
| 1 | String encodeRedirectURL(String url) Encodes the specified URL for use in the sendRedirect method or, if encoding is not needed, returns the URL unchanged. |
| 2 | String encodeURL(String url) Encodes the specified URL by including the session ID in it, or, if encoding is not needed, returns the URL unchanged. |
| 3 | boolean containsHeader(String name) Returns a boolean indicating whether the named response header has already been set. |
| 4 | boolean isCommitted() Returns a boolean indicating if the response has been committed. |
| 5 | void addCookie(Cookie cookie) Adds the specified cookie to the response. |
| 6 | void addDateHeader(String name, long date) Adds a response header with the given name and date-value. |
| 7 | void addHeader(String name, String value) Adds a response header with the given name and value. |
| 8 | void addIntHeader(String name, int value) Adds a response header with the given name and integer value. |
| 9 | void flushBuffer() Forces any content in the buffer to be written to the client. |
| 10 | void reset() Clears any data that exists in the buffer as well as the status code and headers. |
| 11 | void resetBuffer() Clears the content of the underlying buffer in the response without clearing headers or status code. |
| 12 | void sendError(int sc) Sends an error response to the client using the specified status code and clearing the buffer. |
| 13 | void sendError(int sc, String msg) Sends an error response to the client using the specified status. |
| 14 | void sendRedirect(String location) Sends a temporary redirect response to the client using the specified redirect location URL. |
| 15 | void setBufferSize(int size) Sets the preferred buffer size for the body of the response. |
| 16 | void setCharacterEncoding(String charset) Sets the character encoding (MIME charset) of the response being sent to the client, for example, to UTF-8. |
| 17 | void setContentLength(int len) Sets the length of the content body in the response In HTTP servlets, this method sets the HTTP Content-Length header. |
| 18 | void setContentType(String type) Sets the content type of the response being sent to the client, if the response has not been committed yet. |
| 19 | void setDateHeader(String name, long date) Sets a response header with the given name and date-value. |
| 20 | void setHeader(String name, String value) Sets a response header with the given name and value. |
| 21 | void setIntHeader(String name, int value) Sets a response header with the given name and integer value. |
| 22 | void setLocale(Locale loc) Sets the locale of the response, if the response has not been committed yet. |
| 23 | void setStatus(int sc) Sets the status code for this response. |
HTTP Header Response Example:
You already have seen setContentType() method working in previous examples and following example would also use same method, additionally we would use setIntHeader() method to set Refresh header.
// Import required java libraries
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.util.*;
// Extend HttpServlet class
public class Refresh extends HttpServlet {
// Method to handle GET method request.
public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
{
// Set refresh, autoload time as 5 seconds
response.setIntHeader("Refresh", 5);
// Set response content type
response.setContentType("text/html");
// 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;
PrintWriter out = response.getWriter();
String title = "Auto Refresh Header Setting";
String docType =
"<!doctype html public \"-//w3c//dtd html 4.0 " +
"transitional//en\">\n";
out.println(docType +
"<html>\n" +
"<head><title>" + title + "</title></head>\n"+
"<body bgcolor=\"#f0f0f0\">\n" +
"<h1 align=\"center\">" + title + "</h1>\n" +
"<p>Current Time is: " + CT + "</p>\n");
}
// Method to handle POST method request.
public void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
doGet(request, response);
}
}
Now calling the above servlet would display current system time after every 5 seconds as follows. Just run the servlet and wait to see the result:
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