Servlets - Server HTTP Response

As discussed in the previous chapter, when a Web server responds to an HTTP request, 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 −
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 or no-cache etc. Public means document is cacheable, Private means document is for a single user and can only be stored in private (non-shared) caches and nocache 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 keepalive 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This header specifies the time at which the content should be considered out-of-date and thus no longer be cached.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.
There are following methods which can be used to set HTTP response header in your servlet program. These methods are available with HttpServletResponse object.
String encodeRedirectURL(String url)
Encodes the specified URL for use in the sendRedirect method or, if encoding is not needed, returns the URL unchanged.
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.
boolean containsHeader(String name)
Returns a Boolean indicating whether the named response header has already been set.
boolean isCommitted() 
Returns a Boolean indicating if the response has been committed.
void addCookie(Cookie cookie)
Adds the specified cookie to the response.
void addDateHeader(String name, long date)
Adds a response header with the given name and date-value.
void addHeader(String name, String value)
Adds a response header with the given name and value.
void addIntHeader(String name, int value)
Adds a response header with the given name and integer value.
void flushBuffer() 
Forces any content in the buffer to be written to the client.
void reset()
Clears any data that exists in the buffer as well as the status code and headers.
void resetBuffer()
Clears the content of the underlying buffer in the response without clearing headers or status code.
void sendError(int sc)
Sends an error response to the client using the specified status code and clearing the buffer.
void sendError(int sc, String msg)
Sends an error response to the client using the specified status.
void sendRedirect(String location)
Sends a temporary redirect response to the client using the specified redirect location URL.
void setBufferSize(int size) 
Sets the preferred buffer size for the body of the response.
void setCharacterEncoding(String charset)
Sets the character encoding (MIME charset) of the response being sent to the client, for example, to UTF-8.
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.
void setContentType(String type) 
Sets the content type of the response being sent to the client, if the response has not been committed yet.
void setDateHeader(String name, long date) 
Sets a response header with the given name and date-value. 
void setHeader(String name, String value) 
Sets a response header with the given name and value.
void setIntHeader(String name, int value) 
Sets a response header with the given name and integer value
void setLocale(Locale loc)
Sets the locale of the response, if the response has not been committed yet. 
void setStatus(int sc) 
Sets the status code for this response
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 −
Auto Refresh Header Setting
Current Time is: 9:44:50 PM

Current Time is: 9:44:50 PM

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