getHeaderNames() getHeader() Example

getHeaderNames() getHeader() Example


Retrieve Header Information with getHeaderNames() getHeader()

When a client clicks a hyper link or a submit button, we know that the data entered by the user in the Form fields is sent to server and is the default behavior of submit button. Ofcourse, right. But along with a lot of extra information goes to server on the name of headers (attached to request object) like what browser client is using, its supported languages etc. Servlet Programmer can know this extra information using getHeaderNames() getHeader() methods of HttpServletRequest interface.

Apart header data, lot of other data of client system also goes like what client system IP address is, what protocol client is using and on what port number client browser is sending request etc.

This tutorial illustrates how to extract Header information from request object. The methods used are getHeaderNames() getHeader() defined in javax.servlet.http.HttpServletRequest interface.

Let us see first what Servlet API says about these methods:

  • Enumeration getHeaderNames(): Returns an enumeration of all the header names this request contains. If the request has no headers, this method returns an empty enumeration.
    Some servlet containers do not allow servlets to access headers using this method, in which case this method returns null.
  • String getHeader(String name): Returns the value of the specified request header as a String. If the request did not include a header of the specified name, this method returns null. If there are multiple headers with the same name, this method returns the first head in the request. The header name is case insensitive. You can use this method with any request header.
Following code gives a very simple program of extracting header information from request object using getHeaderNames() getHeader().

Client HTML file: Headers.html

web.xml entry for Headers.java servlet

Server Servlet File: Headers.java

Enumeration e = req.getHeaderNames();

The getHeaderNames() method of HttpServletRequest returns an object of Enumeration. The Enumeration object e contains all the header names but not their associated values. In the code, value associated with each header is read and printed.

String name = (String) e.nextElement();
String value = req.getHeader(name);

The getHeaderNames() returns the header names in the form of strings. For this reason, the object returned by nextElement() is casted to String. Pass the header name "name" to getHeader() and the method returns the associated value as a string. In the while loop, with each iteration, one header name and its value are printed. Observe the output screen.


getHeaderNames() getHeader()
Client HTML Screen with a Hyperlink


getHeaderNames() getHeader()
Output screen with Headers Name Vs Header Value
>

In the above screenshot, the first column of table gives the header names and the second column gives the values associated with each header.

Following list gives the meaning of each header name.

  1. host: This header gives the host (server) name and port number as written in the URL (in ACTION attribute).
  2. connection: It specifies whether the client’s browser can handle the persistent HTTP connections. "keep-alive" indicates that the browser can handle persistent connections. On a persistent connection, the browser can retrieve multiple files on a single request (or single connection).
  3. accept: This header informs the Server (or Servlet) what MIME types (like text/html, image/gif etc.) the browser supports (so that the Servlet can send response in the supported MIME type only).
  4. user-agent: Returns the name of the browser and its version etc. This information is used by the Servlet to send data specific to that browser. Suppose, the Servlet would like to send <MARQUEE> tag and if client is having Netscape Navigator browser (which does not support MARQUEE tag), it does not send and instead it may send the same information in <BLINK> tag supported by Netscape. Or a Servlet sends a response to the browser that requires an ActiveX control which the browser may not have. Microsoft IE version 4 does not come with ActiveX controls supported by version 5. For this reason, the Servlet requires the user-agent information.
  5. accept-encoding: This header tells the Servlet what type of compression algorithms are supported by the browser like gzip etc. so that Servlet sends an appropriate compressed file to the browser.
  6. accept language: This informs the Server that what type of languages the browser support like English, Spanish or Japanese etc., so that the Servlet may send response in other languages supported by the browser. Java and Servlets support Internationalization.

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