Reading HTTP request headers from a servlet

When a browser requests a web page, it generally sends some additional headers which specify things such as:

• any cookies set for that page;
• which browser or "agent" is making request, along with information about the operating system, installed browser plugins etc;
• the referrer (generally the link that the user clicked on that provoked the request);
• what types of content the browser can deal with.

It's beyond the scope of this tutorial to give all the details of every possibly HTTP request header. And it's actually quite rare to need to read request headers. If necessary, it is recommended that you look at the HTTP protocol specification if you need more details. However, we'll look at a couple of useful cases here by way of example.

Reading a request header

Usually, we'll know the name of the request header that we want to read. In this case, it is a simple matter of calling getHeader() on the HttpServletRequest object that was passed to our doGet()/doPost() method. For example, here is how to read the user-agent header, which essentially tells us which browser (IE, Mozilla, Safari etc) and operating system is making the request:

public final void doGet(HttpServletRequest req, HttpServletResponse res)
    throws ServletException, IOException {
  String agent = req.getHeader("user-agent");
  if (agent != null && agent.indexOf("MSIE") > -1) {
    // Internet Explorer mode
  } else {
    // Non-Internet Explorer mode
  }
}