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Java Servlets

1 Java Servlets
2 Servlet Overview
3 Servlet Life Cycle
4 Java Servlet Example
5 HttpServlet
6 HttpRequest
7 HttpResponse
8 HttpSession
9 RequestDispatcher
10 ServletContext
11 web.xml Servlet Configuration
12 Cookies and Servlets
13 Servlet Filters
14 GZip Servlet Filter
15 Servlet Concurrency




HttpRequest


The HttpServlet class request processing methods take two parameters.

  1. javax.servlet.http.HttpRequest
  2. javax.servlet.http.HttpResponse

For instance, here is the signature of the HttpServlet.doGet() method:

protected void doGet(
    HttpServletRequest request,
    HttpServletResponse response)
      throws ServletException, IOException {

}

In this text I will look at the HttpRequest object.

The purpose of the HttpRequest object is to represent the HTTP request a browser sends to your web application. Thus, anything the browser may send, is accessible via the HttpRequest.

The HttpRequest object has a lot of methods, so I will just cover the most commonly used here. The rest you can read about in the JavaDoc, if you are interested.

Parameters

The request parameters are parameters that are sent from the browser along with the request. Request parameters are typically sent as part of the URL (in the "query string"), or as part of the body of an HTTP request. For instance:

http://jenkov.com/somePage.html?param1=hello¶m2=world

Notice the "query string" part of the URL: ?param1=hello¶m2=world This part contains two parameters with parameter values:

param1=hello
param2=world

You can access these parameters from the HttpRequest object like this:

protected void doGet(
    HttpServletRequest request,
    HttpServletResponse response)
      throws ServletException, IOException {

    String param1 = request.getParameter("param1");
        String param2 = request.getParameter("param2");

}

You would use the same code, if the request parameters were sent in the body part of the HTTP request. If no parameter exists with the given name, null is returned.

In general, if the browser sends an HTTP GET request, the parameters are included in the query string in the URL. If the browser sends an HTTP POST request, the parameters are included in the body part of the HTTP request.

Headers

The request headers are name, value pairs sent by the browser along with the HTTP request. The request headers contain information about e.g. what browser software is being used, what file types the browser is capable of receiving etc. In short, at lot of meta data around the HTTP request.

You can access the request headers from the HttpRequest object like this:

String contentLength = request.getHeader("Content-Length");    

This example reads the Content-Length header sent by the browser.

The Content-Length header contains the number of bytes sent in the HTTP request body, in case the browser sends an HTTP POST request. If the browser sends an HTTP GET request, the Content-Length header is not used, and the above code will return null.

In general, If no header exists with the name passed to getHeader(), null is returned.

InputStream

If the browser sends an HTTP POST request, request parameters and other potential data is sent to the server in the HTTP request body. It doesn't have to be request parameters that is sent in the HTTP request body. It could be pretty much any data, like a file or a SOAP request (web service request).

To give you access to the request body of an HTTP POST request, you can obtain an InputStream pointing to the HTTP request body. Here is how it is done:

InputStream requestBodyInput = request.getInputStream();    

NOTE: You will have to call this method before calling any getParameter() method, because calling the getParameter() method on an HTTP POST request will cause the servlet engine to parse the HTTP request body for parameters. Once parsed, you cannot access the body as a raw stream of bytes anymore.

What you do with the data read from the InputStream is up to you. The servlet engine does not help you parse or interprete that data. You just get it raw.

Session

It is possible to obtain the session object from the HttpRequest object too.

The session object can hold information about a given user, between requests. So, if you set an object into the session object during one request, it will be available for you to read during any subsequent requests within the same session time scope.

Here is how you access the session object from the HttpRequest object:

HttpSession session = request.getSession();

I will not get into more detail about the session object here. It is covered in more detail in its own text.

ServletContext

You can access the ServletContext object from the HttpRequest object too. The ServletContext contains meta information about the web application. For instance, you can access context parameters set in the web.xml file, you can forward the request to other servlets, and you can store application wide parameters in the ServletContext too.

Here is how you access the ServletContext object from the HttpRequest object:

ServletContext context = request.getSession().getServletContext();            

As you can see, you have to first get the session object, to get access to the ServletContext object.

I will not get into more detail about the ServletContext object here. It will be covered in more detail in its own text.



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