doGet and doPost in Servlets

Question

I've developed an HTML page that sends information to a Servlet. In the Servlet, I am using the methods doGet() and doPost():

public void doGet(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException  {       String id = req.getParameter("realname");      String password = req.getParameter("mypassword"); }  public void doPost(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException {      String id = req.getParameter("realname");     String password = req.getParameter("mypassword"); } 

In the html page code that calls the Servlet is:

<form action="identification" method="post" enctype="multipart/form-data">     User Name: <input type="text" name="realname">     Password: <input type="password" name="mypassword">     <input type="submit" value="Identification"> </form>  

When I use method = "get" in the Servlet, I get the value of id and password, however when using method = "post", id and password are set to null. Why don't I get the values in this case?

Another thing I'd like to know is how to use the data generated or validated by the Servlet. For example, if the Servlet shown above authenticates the user, I'd like to print the user id in my HTML page. I should be able to send the string 'id' as a response and use this info in my HTML page. Is it possible?

Answer 1

Introduction

You should use doGet() when you want to intercept on HTTP GET requests. You should use doPost() when you want to intercept on HTTP POST requests. That's all. Do not port the one to the other or vice versa (such as in Netbeans' unfortunate auto-generated processRequest() method). This makes no utter sense.

GET

Usually, HTTP GET requests are idempotent. I.e. you get exactly the same result everytime you execute the request (leaving authorization/authentication and the time-sensitive nature of the page —search results, last news, etc— outside consideration). We can talk about a bookmarkable request. Clicking a link, clicking a bookmark, entering raw URL in browser address bar, etcetera will all fire a HTTP GET request. If a Servlet is listening on the URL in question, then its doGet() method will be called. It's usually used to preprocess a request. I.e. doing some business stuff before presenting the HTML output from a JSP, such as gathering data for display in a table.

@WebServlet("/products") public class ProductsServlet extends HttpServlet {      @EJB     private ProductService productService;      @Override     protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {         List<Product> products = productService.list();         request.setAttribute("products", products); // Will be available as ${products} in JSP         request.getRequestDispatcher("/WEB-INF/products.jsp").forward(request, response);     }  } 

Note that the JSP file is explicitly placed in /WEB-INF folder in order to prevent endusers being able to access it directly without invoking the preprocessing servlet (and thus end up getting confused by seeing an empty table).

<table>     <c:forEach items="${products}" var="product">         <tr>             <td>${product.name}</td>             <td><a href="product?id=${product.id}">detail</a></td>         </tr>     </c:forEach> </table> 

Also view/edit detail links as shown in last column above are usually idempotent.

@WebServlet("/product") public class ProductServlet extends HttpServlet {      @EJB     private ProductService productService;      @Override     protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {         Product product = productService.find(request.getParameter("id"));         request.setAttribute("product", product); // Will be available as ${product} in JSP         request.getRequestDispatcher("/WEB-INF/product.jsp").forward(request, response);     }  } 

<dl>     <dt>ID</dt>     <dd>${product.id}</dd>     <dt>Name</dt>     <dd>${product.name}</dd>     <dt>Description</dt>     <dd>${product.description}</dd>     <dt>Price</dt>     <dd>${product.price}</dd>     <dt>Image</dt>     <dd><img src="productImage?id=${product.id}" /></dd> </dl> 

POST

HTTP POST requests are not idempotent. If the enduser has submitted a POST form on an URL beforehand, which hasn't performed a redirect, then the URL is not necessarily bookmarkable. The submitted form data is not reflected in the URL. Copypasting the URL into a new browser window/tab may not necessarily yield exactly the same result as after the form submit. Such an URL is then not bookmarkable. If a Servlet is listening on the URL in question, then its doPost() will be called. It's usually used to postprocess a request. I.e. gathering data from a submitted HTML form and doing some business stuff with it (conversion, validation, saving in DB, etcetera). Finally usually the result is presented as HTML from the forwarded JSP page.

<form action="login" method="post">     <input type="text" name="username">     <input type="password" name="password">     <input type="submit" value="login">     <span class="error">${error}</span> </form> 

...which can be used in combination with this piece of Servlet:

@WebServlet("/login") public class LoginServlet extends HttpServlet {      @EJB     private UserService userService;      @Override     protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {         String username = request.getParameter("username");         String password = request.getParameter("password");         User user = userService.find(username, password);          if (user != null) {             request.getSession().setAttribute("user", user);             response.sendRedirect("home");         }         else {             request.setAttribute("error", "Unknown user, please try again");             request.getRequestDispatcher("/login.jsp").forward(request, response);         }     }  } 

You see, if the User is found in DB (i.e. username and password are valid), then the User will be put in session scope (i.e. "logged in") and the servlet will redirect to some main page (this example goes to http://example.com/contextname/home), else it will set an error message and forward the request back to the same JSP page so that the message get displayed by ${error}.

You can if necessary also "hide" the login.jsp in /WEB-INF/login.jsp so that the users can only access it by the servlet. This keeps the URL clean http://example.com/contextname/login. All you need to do is to add a doGet() to the servlet like this:

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {     request.getRequestDispatcher("/WEB-INF/login.jsp").forward(request, response); } 

(and update the same line in doPost() accordingly)

That said, I am not sure if it is just playing around and shooting in the dark, but the code which you posted doesn't look good (such as using compareTo() instead of equals() and digging in the parameternames instead of just using getParameter() and the id and password seems to be declared as servlet instance variables — which is NOT threadsafe). So I would strongly recommend to learn a bit more about basic Java SE API using the Oracle tutorials (check the chapter "Trails Covering the Basics") and how to use JSP/Servlets the right way using those tutorials.

See also:

• Our servlets wiki page
• Java EE web development, where do I start and what skills do I need?
• Servlet returns "HTTP Status 404 The requested resource (/servlet) is not available"
• Show JDBC ResultSet in HTML in JSP page using MVC and DAO pattern

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Update: as per the update of your question (which is pretty major, you should not remove parts of your original question, this would make the answers worthless .. rather add the information in a new block) , it turns out that you're unnecessarily setting form's encoding type to multipart/form-data. This will send the request parameters in a different composition than the (default) application/x-www-form-urlencoded which sends the request parameters as a query string (e.g. name1=value1&name2=value2&name3=value3). You only need multipart/form-data whenever you have a <input type="file"> element in the form to upload files which may be non-character data (binary data). This is not the case in your case, so just remove it and it will work as expected. If you ever need to upload files, then you'll have to set the encoding type so and parse the request body yourself. Usually you use the Apache Commons FileUpload there for, but if you're already on fresh new Servlet 3.0 API, then you can just use builtin facilities starting with HttpServletRequest#getPart(). See also this answer for a concrete example: How to upload files to server using JSP/Servlet?

Answer 2

Both GET and POST are used by the browser to request a single resource from the server. Each resource requires a separate GET or POST request.

1. The GET method is most commonly (and is the default method) used by browsers to retrieve information from servers. When using the GET method the 3rd section of the request packet, which is the request body, remains empty.

The GET method is used in one of two ways: When no method is specified, that is when you or the browser is requesting a simple resource such as an HTML page, an image, etc. When a form is submitted, and you choose method=GET on the HTML tag. If the GET method is used with an HTML form, then the data collected through the form is sent to the server by appending a "?" to the end of the URL, and then adding all name=value pairs (name of the html form field and value entered in that field) separated by an "&" Example: GET /sultans/shop//form1.jsp?name=Sam%20Sultan&iceCream=vanilla HTTP/1.0 optional headeroptional header<< empty line >>>

The name=value form data will be stored in an environment variable called QUERY_STRING. This variable will be sent to a processing program (such as JSP, Java servlet, PHP etc.)

2. The POST method is used when you create an HTML form, and request method=POST as part of the tag. The POST method allows the client to send form data to the server in the request body section of the request (as discussed earlier). The data is encoded and is formatted similar to the GET method, except that the data is sent to the program through the standard input.

Example: POST /sultans/shop//form1.jsp HTTP/1.0 optional headeroptional header<< empty line >>> name=Sam%20Sultan&iceCream=vanilla

When using the post method, the QUERY_STRING environment variable will be empty. Advantages/Disadvantages of GET vs. POST

Advantages of the GET method: Slightly faster Parameters can be entered via a form or by appending them after the URL Page can be bookmarked with its parameters

Disadvantages of the GET method: Can only send 4K worth of data. (You should not use it when using a textarea field) Parameters are visible at the end of the URL

Advantages of the POST method: Parameters are not visible at the end of the URL. (Use for sensitive data) Can send more that 4K worth of data to server

Disadvantages of the POST method: Can cannot be bookmarked with its data