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    lt form action  identification  method  post  enctype  multipart form-data  gt      User Name   lt input type  text  name  realname  gt      Password   lt input type  password  name  mypassword  gt       lt input type  submit  value  Identification  gt   lt  form gt     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

User · Answer

Could it be that you are  passing the data through get  not post    lt form method  get     gt      lt  form gt

User · Answer

If you do  lt form action  identification   gt  for your html form  data will be passed using  Get  by default and hence you can catch this using doGet function in your java servlet code  This way data will be passed under the HTML header and hence will be visible in the URL when submitted  On the other hand if you want to pass data in HTML body  then USE Post   lt form action  identification  method  post  gt  and catch this data in doPost function  This was  data will be passed under the html body and not the html header  and you will not see the data in the URL after submitting the form   Examples from my html    lt body gt     lt form action  StartProcessUrl  method  post  gt                Examples from my java servlet code   protected void doPost HttpServletRequest request  HttpServletResponse response  throws ServletException  IOException              TODO Auto-generated method stub         PrintWriter out   response getWriter             String surname   request getParameter  txtSurname             String firstname   request getParameter  txtForename             String rqNo   request getParameter  txtRQ6             String nhsNo   request getParameter  txtNHSNo              String attachment1   request getParameter  base64textarea1             String attachment2   request getParameter  base64textarea2

User · Answer

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  quot  products quot   public class ProductsServlet extends HttpServlet         EJB     private ProductService productService        Override     protected void doGet HttpServletRequest request  HttpServletResponse response  throws ServletException  IOException           List lt Product gt  products   productService list            request setAttribute  quot products quot   products      Will be available as   products  in JSP         request getRequestDispatcher  quot  WEB-INF products jsp quot   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    lt table gt       lt c forEach items  quot   products  quot  var  quot product quot  gt           lt tr gt               lt td gt   product name  lt  td gt               lt td gt  lt a href  quot product id   product id  quot  gt detail lt  a gt  lt  td gt           lt  tr gt       lt  c forEach gt   lt  table gt   Also view edit detail links as shown in last column above are usually idempotent   WebServlet  quot  product quot   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  quot id quot             request setAttribute  quot product quot   product      Will be available as   product  in JSP         request getRequestDispatcher  quot  WEB-INF product jsp quot   forward request  response              lt dl gt       lt dt gt ID lt  dt gt       lt dd gt   product id  lt  dd gt       lt dt gt Name lt  dt gt       lt dd gt   product name  lt  dd gt       lt dt gt Description lt  dt gt       lt dd gt   product description  lt  dd gt       lt dt gt Price lt  dt gt       lt dd gt   product price  lt  dd gt       lt dt gt Image lt  dt gt       lt dd gt  lt img src  quot productImage id   product id  quot    gt  lt  dd gt   lt  dl gt   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   lt form action  quot login quot  method  quot post quot  gt       lt input type  quot text quot  name  quot username quot  gt       lt input type  quot password quot  name  quot password quot  gt       lt input type  quot submit quot  value  quot login quot  gt       lt span class  quot error quot  gt   error  lt  span gt   lt  form gt      which can be used in combination with this piece of Servlet   WebServlet  quot  login quot   public class LoginServlet extends HttpServlet         EJB     private UserService userService        Override     protected void doPost HttpServletRequest request  HttpServletResponse response  throws ServletException  IOException           String username   request getParameter  quot username quot            String password   request getParameter  quot password quot            User user   userService find username  password            if  user    null                request getSession   setAttribute  quot user quot   user               response sendRedirect  quot home quot                      else               request setAttribute  quot error quot    quot Unknown user  please try again quot                request getRequestDispatcher  quot  login jsp quot   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   quot logged in quot   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  quot hide quot  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  quot  WEB-INF login jsp quot   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  quot Trails Covering the Basics quot   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  quot HTTP Status 404 The requested resource   servlet  is not available quot  Show JDBC ResultSet in HTML in JSP page using MVC and DAO pattern   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 amp name2 value2 amp name3 value3   You only need multipart form-data whenever you have a  lt input type  quot file quot  gt  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

User · Answer

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.

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.)

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

User · Answer

The servlet container s implementation of HttpServlet service   method will automatically forward to doGet   or doPost   as necessary  so you shouldn t need to override the service method

[java] doGet and doPost in Servlets

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