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how to call a jsp page from servlet though post [duplicate]

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?

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>detail</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
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?

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

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.

Could it be that you are passing the data through get, not post?
<form method="get" ..>
..
</form>

If you do <form action="identification" > 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: <form action="identification" method="post"> 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:
<body>
<form action="StartProcessUrl" method="post">
.....
.....
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");
.........
.........

Index.html is not showing when I open my servlet [duplicate]

How do I generate an HTML response in a Java servlet?

You normally forward the request to a JSP for display. JSP is a view technology which provides a template to write plain vanilla HTML/CSS/JS in and provides ability to interact with backend Java code/variables with help of taglibs and EL. You can control the page flow with taglibs like JSTL. You can set any backend data as an attribute in any of the request, session or application scope and use EL (the ${} things) in JSP to access/display them. You can put JSP files in /WEB-INF folder to prevent users from directly accessing them without invoking the preprocessing servlet.
Kickoff example:
#WebServlet("/hello")
public class HelloWorldServlet extends HttpServlet {
#Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
String message = "Hello World";
request.setAttribute("message", message); // This will be available as ${message}
request.getRequestDispatcher("/WEB-INF/hello.jsp").forward(request, response);
}
}
And /WEB-INF/hello.jsp look like:
<!DOCTYPE html>
<html lang="en">
<head>
<title>SO question 2370960</title>
</head>
<body>
<p>Message: ${message}</p>
</body>
</html>
When opening http://localhost:8080/contextpath/hello this will show
Message: Hello World
in the browser.
This keeps the Java code free from HTML clutter and greatly improves maintainability. To learn and practice more with servlets, continue with below links.
Our Servlets wiki page
How do servlets work? Instantiation, sessions, shared variables and multithreading
doGet and doPost in Servlets
Calling a servlet from JSP file on page load
How to transfer data from JSP to servlet when submitting HTML form
Show JDBC ResultSet in HTML in JSP page using MVC and DAO pattern
How to use Servlets and Ajax?
Servlet returns "HTTP Status 404 The requested resource (/servlet) is not available"
Also browse the "Frequent" tab of all questions tagged [servlets] to find frequently asked questions.

You need to have a doGet method as:
public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws IOException, ServletException
{
response.setContentType("text/html");
PrintWriter out = response.getWriter();
out.println("<html>");
out.println("<head>");
out.println("<title>Hola</title>");
out.println("</head>");
out.println("<body bgcolor=\"white\">");
out.println("</body>");
out.println("</html>");
}
You can see this link for a simple hello world servlet

Apart of directly writing HTML on the PrintWriter obtained from the response (which is the standard way of outputting HTML from a Servlet), you can also include an HTML fragment contained in an external file by using a RequestDispatcher:
public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws IOException, ServletException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
out.println("HTML from an external file:");
request.getRequestDispatcher("/pathToFile/fragment.html")
.include(request, response);
out.close();
}

Updating JSP after filtering

Please help me to understand what should I do if I want to update my jsp page after using filter.
For example, my jsp page /index.jsp contains <div id="login-id"> that I would like to update.
I have a filter that looks like:
<filter-mapping>
<filter-name>AccessFilter</filter-name>
<url-pattern>/index.jsp</url-pattern>
</filter-mapping>
public class AccessFilter implements Filter {
private LoginChecker loginChecker;
#Override
public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) throws IOException, ServletException {
//some code goes here where I get name and hash from the cookies
loginChecker = new LoginChecker();
if (loginChecker.isLoggedIn(name, hash)) {
// here I want to send something into my div
} else {
}
chain.doFilter(request, response);
}
I guess that I need to do something like this:
HttpServletResponse resp = (HttpServletResponse) response;
resp.getWriter().write("12345");
chain.doFilter(request, resp);
And add to index like:
<script type="text/javascript" src="http://code.jquery.com/jquery-1.4.2.min.js"></script>
<script type="text/javascript">
$.get('index.jsp', function(data) {
console.log("data = " + data);
});
</script>
But I think I am wrong because I get whole page instead of just "12345".
Thank you.

in your case I believe what you are trying to achieve is actually just retrieving values you generated in your Filter in the JSP documents. If this is correct you can go a route like this.
The class HttpServletRequest features the getter/setter setAttributes and getAttributes which you can use very similarly to what you are used to from POST and GET parameters.
In you filter you are then able to set arbitrary data to the attributes map like this.
request.setAttribute("data", "my data");
If this data is later needed in your JSP you can do something like this:
First make sure that EL Expressions are evaluated in your JSP page by setting the following in the page header:
<%# page isELIgnored="false" %>
Then you can access the request attributes like this:
<p>Hello world this is my JSP and we received ${data} from our filter</p>
To clarify the request attributes. Opposed to session attributes the request attributes only live for the time of a single request. This means an usually empty map is created at the beginning of each HTTP request and then gradually gets filled with additional information during the processing run of the request.

sendRedirect() doens't work

I looked for a solution on this forum, but i didn't find anything that suits my problem.
I have a very simplce code
a jsp page
<html>
<body>
<jsp:include page="/servletName"/>
</body>
</html>
and a servlet
#WebServlet("/servletName")
public class reindirizzaController extends HttpServlet {
private static final long serialVersionUID = 1L;
public reindirizzaController() {
super();
}
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
String redirectURL = "http://www.google.it";
response.sendRedirect(redirectURL);
}
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
}
}
No redirect is done. I'm stuck in the jsp page and i get no error. I also tried to add return; after the response.

Since you are calling the servlet through Include, it does not make you redirect. It simply ignores.
From docs of include()
includes the content of a resource (servlet, JSP page, HTML file) in the response. In essence, this method enables programmatic server-side includes.
The ServletResponse object has its path elements and parameters remain unchanged from the caller's. The included servlet cannot change the response status code or set headers; any attempt to make a change is ignored.

First, it is bad practice to call a servlet from a jsp. As you are using #WebServlet("/servletName"), you can directly call it at http://host/context/servletName
If you really need to call it from a jsp, you must forward to the servlet instead of including it as Suresh Atta explained. So you should use :
<jsp:forward page="/servletName"/>

When you do a JSP include directive, you're essentially plopping the code you're including right on the page. In your example, you would be doing the sendRedirect in your JSP. Even if you got the redirect to work miraculously, you would get an error saying that your response has already been committed. This is because by the time your browser loads the JSP, it is already reading from the server's response. The server cannot send another response while it is already sending a response to your browser.
One way to approach this is instead of doing an include directive, create a form with your servlet path as the action. Something like:
<form name="someForm" action="/servletPath/servletName">
<!-- some stuff here if you want -->
</form>
And then in your body tag, have it submit the form on load:
<body onLoad="document.someForm.submit();">

JSP or Servlet PrintWriter

I recently began implementing java into my website but I've been reading that the method:
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
PrintWriter out = response.getWriter();
out.println("<html>");
//code
out.println("</html>");
out.close();
}
Is outdated and rarely used due to jsp. What are the benefits of doing one versus the other?

The advantage of using JSP over pure servlets is that it is more convenient to write (and to modify) regular HTML than to have plenty of out.println statements that generate the HTML. With JSP, you can mix Java code freely with your HTML code (using tags JSP provides like <%= %>). Your JSP page ultimately compiles to a servlet, the servlet runs, and the response is sent back to the browser.
Pure Servlet:
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
PrintWriter out = response.getWriter();
out.println("<html>");
out.println("<body>")
out.println("<p>The date is: " + (new Java.util.date()).toLocaleString() +"</p>");
out.println("</body>")
out.println("</html>");
out.close();
}
JSP:
<%#page contentType="text/html" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
....
<body>
<p>The date is: <%= (new Java.util.date()).toLocaleString() %></p> //mixing HTML and Java
</body>
</html>

Technically you can write presentation and business logic in both jsp and servlets. It's widely considered a good practice to to implement the MVC pattern in your webapp, so you want to implement the view in the JSP, use the servlets as the controller and EJBs for the model. Generating the html with your servlet break this separation, that's why it's generally to avoid.
I'm not aware of any benefit from generating the html in a servlet.

Servlet intended for Control and Business Logic.
JSP intended for Presentation Logic.