Based on Java Servlet technology, JSP (JavaServer Pages) allows the platform-independent development of dynamic web applications.
JSP (JavaServer Pages)
JSP is a Java view technology running on a server which allows you to write template text in client side languages like HTML, CSS, JavaScript and so on. JSP supports the so-called taglibs which are backed by pieces of Java code with which you can control the page flow and/or output dynamically (programmatically). A well known taglib is JSTL. JSP also supports Expression Language (EL), with syntax like ${} which can be used to access backend data (actually, the attributes which are available in page, request, session and application scopes), mostly in combination with taglibs.
Lifecycle
When a JSP is requested for the first time or when the webapp starts up, the servlet container will compile the JSP file into a class extending HttpServlet and use it during the webapp's lifetime. You can find the generated source code in the server's work directory. In, for example, Tomcat, it's the /work directory. On a JSP request, the servlet container will execute the compiled JSP class and send the generated output (usually just HTML/CSS/JS) through the webserver over the network to the client side, which in turn displays it in the browser.
Hello World
This example uses JSTL and EL to display the user IP and the today's date.
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<%@ taglib uri="http://java.sun.com/jsp/jstl/fmt" prefix="fmt" %>
<jsp:useBean id="date" class="java.util.Date" />
<!DOCTYPE html>
<html lang="en">
<head>
<title>JSP Hello World</title>
</head>
<body>
<h1>Hello</h1>
<p>Welcome, user from <c:out value="${pageContext.request.remoteAddr}" />
<p>It's now <fmt:formatDate value="${date}" pattern="MM/dd/yyyy HH:mm" />
</body>
</html>
Save it as /hello.jsp and open it by http://localhost:8080/contextname/hello.jsp.
If JSTL doesn't work (the JSTL tags are not parsed/executed and still there in generated HTML output when you right-click and View Source in browser), then probably your servlet container doesn't support it out of the box (like Tomcat). You can install it by just dropping jstl-1.2.jar in /WEB-INF/lib. If it still doesn't work, then refer JSTL info page for more detail.
Scriptlets
You can also inline raw Java code in a JSP file using scriptlets (those <% %> things).
<%@ page import="java.text.SimpleDateFormat" %>
<%@ page import="java.util.Date" %>
<!DOCTYPE html>
<html lang="en">
<head>
<title>JSP Hello World</title>
</head>
<body>
<h1>Hello</h1>
<p>Welcome, user from <%= request.getRemoteAddr() %>
<p>It's now <%= new SimpleDateFormat("MM/dd/yyyy HH:mm").format(new Date()) %>
</body>
</html>
Its use is however as per the JSP coding conventions discouraged for other purposes than quick prototyping.
Data pre-loading and form post-processing
To pre-load data for display in a JSP and to post-process a form submit, you'd like to use a Servlet. For more detail, see Servlets tag info page.
JavaScript
It's important to realize that JSP runs in the webserver, producing HTML output, and that JavaScript is part of the HTML output that runs in the browser only. So JSP and JavaScript don't run in sync as you might expect from the coding. To let JavaScript "access" JSP variables, all you need to do is to let JSP/JSTL/EL print it as if it is a Javascript variable. This way any JavaScript function, once executed in the browser, can access it. The below example prints the server side session ID as a JavaScript variable using EL:
<script>var jsessionid = '${pageContext.session.id}';</script>
If you open this page in a browser and do a View Source, then you'll see something like:
<script>var jsessionid = '4C147636FF923CA7EA642F2E10DB95F1';</script>
(note that those single quotes were thus mandatory to represent a JavaScript string value!)
Then, to let JSP "access" JavaScript variables, you need to send the JavaScript variable back to the server using an HTTP request, since that's the only way to send data from the browser to a webserver. You could:
- use the HTML DOM to manipulate a hidden input field and fill it with the data, and if necessary submit the form using
form.submit()so that it's available byrequest.getParameter(). - use
window.locationto do a "redirect" to a new URL with the JavaScript variable as a request parameter. - use
XMLHttpRequestto send an asynchronous (ajax) request with the JS variable as a request parameter. - let JavaScript set it as a cookie so that it's available by
request.getCookies()in subsequent requests.
Facelets
Since Java EE 6, JSP has been succeeded by Facelets as default view technology for the Java EE MVC framework JSF (JavaServer Faces). Since the Java EE 6 tutorial, JSP is not treated in detail any longer. You need to head back to the Java EE 5 tutorial if you want to learn JSP. See also http://stackoverflow.com/questions/4845032/wheres-the-official-jsp-tutorial.
Online resources
- Wikipedia: JavaServer Pages
- Beginning & Intermediate Servlet & JSP Tutorials
- Advanced Servlet and JSP tutorials
- Core Servlets and JavaServer Pages eBook
Frequently Asked Questions
- What is the difference between JSP, Servlet and JSF?
- How to avoid Java code in JSP files?
- Hidden features of JSP/Servlet
- How to introduce Ajax in JSP? - Follow up
- How to upload files to server using JSP/Servlet?
