Interview Questions and Answers

Java Server Pages (JSP) is a platform independent presentation layer technology that comes with SUN s J2EE platform. JSPs are normal HTML pages with Java code pieces embedded in them. JSP pages are saved to *.jsp files. A JSP compiler is used in the background to generate a Servlet from the JSP page.

<%-- JSP Comment --%>
<!-- HTML Comment -->

he JavaServer Pages technology enables you to generate dynamic web content, such as HTML, DHTML, XHTML, and XML files, to include in a Web application. JSP files are one way to implement server-side dynamic page content. JSP files allow a Web server, such as Apache Tomcat, to add content dynamically to your HTML pages before they are sent to a requesting browser.
When you deploy a JSP file to a Web server that provides a servlet engine, it is preprocessed into a servlet that runs on the Web server. This is in contrast with client-side JavaScript� (within SCRIPT tags), which is run in a browser. A JSP page is ideal for tasks that are better suited to execution on the server, such as accessing databases or calling Enterprise Java� beans. You can create and edit a JSP file in the HTML editor by adding your own text and images using HTML, JSP tagging, or JavaScript, including Java source code inside of scriptlet tags. Typically, JSP files have the file extension .jsp. Additionally, the JSP specification suggests that JSP fragment files should have file extension .jspf. If this convention is not followed, the JSP validator will treat JSP fragments as regular standalone JSP files, and compilation errors might be reported.

A JSP page is a text-based document that contains two types of text: static template data, which can be expressed in any text-based format such as HTML, SVG, WML, and XML, and JSP elements, which construct dynamic content.

Implicit objects are objects that are created by the web container and contain information related to a particular request, page, or application. They are:
--request
--response
--pageContext
--session
--application
--out
--config
--page
--exception

There are three scripting language elements:
--declarations
--scriptlets
--expressions

A Java package is a naming context for classes and interfaces. A package is used to create a separate namespace for groups of classes and interfaces. Packages are also used to organize related classes and interfaces into a single API unit and to control accessibility to these classes and interfaces.

The Object class is the highest-level class in the Java class hierarchy. The Class class is used to represent the classes and interfaces that are loaded by a Java program.

There are three scripting language elements: declarations, scriptlets, expressions.

JSP is used mainly for presentation only. A JSP can only be HttpServlet that means the only supported protocol in JSP is HTTP. But a servlet can support any protocol like HTTP, FTP, SMTP etc.

In GET your entire form submission can be encapsulated in one URL, like a hyperlink. query length is limited to 255 characters, not secure, faster, quick and easy. The data is submitted as part of URL.

In POST data is submitted inside body of the HTTP request. The data is not visible on the URL and it is more secure.

A comment that is sent to the client in the viewable page source.The JSP engine handles an output comment as uninterpreted HTML text, returning the comment in the HTML output sent to the client. You can see the comment by viewing the page source from your Web browser.

JSP Syntax:

<!� comment [ <%= expression %> ] �>

Example 1

<!� This is a commnet sent to client on

<%= (new java.util.Date()).toLocaleString() %>

�>

A comments that documents the JSP page but is not sent to the client. The JSP engine ignores a hidden comment, and does not process any code within hidden comment tags. A hidden comment is not sent to the client, either in the displayed JSP page or the HTML page source. The hidden comment is useful when you want to hide or "comment out" part of your JSP page.

You can use any characters in the body of the comment except the closing �%> combination. If you need to use �%> in your comment, you can escape it by typing �%>.

JSP Syntax

<%� comment �%>

Examples

<%@ page %>

<html>

<head><title>A Hidden Comment </title></head>

<body>

<%� This comment will not be visible to the colent in the page source �%>

</body>

</html>

Static resources should always be included using the JSP include directive. This way, the inclusion is performed just once during the translation phase. Do note that you should always supply a relative URL for the file attribute. Although you can also include static resources using the action, this is not advisable as the inclusion is then performed for each and every request.

<%� JSP Comment �%>
<!� HTML Comment �>

An expression tag contains a scripting language expression that is evaluated, converted to a String, and inserted where the expression appears in the JSP file. Because the value of an expression is converted to a String, you can use an expression within text in a JSP file. Like

<%= someexpression %>

<%= (new java.util.Date()).toLocaleString() %>

You cannot use a semicolon to end an expression

A declaration declares one or more variables or methods for use later in the JSP source file.

A declaration must contain at least one complete declarative statement. You can declare any number of variables or methods within one declaration tag, as long as they are separated by semicolons. The declaration must be valid in the scripting language used in the JSP file.

<%! somedeclarations %>

<%! int i = 0; %>

<%! int a, b, c; %>

A scriptlet can contain any number of language statements, variable or method declarations, or expressions that are valid in the page scripting language.Within scriptlet tags, you can

1.Declare variables or methods to use later in the file (see also Declaration).

2.Write expressions valid in the page scripting language (see also Expression).

3.Use any of the JSP implicit objects or any object declared with a <jsp:useBean> tag.

You must write plain text, HTML-encoded text, or other JSP tags outside the scriptlet.

Scriptlets are executed at request time, when the JSP engine processes the client request. If the scriptlet produces output, the output is stored in the out object, from which you can display it.

Because no plug-ins or security policy files are needed on the client systems(applet does). Also, JSP pages enable cleaner and more module application design because they provide a way to separate applications programming from web page design. This means personnel involved in web page design do not need to understand Java programming language syntax to do their jobs.

When you invoke a forward request, the request is sent to another resource on the server, without the client being informed that a different resource is going to process the request. This process occurs completly with in the web container. When a sendRedirtect method is invoked, it causes the web container to return to the browser indicating that a new URL should be requested. Because the browser issues a completly new request any object that are stored as request attributes before the redirect occurs will be lost. This extra round trip a redirect is slower than forward.

The different scope values for <jsp:useBean> are

1. page
2. request
3.session
4.application

THe generated servlet class for a JSP page implements the HttpJspPage interface of the javax.servlet.jsp package. Hte HttpJspPage interface extends the JspPage interface which inturn extends the Servlet interface of the javax.servlet package. the generated servlet class thus implements all the methods of the these three interfaces. The JspPage interface declares only two mehtods � jspInit() and jspDestroy() that must be implemented by all JSP pages regardless of the client-server protocol. However the JSP specification has provided the HttpJspPage interfaec specifically for the JSp pages serving HTTP requests. This interface declares one method _jspService().

The jspInit()- The container calls the jspInit() to initialize te servlet instance.It is called before any other method, and is called only once for a servlet instance.

The _jspservice()- The container calls the _jspservice() for each request, passing it the request and the response objects.

The jspDestroy()- The container calls this when it decides take the instance out of service. It is the last method called n the servlet instance.

You will need to set the appropriate HTTP header attributes to prevent the dynamic content output by the JSP page from being cached by the browser. Just execute the following scriptlet at the beginning of your JSP pages to prevent them from being cached at the browser. You need both the statements to take care of some of the older browser versions.

<%

response.setHeader("Cache-Control","no-store"); //HTTP 1.1

response.setHeader("Pragma","no-cache"); //HTTP 1.0

response.setDateHeader ("Expires", 0); //prevents caching at the proxy server

%>

Variable declared inside declaration part is treated as a global variable.that means after convertion jsp file into servlet that variable will be in outside of service method or it will be declared as instance variable.And the scope is available to complete jsp and to complete in the converted servlet class.where as if u declare a variable inside a scriplet that variable will be declared inside a service method and the scope is with in the service method.

There is a little tool called JSPExecutor that allows you to do just that.

You can use the errorPage attribute of the page directive to have uncaught run-time exceptions automatically forwarded to an error processing page. For example:

<%@ page errorPage="error.jsp" %> redirects the browser to the JSP page error.jsp if an uncaught exception is encountered during request processing. Within error.jsp, if you indicate that it is an error-processing page, via the directive: <%@ page isErrorPage="true" %> Throwable object describing the exception may be accessed within the error page via the exception implicit object. Note: You must always use a relative URL as the value for the errorPage attribute.

You can make your JSPs thread-safe by having them implement the SingleThreadModel interface. This is done by adding the directive <%@ page isThreadSafe="false" %> within your JSP page. With this, instead of a single instance of the servlet generated for your JSP page loaded in memory, you will have N instances of the servlet loaded and initialized, with the service method of each instance effectively synchronized. You can typically control the number of instances (N) that are instantiated for all servlets implementing SingleThreadModel through the admin screen for your JSP engine. More importantly, avoid using the tag for variables. If you do use this tag, then you should set isThreadSafe to true, as mentioned above. Otherwise, all requests to that page will access those variables, causing a nasty race condition. SingleThreadModel is not recommended for normal use. There are many pitfalls, including the example above of not being able to use <%! %>. You should try really hard to make them thread-safe the old fashioned way: by making them thread-safe .

You could make a simple wrapper function, like

<%!

String blanknull(String s) {

return (s == null) ? �� : s;

}

%>

then use it inside your JSP form, like

<input type=�text� name=�lastName� value=�<%=blanknull(lastName)% >� >

Variable declared inside declaration part is treated as a instance variable and will be placed directly at class level in the generated servlet.
<%! int k = 10; %>
Variable declared in a scriptlet will be placed inside _jspService() method of generated servlet.It acts as local variable.
<%
int k = 10;
%>

ServletContext: Gives the information about the container and it represents an application.

PageContext: Gives the information about the Request and it can provide all other implicit JSP objects.