JSP Servlet

Servlet:

Explain the life cycle methods of a Servlet.

                 The javax.servlet.Servlet interface defines the three methods known as life-cycle method.

public void init(ServletConfig config) throws ServletException

public void service( ServletRequest req, ServletResponse res) throws ServletException, IOException

public void destroy()

First the servlet is constructed, then initialized wih the init() method.

Any request from client are handled initially by the service() method before delegating to the doXxx() methods in the case of HttpServlet.

The servlet is removed from service, destroyed with the destroy() methid, then garbaged collected and finalized.

What is the difference between the getRequestDispatcher(String path) method of javax.servlet.ServletRequest interface and javax.servlet.ServletContext interface?

                 The getRequestDispatcher(String path) method of javax.servlet.ServletRequest interface accepts parameter the path to the resource to be included or forwarded to, which can be relative to the request of the calling servlet. If the path begins with a "/" it is interpreted as relative to the current context root.

The getRequestDispatcher(String path) method of javax.servlet.ServletContext interface cannot accepts relative paths. All path must sart with a "/" and are interpreted as relative to curent context root.

Explain the directory structure of a web application.

                 The directory structure of a web application consists of two parts.

A private directory called WEB-INF

A public resource directory which contains public resource folder.

WEB-INF folder consists of

1. web.xml

2. classes directory

3. lib directory

What are the common mechanisms used for session tracking?

                 Cookies

SSL sessions

URL- rewriting

Explain ServletContext.

                 ServletContext interface is a window for a servlet to view it's environment. A servlet can use this interface to get information such as initialization parameters for the web application or servlet container's version. Every web application has one and only one ServletContext and is accessible to all active resource of that application.

What is preinitialization of a servlet?

                 A container doesn't initialize the servlets as soon as it starts up, it initializes a servlet when it receives a request for that servlet first time. This is called lazy loading. The servlet specification defines the element, which can be specified in the deployment descriptor to make the servlet container load and initialize the servlet as soon as it starts up. The process of loading a servlet before any request comes in is called preloading or preinitializing a servlet.

What is the difference between Difference between doGet() and doPost()?

                 A doGet() method is limited with 2k of data to be sent, and doPost() method doesn't have this limitation. A request string for doGet() looks like the following:

doPost() method call doesn't need a long text tail after a servlet name in a request. All parameters are stored in a request itself, not in a request string, and it's impossible to guess the data transmitted to a servlet only looking at a request string.

What is the difference between HttpServlet and GenericServlet?

                 A GenericServlet has a service() method aimed to handle requests. HttpServlet extends GenericServlet and adds support for doGet(), doPost(), doHead() methods (HTTP 1.0) plus doPut(), doOptions(), doDelete(), doTrace() methods (HTTP 1.1).

Both these classes are abstract.

What is the difference between ServletContext and ServletConfig?

 ServletContext: Defines a set of methods that a servlet uses to communicate with its servlet container, for example, to get the MIME type of a file, dispatch requests, or write to a log file.The ServletContext object is contained within the ServletConfig object, which the Web server provides the servlet when the servlet is initialized

ServletConfig: The object created after a servlet is instantiated and its default constructor is read. It is created to pass initialization information to the servlet.

What is a output comment?

                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

Example 1

Displays in the page source:

What is a Hidden Comment?

                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 language="java" %>

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

What is a Expression?

                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

What is a Declaration?

                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; %>

What is a Scriptlet?

                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 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.

What are implicit objects? List them?

                Certain objects that are available for the use in JSP documents without being declared first. These objects are parsed by the JSP engine and inserted into the generated servlet. The implicit objects re listed below

request

response

pageContext

session

application

out

config

page

exception

Difference between forward and sendRedirect?

                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.

What are the different scope valiues for the ?

The different scope values for are

1. page

2. request

3.session

4.application

Explain the life-cycle mehtods in JSP?

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.

How do I prevent the output of my JSP or Servlet pages from being cached by the browser?

                 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

%>

How does JSP handle run-time exceptions?

                 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.

How can I implement a thread-safe JSP page? What are the advantages and Disadvantages of using it?

                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 .

How do I use a scriptlet to initialize a newly instantiated bean?

A jsp:useBean action may optionally have a body. If the body is specified, its contents will be automatically invoked when the specified bean is instantiated. Typically, the body will contain scriptlets or jsp:setProperty tags to initialize the newly instantiated bean, although you are not restricted to using those alone.

The following example shows the “today” property of the Foo bean initialized to the current date when it is instantiated. Note that here, we make use of a JSP expression within the jsp:setProperty action.

value="<%=java.text.DateFormat.getDateInstance().format(new java.util.Date()) %>" / >

<%-- scriptlets calling bean setter methods go here --%>

How can I prevent the word "null" from appearing in my HTML input text fields when I populate them with a resultset that has null values?

You could make a simple wrapper function, like

<%!

String blanknull(String s) {

return (s == null) ? \"\" : s;

}

%>

then use it inside your JSP form, like

What's a better approach for enabling thread-safe servlets and JSPs? SingleThreadModel Interface or Synchronization?

                Although the SingleThreadModel technique is easy to use, and works well for low volume sites, it does not scale well. If you anticipate your users to increase in the future, you may be better off implementing explicit synchronization for your shared data. The key however, is to effectively minimize the amount of code that is synchronzied so that you take maximum advantage of multithreading.

Also, note that SingleThreadModel is pretty resource intensive from the server\'s perspective. The most serious issue however is when the number of concurrent requests exhaust the servlet instance pool. In that case, all the unserviced requests are queued until something becomes free - which results in poor performance. Since the usage is non-deterministic, it may not help much even if you did add more memory and increased the size of the instance pool.

How can I enable session tracking for JSP pages if the browser has disabled cookies?

                We know that session tracking uses cookies by default to associate a session identifier with a unique user. If the browser does not support cookies, or if cookies are disabled, you can still enable session tracking using URL rewriting. URL rewriting essentially includes the session ID within the link itself as a name/value pair. However, for this to be effective, you need to append the session ID for each and every link that is part of your servlet response. Adding the session ID to a link is greatly simplified by means of of a couple of methods: response.encodeURL() associates a session ID with a given URL, and if you are using redirection, response.encodeRedirectURL() can be used by giving the redirected URL as input. Both encodeURL() and encodeRedirectedURL() first determine whether cookies are supported by the browser; if so, the input URL is returned unchanged since the session ID will be persisted as a cookie.

Consider the following example, in which two JSP files, say hello1.jsp and hello2.jsp, interact with each other. Basically, we create a new session within hello1.jsp and place an object within this session. The user can then traverse to hello2.jsp by clicking on the link present within the page. Within hello2.jsp, we simply extract the object that was earlier placed in the session and display its contents. Notice that we invoke the encodeURL() within hello1.jsp on the link used to invoke hello2.jsp; if cookies are disabled, the session ID is automatically appended to the URL, allowing hello2.jsp to still retrieve the session object. Try this example first with cookies enabled. Then disable cookie support, restart the brower, and try again. Each time you should see the maintenance of the session across pages. Do note that to get this example to work with cookies disabled at the browser, your JSP engine has to support URL rewriting.

hello1.jsp

<%@ page session=\"true\" %>

<%

Integer num = new Integer(100);

session.putValue("num",num);

String url =response.encodeURL("hello2.jsp");

%>

\'>hello2.jsp

hello2.jsp

<%@ page session="true" %>

<%

Integer i= (Integer )session.getValue("num");

out.println("Num value in session is " + i.intValue());

%>

What is the difference b/w variable declared inside a declaration part and variable declared in scriplet part?

                 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.

Is there a way to execute a JSP from the comandline or from my own application?

There is a little tool called JSPExecutor that allows you to do just that. The developers (Hendrik Schreiber & Peter Rossbach ) aim was not to write a full blown servlet engine, but to provide means to use JSP for generating source code or reports. Therefore most HTTP-specific features (headers, sessions, etc) are not implemented, i.e. no reponseline or header is generated. Nevertheless you can use it to precompile JSP for your website.

How you will display validation fail errors on jsp page?

                Following tag displays all the errors:

How you will enable front-end validation based on the xml in validation.xml?

                The tag to allow front-end validation based on the xml in validation.xml. For example the code: generates the client side java script for the form \"logonForm\" as defined in the validation.xml file. The when added in the jsp file generates the client site validation script.

How to get data from the velocity page in a action class?

                We can get the values in the action classes by using data.getParameter(\"variable name defined

200-299

Success – The action was successfully received, understood, and accepted.

300-399

Redirection – Further action must be taken in order to complete the request.

400-499

Client Error – The request contains bad syntax or cannot be fulfilled.

500-599

Server Error – The server failed to fulfill an apparently valid request.

200

OK— The request has succeeded.

302

Moved Temporarily — The request resides temporarily under a different URI. If the new URI is a location, the location header field in

the response will give the new URL. This is typically used when the client is being redirected.

400

Bad Request— The server couldn’t understand the request due to malformed syntax.

401

Unauthorized— The request requires authentication and/or authorization.

403

Forbidden— The server understood the request, but for some reason is refusing to fulfill it. The server may or may not reveal why it has refused the request.

404

Not Found— The server has not found anything matching the request  URI.

500

Internal Server Error— The server encountered an unexpected condition  which prevented it from fulfilling the request.

Why mvc:

Layer separation, scalability, security, reusability

MVC1 vs MVC2

MVC1:JSP interacts model directly without controller

MVC2:JSP requests pass to controller and contoller interacts model

Connection pooling

It's a technique to allow multiple clinets to make use of a cached set of shared and reusable connection objects providing access to a database

RiskControlAnalysis(RCA)

Baseline

|

Production

|

Testing

|

if Faullt  then goes to production

|

Analyse the problem with senior

|

Action Team meeting

|

Find solution and prevents in future

Defect Prevention

Load Testing using jmeter

Session

URLRewriting, Cookies,

Session obj creation:

HttpSession obj=request.getSession();

Why session----associate with objects in whole appln.transer data from 1 page to other page or servlet

Web server vs app server

Web server:

Handles http req and resp with html pages

App server:

Connection pooling, multithreading, transaction mgmt

Version Control

Cvs, svn, git

Mercurial,  IBM Clearcase, IBM Clearquest.

Load on startup

May have negative - if negative servlet container load at any time where it needs

if positive from 0 to 128 then it loads from lower order to higher integers

ServletFilters

Servlet Filters are the latest components that are added in Servlet 2.3 specifications. These filters are used basically for intercepting and modifying requests and response from server.  Consider a scenario where you want to check session from the every users request and if it is valid then only you want to let the user access the page. You can acheive this by checking sessions on all the servlet pages (or JSP pages) which users queries or you can do this by using Filter.

package net.viralpatel.servlet.filters;

import java.io.IOException;

import java.util.Date;

import javax.servlet.Filter;

import javax.servlet.FilterChain;

import javax.servlet.FilterConfig;

import javax.servlet.ServletException;

import javax.servlet.ServletRequest;

import javax.servlet.ServletResponse;

import javax.servlet.http.HttpServletRequest;

public class LogFilter implements Filter {

    public void doFilter(ServletRequest req, ServletResponse res,

            FilterChain chain) throws IOException, ServletException {

        HttpServletRequest request = (HttpServletRequest) req;

        //Get the IP address of client machine.

        String ipAddress = request.getRemoteAddr();

        //Log the IP address and current timestamp.

        System.out.println("IP "+ipAddress + ", Time "

                           + new Date().toString());

        chain.doFilter(req, res);

    }

    public void init(FilterConfig config) throws ServletException {

        //Get init parameter

        String testParam = config.getInitParameter("test-param");

        //Print the init parameter

        System.out.println("Test Param: " + testParam);

    }

    public void destroy() {

        //add code to release any resource

    }

}

In this filter example, we have implemented an interface javax.servlet.Filter and override its methods init, doFilter and destroy.

The init() method is used to initialize any code that is used by Filter. Also note that, init() method will get an object of FilterConfig which contains different Filter level information as well as init parameters which is passed from Web.xml (Deployment descriptor).

The doFilter() method will do the actual logging of information. You can modify this method and add your code which can modify request/session/response, add any attribute in request etc.

The destroy() method is called by the container when it wants to garbage collect the filter. This is usually done when filter is not used for long time and server wants to allocate memory for other applications.

Step 4: Create Servlet Filter Mapping in Web.xml

Open web.xml file from WEB-INF directory of your Project and add following entry for filter tag.

    LogFilter

   

        net.viralpatel.servlet.filters.LogFilter

   

   

        test-param

        This parameter is for testing.

   

    LogFilter

    /*

In this entry, we have added LogFilter class in Web xml and mapped it with URL /*. Hence any request from client will generated a call to this filter. Also we have passed a parameter test-param. This is just to show how to pass and retrieve a parameter in servlet filter.