servlets interview questions and answers pdf ~ Oracle SOA BPEL ESB OSB AIA @ Admin

Servlets Faqs-->3

26) What is the difference between the doGet and doPost methods?

doGet is called in response to an HTTP GET request. This happens when users click on a link, or enter a URL into the browser's address bar. It also happens with some HTML FORMs (those with METHOD="GET" specified in the FORM tag).

doPost is called in response to an HTTP POST request. This happens with some HTML FORMs (those with METHOD="POST" specified in the FORM tag).

Both methods are called by the default (superclass) implementation of service in the HttpServlet base class. You should override one or both to perform your servlet's actions. You probably shouldn't override service().

27) What is the difference between encodeRedirectUrl and encodeURL?

encodeURL and encodeRedirectURL are methods of the HttpResponse object. Both rewrite a raw URL to include session data if necessary. (If cookies are on, both are no-ops.)

encodeURL is for normal links inside your HTML pages.

encodeRedirectURL is for a link you're passing to response.sendRedirect(). It has slightly different syntax requirements too gory to get into here.

28) Can I use System.exit() in servlets?

Gack! No no no no no...

At best, you'll get a security exception. At worst, you'll make the servlet engine, or maybe the entire web server, quit. You don't really want to do that, huh? :-)

29) I am opening a single JDBC connection in my init() method. Do I need to synchronize on the Connection or the Statement object?

You shouldn't have to. If your JDBC driver supports multiple connections, then the various createStatement methods will give you a thread-safe, reentrant, independent Statement that should work OK, even if other requests/threads are also accessing other Statements on the same Connection.

Of course, crossing your fingers never hurts... Many early JDBC drivers were not re-entrant. The modern versions of JDBC drivers should work OK, but there are never any guarantees.

Using connection pooling will avoid the whole issue, plus will lead to improved performance. See this FAQ for more information.

30) How can I determine the name and version number of the servlet or JSP engine that I am using?

From within a servlet, you can invoke the ServletContext.getServerInfo() method as follows:

String thisServer= getServletConfig().getServletContext().getServerInfo();

If you are using JSP, you can use this expression:

<%= application.getServerInfo() %>

31) How can I get the absolute URL of a servlet/JSP page at runtime?

You can get all the necessary information to determine the URL from the request object. To reconstruct the absolute URL from the scheme, server name, port, URI and query string you can use the URL class from java.net. The following code fragment will determine your page's absolute URL:

String file = request.getRequestURI();

if (request.getQueryString() != null) {

file += '?' + request.getQueryString();

}

URL reconstructedURL = new URL(request.getScheme(),

request.getServerName(),

request.getServerPort(),

file);

out.println(URL.toString());

32) Why do GenericServlet and HttpServlet implement the Serializable interface?

GenericServlet and HttpServlet implement the Serializable interface so that servlet engines can "hybernate" the servlet state when the servlet is not in use and reinstance it when needed or to duplicate servlet instances for better load balancing. I don't know if or how current servlet engines do this, and it could have serious implications, like breaking references to objects gotten in the init() method without the programmer knowing it. Programmers should be aware of this pitfall and implement servlets which are stateless as possible, delegating data store to Session objects or to the ServletContext. In general stateless servlets are better because they scale much better and are cleaner code.

33) How does one choose between overriding the doGet(), doPost(), and

service() methods?

The differences between the doGet() and doPost() methods are that they are called in the HttpServlet that your servlet extends by its service() method when it recieves a GET or a POST request from a HTTP protocol request.

A GET request is a request to get a resource from the server. This is the case of a browser requesting a web page. It is also possible to specify parameters in the request, but the length of the parameters on the whole is limited. This is the case of a form in a web page declared this way in html: <form method="GET"> or <form>.

A POST request is a request to post (to send) form data to a resource on the server. This is the case of of a form in a web page declared this way in html: <form method="POST">. In this case the size of the parameters can be much greater.

The GenericServlet has a service() method that gets called when a client request is made. This means that it gets called by both incoming requests and the HTTP requests are given to the servlet as they are (you must do the parsing yourself).

The HttpServlet instead has doGet() and doPost() methods that get called when a client request is GET or POST. This means that the parsing of the request is done by the servlet: you have the appropriate method called and have convenience methods to read the request parameters.

NOTE: the doGet() and doPost() methods (as well as other HttpServlet methods) are called by the service() method.

Concluding, if you must respond to GET or POST requests made by a HTTP protocol client (usually a browser) don't hesitate to extend HttpServlet and use its convenience methods.

If you must respond to requests made by a client that is not using the HTTP protocol, you must use service().

34) How do servlets differ from RMI? What are the advantages and disadvantages of each technology?

Servlets extend the server-side functionality of a website. Servlets communicate with other application(s) on that server (or any other server) and perform tasks above and beyond the "normal" static HTML document. A servlet can receive a request to get some information through EJB from one or more databases, then convert this data into a static HTML/WML page for the client to see, for example. Even if the servlet talks to many other applications all over the world to get this information, it still looks like it happened at that website.

RMI (Remote Method Invocation) is just that - a way to invoke methods on remote machines. It is way for an application to talk to another remote machine and execute different methods, all the while appearing as if the action was being performed on the local machine.

Servlets (or JSP) are mainly used for any web-related activity such as online banking, online grocery stores, stock trading, etc. With servlets, you need only to know the web address and the pages displayed to you take care of calling the different servlets (or actions within a servlet) for you. Using RMI, you must bind the RMI server to an IP and port, and the client who wishes to talk to the remote server must know this IP and port, unless of course you used some kind of in-between lookup utility, which you could do with (of all things) servlets.

35) How can we use a servlet as a proxy for communications between two applets?

One way to accomplish this is to have the applets communicate via TCP/IP sockets to the servlet. The servlet would then use a custom protocol to receive and push information between applets. However, this solution does have firewall problems if the system is to be used over and Internet verses an Intranet.

36) How can I design my servlet/JSP so that query results get displayed on several pages, like the results of a search engine? Each page should display, say, 10 records each and when the next link is clicked, I should see the next/previous 10 records and so on.

Use a Java Bean to store the entire result of the search that you have found. The servlet will then set a pointer to the first line to be displayed in the page and the number of lines to display, and force a display of the page. The Action in the form would point back to the servlet in the JSP page which would determine whether a next or previous button has been pressed and reset the pointer to previous pointer + number of lines and redisplay the page. The JSP page would have a scriplet to display data from the Java Bean from the start pointer set to the maximum number of lines with buttons to allow previous or next pages to be selected. These buttons would be displayed based on the page number (i.e. if first then don't display previous button).

37) How do I deal with multi-valued parameters in a servlet?

Instead of using getParameter() with the ServletRequest, as you would with single-valued parameters, use the getParameterValues() method. This returns a String array (or null) containing all the values of the parameter requested.

38) How can I pass data retrieved from a database by a servlet to a JSP page?

One of the better approaches for passing data retrieved from a servlet to a JSP is to use the Model 2 architecture as shown below:

Basically, you need to first design a bean which can act as a wrapper for storing the resultset returned by the database query within the servlet. Once the bean has been instantiated and initialized by invoking its setter methods by the servlet, it can be placed within the request object and forwarded to a display JSP page as follows:

com.foo.dbBean bean = new com.foo.dbBean();

//call setters to initialize bean

req.setAttribute("dbBean", bean);

url="..."; //relative url for display jsp page

ServletContext sc = getServletContext();

RequestDispatcher rd = sc.getRequestDispatcher(url);

rd.forward(req, res);

The bean can then be accessed within the JSP page via the useBean tag as:

<jsp:useBean id="dbBean" class="com.foo.dbBean" scope="request"/>

...

//iterate through the rows within dbBean and

//access the values using a scriptlet

Also, it is best to design your application such that you avoid placing beans into the session unless absolutely necessary. Placing large objects within the session imposes a heavy burden on the performance of the servlet engine. Of course, there may be additional design considerations to take care of - especially if your servlets are running under a clustered or fault-tolerant architecture.

39) How can I use a servlet to generate a site using frames?

In general, look at each frame as a unique document capable of sending its own requests and receiving its own responses. You can create a top servlet (say, FrameServlet) that upon invocation creates the frame layout you desire and sets the SRC parameters for the frame tags to be another servlet, a static page or any other legal value for SRC.

---------------------- SAMPLE ----------------------

public void doGet(HttpServletRequest request,

HttpServletResponse response) throws ServletException, IOException {

response.setContentType("text/html");

PrintWriter out = new PrintWriter (response.getWriter());

out.println("");

out.println("Your Title");

// definingthe three rows of Frames for the main page

// top : frm_1

// middle : frm_2

// bottom : frm_3

out.println("");

out.close();

-------------------------- END ------------------------------------------

Where MenuServlet and DummyServlet provide content and behavior for the frames generated by FrameServlet.

40) What is HTTP tunneling, in the general sense?

HTTP tunneling is a general technique whereby arbitrary data may be sent via an HTTP connection to and from CGI scripts or Java Servlets on a Web server. This is done by serializing the data to be transmitted into a stream of bytes, and sending an HTTP message with content type "application/octet-stream".

HTTP tunneling is also referred to as Firewall tunneling.

41) How do I handle FORMs with multiple form elements (e.g. radio buttons) using the same name?

For radio buttons, the HTML spec assumes that a given group of buttons will have the same NAME and different VALUEs; the browser makes sure that only one button per group name will be selected (at most). So you can just call request.getParameter("groupname").

<input type="radio" name="topping" value="cheese" checked>Cheese

<input type="radio" name="topping" value="pepperoni">Pepperoni

<input type="radio" name="topping" value="anchovies">Anchovies

If the user selects "Pepperoni" then request.getParameter("topping") will return the string "pepperoni".

For lists using the <select multiple> FORM tag, multiple values can be returned for the same parameter name. When that can happen, use request.getParameterValues("param") which returns a String[] you can iterate through.

It's bad form (so to speak), but you can also duplicate other element types, like

Name 1: <input type="text" name="name" value="Dick">

Name 2: <input type="text" name="name" value="Jane">

These also get returned in an array by request.getParameterValues().

42) How do I separate presentation (HTML) from business logic (Java) when using servlets?

Almost anybody who has ever written a servlet can identify with this one. We all know it's bad for to embed HTML code in our java source; it's lame to have to recompile and re-deploy every time you want an HTML element to look a bit different. But what are our choices here? There are two basic options;

1. Use JSP: Java Server Pages allows you to embed Java code or the results of a servlet into your HTML. You could, for instance, define a servlet that gives a stock quote, then use the tag in a JSP page to embed the output. But then, this brings up the same problem; without discipline, your content/presentation and program logic are again meshed. I think the ideal here is to completely separate the two.

2. Use a templating/parsing system: Hmm...I know you're about to rant about re-inventing the wheel, but it's not that bad (see below). Plus, it really does pay to take this approach; you can have a group of programmers working on the Java code, and a group of HTML producers maintaining the interface. So now you probably want to know how to do it...so read on.

Use SSI!

Remember SSI? It hasn't gotten much attention in recent years because of embeddable scripting languages like ASP and JSP, but it still remains a viable option. To leverage it in the servlet world, I believe the best way is to use an API called SSI for Java from Areane. This API will let you emulate SSI commands from a templating system, and much more. It will let you execute any command on any system, including executing java classes! It also comes with several utility classes for creating stateful HTML form elements, tables for use with iteration, and much more. It's also open source, so it's free and you can tweak it to your heart's content! You can read the SSI for Java documentation for detailed info, but the following is an example of its use.

Here's the servlet:

import javax.servlet.*;

import javax.servlet.http.*;

import java.io.*;

import java.util.*;

import com.areane.www.ssi.*;

public class SSITemplatingServlet extends HttpServlet {

private String templateFilesDirectory = "d:\\projects\\idemo\\templates\\"; //Holds path to template files

/**Handles GET requests; defers every request to the POST processor*/

public void doGet(HttpServletRequest req, HttpServletResponse res)

throws ServletException, IOException, FileNotFoundException {doPost(req, res);}

/**Handles all requests. Processes the request,

*saves the values, parses the file, then feeds the file to the out stream*/

public void doPost(HttpServletRequest req, HttpServletResponse res)

throws ServletException, IOException, FileNotFoundException {

HttpSession ses = req.getSession(true);

Properties context = null;

if((context = (Properties)ses.getValue("user.context")) == null) { //if properties doesn't already exist, create it.

context = new Properties();

}

//Write parameters to Properties object

Enumeration paramNames = req.getParameterNames();

String curName, curVal;

while(paramNames.hasMoreElements()) {

curName = (String)paramNames.nextElement();

curVal = req.getParameter(curName);

context.setProperty(curName, curVal);

}

//Save the values to the session

ses.putValue("user.context", context);

//Parse the page and stream to the client

String templateName = req.getParameter("template"); // Get the file name of the template to use

res.setContentType("text/html");

SsiPage page = SsiParser.parse(this.templateFilesDirectory + templateName); //Parsing occurs here

page.write(res.getWriter(), context); //Stream to the client

page = null; //clean up

}

Now, just create a template file, pass the servlet the template file name, and have at it!

43) For an HTML FORM with multiple SUBMIT buttons, how can a servlet ond differently for each button?

The servlet will respond differently for each button based on the html that you have placed in the HTML page. Let's explain.

For a submit button the HTML looks like <input type=submit name="Left" value="left">. A servlet could extract the value of this submit by using the getParameter("Left") from the HttpRequest object. It follows then that if you have HTML within a FORM that appears as:

Then the getParameter("Direction") from the HttpRequest would extract the value pressed by the user, either "left", "right", "up" or "down". A simple comparision in the servlet with the these values could occur and processing based on the submit button would be performed.

Similiarly,for submit buttons with different names on a page, each of these values could be extracted using the getParameter() call and acted on. However, in a situation where there are multiple buttons, common practice would be to use one name and multiple values to identify the button pressed.

44) What is meant by the term "business logic"?

"Business logic" is just a fancy way of saying "code." :-)

More precisely, in a three-tier architecture, business logic is any code that is not specifically related to storing and retrieving data (that's "data storage code"), or to formatting data for display to the user (that's "presentation logic"). It makes sense, for many reasons, to store this business logic in separate objects; the middle tier comprises these objects. However, the divisions between the three layers are often blurry, and business logic is more of an ideal than a reality in most programs. The main point of the term is, you want somewhere to store the logic and "business rules" (another buzzword) of your application, while keeping the division between tiers clear and clean.

45) How can I explicitly unload a servlet or call the destroy method?

In general, you can't. The Servlet API does not specify when a servlet is unloaded or how the destroy method is called. Your servlet engine (ie the implementation of the interfaces in the JSDK) might provide a way to do this, probably through its administration interface/tool (like Webshpere or JWS). Most servlet engines will also destroy and reload your servlet if they see that the class file(s) have been modified.

46) What is a servlet bean?

A servlet bean is a serializable servlet that follows the JavaBeans component architecture, basically offering getter/setter methods.

As long as you subclass GenericServlet/HttpServlet, you are automatically Serializable.

If your web server supports them, when you install the servlet in the web server, you can configure it through a property sheet-like interface.

47) Why do we need to call super.init(config) in the init method of a servlet?

Just do as you're told and you won't get hurt! :-)

Because if you don't, then the config object will get lost. Just extend HttpServlet, use init() (no parameters) and it'll all work ok.

From the Javadoc: init() - A convenience method which can be overridden so that there's no need to call super.init(config).

48) What is a servlet engine?

A "servlet engine" is a program that plugs in to a web server and runs servlets. The term is obsolete; the preferred term now is "servlet container" since that applies both to plug-in engines and to stand-alone web servers that support the Servlet API.

49) Which is the most efficient (i.e. processing speed) way to create a server application that accesses a database: A Servlet using JDBC; a JSP page using a JavaBean to carry out the db access; or JSP combined with a Servlet? Are these my only choices?

Your question really should be broken in two.

1-What is the most efficient way of serving pages from a Java object?. There you have a clear winner in the Servlet. Althought if you are going to change the static content of the page often is going to be a pain because you'll have to change Java code. The second place in speed is for JSP pages. But, depending on your application, the difference in speed between JSP pages and raw servlets can be so small that is not worth the extra work of servlet programming.

2-What is the most efficient way of accessing a database from Java?. If JDBC is the way you want to go the I'd suggest to pick as many drivers as you can (II,III,IV or wathever) and benchmark them. Type I uses a JDBC/ODBC bridge and usually has lousy performance. Again, go for the simplest (usually type IV driver) solution if that meets you performance needs.

For database applications, the performance bottleneck is usually the database, not the web server/engine. In this case, the use of a package that access JDBC with connection pooling at the application level used from JSP pages (with or withouth beans as middleware) is the usual choice. Of course, your applications requirements may vary.

50) How can I change the port of my Java Web Server from 8080 to something else?

It is very simple. JAVA WEB SERVER comes with remote Web administration tool. You can access this with a web browser.

Administration tool is located on port 9090 on your web server. To change port address for web server:

Access tool (http://hostname:9090)
Enter User Id/Password (by default it is admin/admin)
Select service (Web service)
Click on "manage" button. You will get a popup screen with all settings.
Click on network tree node, On right hand side you will get text box for entering port no.
Change port number with desire one.
click on restart button.

Oracle SOA BPEL ESB OSB AIA @ Admin

Tuesday, 26 June 2012