Java Servlets Interview Questions

11) What are all the different kinds of servers? (Such as Web Servers, Application Servers, etc)
 

The servers involved in handling and processing a user's request break down into a few basic types, each of which may have one or more tasks it solves. This flexibility gives developers a great deal of power over how applications will be created and deployed, but also leads to confusion over what server is able to, or should, perform a specific task.

Starting at the basic level, a user is typically submitting a request to a system through a web browser. (We are conveniently ignoring all other types of clients (RMI, CORBA, COM/DCOM, Custom, etc..) for the time being for purposes of clarity.) The web request must be received by a Web Server (otherwise known as an HTTP Server) of some sort. This web server must handle standard HTTP requests and responses, typically returning HTML to the calling user. Code that executes within the server environment may be CGI driven, Servlets, ASP, or some other server-side programming language, but the end result is that the web server will pass back HTML to the user.

The web server may need to execute an application in response to the users request. It may be generating a list of news items, or handling a form submission to a guest book. If the server application is written as a Java Servlet, it will need a place to execute, and this place is typically called a Servlet Engine. Depending on the web server, this engine may be internal, external, or a completely different product. This engine is continually running, unlike a traditional CGI environment where a CGI script is started upon each request to the server. This persistance gives a servlet connection and thread pooling, as well as an easy way to maintain state between each HTTP request. JSP pages are usually tied in with the servlet engine, and would execute within the same space/application as the servlets.

There are many products that handle the web serving and the servlet engine in different manners. Netscape/iPlanet Enterprise Server builds the servlet engine directly into the web server and runs within the same process space. Apache requires that a servlet engine run in an external process, and will communicate to the engine via TCP/IP sockets. Other servers, such as MS IIS don't officially support servlets, and require add-on products to add that capability.

When you move on to Enterprise JavaBeans (and other J2EE components like JMS and CORBA) you move into the application server space. An Application Server is any server that supplies additional functionality related to enterprise computing -- for instance, load balancing, database access classes, transaction processing, messaging, and so on.

EJB Application Servers provide an EJB container, which is the environment that beans will execute in, and this container will manage transactions, thread pools, and other issues as necessary. These application servers are usually stand-alone products, and developers would tie their servlets/JSP pages to the EJB components via remote object access APIs. Depending on the application server, programmers may use CORBA or RMI to talk to their beans, but the baseline standard is to use JNDI to locate and create EJB references as necessary.

Now, one thing that confuses the issue is that many application server providers include some or all of these components in their product. If you look at WebLogic (http://www.beasys.com/) you will find that WebLogic contains a web server, servlet engine, JSP processor, JMS facility, as well as an EJB container. Theoretically a product like this could be used to handle all aspects of site development. In practice, you would most likely use this type of product to manage/serve EJB instances, while dedicated web servers handle the specific HTTP requests.

12) Should I override the service() method?
 

No. It provides a fair bit of housekeeping that you'd just have to do yourself. If you need to do something regardless of whether the request is e.g., a POST or a GET, create a helper method and call that at the beginning of e.g., doPost() and doGet().

13) How can my application get to know when a HttpSession is removed (when it time-outs)?
 

Define a class, say SessionTimeoutNotifier, that implements javax.servlet.http.HttpSessionBindingListener. Create a SessionTimeoutNotifier object and add it to the user session. When the session is removed, SessionTimeoutNotifier.valueUnbound() will be called by the servlet engine. You can implement valueUnbound() to do whatever you want.

14) Why use JSP when we can do the same thing with servlets?
 

[Original question: Why should I use JSP when there is already servlet technology available for serving dynamic content?]

While JSP may be great for serving up dynamic Web content and separating content from presentation, some may still wonder why servlets should be cast aside for JSP. The utility of servlets is not in question. They are excellent for server-side processing, and, with their significant installed base, are here to stay. In fact, architecturally speaking, you can view JSP as a high-level abstraction of servlets that is implemented as an extension of the Servlet 2.1 API. Still, you shouldn't use servlets indiscriminately; they may not be appropriate for everyone. For instance, while page designers can easily write a JSP page using conventional HTML or XML tools, servlets are more suited for back-end developers because they are often written using an IDE -- a process that generally requires a higher level of programming expertise.

When deploying servlets, even developers have to be careful and ensure that there is no tight coupling between presentation and content. You can usually do this by adding a third-party HTML wrapper package like htmlKona to the mix. But even this approach, though providing some flexibility with simple screen changes, still does not shield you from a change in the presentation format itself. For example, if your presentation changed from HTML to DHTML, you would still need to ensure that wrapper packages were compliant with the new format. In a worst-case scenario, if a wrapper package is not available, you may end up hardcoding the presentation within the dynamic content. So, what is the solution? One approach would be to use both JSP and servlet technologies for building application systems.

15) How do I send information and data back and forth between applet and servlet using the HTTP protocol?
 

Use the standard java.net.URL class, or "roll your own" using java.net.Socket. See the HTTP spec at W3C for more detail.

Note: The servlet cannot initiate this connection! If the servlet needs to asynchronously send a message to the applet, then you must open up a persistent socket using java.net.Socket (on the applet side), and java.net.ServerSocket and Threads (on the server side).

16) Can I get the path of the current servlet where it lives on the file system (not its URL)?
 

Try using:

request.getRealPath(request.getServletPath())

An example may be:

 out.println(request.getRealPath(request.getServletPath()));

17) How can I daisy chain servlets together such that the output of one servlet serves as the input to the next?
 

There are two common methods for chaining the output of one servlet to another servlet :

ï‚·  the first method is the aliasing which describes a series of servlets to be executed

ï‚·  the second one is to define a new MIME-Type and associate a servlet as handlers As I don't really use the second one, I'll focus on the aliasing.

To chain servlets together, you have to specify a sequential list of servlets and associate it to an alias. When a request is made to this alias, the first servlet in the list is invoked, processed its task and sent the ouptut to the next servlet in the list as the request object. The output can be sent again to another servlets.

To accomplish this method, you need to configure your servlet engine (JRun, JavaWeb server, JServ ...).
For example to configure JRun for servlet chaining, you select the JSE service (JRun servlet engine) to access to the JSE Service Config panel. You have just to define a new mapping rule where you define your chaining servlet.
Let say /servlets/chainServlet for the virtual path and a comma separated list of servlets as srvA,srvB.
So when you invoke a request like http://localhost/servlets/chainServlet, internally the servlet srvA will be invoked first and its results will be piped into the servlet srvB.

The srvA servlet code should look like :

public class srvA extends HttpServlet {

   ...

   public void doGet (...) {

      PrintWriter out =res.getWriter();

      rest.setContentType("text/html");

      ...

      out.println("Hello Chaining servlet");

      }

}

All the servlet srvB has to do is to open an input stream to the request object and read the data into a BufferedReader object as for example :

BufferedReader b = new BufferedReader( new InputStreamReader(req.getInputStream() ) );

String data = b.readLine();

b.close();

After that you can format your output with the data.

It should work straigthforward with Java Web Server or Jserv too. Just look at in their documentation to define an alias name. Hope that it'll help.

18) Why there are no constructors in servlets?
 

A servlet is just like an applet in the respect that it has an init() method that acts as a constrcutor. Since the servlet environment takes care of instantiating the servlet, an explicit constructor is not needed. Any initialization code you need to run should be placed in the init() method since it gets called when the servlet is first loaded by the servlet container.

19) How to handle multiple concurrent database requests/updates when using JDBC with servlets?
 

All the dbms provide the facility of locks whenever the data is being modified. There can be two scenarios:

1. Multiple database updates on different rows, if you are using servlets the servlets will open multiple connections for different users. In this case there is no need to do additional programming.
2. If database updates are on the same row then the rows are locked automatically by the dbms, hence we have to send requests to the dbms repeatatively until the lock is released by dbms.

This issue is dealt with in the JDBC documentation; look up "Transactions" and "auto-commit". It can get pretty confusing.

20) What is the difference between GenericServlet and HttpServlet?
 

GenericServlet is for servlets that might not use HTTP, like for instance FTP servlets. Of course, it turns out that there's no such thing as FTP servlets, but they were trying to plan for future growth when they designed the spec. Maybe some day there will be another subclass, but for now, always use HttpServlet.