Java Interview Questions - 84

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().

Java Interview Questions - 83

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.  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.

Java Interview Questions - 82

How do I call one servlet from another servlet?

[ Short answer: there are several ways to do this, including

• use a RequestDispatcher
• use a URLConnection or HTTPClient
• send a redirect
• call getServletContext().getServlet(name) (deprecated, doesn't work in 2.1+)

- ]
It depends on what you mean by "call" and what it is you seek to do and why you seek to do it.
If the end result needed is to invoke the methods then the simplest mechanism would be to treat the servlet like any java object , create an instance and call the mehods.
If the idea is to call the service method from the service method of another servlet, AKA forwarding the request, you could use the RequestDispatcher object.
If, however, you want to gain access to the instance of the servlet that has been loaded into memory by the servlet engine, you have to know the alias of the servlet. (How it is defined depends on the engine.) For example, to invoke a servlet in JSDK a servlet can be named by the property

 myname.code=com.sameer.servlets.MyServlet

The code below shows how this named servlet can be accessed in the service method of another servlet

public void service (HttpServletRequest request, HttpServletResponse response)
  throws ServletException, IOException {
   ...
 MyServlet ms=

(MyServlet) getServletConfig().getServletContext().getServlet("myname");
    ...
}

That said, This whole apporach of accessing servlets in another 

servlets has been deprecated in the 2.1 version of the servlet 

API due to the security issues. The cleaner and better apporach

is to just avoid accessing other servlets directly and use the 

RequestDispatcher instead.

Java Interview Questions - 81

How does the performance of JSP pages compare with that of servlets? How does it compare with Perl scripts?

The performance of JSP pages is very close to that of servlets. However, users may experience a perceptible delay when a JSP page is accessed for the very first time. This is because the JSP page undergoes a "translation phase" wherein it is converted into a servlet by the JSP engine. Once this servlet is dynamically compiled and loaded into memory, it follows the servlet life cycle for request processing. Here, the jspInit() method is automatically invoked by the JSP engine upon loading the servlet, followed by the _jspService() method, which is responsible for request processing and replying to the client. Do note that the lifetime of this servlet is non-deterministic - it may be removed from memory at any time by the JSP engine for resource-related reasons. When this happens, the JSP engine automatically invokes the jspDestroy() method allowing the servlet to free any previously allocated resources.
Subsequent client requests to the JSP page do not result in a repeat of the translation phase as long as the servlet is cached in memory, and are directly handled by the servlet's service() method in a concurrent fashion (i.e. the service() method handles each client request within a seperate thread concurrently.)
There have been some recent studies contrasting the performance of servlets with Perl scripts running in a "real-life" environment. The results are favorable to servlets, especially when they are running in a clustered environment.

Java Interview Questions - 79

Can a servlet maintain a JTA UserTransaction object across multiple servlet invocations?

No. A JTA transaction must start and finish within a single invocation (of the service() method). Note that this question does not address servlets that maintain and manipulate JDBC connections, including a connection's transaction handling.

Java Interview Questions - 78

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.

Java Interview Questions - 77

How does a servlet communicate with a JSP page?

The following code snippet shows how a servlet instantiates a bean and initializes it with FORM data posted by a browser. The bean is then placed into the request, and the call is then forwarded to the JSP page, Bean1.jsp, by means of a request dispatcher for downstream processing.

 public void doPost (HttpServletRequest request,
   HttpServletResponse response) {
 
     try {
      govi.FormBean f = new govi.FormBean();
      String id = request.getParameter("id");
      f.setName(request.getParameter("name"));
      f.setAddr(request.getParameter("addr"));
      f.setAge(request.getParameter("age"));
       //use the id to compute 
       //additional bean properties like info 
  //maybe perform a db query, etc.
       // . . .
       f.setPersonalizationInfo(info);
      request.setAttribute("fBean",f);
      getServletConfig().getServletContext().getRequestDispatcher
        ("/jsp/Bean1.jsp").forward(request, response);
    } catch (Exception ex) {
      . . .
    }
  }

The JSP page Bean1.jsp can then process fBean, after first extracting it from the default request scope via the useBean action.

 <jsp:useBean id="fBean" class="govi.FormBean" scope="request"/>
 <jsp:getProperty name="fBean" property="name" />
 <jsp:getProperty name="fBean" property="addr" />
 <jsp:getProperty name="fBean" property="age" />
 <jsp:getProperty name="fBean" property="personalizationInfo" />

Java Interview Questions - 76

How do I upload a file to my servlet or JSP?

On the client side, the client's browser must support form-based upload. Most modern browsers do, but there's no guarantee. For example,

 <FORM ENCTYPE='multipart/form-data'  method='POST' action='/myservlet'>
    <INPUT TYPE='file' NAME='mptest'>
    <INPUT TYPE='submit' VALUE='upload'>
 </FORM>

The input type "file" brings up a button for a file select box on the browser together with a text field that takes the file name once selected. The servlet can use the GET method parameters to decide what to do with the upload while the POST body of the request contains the file data to parse.
When the user clicks the "Upload" button, the client browser locates the local file and sends it using HTTP POST, encoded using the MIME-type multipart/form-data. When it reaches your servlet, your servlet must process the POST data in order to extract the encoded file. You can learn all about this format in RFC 1867.
Unfortunately, there is no method in the Servlet API to do this. Fortunately, there are a number of libraries available that do. Some of these assume that you will be writing the file to disk; others return the data as an InputStream.

• Jason Hunter's MultipartRequest (available from http://www.servlets.com/)
• Apache Jakarta Commons Upload (package org.apache.commons.upload) "makes it easy to add robust, high-performance, file upload capability to your servlets and web applications"
• CParseRFC1867 (available from http://www.servletcentral.com/).
• HttpMultiPartParser by Anil Hemrajani, at the isavvix Code Exchange
• There is a multipart/form parser availailable from Anders Kristensen (http://www-uk.hpl.hp.com/people/ak/java/, ak@hplb.hpl.hp.com) at http://www-uk.hpl.hp.com/people/ak/java/#utils.
• JavaMail also has MIME-parsing routines (see the Purple Servlet References).
• Jun Inamori has written a class called org.apache.tomcat.request.ParseMime which is available in the Tomcat CVS tree.
• JSPSmart has a free set of JSP for doing file upload and download.
• UploadBean by JavaZoom claims to handle most of the hassle of uploading for you, including writing to disk or memory.
• There's an Upload Tag in dotJ

Once you process the form-data stream into the uploaded file, you can then either write it to disk, write it to a database, or process it as an InputStream, depending on your needs. See How can I access or create a file or folder in the current directory from inside a servlet? and other questions in the Servlets:Files Topic for information on writing files from a Servlet.
Please note that you can't access a file on the client system directly from a servlet; that would be a huge security hole. You have to ask the user for permission, and currently form-based upload is the only way to do that.

Java Interview Questions - 75

What is the difference between URL encoding, URL rewriting, HTML escaping, and entity encoding?

URL Encoding is a process of transforming user input to a CGI form so it is fit for travel across the network -- basically, stripping spaces and punctuation and replacing with escape characters. URL Decoding is the reverse process. To perform these operations, call java.net.URLEncoder.encode() and java.net.URLDecoder.decode() (the latter was (finally!) added to JDK 1.2, aka Java 2).
Example: changing "We're #1!" into "We%27re+%231%21"
URL Rewriting is a technique for saving state information on the user's browser between page hits. It's sort of like cookies, only the information gets stored inside the URL, as an additional parameter. The HttpSession API, which is part of the Servlet API, sometimes uses URL Rewriting when cookies are unavailable.
Example: changing <A HREF="nextpage.html"> into
<A HREF="nextpage.html;$sessionid$=DSJFSDKFSLDFEEKOE"> (or whatever the actual syntax is; I forget offhand) (Unfortunately, the method in the Servlet API for doing URL rewriting for session management is called encodeURL(). Sigh...)
There's also a feature of the Apache web server called URL Rewriting; it is enabled by the mod_rewrite module. It rewrites URLs on their way in to the server, allowing you to do things like automatically add a trailing slash to a directory name, or to map old file names to new file names. This has nothing to do with servlets. 

Java Interview Questions - 74

How do I ensure that my servlet is thread-safe?

This is actually a very complex issue. A few guidelines:

1. The init() method is guaranteed to be called once per servlet instance, when the servlet is loaded. You don't have to worry about thread safety inside this method, since it is only called by a single thread, and the web server will wait until that thread exits before sending any more threads into your service() method.
2. Every new client request generates (or allocates) a new thread; that thread calls the service() method of your servlet (which may in turn call doPost(), doGet() and so forth).
3. Under most circumstances, there is only one instance of your servlet, no matter how many client requests are in process. That means that at any given moment, there may be many threads running inside the service() method of your solo instance, all sharing the same instance data and potentially stepping on each other's toes. This means that you should be careful to synchronize access to shared data (instance variables) using the synchronized keyword.
(Note that the server will also allocate a new instance if you register the servlet with a new name and, e.g., new init parameters.)
4. Note that you need not (and should not) synchronize on local data or parameters. And especially you shouldn't synchronize the service() method! (Or doPost(), doGet() et al.)
5. A simple solution to synchronizing is to always synchronize on the servlet instance itself using &quot;synchronized (this) { ... }&quot;. However, this can lead to performance bottlenecks; you're usually better off synchronizing on the data objects themselves.
6. If you absolutely can't deal with synchronizing, you can declare that your servlet &quot;implements SingleThreadModel&quot;. This empty interface tells the web server to only send one client request at a time into your servlet. From the JavaDoc: &quot;If the target servlet is flagged with this interface, the servlet programmer is guaranteed that no two threads will execute concurrently the service method of that servlet. This guarantee is ensured by maintaining a pool of servlet instances for each such servlet, and dispatching each service call to a free servlet. In essence, if the servlet implements this interface, the servlet will be thread safe.&quot; Note that this is not an ideal solution, since performance may suffer (depending on the size of the instance pool), plus it's more difficult to share data across instances than within a single instance.
See also What's a better approach for enabling thread-safe servlets and JSPs? SingleThreadModel Interface or Synchronization?
7. To share data across successive or concurrent requests, you can use either instance variables or class-static variables, or use Session Tracking.
8. The destroy() method is not necessarily as clean as the init() method. The server calls destroy either after all service calls have been completed, or after a certain number of seconds have passed, whichever comes first. This means that other threads might be running service requests at the same time as your destroy() method is called! So be sure to synchronize, and/or wait for the other requests to quit. Sun's Servlet Tutorial has an example of how to do this with reference counting.
9. destroy() can not throw an exception, so if something bad happens, call log() with a helpful message (like the exception). See the &quot;closing a JDBC connection&quot; example in Sun's Tutorial.

Java Interview Questions - 73

How do I support both GET and POST protocol from the same Servlet?

The easy way is, just support POST, then have your doGet method call your doPost method:

 public void doGet(HttpServletRequest req, HttpServletResponse res)
 throws ServletException, IOException
 {
    doPost(req, res); 
 }

Java Interview Questions - 72

Is it the "servlets" directory or the "servlet" directory?

For Java Web Server:

• on the file system, it's "servlets"
c:\JavaWebServer1.1\servlets\DateServlet.class
• in a URL path, it's "servlet"
http://www.stinky.com/servlet/DateServlet

Java Interview Questions - 53

Q:

What is the difference between ServletContext and ServletConfig?

A:

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.

Java Interview Questions - 52

Q:	What is the difference between HttpServlet and GenericServlet?
A:	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.

Java Interview Questions - 51

Q:

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

A:

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: http://www.allapplabs.com/svt1?p1=v1&p2=v2&...&pN=vN 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.

Java Interview Questions - 50

Q:

What is preinitialization of a servlet?

A:

A container doesnot initialize the servlets ass 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 <load-on-startup> 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.

Java Interview Questions - 49

Q:	Explain ServletContext.
A:	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 applicationor servlet container's version. Every web application has one and only one ServletContext and is accessible to all active resource of that application.

Java Interview Questions - 48

Q:	What are the common mechanisms used for session tracking?
A:	Cookies SSL sessions URL- rewriting

Java Questions - 47

Q:	Explain the directory structure of a web application.
A:	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

Java Questions - 46

Q:

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

A:

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.