Servlets

What are the different types of session tracking?

Different types are: • URL rewriting • Hidden Form Fields • Cookies • Secure Socket Layer (SSL) Sessions

What is the <load-on-startup> element?

The <load-on-startup> element of a deployment descriptor is used to load a servlet file when the server starts instead of waiting for the first request. It is also used to specify the order in which the files are to be loaded.

How can destroyed session in servlet?

Using session.invalidate() method.

What is meant by a Web Application ?

A Web application is a dynamic extension of a Web or application server. There are two types of web applications: presentation-oriented and service-oriented. A presentation-oriented Web application generates interactive web pages, which contain various types of markup language and dynamic content in response to requests. On the other hand, a service-oriented web application implements the endpoint of a web service. In general, a Web application can be seen as a collection of servlets installed under a specific subset of the server's URL namespace.

What is servlet lazy loading?

A container does not 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.

What is the preinitialization of servlet?

A container does not 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 a cookie ? What is the difference between session and cookie ?

A cookie is a bit of information that the Web server sends to the browser. The browser stores the cookies for each Web server in a local file. In a future request, the browser, along with the request, sends all stored cookies for that specific Web server.The differences between session and a cookie are the following: • The session should work, regardless of the settings on the client browser. The client may have chosen to disable cookies. However, the sessions still work, as the client has no ability to disable them in the server side. • The session and cookies also differ in the amount of information the can store. The HTTP session is capable of storing any Java object, while a cookie can only store String objects.

What is a deployment descriptor?

A deployment descriptor is an XML document with an .xml extension. It defines a component's deployment settings. It declares transaction attributes and security authorization for an enterprise bean. The information provided by a deployment descriptor is declarative and therefore it can be modified without changing the source code of a bean.

What is the ServletContext?

A servlet context object contains the information about the Web application of which the servlet is a part. It also provides access to the resources common to all the servlets in the application. Each Web application in a container has a single servlet context associated with it.

What is a Servlet ?

A servlet is a Java programming language class that is used to extend the capabilities of servers that host applications accessed by means of a request- response programming model. Before the servlet, CGI scripting language was used as server side programming language.

What is session?

A session refers to all the requests that a single client might make to a server in the course of viewing any pages associated with a given application. Sessions are specific to both the individual user and the application.

When to use doGet() and when doPost()?

Always prefer to use GET (As because GET is faster than POST), except mentioned in the following reason: •If data is sensitive. •Data is greater than 1024 characters. •If your application don't need bookmarks.

What are the advantages of storing session state in cookies?

Cookies are usually persistent, so for low-security sites, user data that needs to be stored long-term (such as a user ID, historical information, etc.) can be maintained easily with no server interaction. For small- and medium-sized session data, the entire session data (instead of just the session ID) can be kept in the cookie.

Why do we need constructor in servlet if we use the init ()?

Even though there is an init method in a servlet which gets called to initialize it, a constructor is still required to instantiate the servlet since a servlet is still a class and all classes must have a constructor (implicit or explicit).

What is filter?

Filters are Java components that are used to intercept an incoming request to a Web resource and a response sent back from the resource. It is used to abstract any useful information contained in the request or response.

What is the difference between GenericServlet and HttpServlet ?

GenericServlet is a generalized and protocol-independent servlet that implements the Servlet and ServletConfig interfaces. Those servlets extending the GenericServlet class shall override the service method. Finally, in order to develop an HTTP servlet for use on the Web that serves requests using the HTTP protocol, your servlet must extend the HttpServlet instead.

What is the generic servlet class?

GenericServlet is an abstract class that implements the Servlet interface and the ServletConfig interface. In addition to the methods declared in these two interfaces, this class also provides simple versions of the lifecycle methods init () and destroy (), and implements the log method declared in the ServletContext interface.

What is HTTP Tunneling ?

HTTP Tunneling is a technique by which, communications performed using various network protocols are encapsulated using the HTTP or HTTPS protocols. The HTTP protocol therefore acts as a wrapper for a channel that the network protocol being tunneled uses to communicate. The masking of other protocol requests as HTTP requests is HTTP Tunneling.

What is the need of session tracking in web application?

HTTP is a stateless protocol. Every request is treated as new request. For web applications to be more realistic they have to retain information across multiple requests. Such information which is part of the application is referred as "state". To keep track of this state we need session tracking.

How do I use cookies to store session state on client?

In a servlet, the HttpServletResponse and HttpServletRequest objects passed to method HttpServlet. Service () can be used to create cookies on the client and use cookie information transmitted during client requests. JSPs can also use cookies, in scriptlet code or, preferably, from within custom tag code. • To set a cookie on the client, use the addCookie() method in class HttpServletResponse. Multiple cookies may be set for the same request, and a single cookie name may have multiple values. • To get all of the cookies associated with a single HTTP request, use the getCookies() method of class HttpServletRequest

What are the type of protocols supported by the HttpServlet?

It extends the GenericServlet base class and provides a framework for handling the HTTP protocol. So, HttpServlet only supports HTTP and HTTPS protocol.

What is the life cycle of servlet?

Life cycle of Servlet: •Servlet class loading •Servlet instantiation •Initialization (call the init method) •Request handling (call the service method) •Removal from service (call the destroy method)

Explain the life cycle of a Servlet.

On every client's request, the Servlet Engine loads the servlets and invokes its init methods, in order for the servlet to be initialized. Then, the Servlet object handles all subsequent requests coming from that client, by invoking the service method for each request separately. Finally, the servlet is removed by calling the server's destroy method.

What is a Server Side Include (SSI) ?

Server Side Includes (SSI) is a simple interpreted server-side scripting language, used almost exclusively for the Web, and is embedded with a servlet tag. The most frequent use of SSI is to include the contents of one or more files into a Web page on a Web server. When a Web page is accessed by a browser, the Web server replaces the servlet tag in that Web page with the hyper text generated by the corresponding servlet.

What is servlet chaining?

Servlet Chaining is a method where the output of one servlet is piped into a second servlet. The output of the second servlet could be piped into a third servlet, and so on. The last servlet in the chain returns the output to the Web browser

What is Servlet Chaining ?

Servlet Chaining is the method where the output of one servlet is sent to a second servlet. The output of the second servlet can be sent to a third servlet, and so on. The last servlet in the chain is responsible for sending the response to the client.

When the servlet is unloaded?

Servlet gets unloaded when: •Server shuts down. •Administrator manually unloads.

What is the session tracking?

Session tracking is a mechanism that servlets use to maintain state about a series of requests from the same user (requests originating from the same browser) across some period of time.

What is the structure of the HTTP response ?

The HTTP response consists of three parts: • Status Code: describes the status of the response. It can be used to check if the request has been successfully completed. In case the request failed, the status code can be used to find out the reason behind the failure. If your servlet does not return a status code, the success status code, HttpServletResponse.SC_OK, is returned by default. • HTTP Headers: they contain more information about the response. For example, the headers may specify the date/time after which the response is considered stale, or the form of encoding used to safely transfer the entity to the user. • Body: it contains the content of the response. The body may contain HTML code, an image, etc. The body consists of the data bytes transmitted in an HTTP transaction message immediately following the headers.

Why HttpServlet class is declared abstract?

The HttpServlet class is declared abstract because the default implementations of the main service methods do nothing and must be overridden. This is a convenience implementation of the Servlet interface, which means that developers do not need to implement all service methods. If your servlet is required to handle doGet () requests for example, there is no need to write a doPost () method too.

What is the use of servlet wrapper classes?

The HttpServletRequestWrapper and HttpServletResponseWrapper classes are designed to make it easy for developers to create custom implementations of the servlet request and response types. The classes are constructed with the standard HttpServletRequest and HttpServletResponse instances respectively and their default behaviour is to pass all method calls directly to the underlying objects.

What is the difference between forward() and include()?

The RequestDispatcher include() method inserts the contents of the specified resource directly in the flow of the servlet response, as if it were part of the calling servlet. The RequestDispatcher forward() method is used to show a different resource in place of the servlet that was originally called.

What is the difference between the ServletConfig and ServletContext interface?

The ServletConfig interface is implemented by the servlet container in order to pass configuration information to a servlet. The server passes an object that implements the ServletConfig interface to the servlet's init () method. A ServletContext defines a set of methods that a servlet uses to communicate with its servlet container.

How do you find out what client machine is making a request to your servlet ?

The ServletRequest class has functions for finding out the IP address or host name of the client machine. getRemoteAddr() gets the IP address of the client machine and getRemoteHost() gets the host name of the client machine.

What is URL Encoding and URL Decoding ?

The URL encoding procedure is responsible for replacing all the spaces and every other extra special character of a URL, into their corresponding Hex representation. In correspondence, URL decoding is the exact opposite procedure.

What are the advantages of jsp over servlet?

The advantage of JSP is that they are document-centric. Servlets, on the other hand, look and act like programs. A Java Server Page can contain Java program fragments that instantiate and execute Java classes, but these occur inside an HTML template file and are primarily used to generate dynamic content. Some of the JSP functionality can be achieved on the client, using JavaScript. The power of JSP is that it is server-based and provides a framework for Web application development.

Which protocol will be used by browser and servlet to communicate ?

The browser communicates with a servlet by using the HTTP protocol.

What is servlet interface?

The central abstraction in the Servlet API is the Servlet interface. All servlets implement this interface, either directly or more commonly by extending a class that implements it.

Explain the architechure of a Servlet

The core abstraction that must be implemented by all servlets is the javax.servlet.Servlet interface. Each servlet must implement it either directly or indirectly, either by extending javax.servlet.GenericServlet or javax.servlet.http.HTTPServlet. Finally, each servlet is able to serve multiple requests in parallel using multithreading.

What is the difference between forward () and sendRedirect ()?

The difference is: •A forward is performed internally by the servlet. A redirect is a two step process, where the web application instructs the browser to fetch a second URL, which differs from the original. •The browser is completely unaware that it has taken place, so its original URL remains intact. But in sendRedirect, the browser, in this case, is doing the work and knows that it's making a new request.

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

The difference is: •In doGet() the parameters are appended to the URL and sent along with header information. In doPost (),send the information through a socket back to the webserver and it won't show up in the URL bar. •The amount of information you can send back using a GET is restricted as URLs can only be 1024 characters. You can send much more information to the server by using post and it's not restricted to textual data either. It is possible to send files and even binary data such as serialized Java objects! •DoGet() is a request for information.It does not change anything on the server. (doGet () should be idempotent). doPost () provides information (such as placing an order for merchandise) that the server is expected to remember.

What is the difference between GenericServlet and HttpServlet?

The difference is: •The GenericServlet is an abstract class that is extended by HttpServlet to provide HTTP protocol-specific methods. But HttpServlet extends the GenericServlet base class and provides a framework for handling the HTTP protocol. •The GenericServlet does not include protocol-specific methods for handling request parameters, cookies, sessions and setting response headers. The HttpServlet subclass passes generic service method requests to the relevant doGet () or doPost () method. •GenericServlet is not specific to any protocol. HttpServlet only supports HTTP and HTTPS protocol.

How do I support both doGet () and doPost () from same servlet?

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

What's the difference between sendRedirect and forward methods ?

The sendRedirect method creates a new request, while the forward method just forwards a request to a new target. The previous request scope objects are not available after a redirect, because it results in a new request. On the other hand, the previous request scope objects are available after forwarding. FInally, in general, the sendRedirect method is considered to be slower compare to the forward method.

How servlet is loaded?

The servlet is loaded by: •First request is made. •Server starts up (auto-load). •There is only a single instance which answers all requests concurrently. This saves memory and allows a Servlet to easily manage persistent data. •Administrator manually loads.

What is URL rewriting?

URL rewriting is a method of session tracking in which some extra data is appended at the end of each URL. This extra data identifies the session. The server can associate this session identifier with the data it has stored about that session.

What is the use of servlet?

Uses of servlet includes: •Processing and storing data submitted by an HTML form. •Providing dynamic content. •A Servlet can handle multiple request concurrently and be used to develop high performance system •Managing state information on top of the stateless HTTP

Should I override the service () method?

We never override the service method, since the HTTP Servlets have already taken care of it. The default service function invokes the doXXX() method corresponding to the method of the HTTP request. For example, if the HTTP request method is GET, doGet () method is called by default. A servlet should override the doXXX() method for the HTTP methods that servlet supports. Because HTTP service method checks the request method and calls the appropriate handler method, it is not necessary to override the service method itself. Only override the appropriate doXXX() method.

Can servlet have a constructor?

Yes.

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

doGET: The GET method appends the name-value pairs on the request's URL. Thus, there is a limit on the number of characters and subsequently on the number of values that can be used in a client's request. Furthermore, the values of the request are made visible and thus, sensitive information must not be passed in that way. doPOST: The POST method overcomes the limit imposed by the GET request, by sending the values of the request inside its body. Also, there is no limitations on the number of values to be sent across. Finally, the sensitive information passed through a POST request is not visible to an external client.

What is the difference between an Applet and a Servlet ?

n Applet is a client side java program that runs within a Web browser on the client machine. On the other hand, a servlet is a server side component that runs on the web server.An applet can use the user interface classes, while a servlet does not have a user interface. Instead, a servlet waits for client's HTTP requests and generates a response in every request.