面试题与答案

A servlet is a Java technology-based Web component, managed by a container called servlet container or servlet engine, that generates dynamic content and interacts with web clients via a request/response paradigm.

Because servlets are platform-independent Java classes that are compiled to platform-neutral byte code that can be loaded dynamically into and run by a Java technology-enabled Web server.

The servlet container is a part of a Web server or application server that provides the network services over which requests and responses are sent, decodes MIME-based requests, and formats MIME-based responses. A servlet container also contains and manages servlets through their lifecycle.

The servlet container determines which servlet to invoke based on the configuration of its servlets, and calls it with objects representing the request and response.

In GET your entire form submission can be encapsulated in one URL, like a hyperlink. query length is limited to 260 characters, not secure, faster, quick and easy.
In POST Your name/value pairs inside the body of the HTTP request, which makes for a cleaner URL and imposes no size limitations on the form's output. It is used to send a chunk of data to the server to be processed, more versatile, most secure.

The session is an object used by a servlet to track a user's interaction with a Web application across multiple HTTP requests.

The servlet mapping defines an association between a URL pattern and a servlet. The mapping is used to map requests to servlets.

The servlet context is an object that contains a servlet's view of the Web application within which the servlet is running. Using the context, a servlet can log events, obtain URL references to resources, and set and store attributes that other servlets in the context can use. (answer supplied by Sun's tutorial).

Servlet interface.

An instance of servlet is created when the servlet is loaded for the first time in the container. Init() method is used to configure this servlet instance. This method is called only once in the life time of a servlet, hence it makes sense to write all those configuration details about a servlet which are required for the whole life of a servlet in this method.

Two important packages are required to build servlet "javax.servlet" and "javax.servlet.http". They form the core of Servlet API. Servlets are not part of core Java but are standard extensions provided by Tomcat.

ServletContext Interface:  It gives information about the environment. It represents a Servlet's view of the Web Application.Using this interface servlet can access raw input streams to Web Application resources, virtual directory translation, a common mechanism for logging information, and an application scope for binding objects.

HttpServlet class extends GenericServlet class which is an abstract class to provide HTTP protocol-specific functionalities. Most of the java application developers extend HttpServlet class as it provides more HTTP protocol-specific functionalities. You can see in HttpServlet class doGet (), doPOst () methods which are more targeted towards HTTP protocol specific functionalities. For instance we can inherit from GenericServlet class to make something like MobileServlet. So GenericServlet class should be used when we want to write protocol specific implementation which is not available. But when we know we are making an internet application where HTTP is the major protocol its better to use HttpServlet.

At the top of all is the main servlet interface which is implemented by the generic servlet. But generic servlet does not provide implementation specific to any protocol. HTTP servlet further inherits from the generic servlet and provides HTTP implementation like "Get" and "Post". Finally comes our custom servlet which inherits from HTTP Servlet.

A protocol is stateless if it can remember difference between one client request and the other. HTTP is a stateless protocol because each request is executed independently without any knowledge of the requests that came before it.

Following are the different ways of maintaining state�s between stateless requests:-
>> URL rewriting
>> Cookies
>> Hidden fields
>> Sessions