What is servlets in Java

A servlet is a Java programming language class used to extend the capabilities of servers that host applications accessed by means of a request-response programming model. Although servlets can respond to any type of request, they are commonly used to extend the applications hosted by web servers. For such applications, Java Servlet technology defines HTTP-specific servlet classes.

The javax.servlet and javax.servlet.http packages provide interfaces and classes for writing servlets. All servlets must implement the Servlet interface, which defines lifecycle methods. When implementing a generic service, you can use or extend the GenericServlet class provided with the Java Servlet API. The HttpServlet class provides methods, such as doGet and doPost, for handling HTTP-specific services.

Servlet Lifecycle

The lifecycle of a servlet is controlled by the container in which the servlet has been deployed. When a request is mapped to a servlet, the container performs the following steps.

1. If an instance of the servlet does not exist, the web container
   
   1. Loads the servlet class.
   2. Creates an instance of the servlet class.
   3. Initializes the servlet instance by calling the init method. Initialization is covered in Creating and Initializing a Servlet.
2. Invokes the service method, passing request and response objects. Service methods are discussed in Writing Service Methods.

If it needs to remove the servlet, the container finalizes the servlet by calling the servlet’s destroy method. For more information, see Finalizing a Servlet.

Handling Servlet Lifecycle Events

You can monitor and react to events in a servlet’s lifecycle by defining listener objects whose methods get invoked when lifecycle events occur. To use these listener objects, you must define and specify the listener class.

Defining the Listener Class

You define a listener class as an implementation of a listener interface. Table 15-1 lists the events that can be monitored and the corresponding interface that must be implemented. When a listener method is invoked, it is passed an event that contains information appropriate to the event. For example, the methods in theHttpSessionListener interface are passed an HttpSessionEvent, which contains an HttpSession.

Table 15-1 Servlet Lifecycle Events

Object	Event	Listener Interface and Event Class
Web context	Initialization and destruction	javax.servlet.ServletContextListener and ServletContextEvent
Web context	Attribute added, removed, or replaced	javax.servlet.ServletContextAttributeListener and ServletContextAttributeEvent
Session	Creation, invalidation, activation, passivation, and timeout	javax.servlet.http.HttpSessionListener, javax.servlet.http.HttpSessionActivationListener, and HttpSessionEvent
Session	Attribute added, removed, or replaced	javax.servlet.http.HttpSessionAttributeListener and HttpSessionBindingEvent
Request	A servlet request has started being processed by web components	javax.servlet.ServletRequestListener and ServletRequestEvent
Request	Attribute added, removed, or replaced	javax.servlet.ServletRequestAttributeListener and ServletRequestAttributeEvent

Use the @WebListener annotation to define a listener to get events for various operations on the particular web application context. Classes annotated with @WebListener must implement one of the following interfaces:

javax.servlet.ServletContextListener
javax.servlet.ServletContextAttributeListener
javax.servlet.ServletRequestListener
javax.servlet.ServletRequestAttributeListener
javax.servlet..http.HttpSessionListener
javax.servlet..http.HttpSessionAttributeListener

For example, the following code snippet defines a listener that implements two of these interfaces:

import javax.servlet.ServletContextAttributeListener;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;

@WebListener()
public class SimpleServletListener implements ServletContextListener,
        ServletContextAttributeListener {
    ...

Handling Servlet Errors

Any number of exceptions can occur when a servlet executes. When an exception occurs, the web container generates a default page containing the following message:

A Servlet Exception Has Occurred

But you can also specify that the container should return a specific error page for a given exception.

Creating and Initializing a Servlet

Use the @WebServlet annotation to define a servlet component in a web application. This annotation is specified on a class and contains metadata about the servlet being declared. The annotated servlet must specify at least one URL pattern. This is done by using the urlPatterns or value attribute on the annotation. All other attributes are optional, with default settings. Use the value attribute when the only attribute on the annotation is the URL pattern; otherwise use the urlPatterns attribute when other attributes are also used.

Classes annotated with @WebServlet must extend the javax.servlet.http.HttpServlet class. For example, the following code snippet defines a servlet with the URL pattern /report:

import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;

@WebServlet("/report")
public class MoodServlet extends HttpServlet {
    ...

The web container initializes a servlet after loading and instantiating the servlet class and before delivering requests from clients. To customize this process to allow the servlet to read persistent configuration data, initialize resources, and perform any other one-time activities, you can either override the init method of the Servlet interface or specify the initParams attribute of the @WebServlet annotation. The initParams attribute contains a @WebInitParam annotation. If it cannot complete its initialization process, a servlet throws an UnavailableException.

Use an initialization parameter to provide data needed by a particular servlet. By contrast, a context parameter provides data that is available to all components of a web application.