Servlet - API (Servlet Interface)

The Servlet API defines two main packages:

• javax.servlet — Defines classes and interfaces for protocol-independent servlets (Generic Servlets)

• javax.servlet.http — Defines classes and interfaces for HTTP protocol-based servlets

Important interfaces of javax.servlet package:

Important classes of javax.servlet package:

The javax.servlet.http package provides HTTP-specific servlet classes and interfaces. It extends the generic servlet types with HTTP methods and session management.

For every request, the web container creates one ServletRequest object. This object holds end-user provided information such as form fields, query parameters, and headers.

The servlet can use this request object to retrieve all information submitted by the client.

ServletResponse (I): For every request, the web container creates a response object. The servlet can use this response object to prepare and send a response to the end user.

ServletConfig (I): For every servlet, the web container creates a separate config object to hold its configuration information. The servlet can use this config object to get its configuration information.

Note: ServletConfig is per servlet, whereas ServletContext is per web application.

ServletContext (I): For every web application, the web container creates a separate context object to hold application-level configuration information. The servlet can use this context object to get application-level configuration information.

RequestDispatcher (I): We can use RequestDispatcher to dispatch a request from one servlet to another servlet. This enables request forwarding and includes.

A single servlet object can be accessed by multiple threads simultaneously, which may cause data inconsistency problems — servlets are not thread-safe by default.

If a servlet class implements SingleThreadModel, then the servlet object can be accessed by only one thread at a time.

Advantages:

• Threads are executed one by one, resolving data inconsistency problems

Disadvantages:

• Increases thread waiting time and creates performance problems

SingleThreadModel is deprecated in Servlet 2.4. Instead, use the synchronized keyword. SingleThreadModel is a marker interface — it does not contain any methods; the JVM provides the required ability internally.

Every servlet in Java must implement the Servlet interface either directly or indirectly. The Servlet interface defines the following 5 methods:

init() method: Called automatically by the web container after servlet instantiation immediately to perform initialization activities. Once init() completes, the servlet is in a position to provide service.

service() method: Executed automatically by the web container for every request to provide the required response. All service logic must be written in this method.

destroy() method: Executed only once by the web container to perform cleanup activities just before taking the servlet object out of service. Once destroy() completes, the web container destroys that servlet object. This usually happens at server shutdown or application undeployment.

Note: init(), service(), and destroy() are called lifecycle methods of the servlet.

getServletConfig(): Returns the ServletConfig object. By using this object, the servlet can get its configuration information.

getServletInfo(): Returns information about the servlet such as author, version, and copyright information.

Callback vs Inline Methods:

Steps to develop a first web application:

• Step 1: Develop a servlet by implementing the Servlet interface — provide implementation for all 5 methods