A Jakarta Servlet (formerly Java Servlet) is a
Java
Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's mo ...
software component that extends the capabilities of a
server
Server may refer to:
Computing
*Server (computing), a computer program or a device that provides functionality for other programs or devices, called clients
Role
* Waiting staff, those who work at a restaurant or a bar attending customers and su ...
. Although servlets can respond to many types of requests, they most commonly implement
web containers for hosting
web application
A web application (or web app) is application software that is accessed using a web browser. Web applications are delivered on the World Wide Web to users with an active network connection.
History
In earlier computing models like client-serv ...
s on
web server
A web server is computer software and underlying hardware that accepts requests via HTTP (the network protocol created to distribute web content) or its secure variant HTTPS. A user agent, commonly a web browser or web crawler, initia ...
s and thus qualify as a server-side servlet
web API
A web API is an application programming interface for either a web server or a web browser. It is a web development concept, usually limited to a web application's client-side (including any web frameworks being used), and thus usually does not ...
. Such web servlets are the
Java
Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's mo ...
counterpart to other
dynamic web content
A server-side dynamic web page is a web page whose construction is controlled by an application server processing server-side scripts. In server-side scripting, parameters determine how the assembly of every new web page proceeds, and includin ...
technologies such as
PHP and
ASP.NET.
Introduction
A Jakarta Servlet processes or stores a
Java class in
Jakarta EE that conforms to the Jakarta Servlet API, a standard for implementing Java classes that respond to requests. Servlets could in principle communicate over any
client–server protocol, but they are most often used with
HTTP
The Hypertext Transfer Protocol (HTTP) is an application layer protocol in the Internet protocol suite model for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide We ...
. Thus "servlet" is often used as shorthand for "HTTP servlet".
Thus, a
software developer
Software development is the process of conceiving, specifying, designing, programming, documenting, testing, and bug fixing involved in creating and maintaining applications, frameworks, or other software components. Software development invo ...
may use a servlet to add
dynamic content to a
web server
A web server is computer software and underlying hardware that accepts requests via HTTP (the network protocol created to distribute web content) or its secure variant HTTPS. A user agent, commonly a web browser or web crawler, initia ...
using the
Java platform. The generated content is commonly
HTML
The HyperText Markup Language or HTML is the standard markup language for documents designed to be displayed in a web browser. It can be assisted by technologies such as Cascading Style Sheets (CSS) and scripting languages such as JavaS ...
, but may be other data such as
XML
Extensible Markup Language (XML) is a markup language and file format for storing, transmitting, and reconstructing arbitrary data. It defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. T ...
and more commonly,
JSON
JSON (JavaScript Object Notation, pronounced ; also ) is an open standard file format and data interchange format that uses human-readable text to store and transmit data objects consisting of attribute–value pairs and arrays (or other se ...
. Servlets can maintain
state in
session variables across many server transactions by using
HTTP cookie
HTTP cookies (also called web cookies, Internet cookies, browser cookies, or simply cookies) are small blocks of data created by a web server while a user is browsing a website and placed on the user's computer or other device by the user's ...
s, or
URL mapping.
The Jakarta Servlet API has, to some extent, been superseded by two standard Java technologies for web services:
* the
Jakarta RESTful Web Services (JAX-RS 2.0) useful for AJAX, JSON and REST services, and
* the
Jakarta XML Web Services (JAX-WS) useful for
SOAP
Soap is a salt of a fatty acid used in a variety of cleansing and lubricating products. In a domestic setting, soaps are surfactants usually used for washing, bathing, and other types of housekeeping. In industrial settings, soaps are us ...
Web Services.
To deploy and run a servlet, a
web container must be used. A web container (also known as a servlet container) is essentially the component of a web server that interacts with the servlets. The web container is responsible for managing the lifecycle of servlets, mapping a URL to a particular servlet and ensuring that the URL requester has the correct access rights.
The Servlet
API, contained in the
Java package hierarchy , defines the expected interactions of the web container and a servlet.
A is an
object
Object may refer to:
General meanings
* Object (philosophy), a thing, being, or concept
** Object (abstract), an object which does not exist at any particular time or place
** Physical object, an identifiable collection of matter
* Goal, an ...
that receives a request and generates a response based on that request. The basic Servlet package defines Java objects to represent servlet requests and responses, as well as objects to reflect the servlet's configuration parameters and execution environment. The package defines
HTTP
The Hypertext Transfer Protocol (HTTP) is an application layer protocol in the Internet protocol suite model for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide We ...
-specific subclasses of the generic servlet elements, including session management objects that track multiple requests and responses between the web server and a client. Servlets may be packaged in a
WAR file as a
web application
A web application (or web app) is application software that is accessed using a web browser. Web applications are delivered on the World Wide Web to users with an active network connection.
History
In earlier computing models like client-serv ...
.
Servlets can be generated automatically from
Jakarta Server Pages
Jakarta Server Pages (JSP; formerly JavaServer Pages) is a collection of technologies that helps software developers create dynamically generated web pages based on HTML, XML, SOAP, or other document types. Released in 1999 by Sun Microsystems, ...
(JSP) by the
Jakarta Server Pages compiler. The difference between servlets and JSP is that servlets typically embed HTML inside Java code, while JSPs embed Java code in HTML. While the direct usage of servlets to generate HTML (as shown in the example below) has become rare, the higher level MVC web framework in Jakarta EE (
JSF) still explicitly uses the servlet technology for the low level request/response handling via the . A somewhat older usage is to use servlets in conjunction with JSPs in a pattern called "
Model 2", which is a flavor of the
model–view–controller
Model–view–controller (MVC) is a software architectural pattern commonly used for developing user interfaces that divide the related program logic into three interconnected elements. This is done to separate internal representations of info ...
.
History
The Java Servlet API was first publicly announced at the inaugural
JavaOne
__NOTOC__
JavaOne is an annual conference first organized in 1996 by Sun Microsystems to discuss Java technologies, primarily among Java developers. It was held in San Francisco, California, typically running from a Monday to Thursday in summer ...
conference in May 1996. About two months after the announcements at the conference, the first public implementation was made available on the JavaSoft website. This was the first alpha of the Java Web Server (JWS; then known by its codename ''Jeeves'') which would eventually be shipped as a product on June 5, 1997.
In his blog on
java.net, Sun veteran and
GlassFish lead Jim Driscoll details the history of servlet technology.
James Gosling first thought of servlets in the early days of
Java
Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's mo ...
, but the concept did not become a product until December 1996 when Sun shipped JWS.
This was before what is now the
Jakarta EE was made into a specification.
The Servlet1 specification was created by Pavni Diwanji
while she worked at
Sun Microsystems
Sun Microsystems, Inc. (Sun for short) was an American technology company that sold computers, computer components, software, and information technology services and created the Java programming language, the Solaris operating system, ZFS, t ...
, with version 1.0 finalized in June 1997. Starting with version 2.2, the specification was developed under the
Java Community Process.
Life cycle of a servlet
Three methods are central to the life cycle of a servlet. These are
init()
,
service()
, and
destroy()
.
They are implemented by every servlet and are invoked at specific times by the server.
* During initialization stage of the servlet
life cycle, the web container initializes the servlet instance by calling th
init()
method, passing an object implementing th
interface. This configuration object allows the servlet to access
name-value initialization parameters from the web application.
* After initialization, the servlet instance can service client requests. Each
request is serviced in its own separate thread. The web container calls the
service()
method of the servlet for every request. The
service()
method determines the kind of request being made and dispatches it to an appropriate method to handle the request. The developer of the servlet must provide an implementation for these methods. If a request is made for a method that is not implemented by the servlet, the method of the parent class is called, typically resulting in an error being returned to the requester.
* Finally, the web container calls the
destroy()
method that takes the servlet out of service. The
destroy()
method, like
init()
, is called only once in the lifecycle of a servlet.
The following is a typical user scenario of these methods.
# Assume that a user requests to visit a
URL.
#* The browser then generates an HTTP request for this URL.
#* This request is then sent to the appropriate server.
# The HTTP request is received by the web server and forwarded to the servlet container.
#* The container maps this request to a particular servlet.
#* The servlet is dynamically retrieved and loaded into the address space of the container.
# The container invokes the
init()
method of the servlet.
#* This method is invoked only when the servlet is first loaded into memory.
#* It is possible to pass initialization parameters to the servlet so that it may configure itself.
# The container invokes the
service()
method of the servlet.
#* This method is called to process the HTTP request.
#* The servlet may read data that has been provided in the HTTP request.
#* The servlet may also formulate an HTTP response for the client.
# The servlet remains in the container's address space and is available to process any other HTTP requests received from clients.
#* The
service()
method is called for each HTTP request.
# The container may, at some point, decide to unload the servlet from its memory.
#* The algorithms by which this decision is made are specific to each container.
# The container calls the servlet's
destroy()
method to relinquish any resources such as file handles that are allocated for the servlet; important data may be saved to a persistent store.
# The memory allocated for the servlet and its objects can then be garbage collected.
Example
The following example servlet prints how many times its
service()
method was called.
Note that
HttpServlet
is a subclass of
GenericServlet
, an implementation of the
Servlet
interface.
The
service()
method of
HttpServlet
class dispatches requests to the methods
doGet()
,
doPost()
,
doPut()
,
doDelete()
, and so on; according to the HTTP request. In the example below
service()
is overridden and does not distinguish which HTTP request method it serves.
import java.io.IOException;
import jakarta.servlet.ServletConfig;
import jakarta.servlet.ServletException;
import jakarta.servlet.http.HttpServlet;
import jakarta.servlet.http.HttpServletRequest;
import jakarta.servlet.http.HttpServletResponse;
public class ServletLifeCycleExample extends HttpServlet
Container servers
The specification for Servlet technology has been implemented in many products. See a list of implementations on the
web container page.
There are also other types of servlet containers such as those for SIP servlets, e.g.,
SailFin
SailFin was an open-source Java application server project led by Sun Microsystems. It implements the JCP SIP Servlet 1.1 (JSR 289) specification integrated with the open-source Java EE application server GlassFish.
SailFin effectively extends ...
.
References
External links
*
Servlets.com
{{Authority control
Articles with example Java code
Servlet
Servlet