Original author(s) | Pavni Diwanji |
---|---|
Developer(s) | Eclipse Foundation |
Initial release | December 1996 |
Stable release | 5.0
/ September 7, 2020 |
Repository | |
Written in | Java |
Platform | Jakarta EE 9 |
Size | 2.56 MB |
Type | Software component for Web API |
License | Eclipse Public License |
Website | jakarta |
A Jakarta Servlet (formerly Java Servlet) is a Java software component that extends the capabilities of a server. Although servlets can respond to many types of requests, they most commonly implement web containers for hosting web applications on web servers and thus qualify as a server-side servlet web API. Such web servlets are the Java counterpart to other dynamic web content technologies such as PHP and ASP.NET.
A Jakarta Servlet processes or stores a Java class in Jakarta EE that conforms to the Jakarta Servlet API,[1] 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. Thus "servlet" is often used as shorthand for "HTTP servlet".[2] Thus, a software developer may use a servlet to add dynamic content to a web server using the Java platform. The generated content is commonly HTML, but may be other data such as XML and more commonly, JSON. Servlets can maintain state in session variables across many server transactions by using HTTP cookies, or URL mapping.
The Jakarta Servlet API has, to some extent, been superseded by two standard Java technologies for web services: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 javax.servlet
, defines the expected interactions of the web container and a servlet.[2]
A Servlet
is an object 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 javax.servlet.http
defines HTTP-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.
Servlets can be generated automatically from Jakarta Server Pages (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 FacesServlet
. 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.
The current version of Servlet is 4.0.[3]
The Java Servlet API was first publicly announced at the inaugural JavaOne conference in May 1996.[4][5] 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)[6] which would eventually be shipped as a product on June 5, 1997.[7]
In his blog on java.net, Sun veteran and GlassFish lead Jim Driscoll details the history of servlet technology.[8]James Gosling first thought of servlets in the early days of Java, but the concept did not become a product until December 1996 when Sun shipped JWS.[9][10][11] This was before what is now the Jakarta EE was made into a specification.
The Servlet1 specification was created by Pavni Diwanji[12][13] while she worked at Sun Microsystems, with version 1.0 finalized in June 1997. Starting with version 2.2, the specification was developed under the Java Community Process.
Servlet API version | Released | Specification | Platform | Important Changes |
---|---|---|---|---|
Jakarta Servlet 5.0.0 M1 | Jun 12, 2020 | 5.0 | Jakarta EE 9 | API moved from package javax.servlet to jakarta.servlet
|
Jakarta Servlet 4.0.3 | Aug 13, 2019 | 4.0 | Jakarta EE 8 | Renamed from "Java" trademark |
Java Servlet 4.0 | Sep 2017 | JSR 369 | Java EE 8 | HTTP/2 |
Java Servlet 3.1 | May 2013 | JSR 340 | Java EE 7 | Non-blocking I/O, HTTP protocol upgrade mechanism (WebSocket)[14] |
Java Servlet 3.0 | December 2009 | JSR 315 | Java EE 6, Java SE 6 | Pluggability, Ease of development, Async Servlet, Security, File Uploading |
Java Servlet 2.5 | September 2005 | JSR 154 | Java EE 5, Java SE 5 | Requires Java SE 5, supports annotation |
Java Servlet 2.4 | November 2003 | JSR 154 | J2EE 1.4, J2SE 1.3 | web.xml uses XML Schema |
Java Servlet 2.3 | August 2001 | JSR 53 | J2EE 1.3, J2SE 1.2 | Addition of Filter
|
Java Servlet 2.2 | August 1999 | JSR 902, JSR 903 | J2EE 1.2, J2SE 1.2 | Becomes part of J2EE, introduced independent web applications in .war files |
Java Servlet 2.1 | November 1998 | 2.1a | Unspecified | First official specification, added RequestDispatcher , ServletContext
|
Java Servlet 2.0 | December 1997 | N/A | JDK 1.1 | Part of April 1998 Java Servlet Development Kit 2.0[15] |
Java Servlet 1.0 | December 1996 | N/A | Part of June 1997 Java Servlet Development Kit (JSDK) 1.0[9] |
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.
init()
method, passing an object implementing the javax.servlet.ServletConfig
interface. This configuration object allows the servlet to access name-value initialization parameters from the web application.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.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.
init()
method of the servlet.
service()
method of the servlet.
service()
method is called for each HTTP request.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 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 javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class ServletLifeCycleExample extends HttpServlet {
private Integer sharedCounter;
@Override
public void init(final ServletConfig config) throws ServletException {
super.init(config);
getServletContext().log("init() called");
sharedCounter = 0;
}
@Override
protected void service(final HttpServletRequest request, final HttpServletResponse response) throws ServletException, IOException {
getServletContext().log("service() called");
int localCounter;
synchronized (sharedCounter) {
sharedCounter++;
localCounter = sharedCounter;
}
response.getWriter().write("Incrementing the count to " + localCounter); // accessing a local variable
}
@Override
public void destroy() {
getServletContext().log("destroy() called");
}
}
The specification for Servlet technology has been implemented in many products. See a list of implementations on the web container page.
We cover Version 2.0 of the Servlet API, which was introduced as part of the Java Web Server 1.1 in December 1997 and clarified by the release of the Java Servlet Development Kit 2.0 in April 1998.
By: Wikipedia.org
Edited: 2021-06-18 14:12:07
Source: Wikipedia.org