From Wikipedia, the free encyclopedia
A servlet is a Java programming language class used to extend the capabilities ofservers that can be accessed by a host application via a request-responseprogramming model. Although servlets can respond to any type of request, they are commonly used to extend the applications hosted by Web servers. Thus, it can be thought of as a Java Applet that runs on a server instead of a browser.
A Servlet is a Java-based server-side web technology. As the name implies, it serves a client request and receives a response from the server. Technically speaking, aServlet is a Java class in Java EE that conforms to the Java Servlet API, a protocol by which a Java class may respond to requests. They are not tied to a specific client-server protocol, but are most often used with the HTTP protocol. Therefore, the word "Servlet" is often used in the meaning of "HTTP Servlet". Thus, a software developer may use a servlet to adddynamic content to a Web server using the Java platform. The generated content is commonly HTML, but may be other data such as XML. Servlets are the Java counterpart to non-Java dynamic Web content technologies such as PHP and ASP.NET. Servlets can maintain state in session variables across many server transactions by using HTTP cookies, or URL rewriting.
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.
Servletis 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.httpdefines 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 JavaServer Pages (JSP) by the JavaServer 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 Java 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 pattern.
The complete servlet specification was created by Sun Microsystems, with version 1.0 finalized in June 1997. Starting with version 2.3, the servlet specification was developed under the Java Community Process. JSR 53 defined both the Servlet 2.3 and JavaServer Page 1.2 specifications. JSR 154 specifies the Servlet 2.4 and 2.5 specifications. As of March 26, 2010, the current version of the servlet specification is 3.0.
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, but the concept did not become a product until Sun shipped the Java Web Server product. This was before what is now the Java Platform, Enterprise Edition was made into a specification.
|Servlet API version||Released||Platform||Important Changes|
|Servlet 3.0||December 2009||JavaEE 6, JavaSE 6||Pluggability, Ease of development, Async Servlet, Security, File Uploading|
|Servlet 2.5||September 2005||JavaEE 5, JavaSE 5||Requires JavaSE 5, supports annotation|
|Servlet 2.4||November 2003||J2EE 1.4, J2SE 1.3||web.xml uses XML Schema|
|Servlet 2.3||August 2001||J2EE 1.3, J2SE 1.2||Addition of |
|Servlet 2.2||August 1999||J2EE 1.2, J2SE 1.2||Becomes part of J2EE, introduced independent web applications in .war files|
|Servlet 2.1||November 1998||Unspecified||First official specification, added |
|Servlet 2.0||JDK 1.1||Part of Java Servlet Development Kit 2.0|
|Servlet 1.0||June 1997|
Advantages over CGI
The advantages of using servlets are their fast performance and ease of use combined with more power over traditional CGI (Common Gateway Interface). Traditional CGI scripts written in Java have a number of disadvantages when it comes to performance:
- When an HTTP request is made, a new process is created for each call of the CGI script. This overhead of process creation can be very system-intensive, especially when the script does relatively fast operations. Thus, process creation will take more time than CGI script execution. Java servlets solve this, as a servlet is not a separate process. Each request to be handled by a servlet is handled by a separate Java thread within the Web server process, omitting separate process forking by the HTTP daemon.
- Simultaneous CGI request causes the CGI script to be copied and loaded into memory as many times as there are requests. However, with servlets, there are the same amount of threads as requests, but there will only be one copy of the servlet class created in memory that stays there also between requests.
- Only a single instance answers all requests concurrently. This reduces memory usage and makes the management of persistent data easy.
- A servlet can be run by a servlet engine in a restrictive environment, called a sandbox. This is similar to an applet that runs in the sandbox of the Web browser. This makes a restrictive use of potentially harmful servlets possible.
Life cycle of a servlet
- During initialization stage of the Servlet life cycle, the web container initializes the servlet instance by calling the init() method. The container passes an object implementing the ServletConfiginterface via the init() method. This configuration object allows the servlet to access name-value initialization parameters from the web application.
- After initialization, the servlet 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 for a method that is not implemented by the servlet is made, 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
init(), is called only once in the lifecycle 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. Let us consider a typical user scenario to understand when these methods are called.
- Assume that a user enters a Uniform Resource Locator (URL) to a web browser.
- 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.
- The server maps this request to a particular servlet.
- The servlet is dynamically retrieved and loaded into the address space of the server.
- The server 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 it may configure itself.
- The server invokes the
service()method of the servlet.
- This method is called to process the HTTP request.
- You will see that it is possible for the servlet to read data that has been provided in the HTTP request.
- It may also formulate an HTTP response for the client.
- The servlet remains in the server’s address space and is available to process any other HTTP requests received from clients.
service()method is called for each HTTP request.
- The server may, at some point, decide to unload the servlet from its memory.
- The algorithms by which this decision is made are specific to each server.
- The server calls the
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.
The following example servlet prints how many times its
service()method was called.
HttpServletis a subclass of
GenericServlet, an implementation of the
HttpServletclass dispatches requests to the methods
doDelete(), and so on; according to the HTTP request. In the example below method
service()is overridden and does not distinguish which HTTP request method it serves.
Servlets are most often used to
- process or store data that was submitted from an HTML form
- provide dynamic content such as the results of a database query
- manage state information that does not exist in the stateless HTTP protocol, such as filling the articles into the shopping cart of the appropriate customer.
- "servlet". http://www.webopedia.com/: WEBOPEDIA. Retrieved 2011-04-27. "A small java program that runs on a server. The term usually refers to a Java applet that runs within a Web server environment. This is analogous to a Java applet that runs within a Web browser environment."
-  1.1 What is a servlet?
- Official servlets tutorials from Sun/Oracle
- Beginning and intermediate servlet and JSP tutorials (with source code)
- Oracle's servlet technology overview
- Servlet specifications download page
- New features added to Servlet 3.0
- New features added to Servlet 2.5 at JavaWorld