SERVLET
Servlet technology is used to create a web application (resides at the server-side and generates a dynamic web page).
Servlet technology is robust and scalable because of java language. Before Servlet, CGI (Common Gateway Interface) scripting language was common as a server-side programming language. However, there were many disadvantages to this technology. We have discussed these disadvantages below.
There are many interfaces and classes in the Servlet API such as Servlet, GenericServlet, HttpServlet, ServletRequest, ServletResponse, etc.
What is a Servlet?
Servlet can be described in many ways, depending on the context.
- Servlet is a technology which is used to create a web application.
- Servlet is an API that provides many interfaces and classes including documentation.
- Servlet is an interface that must be implemented for creating any Servlet.
- Servlet is a class that extends the capabilities of the servers and responds to the incoming requests. It can respond to any requests.
- Servlet is a web component that is deployed on the server to create a dynamic web page.
What is a web application?
A web application is an application accessible from the web. A web application is composed of web components like Servlet, JSP, Filter, etc. and other elements such as HTML, CSS, and JavaScript. The web components typically execute in Web Server and respond to the HTTP request.
CGI (Common Gateway Interface)
CGI technology enables the web server to call an external program and pass HTTP request information to the external program to process the request. For each request, it starts a new process.
Disadvantages of CGI
There are many problems in CGI technology:
1. If the number of clients increases, it takes more time for sending the response.
2. For each request, it starts a process, and the web server is limited to start processes.
3. It uses platform dependent language e.g. C, C++, perl.
Advantages of Servlet
There are many advantages of Servlet over CGI.
The web container creates threads for handling the multiple requests to the Servlet. Threads have many benefits over the Processes such as they share a common memory area, lightweight, cost of communication between the threads are low. The advantages of Servlet are as follows:
1. Better performance: because it creates a thread for each request, not process.
2. Portability: because it uses Java language.
3. Robust: JVM manages Servlets, so we don't need to worry about the memory leak, garbage collection, etc.
4. Secure: because it uses java language.
Servlet API
The javax.servlet and javax.servlet.http packages represent interfaces and classes for servlet api.
The javax.servlet package contains many interfaces and classes that are used by the servlet or web container. These are not specific to any protocol.
The javax.servlet.http package contains interfaces and classes that are responsible for http requests only.
Interfaces in javax.servlet package
There are many interfaces in javax.servlet package. They are as follows:
1. Servlet
2. ServletRequest
3. ServletResponse
4. RequestDispatcher
5. ServletConfig
6. ServletContext
7. SingleThreadModel
8. Filter
9. FilterConfig
10. FilterChain
11. ServletRequestListener
12. ServletRequestAttributeListener
13. ServletContextListener
14. ServletContextAttributeListener
Classes in javax.servlet package
There are many classes in javax.servlet package. They are as follows:
1. GenericServlet
2. ServletInputStream
3. ServletOutputStream
4. ServletRequestWrapper
5. ServletResponseWrapper
6. ServletRequestEvent
7. ServletContextEvent
8. ServletRequestAttributeEvent
9. ServletContextAttributeEvent
10. ServletException
11. UnavailableException
Interfaces in javax.servlet.http package
There are many interfaces in javax.servlet.http package. They are as follows:
1. HttpServletRequest
2. HttpServletResponse
3. HttpSession
4. HttpSessionListener
5. HttpSessionAttributeListener
6. HttpSessionBindingListener
7. HttpSessionActivationListener
8. HttpSessionContext (deprecated now)
Classes in javax.servlet.http package
There are many classes in javax.servlet.http package. They are as follows:
1. HttpServlet
2. Cookie
3. HttpServletRequestWrapper
4. HttpServletResponseWrapper
5. HttpSessionEvent
6. HttpSessionBindingEvent
7. HttpUtils (deprecated now)
Servlet Interface
Servlet interface provides common behavior to all the servlets.Servlet interface defines methods that all servlets must implement.
Servlet interface needs to be implemented for creating any servlet (either directly or indirectly). It provides 3 life cycle methods that are used to initialize the servlet, to service the requests, and to destroy the servlet and 2 non-life cycle methods.
Methods of Servlet interface
There are 5 methods in Servlet interface. The init, service and destroy are the life cycle methods of servlet. These are invoked by the web container.
Method | Description |
public void init(ServletConfig config) | initializes the servlet. It is the life cycle method of servlet and invoked by the web container only once. |
public void service(ServletRequest request,ServletResponse response) | provides response for the incoming request. It is invoked at each request by the web container. |
public void destroy() | is invoked only once and indicates that servlet is being destroyed. |
public ServletConfig getServletConfig() | returns the object of ServletConfig. |
public String getServletInfo() | returns information about servlet such as writer, copyright, version etc. |
Servlet Example by implementing Servlet interface
import java.io.*; import javax.servlet.*;
public class First implements Servlet{
ServletConfig config=null;
public void init(ServletConfig config){ this.config=config;
System.out.println("servlet is initialized");
}
public void service(ServletRequest req,ServletResponse res) throws IOException,ServletException{
res.setContentType("text/html");
PrintWriter out=res.getWriter(); out.print("<html><body>"); out.print("<b>hello simple servlet</b>"); out.print("</body></html>");
}
public void destroy(){System.out.println("servlet is destroyed");} public ServletConfig getServletConfig(){return config;} public String getServletInfo(){return "copyright 2007-1010";}
}
GenericServlet class
GenericServlet class implements Servlet, ServletConfig and Serializableinterfaces. It provides the implementation of all the methods of these interfaces except the service method.
GenericServlet class can handle any type of request so it is protocol-independent.
You may create a generic servlet by inheriting the GenericServlet class and providing the implementation of the service method.
Methods of GenericServlet class
There are many methods in GenericServlet class. They are as follows:
1. public void init(ServletConfig config) is used to initialize the servlet.
2. public abstract void service(ServletRequest request, ServletResponse response) provides service for the incoming request. It is invoked at each time when user requests for a servlet.
3. public void destroy() is invoked only once throughout the life cycle and indicates that servlet is being destroyed.
4. public ServletConfig getServletConfig() returns the object of ServletConfig.
5. public String getServletInfo() returns information about servlet such as writer, copyright, version etc.
6. public void init() it is a convenient method for the servlet programmers, now there is no need to call super.init(config)
7. public ServletContext getServletContext() returns the object of ServletContext.
8. public String getInitParameter(String name) returns the parameter value for the given parameter name.
9. public Enumeration getInitParameterNames() returns all the parameters defined in the web.xml file.
10. public String getServletName() returns the name of the servlet object.
11. public void log(String msg) writes the given message in the servlet log file.
12. public void log(String msg,Throwable t) writes the explanatory message in the servlet log file and a stack trace.
import java.io.*; import javax.servlet.*;
public class First extends GenericServlet{ public void service(ServletRequest req,ServletResponse res) throws IOException,ServletException{ res.setContentType("text/html");
PrintWriter out=res.getWriter(); out.print("<html><body>"); out.print("<b>hello generic servlet</b>"); out.print("</body></html>");
}
}
HttpServlet class
The HttpServlet class extends the GenericServlet class and implements Serializable interface. It provides methods such as doGet, doPost, doHead, doTrace etc.
Methods of HttpServlet class
There are many methods in HttpServlet class. They are as follows:
1. public void service(ServletRequest req,ServletResponse res) dispatches the request to the protected service method by converting the request and response object into http type.
2. protected void service(HttpServletRequest req, HttpServletResponse res) receives the request from the service method, and dispatches the request to the doXXX() method depending on the incoming http request type.
3. protected void doGet(HttpServletRequest req, HttpServletResponse res) handles the GET request. It is invoked by the web container.
4. protected void doPost(HttpServletRequest req, HttpServletResponse res) handles the POST request. It is invoked by the web container.
5. protected void doHead(HttpServletRequest req, HttpServletResponse res) handles the HEAD request. It is invoked by the web container.
6. protected void doOptions(HttpServletRequest req, HttpServletResponse res) handles the OPTIONS request. It is invoked by the web container.
7. protected void doPut(HttpServletRequest req, HttpServletResponse res) handles the PUT request. It is invoked by the web container.
8. protected void doTrace(HttpServletRequest req, HttpServletResponse res) handles the TRACE request. It is invoked by the web container.
9. protected void doDelete(HttpServletRequest req, HttpServletResponse res) handles the DELETE request. It is invoked by the web container.
10. protected long getLastModified(HttpServletRequest req) returns the time when HttpServletRequest was last modified since midnight January 1, 1970 GMT.
Life Cycle of a Servlet (Servlet Life Cycle)
The web container maintains the life cycle of a servlet instance. Let's see the life cycle of the servlet:
1. Servlet class is loaded.
2. Servlet instance is created.
3. init method is invoked.
4. service method is invoked.
5. destroy method is invoked.
1) Servlet class is loaded
The classloader is responsible to load the servlet class. The servlet class is loaded when the first request for the servlet is received by the web container.
2) Servlet instance is created
The web container creates the instance of a servlet after loading the servlet class. The servlet instance is created only once in the servlet life cycle.
3) init method is invoked
The web container calls the init method only once after creating the servlet instance. The init method is servlet. It is the life cycle method of the javax.servlet.Servlet interface. Syntax of the init method is give
1. public void init(ServletConfig config) throws ServletException
4) service method is invoked
The web container calls the service method each time when request for the servlet is received. If servlet is not initialized, it follows the first three steps as described above then calls the service method. If servlet is initialized, it calls the service method. Notice that servlet is initialized only once. The syntax of the service method of the Servlet interface is given below:
1. public void service(ServletRequest request, ServletResponse response)
2. throws ServletException, IOException
5) destroy method is invoked
The web container calls the destroy method before removing the servlet instance from the service. It gives the servlet an opportunity to clean up any resource for example memory, thread etc. The syntax of the destroy method of the Servlet interface is given below:
1. public void destroy()
Steps to create a servlet example
There are given 6 steps to create a servlet example. These steps are required for all the servers.
The servlet example can be created by three ways:
1. By implementing Servlet interface,
2. By inheriting GenericServlet class, (or)
3. By inheriting HttpServlet class
The mostly used approach is by extending HttpServlet because it provides http request specific method such as doGet(), doPost(), doHead() etc.
Here, we are going to use apache tomcat server in this example. The steps are as follows:
1. Create a directory structure
2. Create a Servlet
3. Compile the Servlet
4. Create a deployment descriptor
5. Start the server and deploy the project
6. Access the servlet
1)Create a directory structures
The directory structure defines that where to put the different types of files so that web container may get the information and respond to the client.
The Sun Microsystem defines a unique standard to be followed by all the server vendors.
Let's see the directory structure that must be followed to create the servlet.
2)Create a Servlet
There are three ways to create the servlet.
1. By implementing the Servlet interface
2. By inheriting the GenericServlet class
3. By inheriting the HttpServlet class
The HttpServlet class is widely used to create the servlet because it provides methods to handle http req doGet(), doPost, doHead() etc.
In this example we are going to create a servlet that extends the HttpServlet class. In this example, we
HttpServlet class and providing the implementation of the doGet() method. Notice that get request is the
DemoServlet.java
1. import javax.servlet.http.*;
2. import javax.servlet.*;
3. import java.io.*;
4. public class DemoServlet extends HttpServlet{
5. public void doGet(HttpServletRequest req,HttpServletResponse res)
6. throws ServletException,IOException
7. {
8. res.setContentType("text/html");//setting the content type 9. PrintWriter pw=res.getWriter();//get the stream to write the data
10.
11. //writing html in the stream
12. pw.println("<html><body>");
13. pw.println("Welcome to servlet");
14. pw.println("</body></html>");
15.
16. pw.close();//closing the stream
17. }}
3)Compile the servlet
For compiling the Servlet, jar file is required to be loaded. Different Servers provide different jar files:
Two ways to load the jar file
1. set classpath
2. paste the jar file in JRE/lib/ext folder
Put the java file in any folder. After compiling the java file, paste the class file of servlet in WEB-INF/classes directory.
4)Create the deployment descriptor (web.xml file)
Jar file | Server |
1) servlet-api.jar | Apache Tomcat |
2) weblogic.jar | Weblogic |
3) javaee.jar | Glassfish |
4) javaee.jar | JBoss |
The deployment descriptor is an xml file, from which Web Container gets the information about the servet to be invoked.
The web container uses the Parser to get the information from the web.xml file. There are many xml parsers such as SAX, DOM and Pull.
There are many elements in the web.xml file. Here is given some necessary elements to run the simple servlet program.
web.xml file
1. <web-app>
2.
3. <servlet>
4. <servlet-name>sonoojaiswal</servlet-name>
5. <servlet-class>DemoServlet</servlet-class>
6. </servlet>
7.
8. <servlet-mapping>
9. <servlet-name>sonoojaiswal</servlet-name>
10. <url-pattern>/welcome</url-pattern>
11. </servlet-mapping>
12.
13. </web-app>
Description of the elements of web.xml file
There are too many elements in the web.xml file. Here is the illustration of some elements that is used in the above web.xml file. The elements are as follows:
<web-app> represents the whole application.
<servlet> is sub element of <web-app> and represents the servlet.
<servlet-name> is sub element of <servlet> represents the name of the servlet.
<servlet-class> is sub element of <servlet> represents the class of the servlet.
<servlet-mapping> is sub element of <web-app>. It is used to map the servlet.
<url-pattern> is sub element of <servlet-mapping>. This pattern is used at client side to invoke the se
5)Start the Server and deploy the project
To start Apache Tomcat server, double click on the startup.bat file under apache-tomcat/bin directory.
One Time Configuration for Apache Tomcat Server
You need to perform 2 tasks:
1. set JAVA_HOME or JRE_HOME in environment variable (It is required to start server).
2. Change the port number of tomcat (optional). It is required if another server is running on same port (8080).
How Servlet works?
It is important to learn how servlet works for understanding the servlet well. Here, we are going to get the internal detail about the first servlet program.
The server checks if the servlet is requested for the first time.
If yes, web container does the following tasks:
o loads the servlet class. o instantiates the servlet class. o calls the init method passing the ServletConfig object else o calls the service method passing request and response objects
The web container calls the destroy method when it needs to remove the servlet such as at time of stopping server or undeploying the project.
How web container handles the servlet request?
o maps the request with the servlet in the web.xml file. o creates request and response objects for this request o calls the service method on the thread
o The public service method internally calls the protected service method o The protected service method calls the doGet method depending on the type of request.
o The doGet method generates the response and it is passed to the client.
o After sending the response, the web container deletes the request and response objects. The thread is contained in the thread pool or deleted depends on the server implementation.
What is written inside the public service method?
The public service method converts the ServletRequest object into the HttpServletRequest type and ServletResponse object into the HttpServletResponse type. Then, calls the service method passing these objects. Let's see the internal code:
public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException
{
HttpServletRequest request; HttpServletResponse response; try
{
request = (HttpServletRequest)req; response = (HttpServletResponse)res;
}
catch(ClassCastException e)
{
throw new ServletException("non-HTTP request or response");
}
service(request, response);
}
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