Jsp Expression:

• Jsp Expression is one of the three scripting elements.
• it is used to embed java code into the js.
• A jsp expression starts with "<%=" and ends with "%>".
• <%=a+b%>
• within a Jsp expression only one java expression that too without semicolon is placed.
• When Jsp expression is evaluated two thing will happen in the backend
  
  1. Java expression is evaluated
  2. The result is sent to the browser stream.
•  we can have any number of jsp expressions in ajsp.
• During translation phase each java expression of a jsp expression is placed in into _jspService()
  method of page implemented class

Jsp Scriplet:

• Jsp scriplet is one of the three scripting elements.
• it is used to embed java code into the js.
• A jsp scriplet starts with "<%" and ends with "%>".
• <% java code%>
• To serve the client request what ever java code required that is placed in servlet.
• In servlet programming whatever code we write in the service() method of user defined servlet all that code can be placed in a jsp scriplet.
• During the translation phase java code placed in the scriplet is written in to _jspService() method of page implementation class.
• in jsp we can have any number of scriplets.

• We can deploy multiple web applications in a servlet container.
• Each web application contains its own resources in a separate  environment. This environment called ad Web Application context or ServletContext.
• The resources belongs to the one web application context are not available to the other web application context.
• A Servlet context contains zero or more number of servlets. For every servlet object the container creates separate servletConfig object.
• ServletContext is an interface. this interface implemented by the container by the container provider.
• The implemented class allows all servlets in the web application to communicate with the container.
• We can store common data into this application and also we can share that data even after request response objects deletion in the memory.

 Configure Context parameters in web.xml file

Methods of ServletContext interface:

1. public String getInitParameter(String name): Returns a String containing a value of the method context-wide initialization  parameter, or null if if parameter does not exist.
2. public Enumeration getInitParameterNames():Returns the names of the context's initialization parameters as enumeration of string objects , or empty enumeration if context has no initialization parameters.
3. public void setAttribute(String name,Object object):sets the given object in the application scope.
4. public Object getAttribute(String name):Returns the servlet container attribute with the given name , or null if there is no attribute by that name.
5. public Enumeration getInitParameterNames():Returns the names of the context's initialization parameters as an Enumeration of String objects.
6. public void removeAttribute(String name):Removes the attribute with the given name from the servlet context.
7. public int getMajorVersion(): Returns the major version of the Java Servlet API that is servlet container supports.
8. public int getMinorVersion(): Returns the minor version of the Java Servlet API that is servlet container supports.
9. public void log(Java.lang.String msg): Writes specified message to a servlet log file, usually an event log.

• Request parameters are used by the servlet  to receive data from client to process the request.
• Some specific data must be supplied to the servlet at the time of initialization of the servlet and this data is specific to the client.But data is not supplied by the client via request parameters
• Use initialization parameters to supply the data to the servlet at the time of initialization of the servlet. initialization parameters are set in deployment descriptor   (Web.xml).
• Servlet can access these parameters during the run time
• Request parameters change form Request to request 
• Initialization parameters change from servlet to servlet.

Configure  Initialization parameters in Web.xml file:

ServletConfig Methods :

1. public String getInitParameter(String name): Returns a String containing value of the named parameter, or null if the parameter does not exist.
2. public Enumeration getInitParameterNames(): Returns names of the servlets initialization parameters as an enumeration of String objects, or an empty enumeration if servelt has no initialization
3. public String getServletName():Returns the name of the servlet.
4. public ServletContext getServletContext():Returns an object of ServletContext.

 

How to get Initialization parameters:

• To access initialization parameters of a servlet we use servelt config object.
• ServletConfig is an interface defined by javax.servlet package and this interface is implemented by container.
• ServletConfig is the object of the class that implements ServletConfig interface and this object is created by servlet conatainer with servlet init() method parameters.
• For each servlet One ServletConfig object is created by the container.

        ServletConfig sc=getServletConfig();
        String s=sc.getInitParameter("email");
     

To get all values:

       ServletConfig config=getServletConfig(); 
       Enumeration<String> e=config.getInitParameterNames(); 
       String names=""; 
       while(e.hasMoreElements()){ 
       str=e.nextElement(); 
       out.print("<br>Name: "+str); 
       out.print(" value: "+config.getInitParameter(names)); 
    }

• One servlet delegating request processing duty to other servlet is known as request dispatching.
• To implement inter-servlet communication and servlet -jsp communication we need request dispatching.
• When servlet receives a simple client request it need to communicate with any other servlet. if the incoming client request is complex one.
• A servlet process some portion of the request and remaining portion it need to communicate with other servlet known as inter-servlet communication.

Implementing Request dispatching:

1. public void include(ServletRequest request,ServletResponse response)throws ServletException,java.io.IOException:Includes the content of a resource (servlet, JSP page, or HTML file) in the response.
2. public void forward(ServletRequest request,ServletResponse response)throws ServletException,java.io.IOException:Forwards a request from a servlet to another resource (servlet, JSP file, or HTML file) on the server.

By calling any of the above two methods request dispatching is implemented

           rd.forward(req, res) or rd.include(req, res)

forward();

• In case of forward mechanism of  request dispatching one servlet receives the control , performs a portion of processing and switches the control to another servlet for the remaining portion of the processing
•  That second servlet performs remaining portion of the request processing , produces the response page and handle over to the same web server.

• In case of include mechanism of  request dispatching one servlet receives the control , performs a portion of processing and switches the control to another servlet for the remaining portion of the processing. gets control back from the next servlet and produces the dynamic web page on the web server.

Initialization:

• Servlet engine creates servletConfig object
• Servlet engine calls the init() method on the servlet instance by supplying servletconfig object
  as argument.
• Once init method completely executed servlet is ready to serve the client request.

Servicing : 

• Servlet engine creates servlet request and servlet response object based on the web server
  provided client information.
• Servlet engine calls service method on the servlet instance by supplying two object reference as arguments
• Once service method is completely executed , client request is served and request-response cycle is complete.
• Within the service method request object is used to capture the user input
• Response object is used to build the dynamic page and hand over the same to the web server

Exceptions:

• ServletException
• UnavilableException

 Interfaces present in javax.servlet.http:  

• HttpServletRequest 
• HttpServletResponse
• HttpSession
• HttpSessionBindingListener
• HttpSessionContext
• HttpSessionListener
• HttpSessionActivationListener
• HttpSessionAttributeListener

Classes present in javax.servlet.http:

• Cookie
• HttpServlet
• HttpServletRequestWrapper
• HttpServletResponseWrapper
• HttpSessionBindingEvent
• HttpSessionEvent
• HttpUtils