My Code Snapshots

When first time a JSP page is request necessary servlet code is generated and loaded in the servlet container. Now until the JSP page is not changed the compiled servlet code serves any request which comes from the browser. When you again change the JSP page the JSP engine again compiles a servlet code for the same.

JSP page is first initialized by jspInit() method. This initializes the JSP in much the same way as servlets are initialized, when the first request is intercepted and just after translation.

Every time a request comes to the JSP, the container generated _jspService() method is invoked, the request is processed, and response generated.

When the JSP is destroyed by the server, the jspDestroy() method is called and this can be used for clean up purposes.

EL stands for expression language. An expression language makes it possible to easily access application data.In the below expression amountofwine variable value will be rendered. There are ${amount} litres of milk in the bottle.

EL parser searches the attribute in following order:
Page
Request
Session (if it exists)
Application
If no match is found for then it displays empty string.

Following are the implicit EL objects:-
PageContext: The context for the JSP page.
Provides access to various objects for instance:-
ervletContext: The context for the JSP page's servlet and any web components contained n the same application.
ession: The session object for the client.
equest: The request triggering the execution of the JSP page.
esponse: The response returned by the JSP page. See Constructing Responses.
n addition, several implicit objects are available that allow easy access to the following objects:
param: Maps a request parameter name to a single value
paramValues: Maps a request parameter name to an array of values
header: Maps a request header name to a single value
headerValues: Maps a request header name to an array of values
cookie: Maps a cookie name to a single cookie

initParam: Maps a context initialization parameter name to a single value
Finally, there are objects that allow access to the various scoped variables described in Using Scope Objects.
pageScope: Maps page-scoped variable names to their values
requestScope: Maps request-scoped variable names to their values
sessionScope: Maps session-scoped variable names to their values
applicationScope: Maps application-scoped variable names to their values
Browser: ${header["user-agent"]}

We can disable using isELIgnored attribute of the page directive:
<%@ page isELIgnored ="true|false" %> .

JSTL is also called as JSP tag libraries. They are collection of custom actions which can be accessed as JSP tags.

Tags are classified in to four groups:-
Core tags
Formatting tags
XML tags
SQL tags

JSP provides three tags to work with beans:-
< jsp:useBean id=“bean name” class=“bean class” scope = “page | request | session |application ”/>

Bean name = the name that refers to the bean. Bean class = name of the java class that defines the bean.
< jsp:setProperty name = “id” property = “someProperty” value = “someValue” / > id = the name of the bean as specified in the useBean tag. property = name of the property to be passed to the bean. value = value of that particular property .
< jsp:getProperty name = “id” property = “someProperty” />

Here the property is the name of the property whose value is to be obtained from the bean.Below is a code snippet which shows how MyUserClass is used and the values accessed.
< jsp:useBean id="user" class="MyUserClass" scope="session"/>
< HTML>
< BODY>
You entered< BR>
Name: <%= user.getUsername() %>< BR>
Email: <%= user.getEmail() %>< BR>

It includes the output of one JSP in to other JSP file at the location of the tag. Below is the syntax for the same:-
< jsp:include page="..some.url.." flush="true or false"/>-
page: A URL that is relative to the current JSP page at request time-
flush: Determines if the buffer for the output is flushed immediately, before the included page's output. 

It forwards the current request to another JSP page. Below is the syntax for the same:-
< jsp:forward page="...url..." />
We can also forward parameter to the other page using the param tag
< jsp:forward page="..url...">
< jsp:param ..../>

JSP directives do not produce any output. They are used to set global values like class declaration, content type etc. Directives have scope for entire JSP file. They start with <%@ and ends with %>. There are three main directives that can be used in JSP:-
page directive
include directive
taglib directive

WE can prevent pages from caching JSP pages output using the below code snippet. <%response.setHeader("Cache-Control","no-cache"); //HTTP 1.1 response.setHeader("Pragma","no-cache"); //HTTP 1.0 response.setDateHeader ("Expires", 0); //prevents caching at the proxy server %>

OSCache is an open-source caching library that's available free of charge from the OpenSymphony organization . OSCache has a set of JSP tags that make it easy to implement page caching in your JSP application.
Following are some Cache techniques it fulfills:-
Cache entry
An object that's stored into a page cache is known as a cache entry. In a JSP application, a cache entry is typically the output of a JSP page, a portion of a JSP page, or a servlet.

Cache key
A page cache is like a hash table. When you save a cache entry in a page cache, you must provide a cache key to identify the cache data. You can use keys like URI, other parameters like username, ipaddress to indentify cache data.
Cache duration
This is the period of time that a cache entry will remain in a page cache before it expires. When a cache entry expires, it's removed from the cache and will be regenerated again.

Cache scope
This defines at what scope the data is stored application or session scope. <%= new java.util.Date().toString() %> 
The above tag says that refresh after every 60 seconds the user requests data. So if user1 s requesting the page it will display fresh date and if an other user requests with in 60 seconds it will show same data. If any other user requests the page after 60 second he will again see refreshed date.

Page directive is used to define page attributes the JSP file. Below is a sample of the same:- <% @ page language="Java" import="java.rmi.*,java.util.*" session="true" buffer="12kb" autoFlush="true" errorPage="error.jsp" %>
To summarize some of the important page attributes:-
import :- Comma separated list of packages or classes, just like import statements in usual Java code.
session :- Specifies whether this page can use HTTP session. If set "true" session (which refers to the javax.servlet.http.HttpSession) is available and can be used to access the current/new session for the page. If "false", the page does not participate in a session and the implicit session object is unavailable.
buffer :- If a buffer size is specified (such as "50kb") then output is buffered with a buffer size not less than that value.
isThreadSafe :- Defines the level of thread safety implemented in the page. If set "true" the JSP engine may send multiple client requests to the page at the same time. If "false" then the JSP engine queues up client requests sent to the page for processing, and processes them one request at a time, in the order they were received. This is the same as implementing the javax.servlet.SingleThreadModel interface in a servlet.
rrorPage: - Defines a URL to another JSP page, which is invoked if an unchecked runtime exception is thrown. The page implementation catches the instance of the Throwable object and passes it to the error page processing.

JSP engines will always instantiate a new tag handler instance every time a tag is encountered in a JSP page. A pool of tag instances are maintained and reusing them where possible. When a tag is encountered, the JSP engine will try to find a Tag instance that is not being used and use the same and then release it.

JavaBeans and taglib fundamentals were introduced for reusability. But following are the major differences between them:-
Taglib are for generating presentation elements while JavaBeans are good for storing information and state.
Use custom tags to implement actions and JavaBeans to present information.

Page Scope
Objects with page scope are accessible only within the page. Data only is valid for the current response. Once the response is sent back to the browser then data is no more valid. Even if request is passed from one page to other the data is lost. Request Scope
Objects with request scope are accessible from pages processing the same request in which they were created. Once the container has processed the request data is invalid. Even if the request is forwarded to another page, the data is still available though not if a redirect is required. Session Scope
Objects with session scope are accessible in same session. Session is the time users spend using the application, which ends when they close their browser or when they go to another Web site. So, for example, when users log in, their username could be stored in the session and displayed on every page they access. This data lasts until they leave the Web site or log out. Application Scope
Application scope objects are basically global object and accessible to all JSP pages which lie in the same application. This creates a global object that's available to all pages. Application scope variables are typically created and populated when an application starts and then used as read-only for the rest of the application.

Below are different implicit objects of JSP
pageContext :- The PageContext object.
pageScope :- A Map of all the objects that have page scope.
requestScope :- A Map of all the objects that have request scope.
sessionScope :- A Map of all the objects that have session scope.
applicationScope :- A Map of all the objects that have application scope.
param :- A Map of all the form parameters that were passed to your JSP page (for example, the HTML < input name="someName" type="text"/> is passed to your JSP page as a form parameter).
paramValues :- HTML allows for multiple values for a single form parameter. This is a Map of all the parameters, just like param, but in this object the values are an array containing all of the values for a given parameter in the event that there's more than one. header :- A Map of all the request headers.
headerValues :- For the same reasons as paramValues, a headerValues object is provided.
cookie :- A Map of all the cookies passed to your JSP. The value returned is a Cookie object.
initParam :- A Map that maps context initialization parameter names to their parameter values.