JSP Implicit Objects

JSP Implicit Objects

JSP Implicit Objects are the Java objects that the JSP Container makes available to developers in each page and developer can call them directly without declaring them explicitly. These objects are created by JSP Engine during translation phase (while translating JSP to Servlet). They are being created inside service method so the developer can directly use them within Scriptlet without initializing and declaring them.

There are total 8+1 implicit objects available in JSP.

The request Object

The request object is an instance of a javax.servlet.http.HttpServletRequest object. Each time a client requests a page the JSP engine creates a new object to represent that request.

The request object provides methods to get HTTP header information including form data, cookies, HTTP methods etc.

The response Object:

The response object is an instance of a javax.servlet.http.HttpServletResponse object. The server creates the response object to represent the response to the client.

The out Object:

The out object is an instance of a javax.servlet.jsp.JspWriter object and is used to send content in a response.

The JspWriter object contains most of the methods as the java.io.PrintWriter class.

The session Object:

The session object is an instance of javax.servlet.http.HttpSession and behaves exactly the same way that session objects behave under Java Servlets.

The session object is used to track client session between client requests.

The application Object:

The application object is an instance of a javax.servlet.ServletContext.

Application object is a representation of the JSP page through its entire lifecycle. This object is created when the JSP page is initialized and will be removed when the JSP page is removed by the jspDestroy() method.

The config Object:

The config object is an instantiation of javax.servlet.ServletConfig.

The config object allows the JSP programmer access to the Servlet or JSP engine initialization parameters such as the paths or file locations etc.

The pageContext Object:

The pageContext object is an instance of  javax.servlet.jsp.PageContext. The pageContext object is used to represent the entire JSP page.

This object stores references to the request and response objects for each request. The application, config, session, and out objects are derived by accessing attributes of pageContext object.

The page Object:

Page object is a reference to the current Servlet instance (Converted Servlet, generated during translation phase from a JSP page). We can simply use this in place of it.

The exception Object

Exception object is used in exception handling for displaying the error messages. This object is only available to the JSP pages, which has isErrorPage set to true.

JSP Implicit Objects

JSP Lifecycle

JSP Lifecycle

The lifecycle of a JSP is the entire process from its creation till the destruction which is similar to a servlet life cycle with an additional step, which translates the JSP into a Servlet.

Following are the JSP Lifecycle phases.

1.     Translation of JSP to Servlet code.

2.     Compilation of Servlet to bytecode.

3.     Loading Servlet class.

4.     Creating servlet instance.

5.     Initialization by calling jspInit() method

6.     Request Processing by calling _jspService() method

7.     Destroying by calling jspDestroy() method

JSP Lifecycle

JSP Translation and Compilation

When a user request for a JSP, the JSP engine first checks to see whether it needs to compile the page. If the page has never been compiled, or if the JSP has been modified since it was last compiled, the JSP engine does the following

1.   Parse the JSP

2.   Transforms the JSP to a Servlet

3.   Compile the Servlet (.java) file to a class file.

JSP Initialization

Before servicing any request the web container loads the JSP and invokes jspInit() method. Initialization is performed only once, just as with servlets init() method. If you need to perform any JSP-specific initialization, you need yo override the jspInit() method.

JSP Service

Once the JSP page has been loaded and initialized, the JSP engine invokes the _jspService() method in the JSP.

The _jspService() method takes an HttpServletRequest and an HttpServletResponse as its parameters.

The _jspService() method of a JSP is invoked once per request and is responsible for generating the response for that request.

JSP Destruction

The destruction phase of the JSP life cycle represents when a JSP is being removed from use by a container.

The jspDestroy() method is the JSP similar to the destroy method for servlets. We need to Override jspDestroy() method when we need to perform any cleanup operation, such as releasing database connections or closing files which were opened.

Difference Between Servlet and JSP

This post highlights the difference between JSP and Servlet technologies.

Java Servlet technology and JavaServer Pages (JSP pages) are server-side technologies that become the standard way to develop web applications.

Most importantly, if used effectively by following best practices, servlets and JSP pages help separate presentation from content.

Servlets support a request and response programming model. When a client sends a request to the server, the server sends the request to the servlet. The servlet then constructs a response that the server sends back to the client. When a client request is made, the service method is called and passed a request and response object. The servlet first determines whether the request is a GET or POST operation. It then calls one of the following methods: doGet or doPost. The doGet method is called if the request is GET, and doPost is called if the request is POST.

Adding more, Servlets are Java classes that can generate dynamic HTML content using print statements.

out.println("<html><head><title>"+thisIsMyMessage+"</title></head>");

So, you can embed HTML code into Java code.

A JSP page is basically a web page with traditional HTML and bits of Java code. The file extension of a JSP page is .jsp rather than .html or .htm, which tells the server that this page requires special handling that will be accomplished by a server extension. When a JSP page is called, it will be compiled (by the JSP engine) into a Java servlet.

Importantly, we can add dynamic content or Java Code inside an HTML Tag using JSP’s Scriplets.

And scriptlets, this is code fragments like:

<% String message = "Hello !"%>

<H2> Welcome and  <%=message%>! </H2>

So, you can embed Java code into HTML code.

SERVLET	JSP
A servlet is a server-side program and written purely on Java.	JSPs are HTML pages with .jsp extension. JSP’s are extension of servlets to minimize the effort of developers to write User Interfaces using Java programming.
Executes inside a Web server, such as Tomcat	A JSP program is compiled into a Java servlet before execution. Once compiled into a servlet, it's life cycle will be same as of servlet. But, JSP has it's own API for the lifecycle.
Servlets run faster than JSP	JSP runs slower because it has the transition phase for converting from JSP page to a Servlet file.
Servlet has the life cycle methods init(), service() and destroy()	JSP has the life cycle methods of jspInit(), _jspService() and jspDestroy()
Difficult to write as one has to write HTML tags within quotes(“<HTML>”) in Java. Mixing HTML content inside Java is tedious.	Easier to write than servlets as it is similar to HTML
Written in Java, with a few additional APIs specific to this kind of processing. Since it is written in Java, it follows all the Object Oriented programming techniques.	One of the key advantage is we can build custom tags using JSP API ,  which can be available as the re-usable components with lot of flexibility
In MVC architecture Servlet acts as controller.	In MVC architecture JSP acts as view.
Servlet advantages include: ·  Performance: get loaded upon first request and remains in memory indefinitely. ·  Simplicity: Run inside controlled server environment. ·  Session Management : overcomes HTTP's stateless nature ·  Java Technology : network access, Database connectivity, j2ee integration	JSP Provides an extensive infrastructure for: ·         Tracking sessions. ·         Managing cookies. ·         JSP is Extensible: Can create custom tags to extend the JSP functionality. ·         Separation of roles: Clearly identifies separation of roles for Developers, Content Authors/ Graphic Designers/ Web Masters.