Life Cycle of a JSP Page

JSP stands for JAVA Server Pages. It is JAVA Code inside HTML. It is useful technology for presentation layer or View in Model-View-Controller architecture.

Using JSP, developer can generate dynamic content on web pages. Major advantage of using JSP is that it separates business logic from presentation. Business logic can be implemented in JAVA and dynamic content is sent to JSP to display it on browser.

Life Cycle of JSP:

The lifecycle of JSP is controlled by three methods which are automatically called when a JSP is requested and when the JSP terminates normally.

These are:
jspInit () , _jspService() , jspDestroy().

jspInit() method is identical to the init() method in a Java Servlet and in applet.

It is called first when the JSP is requested and is used to initialize objects and variables that are used throughout the life of the JSP.

_jspService() method is automatically called and retrieves a connection to HTTP.
It will call doGet or doPost() method of servlet created.

jspDestroy() method is identical to the destroy() method in Servlet.
The destroy() method is automatically called when the JSP terminates normally.

A JSP life cycle can be defined as the entire process from its creation till the destruction which is similar to a servlet life cycle with an additional step which is required to compile a JSP into servlet.

The following are the paths followed by a JSP

1. Compilation
2. Initialization
3. Execution
4. Cleanup

JSP Compilation:
When a browser asks 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 compiles the page.

The compilation process involves three steps:

1. Parsing the JSP.
2. Turning the JSP into a servlet.
3. Compiling the servlet.

JSP Initialization:
When a container loads a JSP it invokes the jspInit() method before servicing any requests. If you need to perform JSP-specific initialization, override the jspInit() method:

public void jspInit(){
  // Initialization code...
}

Typically initialization is performed only once and as with the servlet init method, you generally initialize database connections, open files, and create lookup tables in the jspInit method.

JSP Execution:
This phase of the JSP life cycle represents all interactions with requests until the JSP is destroyed.

Whenever a browser requests a JSP and the 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 as follows:

void _jspService(HttpServletRequest request, 
                 HttpServletResponse response)
{
   // Service handling code...
}

The _jspService() method of a JSP is invoked once per a request and is responsible for generating the response for that request and this method is also responsible for generating responses to all seven of the HTTP methods ie. GET, POST, DELETE etc.

JSP Cleanup:
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 equivalent of the destroy method for servlets. Override jspDestroy when you need to perform any cleanup, such as releasing database connections or closing open files.

The jspDestroy() method has the following form:

public void jspDestroy()
{
   // Your cleanup code goes here.
}