JSP - Life Cycle

The key to understanding the low-level functionality of JSP is to understand the simple life cycle they follow.
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

• Compilation
• Initialization
• Execution
• Cleanup

The four major phases of JSP life cycle are very similar to Servlet Life Cycle and they are as follows:

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:

• Parsing the JSP.
• Turning the JSP into a servlet.
• 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.
}