JSP Life Cycle
In this section we will discuss about life cycle of JSP.
Like the each Java based web technology JSP also follows a life cycle for the execution of an application. Life cycle of any technology expresses that the execution of any application should be passed through the specified phases. In the core of the JSP, Java Servlet technology is executed therefore, JSP life cycle is likely to Java Servlet technology. A Servlet container controls the following processes when a request is mapped to a Servlet :
- Loading of Servlet class.
- Instantiation of Servlet class.
- Calling of init() method to initialize a Servlet instance.
- Calling of service method by passing the request and response objects.
But, what for a request mapped to a JSP page. It is quite different of request mapping to a Servlet and request mapping to a JSP page. As we have discussed above a Servlet container controls the processes when a request is mapped to a Servlet but, when a request is mapped to a JSP page a special Servlet controls as, at first it checks the version of Servlet and JSP that whether the Servlet of JSP page is older than the JSP page or not. If it is older then the JSP page is translated into the Servlet class and then it is compiled. In JSP build process is performed automatically whereas, in Servlet it is not performed automatically. Life Cycle of JSP helps in understand its low-level functionality. JSP life cycle explains the process from its creation to destruction. As we have discussed already, the life cycle of JSP is likely to Servlet life cycle are described as follows :
- Translation of JSP page : This is the first step of JSP life cycle. This process is started when a request is mapped to a JSP page and and special Servlet checks whether the Servlet of JSP page is older than or not. If it is older then it translates the JSP page into Servlet class. In this phase container manages the translation of JSP pages and tag files, interprets the standard directives and actions, checks for the syntaxes and many more operations. Directives on the JSP page specifies how, the translation and execution of JSP pages would be performed by the web container, Scriptlet specifies that the element written inside it would be inserted into JSP pages servlet class, and the action tags <jsp:XXX.../> on form specifies the calling of Java Servlet API. An exception ParseException may be generated at this phase by a server when a JSP page is requested for the first time and encountered an error in JSP page translation.
- Compilation of JSP page's servlet class : This is the second step of JSP life cycle when a request is mapped to a JSP page first time. A JSP page is entered into this phase after its successfully translation as in first phase. In this phase the translated Servlet class is compiled. An exception JasperException with the message containing the JSP page's servlet name and the encountered error line may be generated at this phase by a server when a JSP page's servlet encountered and error while compiling. After successfully compilation of the Servlet class it may entered into the following sub phases :
- Container will check whether the instance of the JSP page's servlet is existing. If not, then the container will control the following operations :
- Loading of JSP page's servlet class.
- Instantiation of the Servlet class.
- Calling of jspInit() method to initialize a servlet instance. This method can be overridden if there is a requirement to do JSP specific initialization.
- Container will call the _jspService() method by passing the request and response object.
- Destruction of JSP page's servlet : This is the last phase of JSP life cycle. In this phase the jspDestroy method is invoked by the container if it doesn't required to keep the JSP page's servlet. This method can be overridden if there is requirement to perform JSP specific destruction.
JSP life cycle methods are as follows :
- jspInit()
- _jspService()
- jspDestroy()
Following image is demonstrating the life cycle of JSP.
