YouTipJsp Life Cycle
The key to understanding the underlying functionality of JSP is to understand the life cycle they follow.
The JSP life cycle is the entire process from creation to destruction, similar to the servlet life cycle, except that the JSP life cycle also includes compiling the JSP file into a servlet.
The following are the stages traversed during the JSP life cycle:
* **Compilation Stage:**
The servlet container compiles the servlet source file and generates a servlet class.
* **Initialization Stage:**
Loads the servlet class corresponding to the JSP, creates its instance, and calls its initialization method.
* **Execution Stage:**
Calls the service method of the servlet instance corresponding to the JSP.
* **Destruction Stage:**
Calls the destruction method of the servlet instance corresponding to the JSP, and then destroys the servlet instance.
It is obvious that the four main stages of the JSP life cycle are very similar to the servlet life cycle. The following diagram illustrates this:
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## JSP Compilation
When a browser requests a JSP page, the JSP engine first checks whether the file needs to be compiled. If the file has not been compiled, or has been changed since the last compilation, the JSP file is compiled.
The compilation process includes three steps:
* Parsing the JSP file.
* Converting the JSP file into a servlet.
* Compiling the servlet.
* * *
## JSP Initialization
After the container loads the JSP file, it will call the `jspInit()` method before providing any services for requests. If you need to perform custom JSP initialization tasks, simply override the `jspInit()` method, like this:
public void jspInit(){ // Initialization code}
Generally, the program is initialized only once, and so is the servlet. Typically, you can initialize database connections, open files, and create query tables in the `jspInit()` method.
* * *
## JSP Execution
This stage describes all request-related interactive behaviors in the JSP life cycle until destruction.
After the JSP page is initialized, the JSP engine will call the `_jspService()` method.
The `_jspService()` method requires an `HttpServletRequest` object and an `HttpServletResponse` object as its parameters, like this:
void _jspService(HttpServletRequest request, HttpServletResponse response){ // Server-side processing code}
The `_jspService()` method is called once for each request and is responsible for generating the corresponding response. It also handles responses for all seven HTTP methods, such as GET, POST, DELETE, etc.
* * *
## JSP Cleanup
The destruction stage of the JSP life cycle describes everything that happens when a JSP page is removed from the container.
The `jspDestroy()` method in JSP is equivalent to the destruction method in a servlet. Override the `jspDestroy()` method when you need to perform any cleanup work, such as releasing database connections or closing folders.
The format of the `jspDestroy()` method is as follows:
public void jspDestroy(){ // Cleanup code}
### Example
The following is an example of JSP life cycle code:
life.jsp