If you are getting involved in Java EE development you have to have good knowledge about the JSP and Servlets. Servlets are server side components that provide a powerful mechanism for developing server side programs. JavaServer Pages (JSP) is a technology that helps software developers create dynamically generated web pages based on HTML, XML, or other document types.
First look at how request and response handle in servlet container. Servlet container create one instance from each servlet but its going to serve for multiple threads. See the following diagram. You can see HTTP Request comes to the container and it will create the HTTPServletRequest, HTTPServletResponse objects. Finally put those two objects in new thread and let the thread to access the instance of specified servlet.
According to the above process you can see we have to be careful of the thread safety of instance variable in servlet. Because each and every thread is gong to access the single Servlet instance. If you have some instance variable in the servlet which is going to modify with in the thread you have to make it thread safe.
Look at the Servlet life cycle
init() – this method is going to run only one time
service() – this method is going to run each time the request comes to the servlet. And it will identify either doGet or doPost to execute according to the request.
JSP life cycle is same as Servlet, only additional thing is its going to translate to the servlet. Ultimately its acts as Servlet. JSP you can override two methods which is jspInit() and jspDestroy(). But you cannot override the jspService() method.
Lets talk about Scope variables.
1. Application Context – In other words we can say this is servletContext. You can use servletContext to store the global data which is going to share through out the application. This is not thread safe. You can easily access this context as follows.
2. Session Context – Session management is handling by the servlet container and you can store the user specific data on session. This object is not thread safe.
request.getSession(); //going to create the session if session is not exist.
request.getSession(false); // Not going to create the session.
3. Request Attributes – You can use this scope to carry the data in server side from one resource to another. As an example while you are doing the request forwarding. This is thread safe because one request is handle by one thread.
request.getAttribute("attribute_name"); // return the Object you have to cast it
We have two different types of init parameters. One is available for the whole application and the other one is only for specific servlet. These parameters are deployment time constants. That means once the application is deployed you cannot change it. These parameters are going to declare in web.xml
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" version="3.0">
1. Global init parameter
2. Specified int parameter