Java Souls

Some of questions I came around

Explain about the Request Packet flow from the browser to the DB ? Explain in detail not just high level?

What are different Objects/methods inside a Servlet? JSP Implicit Objects are the Java objects that the JSP Container makes available to developers in each page and developer can call them directly without being explicitly declared. JSP Implicit Objects are also called pre-defined variables.

JSP supports nine Implicit Objects which are listed below:

Object	Description
request	This is the HttpServletRequest object associated with the request.
response	This is the HttpServletResponse object associated with the response to the client.
out	This is the PrintWriter object used to send output to the client.
session	This is the HttpSession object associated with the request.
application	This is the ServletContext object associated with application context.
config	This is the ServletConfig object associated with the page.
pageContext	This encapsulates use of server-specific features like higher performance JspWriters.
page	This is simply a synonym for this, and is used to call the methods defined by the translated servlet class.
Exception	The Exception object allows the exception data to be accessed by designated JSP.

The request Object:

The request object is an instance of a javax.servlet.http.HttpServletRequest object. Each time a client requests a page the JSP engine creates a new object to represent that request.

The request object provides methods to get HTTP header information including form data, cookies, HTTP methods etc.

We would see complete set of methods associated with request object in coming chapter: JSP - Client Request.

The response Object:

The response object is an instance of a javax.servlet.http.HttpServletResponse object. Just as the server creates the request object, it also creates an object to represent the response to the client.

The response object also defines the interfaces that deal with creating new HTTP headers. Through this object the JSP programmer can add new cookies or date stamps, HTTP status codes etc.

We would see complete set of methods associated with response object in coming chapter: JSP - Server Response.

The out Object:

The out implicit object is an instance of a javax.servlet.jsp.JspWriter object and is used to send content in a response.

The initial JspWriter object is instantiated differently depending on whether the page is buffered or not. Buffering can be easily turned off by using the buffered='false' attribute of the page directive.

The JspWriter object contains most of the same methods as the java.io.PrintWriter class. However, JspWriter has some additional methods designed to deal with buffering. Unlike the PrintWriter object, JspWriter throws IOExceptions.

Following are the important methods which we would use to write boolean char, int, double, object, String etc.

Method	Description
out.print(dataType dt)	Print a data type value
out.println(dataType dt)	Print a data type value then terminate the line with new line character.
out.flush()	Flush the stream.

The session Object:

The session object is an instance of javax.servlet.http.HttpSession and behaves exactly the same way that session objects behave under Java Servlets.

The session object is used to track client session between client requests. We would see complete usage of session object in coming chapter: JSP - Session Tracking.

The application Object:

The application object is direct wrapper around the ServletContext object for the generated Servlet and in reality an instance of a javax.servlet.ServletContext object.

This object is a representation of the JSP page through its entire lifecycle. This object is created when the JSP page is initialized and will be removed when the JSP page is removed by the jspDestroy() method.

By adding an attribute to application, you can ensure that all JSP files that make up your web application have access to it.

You can check a simple use of Application Object in chapter: JSP - Hits Counter

The config Object:

The config object is an instantiation of javax.servlet.ServletConfig and is a direct wrapper around the ServletConfig object for the generated servlet.

This object allows the JSP programmer access to the Servlet or JSP engine initialization parameters such as the paths or file locations etc.

The following config method is the only one you might ever use, and its usage is trivial:

 
config.getServletName();

This returns the servlet name, which is the string contained in the <servlet-name> element defined in the WEB-INF\web.xml file

The pageContext Object:

The pageContext object is an instance of a javax.servlet.jsp.PageContext object. The pageContext object is used to represent the entire JSP page.

This object is intended as a means to access information about the page while avoiding most of the implementation details.

This object stores references to the request and response objects for each request. The application, config, session, and out objects are derived by accessing attributes of this object.

The pageContext object also contains information about the directives issued to the JSP page, including the buffering information, the errorPageURL, and page scope.

The PageContext class defines several fields, including PAGE_SCOPE, REQUEST_SCOPE, SESSION_SCOPE, and APPLICATION_SCOPE, which identify the four scopes. It also supports more than 40 methods, about half of which are inherited from the javax.servlet.jsp. JspContext class.

One of the important methods is removeAttribute, which accepts either one or two arguments. For example, pageContext.removeAttribute ("attrName") removes the attribute from all scopes, while the following code only removes it from the page scope:

 
pageContext.removeAttribute("attrName", PAGE_SCOPE);

You can check a very good usage of pageContext in coming chapter: JSP - File Uploading.

The page Object:

This object is an actual reference to the instance of the page. It can be thought of as an object that represents the entire JSP page.

The page object is really a direct synonym for the this object.

The exception Object:

The exception object is a wrapper containing the exception thrown from the previous page. It is typically used to generate an appropriate response to the error condition.

What are the different Marker interfaces and Does the Serializable contains the methods how you will persist an object ?
What is complete Memory Layout on java ?
If you are given parking plot and Ask you to design a software, how will you do that ? How you will gather requirements from the owner?
Does the Web server contains the web.xml and how it does the Load balancing (J2EE Servers and Application servers)
What is XSS/CSRF ? Explain in detailed ?
Differences of JSPs and Servlets ?
JSP is a Java view technology running on the server machine which allows you to write template text in (the client side languages like HTML, CSS, JavaScript and so on). JSP supports the so-called taglibs which are backed by pieces of Java code with which you can control the page flow and/or output dynamically (programmatically). A well known taglib is JSTL. JSP also supports Expression Language which can be used to access backend data (actually, the attributes which are available in page, request, session and application scopes), mostly in combination with taglibs. When a JSP is requested for the first time or when the webapp starts up, the servlet container will compile it into a class extending HttpServlet and use it during the webapp's lifetime. You can find the generated source code in the server's work directory. In for example Tomcat, it's the /work directory. On a JSP request, the servletcontainer will execute the compiled JSP class and send the generated output (usually just HTML/CSS/JS) through the webserver over network to the client side which in turn displays it in the webbrowser.

Servlets
Servlet is an Java application programming interface (API) running on the server machine which can intercept on the requests made by the client and can generate/send a response accordingly. A well known example is the HttpServlet which provides methods to hook on HTTP requests using the popular HTTP methods such as GET and POST. You can configure HttpServlets to listen on a certain HTTP URL pattern, which is configureable in web.xml, or more recently with Java EE 6, with @WebServlet annotation.
When a Servlet is requested for the first time or when the webapp starts up, the servlet container will create an instance of it and keep it in memory during webapp's lifetime. The same instance will be reused for every incoming request whose URL matches the servlet's URL pattern. You can access the request data by HttpServletRequest and handle the response by HttpServletResponse. Both objects are available as method arguments inside any of the overridden methods of HttpServlet, such as doGet() and doPost().

JSF (JavaServer Faces)
JSF is a component based MVC framework which is built on top of the Servlet API and provides components in flavor of taglibs which can be used in JSP or any other Java based view technology such as Facelets. Facelets is mucha more suited to JSF than JSP. It namely provides great templating capabilities such as composite components, while JSP basically only offers the <jsp:include> for templating, so that you're forced to create custom components with raw Java code (which is a bit opaque and a lot of tedious work in JSF) when you want to replace a repeated group of components by a single component. If you can, I recommend to drop JSP and go for Facelets when you want to develop with JSF.
As being a MVC (Model-View-Controller) framework, JSF provides the FacesServlet as the sole request-response Controller. It takes all the standard and tedious HTTP request/response work from your hands, such as gathering user input, validating/converting them, putting them in model objects, invoking actions and rendering the response. This way you end up with basically a JSP or Facelets (XHTML) page for View and a Javabean class as Model. The JSF components are been used to bind the view with the model (such as your ASP.NET web control does) and the FacesServlet uses the JSF component tree to do all the work.

Related questions
What is the main-stream Java alternative to ASP.NET / PHP?
Java EE web development, what skills do I need?
How do servlets work? Instantiation, session variables and multithreading
What is a Javabean and where are they used?
How to avoid Java code in JSP files?

ServletContext

When the servletcontainer (like Apache Tomcat) starts up, it will deploy and load all webapplications. When a webapplication get loaded, the servletcontainer will create the ServletContext once and keep in server's memory. The webapp's web.xml will be parsed and every Servlet, Filter and Listener found in web.xml will be created once and kept in server's memory as well. When the servletcontainer shuts down, it will unload all webapplications and the ServletContext and all Servlet, Filter and Listener instances will be trashed.

HttpServletRequest and HttpServletResponse

The servletcontainer is attached to a webserver which listens on HTTP requests on a certain port number, which is usually 80. When a client (user with a webbrowser) sends a HTTP request, the servletcontainer will create new HttpServletRequest and HttpServletResponse objects and pass it through the methods of the already-created Filter and Servlet instances whose url-pattern matches the request URL, all in the same thread.
The request object provides access to all information of the HTTP request, such as the request headers and the request body. The response object provides facility to control and send the HTTP response the way you want, such as setting headers and the body (usually with HTML content from a JSP file). When the HTTP response is committed and finished, then both the request and response objects will be trashed.

HttpSession

When a client visits the webapp for the first time and/or the HttpSession is to be obtained for the first time by request.getSession(), then the servletcontainer will create it, generate a long and unique ID (which you can get by session.getId()) and store it in server's memory. The servletcontainer will also set a Cookie in the HTTP response with JSESSIONID as cookie name and the unique session ID as cookie value.
As per the HTTP cookie specification (a contract a decent webbrowser and webserver has to adhere), the client (the webbrowser) is required to send this cookie back in the subsequent requests as long as the cookie is valid. Using a HTTP header checker tool like Firebug you can check them. The servletcontainer will determine every incoming HTTP request header for the presence of the cookie with the name JSESSIONID and use its value (the session ID) to get the associated HttpSession from server's memory.
The HttpSession lives until it has not been used for more than the <session-timeout> time, a setting you can specify in web.xml, which defaults to 30 minutes. So when the client doesn't visit the webapp anymore for over 30 minutes, then the servletcontainer will trash the session. Every subsequent request, even though with the cookie specified, will not have access to the same session anymore. The servletcontainer will create a new one.
On the other hand, the session cookie on the client side has a default lifetime which is as long as the browser instance is running. So when the client closes the browser instance (all tabs/windows), then the session will be trashed at the client side. In a new browser instance the cookie associated with the session won't be sent anymore. A new request.getSession() would return a brand new HttpSession and set a cookie with a brand new session ID.

In a nutshell

The ServletContext lives as long as the webapp lives. It's been shared among all requests in all sessions.
The HttpSession lives as long as the client is interacting with the webapp with the same browser instance and the session hasn't timed out at the server side yet. It's been shared among all requests in the same session.
The HttpServletRequest and HttpServletResponse lives as long as the client has sent it until the complete response (the webpage) is arrived. It is not being shared elsewhere.
Any Servlet, Filter and Listener lives as long as the webapp lives. They are being shared among all requests in all sessions.
Any attribute which you set in ServletContext, HttpServletRequest and HttpSession will live as long as the object in question lives.

Threadsafety

That said, your major concern is possibly threadsafety. You should now have learnt that Servlets and filters are shared among all requests. That's the nice thing of Java, it's multithreaded and different threads (read: HTTP requests) can make use of the same instance. It would otherwise have been too expensive to recreate it on every request.
But you should also realize that you should never assign any request or session scoped data as an instance variable of a servlet or filter. It will be shared among all other requests in other sessions. That's threadunsafe! The below example illustrates that:

public class MyServlet extends HttpServlet {

    private Object thisIsNOTThreadSafe;

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        Object thisIsThreadSafe;

        thisIsNOTThreadSafe = request.getParameter("foo"); // BAD!! Shared among all requests!
        thisIsThreadSafe = request.getParameter("foo"); // OK, this is thread safe.
    } }

A: JavaServer Faces technology is a framework for building user interfaces for web applications. JavaServer Faces technology includes:
A set of APIs for: representing UI components and managing their state, handling events and input validation, defining page navigation, and supporting internationalization and accessibility.
A JavaServer Pages (JSP) custom tag library for expressing a JavaServer Faces interface within a JSP page.

JSP is a specialized kind of servlet.

JSF is a set of tags you can use with JSP.