Question

A)         FacesContext facesContext = FacesContext.getCurrentInstance();
           ExternalContext externalContext=facesContext.getExternalContext();
           HttpSession session = (HttpSession) externalContext.getSession(false);

               if(session.isNew()) {            //  java.lang.NullPointerException

B)         HttpServletRequest req1 = (HttpServletRequest)FacesContext.getCurrentInstance()
                                    .getExternalContext().getRequest();
           HttpSession session1=req1.getSession();

             if(session1.isNew()) {            // no Exception

why case A is throwing NullPointerException where as case B is not.

Answer 1

First, it’s important to understand when and why a NullPointerException is been thrown. The way you put the question indicates that you don’t understand it. You asked "Why does it throw NullPointerException?". You didn’t ask "Why does it return null?".

As its javadoc indicates, the NullPointerException will be thrown when you try to access a variable or to invoke a method using the period . operator on an object reference which is actually null.

E.g.

SomeObject someObject = null;
someObject.doSomething(); // NullPointerException!

In your particular case, you were trying to invoke the method isNew() on a null object. This is thus not possible. The null reference has no methods at all. It simply points to nothing. You should be doing a null-check instead.

HttpSession session = (HttpSession) externalContext.getSession(false);

if (session == null) {
    // There's no session been created during current nor previous requests.
}
else if (session.isNew()) {
    // The session has been created during the current request.
}
else {
    // The session has been created during one of the previous requests.
}

The getSession() call with false argument may namely return null when the session hasn’t been created yet. See also the javadoc:

getSession

public abstract java.lang.Object getSession(boolean create)

If the create parameter is true, create (if necessary) and return a session instance associated with the current request. If the create parameter is false return any existing session instance associated with the current request, or return null if there is no such session.

See the emphasized part.

The HttpServletRequest#getSession() call which doesn’t take any argument, uses by default true as create argument. See also the javadoc:

getSession

HttpSession getSession()

Returns the current session associated with this request, or if the request does not have a session, creates one.

See the emphasized part.

I hope that you’ll take this as a hint to consult the javadocs better. They contain more than often already the answers to your questions as they describe very precisely what the classes and methods do.