Page is still executed even after response.sendRedirect()

In the beginning of a JSP page, I checked for a variable called "sessionLogin" everytime a user click a submit button in a form. And if this variable is empty or NULL then I redirect to another page called "invalidLogin.jsp".

I have checked and the redirection is working ok. But the second if statement below keeps getting executed even though the page is redirected.
I don't know why. Could someone please advise? I'm probably missing something trivial here.

The sendRedirect method does not halt the execution of your method.
You should either branch your code in such a way that the call to sendRedirect is the last statement in your method or explicitly call return; after calling sendRedirect.

The same applies to RequestDispatcher.forward.

But I've done a test below and the page is redirected to test2.jsp immediately and the alert statement in test1.jsp is not being executed. Why is that?

Because the sendRedirect method was called.

Remember, these are just Java methods calling one another.
The rules don't change just because they're servlets (or servlets generated by JSPs).

sendRedirect gets called, as soon as it's done, the code under is executed.

The alert() is JavaScript and has nothing to do with your JSP. It is not executed at the same time as the JSP which executes on the server.

Perhaps this article would be helpful.

Thanks for the help.

Let me read the article.