Type the above code into a text file. Name the file helloworld.jsp.
Place it in correct directory on your JSP web server and call it via your browser.
Notice that when page reload in the browser, it comes up with the current time.
The character sequence <%= and %> enclose Java expression, which are evaluated at run time.

JSP allow you to write block of Java code inside the JSP You do this by placing your Java code between <% and %> character (just like expressions, but without the = sign at the start of the sequence.) This block of code is known as a "scriptlet".  By itself, a scriptlet doesn't contribute any HTML (as we will see down below.)  A scriptlet contains Java code that is executed every time the JSP is invoked. 
Here is a modified version of our JSP, adding in a scriptlet.

When above example execute, you will notice the output from the "System.out.println" on the server log.  This is a convenient way to do simple debugging.

By itself a scriptlet does not generate HTML.  If a scriptlet want to generate HTML, it can use a variable called "out".  This variable does not need to be declared.  It is already predefined for scriptlet, along with some other variables.  The following example shows how the scriptlet can generate HTML output.

Here we generate HTML directly by printing to the "out" variable.

A "request" is a server-side processing which refers to the transaction between a browser and the server.  When someone enters a URL, the browser sends a "request" to the server for that URL, and shows returned data.  As a part of this "request", various data is available, including the file the browser wants from the server, and if the request is coming from pressing a SUBMIT button, the information the user has entered in the form fields.
The JSP "request" variable is used to obtain information from the request as sent by the browser.  For instance, you can find out the name of the client's host (if available, otherwise the IP address will be returned.)  Let us modify the code as shown:

A similar variable is "response".  This can be used to affect the response being sent to the browser.  For instance, you can call response.sendRedirect( anotherUrl ); to send a response to the browser that it should load a different URL.  This response will actualy goes all the way to the browser.  The browser will then send a different request, to "anotherUrl".