Expressions become the argument to an out.print()

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In other words, the Container takes everything you type between the <%= and %> and puts it in as the argument to a statement that prints to the implicit response PrintWriter out.

When the Container sees this:

<%= Counter.getCount() %>

It turns it into this:

out.print(Counter.getCount());

If you did put a semicolon in your expression:

<%= Counter.getCount(); %>

That would be bad. It would mean this:

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Note

NEVER end an expression with a semicolon!

<%= neverPutASemicolonInHere %>

<%= becauseThisIsAnArgumentToPrint() %>

There are no Dumb Questions

Q:

Q: Well, if you’re supposed to use expressions INSTEAD of putting out.println() into a scriptlet, then why is the implicit “out” there?

A:

A: You probably won’t use the implicit out variable from within your JSP page, but you might pass it to something else... some other object that’s part of your app that does not have direct access to the output stream for the response.

Q:

Q: In an expression, what happens if the method doesn’t return anything?

A:

A: You’ll get an error!! You cannot, MUST NOT use a method with a void return type as an expression. The Container is smart enough to figure out that there won’t be anything to print if the method has a void return type!

Q:

Q: Why does the import directive start with the word “page”? Why is it <%@ page import...%> instead of just <%@ import... %>.

A:

A: Good question! Rather than having a whole big pile of different directives, the JSP spec has just three JSP directives, but the directives can have attributes. What you called “the import directive” is actually “the import attribute of the page directive”.

Q:

Q: What are the other attributes for the page directive?

A:

A: Remember, the page directive is about giving the Container information it needs when translating your JSP into a servlet. The attributes we care about (besides import) are session, contentType, and isELIgnored (we’ll come back to these later in the chapter).

Sharpen your pencil

Decide which of the following expressions are and are not valid, and why. We haven’t covered every example here, so make your best guess based on what you know about how expressions work. (Answers are later in this chapter so do this NOW.)

Valid? (Check if valid, and if not, explain why not.)

<%= 27 %>

<%= ((Math.random() + 5)*2); %>

<%= “27” %>

<%= Math.random() %>

<%= String s = “foo” %>

<%= new String[3] %>

<% = 42*20 %>

<%= 5 > 3 %>

<%= false %>

<%= new Counter() %>

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