You are looking in all the wrong places! The answer isn't in the Javadocs for String class or elsewhere.
You are using a scriptlet tag which means you only put Pure Java Code in there (between <% and %> tags.)
You are trying to print a String value, which can be simply achieved through
putValue() method belongs to interface HttpSession and is deprecated (i.e. replaced by setAttribute(java.lang.String, java.lang.Object) method).
As far as your printing-the-string-value problem is concerned, there's your answer. A call to putValue() won't work because the method doesn't belong to String class as already mentioned in an earlier post.