This question can be used to explore a candidate's understanding of basic JSP tags. It is appropriate for all levels of experience in JSP.

JSP supports two different ways of merging the content of one file into another - the include directive and the include action. How do these differ and why would you choose one over the other?

JSP provides two distinct ways to include the contents of a file. A beginning JSP developer should be familiar with at least the first one, but a developer with some good JSP experience should understand the use of each of the tags.

The first tag is < jsp:include >, the JSP include action. The include action will insert the rendered output of the referenced file at request time, after the included page has been processed by the JSP container. Thus if the page specified in the include tag is another JSP, it will be passed the current request, evaluated on its own, and the resulting output merged into the source page at the point of the include action tag. The contents of the included file must be stand-alone, because it will be compiled and rendered by the container on its own. This type of include is called a request-time include.

The second type of include is , the include directive. The include directive is known as a translation-time include. Its work is done during the container's initial parsing of the page and merges in the actual source contents of the included page, rather than its rendered content. Thus the included file doesn't necessarily need to be able to stand alone; it can contain code fragments and bean or variable references which expect to be evaluated in the context of the source page.

The include action is appropriate for including shared segments of content that can be rendered on their own. A typical usage would be including standard headers, footers, or navigation elements into each page. The use of the include directive however, should be limited to reusing fragments of JSP code that must be shared between multiple pages. In practice the include directive is used far less often than the include action.

About the author

Duane Fields is a Java developer, author, and Internet technologist with nearly a decade of professional experience in the design and development of leading edge Internet products and services. Duane is also a respected member of the Java development community and is frequently invited to speak at industry conferences and events. He has co-authored two books and published numerous articles on many aspects of web application development from Java to Relational Databases. The newly expanded, second edition of his best selling book "Web Development with JavaServer Pages" was released in December of 2001.