JSP : Java Glossary

Part of the magic of JSP is that you can replace JSP source code on the fly while the server is active. The next time a request comes for that JSP code, will be automatically recompiled. You don’t have to shut down the server ever time you change the text or JSP markup of a page. This works because of clever use of classloaders to see that obsolete classes are garbage collected. Normally, you use this feature only for testing. You normally don’t update your code while a production server is running.

Multiple Languages

• HTML.
• Java scriptlets — a mixture of HTML and encapsulated Java.
• XML, a JSP glue.
• taglibs, e.g. JSTL, JSF, Jakarta Taglibs, ColdTags, Unified Expression Language (merger of JSTL and JSF), Velocity.
• Struts.

Hello.jsp

Note how it looks like ordinary HTML with dynamically generated variable content Java code enclosed in <%=… %>. Inside those delimeters is a Java expression, called a scriptlet, that when evaluated will produce a String to insert at that point in the HTML file. Have a look at how various JSP files expand into Java source code to generate and glue the various bits of text together. In addition there are other things called tags you can embed with many different syntaxes. Unfortunately, the documentation on how to use them reads like the ramblings of someone with ADD. They keep wandering off on tangents before they have explained the core functionality. Further they scrupulously avoid using examples to explain the abstract gibberish. This is ad hoc language creation at its worst.

By default the Servlet generated is thread-safe so the same code can process several requests simultaneously. Alternatively, you can declare your code with <%@ page isThreadSafe="false" %> so that your Servlet will only be used to process one message at a time.

Note that the <%=…%> delimiters used by JSP are not legitimate HTML. They must be expanded and removed before feeding the page to a browser. (In contrast, static macro markup are legitimate HTML even before expansion and are not removed on expansion.)

There are many different sorts of beast you embed in your HTML. Among them are the following tags:

• <%=… %>
• <jsp:… />
• <%@ page… %>
• <%@ include… %>
• <c:forEach var="row" items="${rs.rows}">
• <tr class="${(stat.index & 1) == 0 ? "evenRow" : "oddRow"}">

Tags

There are a number of general purpose tag libraries (taglibs) available on the net that you can access simply by giving the URL of their *.tld files.

Here is how you would invoke a custom tag inside a jsp file:

You can have one-liner bodyless tags or begin-end tags enclosing a body. The body is like a large parameter, often of text to format. The syntax is borrowed from XML.

You must enclose parameters in either " or '. If the attribute value itself contains "s, you must enclose the parameter value in '. If the attribute value itself contains 's, you must enclose it in ". What do you do if a string contains both " and '? You must use the entity &quot; for embedded " and surround the string in "s, e. g.:
<mytag:album title="Sargeant Pepper's Lonely Hearts Club Band">
<mytag:album title='"The Wall"'>
<mytag:album title="Peter's &quot;Weird Songs&quot;">

Two Types of Include

<%@ include> is a macro-style, in-line, translation/compile time include that inserts some JSP/HTML source code into the current JSP program.

Potential of JSP

• Serve different style sheets to lets your viewers choose colour schemes/skins and type size.
• Adjust the layouts to the size of the browser screen, anywhere from a cell phone to a dual airport sized plasma display.
• Prepare a version of the website for download and offline viewing with the Replicator or for local HTML validation.
• Depending on speed of the connection, select resolution/size of images.
• Display prices in local currency units.
• Display dates in local date format.
• Automatically applying the correct width and heights on all image references.
• Move ads around, change colours, change them, alternate them with PSA ads, quotations and other messages, so that people don’t tune them out completely.
• Customise google links to the national server site.
• Show temperatures is F or C depending on country.
• Display units of measure in imperial or metric (or both) depending on country.

Alternatives To JSP

1. Ones that also rely 100% on server side computing, such as Velocity, Tea and Freemarker and other frameworks. There are scores of lemmings all going for the fundamentally flawed, but easy to implement, 100% server side solution. You can see a list of them under servlet wombs.
2. Ones that use a combination of server side and client side computing such as Canoo. Flash is about the only mainstream alternative, (unfortunately non-Java), for the combined solution. I have written up a JSP Replacer student project for an all-Java server-controlled client-side validation environment.

Gotchas

• Your UEL markup will be ignored unless you place a page directive at the top of the page like this:
  <%@ page isELIgnored="false" %>

Advantages

• The big advantage is in maintaining boilerplate. You change it in only one place and it is automatically propagated. This is true of static macros as well.
• There is no extra overhead for having every page different, e.g. with a random quote of the day.
• You can collect information as well as disseminate it.
• When a user is downloading a page, that does not stop you from uploading a new version of it simultaneously.
• There is less uploading traffic. You don’t have to upload all the expanded boiler plate. You don’t have to reupload your html files simply because the expanded bolerplate of static macros (or hand maintenance) has changed.
• You can dynamically change what you send out based on facts you know about the user, such as time zone, country, language.
• You can dynamically change what you send out based on time and date, e.g. noting anniversaries of births, deaths or marriages. You can do this with static macros, but it means updloading every time the display changes.

Disadvantages

• JGuru on alternatives to JSP
• Jason Hunter’s essay
• Reaction to Jason Hunter’s essay
• IBM Developer Works essay
• Hybrid Server Pages. This is pure server side JSP replacement. It attempts to smooth the integration of HTML and Java by creating a new integrating language, rather than trying to embed Java in perfectly standard HTML or HTML in perfectly standard Java.

• JSP is like a 400 year old house where each generation that lived in it tacked on an extra room. There is no overall plan, just a hodge-podge of architectural styles kludged together. The syntax is, to be kind, appalling.
  ~ Roedy (born: 1948-02-04 age: 61)

• Muddling your HTML and procedural code together means you expose your code to the web designers who know only HTML. They can easily damage it because they don’t understand the markup and scriptlets.
• Muddying your HTML and procedural code dilutes your code with reams of irrelevant text.
• You must be much more careful with JSP with testing before you post — one slip and you take down your entire site until you fix the problem. With static content, or static macros, you usually muck up at most one page at a time.
• The tag syntax for doing things like conditionals or loops is clumsy compared with the way you do it in plain Java.
• It may be unwise to write a lot of code in some non-standard Mickey Mouse markup language. What happens when the world inevitably abandons it for something better?
• Your entire website must fit on the heap, double its usual size because of 16-bit characters. The hardware requirements to serve such a website are obviously bigger than to serve the same information statically. In addition, the javac.exe compiler must be in RAM all the time.
• You need to take your server down periodically, especially if you have made a number of changes.
• Your server is more delicate than a simple HTML server, more apt to crash. Your coding bugs can take it down. It is more susceptible to security holes and others exploiting your code to turn against you.
• URLs don’t have the familiar *.html extension. When you flip from a static html to JSP, all the bookmarks people have into your site will stop working unless you create redirects for all of your old URLs.

Outstanding Questions

• Do you have to rename all links on the website to be *.jsp or can *.html be bulk re-routed?
• Con you invent your own tags with your own internal grammar?
• To add a new JSP page, what do you have to do?

Books

	recommend book⇒Head First Servlets and JSP: Passing the Sun Certified Web Component Developer Exam
		paperback	hardcover
	ISBN13:	978-0-596-00540-5	978-0-596-51668-0
	publisher:	O’Reilly
	published:	2008-08-07
	by:	Bryan Basham, Kathy Sierra, Bert Bates
A very complete book, partly because it aims to prepare you for the Sun exam. It is also a difficult book.
abe books.co.uk abe books.ca amazon.co.uk amazon.ca abe books.de chapters.indigo.ca amazon.de abe books.com abe books.fr amazon.com amazon.fr barnes and noble.com abe books.it powells.com iberlibro.com sony e-books abe books anz o’reilly safari

	recommend book⇒Core Web Programming, Second Edition
		paperback
	ISBN13:	978-0-13-089793-0
	publisher:	Prentice Hall
	published:	2001-06-03
	by:	Marty Hall and Gary Cornell
1250 pages. This is a great doorstop of a book. It has a few chapters on client-server programming in Java, and a section of that is on CGI. I have looked at hundreds of Java books and found nothing that deals in depth with client side Java talking to CGI, except Marty’s book. It is really very simple and he does an excellent job of explaining it. Marty has posted all the source code examples from the book for anyone to use. These contain updates and errata fixes you don’t get on the CD-ROM that comes with the book.
abe books.co.uk abe books.ca amazon.co.uk amazon.ca abe books.de chapters.indigo.ca amazon.de abe books.com abe books.fr amazon.com amazon.fr barnes and noble.com abe books.it powells.com iberlibro.com sony e-books abe books anz

	recommend book⇒Core Servlets and Java Server Pages
		paperback
	ISBN13:	978-0-13-089340-6
	publisher:	Prentice Hall
	published:	2000-05-26
	by:	Marty Hall
Complete text of the book available on line in pdf format.
abe books.co.uk abe books.ca amazon.co.uk amazon.ca abe books.de chapters.indigo.ca amazon.de abe books.com abe books.fr amazon.com amazon.fr barnes and noble.com abe books.it powells.com iberlibro.com sony e-books abe books anz

	recommend book⇒More Servlets and Java Server Pages
		paperback
	ISBN13:	978-0-13-067614-6
	publisher:	Pearson Education
	published:	2001-12-26
	by:	Marty Hall
Complete text of the book available on line in pdf format.
abe books.co.uk abe books.ca amazon.co.uk amazon.ca abe books.de chapters.indigo.ca amazon.de abe books.com abe books.fr amazon.com amazon.fr barnes and noble.com abe books.it powells.com iberlibro.com sony e-books abe books anz

	recommend book⇒Web Development with JavaServer Pages
		paperback
	ISBN13:	978-1-930110-12-0
	publisher:	Manning Publications
	published:	2001-09-15
	by:	Duane K. Fields, Mark A. Kolb, Shawn Bayern
A very thorough treatment.
abe books.co.uk abe books.ca amazon.co.uk amazon.ca abe books.de chapters.indigo.ca amazon.de abe books.com abe books.fr amazon.com amazon.fr barnes and noble.com abe books.it powells.com iberlibro.com sony e-books abe books anz

	recommend book⇒Java Servlet Programming
		paperback
	ISBN13:	978-0-596-00040-0
	publisher:	O’Reilly
	published:	2001-01-15
	by:	Jason Hunter, William Crawford
highly recommended. Covers basics of JSP too.
abe books.co.uk abe books.ca amazon.co.uk amazon.ca abe books.de chapters.indigo.ca amazon.de abe books.com abe books.fr amazon.com amazon.fr barnes and noble.com abe books.it powells.com iberlibro.com sony e-books abe books anz o’reilly safari

	recommend book⇒Murach’s Java Servlets and JSP
		paperback
	ISBN13:	978-1-890774-44-8
	publisher:	Mike Murach
	published:	2008-01-21
	by:	Joel Murach and Andrea Steelman
Presumes Tomcat and Netbeans. That is a disadvantage if you use other tools but a boon if you use them, since so many problems are about the details of configuring and implementation, not the code itself.
abe books.co.uk abe books.ca amazon.co.uk amazon.ca abe books.de chapters.indigo.ca amazon.de abe books.com abe books.fr amazon.com amazon.fr barnes and noble.com abe books.it powells.com iberlibro.com sony e-books abe books anz

	recommend book⇒Java 2 Enterprise Edition BibleJava 2 Enterprise Edition Bible
		paperback
	ISBN13:	978-0-7645-0882-0
	publisher:	Wiley
	published:	2002-04-01
	by:	Justin Couch and Daniel H. Steinberg
abe books.co.uk abe books.ca amazon.co.uk amazon.ca abe books.de chapters.indigo.ca amazon.de abe books.com abe books.fr amazon.com amazon.fr barnes and noble.com abe books.it powells.com iberlibro.com sony e-books abe books anz

	recommend book⇒Pure JSP: Java Server Pages
		paperback
	ISBN13:	978-0-672-31902-0
	publisher:	Sams
	published:	2000-06-08
	by:	James Goodwill
I would not recommend this book. It seems as it were constructed by copy/paste from the spec. It tries to explain things abstractly without even explaining the conventions of his BNF notation. It needs to be rewritten replacing the BNF with concrete, realistic examples. Reading this book is likely listening to a politician trying to avoid a question. It does have examples, but beats around the bush needlessly before getting to the point. Does not cover tags or UEL.
abe books.co.uk abe books.ca amazon.co.uk amazon.ca abe books.de chapters.indigo.ca amazon.de abe books.com abe books.fr amazon.com amazon.fr barnes and noble.com abe books.it powells.com iberlibro.com sony e-books abe books anz

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