JSP 2.0 and JSTL: Principles and Patterns

1. JSP 2.0 and JSTL: Principles and Patterns Shawn Bayern Research Programmer, Yale University JSTL reference-implementation lead Author, JSTL in Action Web Development with JavaServer Pages

2. The J2EE Presentation Tier • Four key questions to answer today: • What justifies JSP 2.0? • What are its major new features? • What is JSTL? • What do these new technologies suggest about development patterns and best practices?

3. Crash course on the J2EE Presentation Tier J2EE • Current standards: • JSP 1.2 • Servlet 2.3 • JSTL 1.0 JSTL Servlets JSP JSTL • In a few months: • JSP 2.0 • Servlet 2.4 • JSTL 1.1

4. Crash course on the J2EE presentation tier • Servlets • Java classes that handle requests by producing responses (e.g., HTTP requests and responses) • JavaServer Pages (JSP) • HTML-like pages with some dynamic content. • They turn into servlets automatically. • JSP Standard Tag Library (JSTL) • Set of standard components for JSP. • It is used inside JSP pages.

5. Organization of the platform Your web pages Your application JSTL JavaServer Pages (JSP) Java Servlet API Java language

6. What kinds of things go in JSP pages? Scriptlets <% getFoo(request); printFoo(out); String a = ”goat”; %> <% if (a.equals(”pig”) {%> Oink! <% } %> • Java (and more?) embedded within template text • Access to implicit objects: request, response, etc. • Conditional blocks, loops—manually constructed

7. What kinds of things go in JSP pages? Tag libraries <foo:bar/> <c:if test=”${c}”> c is true </c:if> <c:forEach> Round and round we go </c:forEach> • XML tags • Invoke Java logic behind the scenes. • May access body, e.g., for iteration, conditional inclusion—or just as arbitrary parameter • May access PageContext • Libraries and prefixes

8. Question 1 • Why JSP 2.0? (Or, what’s wrong with the current version of JSP?)

9. Why we like JSP in the first place () • Open standard with support from many vendors • The performance and scalability of servlets (for JSP pages compile into servlets) • Extensibility (custom tags) • Easy integration with other J2EE and Java technologies (Servlets, EJB)

10. What’s irritating about JSP? () • The tag-extension protocol is too complicated Tag handler doStartTag() doEndTag()  Too hard for Gosling, even? doInitBody() doAfterBody() doCatch() doFinally() release()

11. What’s irritating about JSP? () • Also, tags don’t support certain kinds of code reuse. <font color=”<%=statusColor%>”> <% for (…) { %> <%= customer %>: <%= hatSize %> <% } %> </font>  out.println(…); for(…) { out.println(…); … }

12. What’s bad about JSP? () • The general consensus says… Scriplets • They complicate abstraction and code reuse. • They make it harder for nonprogrammers to maintain pages

13. Question 2 • What new features does JSP 2.0 offer? (Or, how does it fix the issues we just raised?)

14. How does JSP 2.0 address these issues? • Expression language • Tag files • Simplified Tag API (SimpleTag versus Tag) • Improved XML syntax Also, though it’s not really part of JSP, JSTL improves things too. The end result: JSP pages become easier to write and maintain.

15. The JSP Expression Language (EL): Goals and principles • The major goal: simplicity. • The language should be usable by nonprogrammers. • Inspirations: JavaScript, XPath • But it’s much simpler than even these basic expression languages. • Quick: what does //foo = ‘bar’ mean in XPath? • Or what happens with age + 3 in ECMAScript?

16. XPath (//foo = ‘bar’) • “If one object to be compared is a node-set and the other is a string, then the comparison will be true if and only if there is a node in the node-set such that the result of performing the comparison on the string-value of the node and the other string is true.”

17. ECMAScript (age + 3) • Page 62 of 188: The Addition operator ( + ) The addition operator either performs string concatenation or numeric addition. The production AdditiveExpression : AdditiveExpression + MultiplicativeExpression is evaluated as follows: 1. Evaluate AdditiveExpression. 2. Call GetValue(Result(1)). 3. Evaluate MultiplicativeExpression. 4. Call GetValue(Result(3)). 5. Call ToPrimitive(Result(2)). 6. Call ToPrimitive(Result(4)). 7. If Type(Result(5)) is String or Type(Result(6)) is String, go to step 12. (Note that this step differs from step 3 in the comparison algorithm for the relational operators, by using or instead of and.) 8. Call ToNumber(Result(5)). 9. Call ToNumber(Result(6)). 10. Apply the addition operation to Result(8) and Result(9). See the note below (11.6.3). 11. Return Result(10). 12. Call ToString(Result(5)). 13. Call ToString(Result(6)). 14. Concatenate Result(12) followed by Result(13). 15. Return Result(14). 29. One-half of self-employment tax. Attach Schedule SE. 30. Self-employed health insurance deduction (see page 33) 31. Self-employed SEP, SIMPLE, and qualified plans 32. Penalty on early withdrawal of savings 33. Alimony paid 34. Add lines 23 through 33a 35. Subtract line 34 from line 22. This is your adjusted gross income.

18. The JSP Expression Language (EL): Key syntax • Expressions appear between ${ and }. • Note that ${ and } may contain whole expressions, not just variable names, as in the Bourne shell (and its dozen derivatives.) • E.g., ${myExpression + 2} • Expressions’ default targets are scoped attributes (page, request, session, application) • ${duck}≡ pageContext.findAttribute(“duck”)

19. The JSP Expression Language (EL): Key syntax • The . and [] operators refer to JavaBean-style properties and Map elements: • ${duck.beakColor}can resolve to ((Duck) pageContext.getAttribute(”duck”)).getBeakColor() • Note the automatic type-cast. • This is one of the great features of the EL: users do not need to concern themselves with types in most cases (even though the underlying types of data objects are preserved.)

20. The JSP Expression Language (EL): advanced data access • Expressions may also refer to cookies, request parameters, and other data: • ${cookie.crumb} • ${param.password} • ${header[“User-Agent”]} • ${pageContext.request.remoteUser}

21. The JSP Expression Language (EL): more syntax • The EL supports • Arithmetic ${age + 3} • Comparisons ${age > 21} • Equality checks ${age = 55} • Logical operations ${young or beautiful} • Emptiness detection ${empty a} • ‘a’ is empty String (“”), empty List, null, etc. Useful for ${empty param.x}

22. The JSP Expression Language: Uses • JSTL 1.0 introduced the EL, but it could be used only within tags. • In JSP 2.0, it can be used almost anywhere <font color=”${color}”> Hi, ${user}. You are <user:age style=”${style}”/> years old. </font>

23. Tag Files: nature and purpose • Solve difficulty of reusing text/HTML within a tag. • And makes it much easier to write simple tags, since you can do so in JSP instead of Java. • Stand-alone file with <%@ tag %> directive instead of traditional <%@ page %> directive.

24. JSP 2.0 tag files <%@ tag name=”tableTag” %> <%@ attribute name=”items” %> <table width=”…” bgcolor=”…”> <th> <td>Name</td> <td>IQ</td> </th> <c:forEach var=”i” items=”${items}”> <tr> <td>${i.fullName}</td> <td>${i.IQ}</td> </tr> </c:forEach> </table>

25. Using the new tag… Your shopping cart: <my:tableTag items=”${cart}” /> Your wish list: <my:tableTag items=”${wishList}” /> Things we want you to buy: <my:tableTag items=”${pressuredSales}” />

26. Old tag handler Tag handler doStartTag() doEndTag() Tag body Tag attributes  doInitBody() doAfterBody() doCatch() doFinally() release()

27. SimpleTag handler Tag handler doTag() Tag body (no scriptlets) Tag attributes 

28. JSP 2.0: Improved XML syntax Old style: JSP as document New style: JSP as namespace • Other changes • <jsp:element>, <jsp:attribute> • Avoids <a href=”<c:url value=”…”/>”/> <jsp:root> xmlns:jsp=“http://java.sun.com/JSP/Page”

29. Question 3 • What is JSTL? What features does it offer?

30. JSTL design principles • JSTL 1.0: Keep it simple! • Targeted tags • Could have a single <go> tag: • <go action=“forEach” argument1=“${myData}”> • Instead, single-purpose tags, tightly focused • Design intended for page author • Perhaps something of a fantasy, like the legal “reasonable person.” But a helpful guide nonetheless.

31. The parable of Mike and Phillipe Mike Phillipe Credit: Pierre Delisle (spec lead)

32. JSTL 1.0 features • Control flow • Iteration, conditions • URL management • Retrieve data, add session IDs • Text formatting and internationalization • Dates and numbers • Localized messages • XML manipulation • XPath, XSLT • Database access • Queries, updates

33. JSTL 1.0 libraries

34. JSTL features:managing variables • Outputting values with EL <c:out value=”${user.IQ}” /> • Storing data <c:set var=”user” scope=”session”> // arbitrary text </c:set> Note the use of “var” and “scope”: a JSTL convention

35. JSTL features:iteration • Iteration <c:forEach items=”${list}” begin=”5” end=”20” step=”4” var=”item”> <c:out value=”${item}”/> </c:forEach> • “paging”

36. Conditional evaluation <c:if test=”${a == b}”> a equals b </c:if> Mutually exclusive conditionals <c:choose> <c:when test=”${a == b}”> a equals b </c:when> <c:when test=”${a == c}”> a equals c </c:when> <c:otherwise> I don’t know what ’a’ equals. </c:otherwise> </c:choose> JSTL features:conditional logic

37. JSTL features:URL management • Retrieving data <c:import var=”cnn” url=”http://www.cnn.com/cnn.rss”/> • Data exposed as String or Reader • All core URLs supported (HTTP, FTP, HTTPS with JSSE) • Local, cross-context imports supported • Printing URLs <c:url value=”/foo.jsp”> • Redirection <c:redirect url=”/foo.jsp”>

38. JSTL features:text formatting • Locale-sensitive formatting and parsing • Numbers • Dates • Internationalization • Message bundles • Message argument substitution • “Hi {0}. I would like to {1} your money today. I will use it to buy myself a big {2}.” <fmt:formatNumber type=“currency” value=“${salary}” /> <fmt:message key=“welcome” />

39. JSTL features:XML manipulation • Use of XPath to access, display pieces of XML documents <c:import url=”http://www.cnn.com/cnn.rss” var=”cnn”/> <x:parse xml=”${cnn}” var=“dom”> <x:out value=”$dom//item[1]/title”/> • Chaining XSLT transformations <x:transform xslt=”${xsl2}” /> <x:transform xml=”${xml}” xslt=”${xsl}” /> </x:transform>

40. Advantages of JSTL XML/XPath support • Why not always use XSLT? • JSTL integrates XPath with convenient, standard access to Java/JSP code. • E.g., parse an article URL out of a document, then follow the URL and parse its contents. • JSP/JSTL may be more familiar and convenient for simple tasks. • Functional versus imperative programming

41. JSTL features:database manipulation • Queries (and ResultSet caching) • Updates / inserts • Transactions (<sql:transaction>) • Parametric (PreparedStatement) argument substitution (<sql:param>) • DataSource-based connection management <sql:query sql=“SELECT * FROM USERS” var=“result” /> <c:forEach items=“${result.rows}”> … </c:forEach>

42. SQL tags: the debate JSP page Back-end Java code Tag library Database Tag library JSTL SQL tags

43. SQL Tags:The expert group’s conclusion • SQL tags are needed because… • many nonstandard offerings exist • it is not JSTL’s role to dictate a choice of framework • As popular as MVC is, it’s not universal. • Even in an MVC application, not all data is worth handling carefully. • prototyping is important • users ask for it! • The JSTL specification recommends avoidance of SQL tags in large applications.

44. JSTL programmer support • JSTL also supports Java developers • Simplifies tag development • IteratorTagSupport, ConditionalTagSupport • Instead of writing whole tag handler (doStartTag(),doEndTag()), simply override a few methods: • protected boolean condition() • protected Object next() • Still, JSP 2.0 is probably easier. • Ugly JSP 1.1 tag protocol  Ugly JSP 1.1 tag protocol with assistance from JSTL 1.0  Nice JSP 2.0 tag protocol.

45. JSTL programmer support • JSTL API allows registrations of defaults… • DataSource • Limit on size of results • Localization context (Locale, etc.) • Time zone

46. Question 4 • How do the technologies work together?

47. “Models” of JSP development • Origin of the terms “model 1” and “model 2.” • JSP 0.92 spec: “You can apply the JavaServer Pages technology in two ways . . . Model 1: A request sent to a JavaServer Pages file. . . . Model 2: A request sent to a Java Servlet.”

48. JSTL works with either model • Core tags, formatting tags are ideal for applications that use either model. • XML-manipulation tags are on the borderline. Ideally, they pull data out of XML files that servlets or other back-end logic sends them. • SQL tags are probably most useful in model-1 applications.

49. Simple “model 2” example public void doGet(HttpServletRequest request HttpServletResponse response) { // business logic that results in object ’data’ request.setAttribute(”d”, data); sc.getRequestDispatcher(”/view.jsp”); } view.jsp We have some data to display: <b>${d.property1}</b> • In this case, the data passed is a simple bean-style object. It could also be an XML document; we’d then use JSTL’s XML-manipulation tags.

50. Transferring data • Use XML documents (i.e., DOM objects) and the JSTL <x:*> tags • Use JavaBean-style classes public class MyData { public String getCustomerName() { return X; } public Date getCustomerBirthday() { return Y; } } ${myDataInstance.customerName}