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JSP Custom Tags

JSP Custom Tags. Agenda, Part I. Tags? Actions? Huh? Tag Interface Tag Attributes Tags and Scripting Variables. Agenda, Part II. Body Tags Nested Tags Iterating Tags Tags and the JSP Container Packaging and Deployment. Tags? Actions? Huh?. A brief introduction to custom tags.

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JSP Custom Tags

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  1. JSP Custom Tags

  2. Agenda, Part I • Tags? Actions? Huh? • Tag Interface • Tag Attributes • Tags and Scripting Variables JSP Custom Tags

  3. Agenda, Part II • Body Tags • Nested Tags • Iterating Tags • Tags and the JSP Container • Packaging and Deployment JSP Custom Tags

  4. Tags? Actions? Huh? A brief introduction to custom tags. JSP Custom Tags

  5. Definitions • A “tag” is an XML element that can be used in a JSP to incorporate functionality without using scriptlet code. • An “action” is the task performed by the tag. JSP Custom Tags

  6. Definitions, cont. • A “tag handler” is basically a JavaBean that implements the Tag interface and has property setter methods corresponding to each tag attribute. The JSP container uses this class to process the tag. • A “tag library” or “taglib” is a collection of one or more custom tags. JSP Custom Tags

  7. Tag Syntax A custom tag consists of a start tag with a taglib-identifying prefix (possibly containing attributes), a body (which can be empty) and an end tag: <log:logMessage categoryName="testCat" priority="fatal"> The body contents… </log:logMessage> JSP Custom Tags

  8. Alternate Tag Syntax If a tag does not contain any body contents, it can be used like this: <log:logMessage categoryName="testCat" priority="fatal" message="test"/> JSP Custom Tags

  9. The <%@taglib%> Directive Before using a tag library, you must inform the JSP container about the tag library using the <%@taglib%> directive before the first use of a tag: <%@ taglib uri="/logtags" prefix="log" %> The taglib directive also allows you to specify a prefix to use with a given tag library in its use in your JSP. JSP Custom Tags

  10. Tag Libraries • One or more tags can be packaged together as a “tag library.” • A tag library is described in a tag library descriptor (.TLD) file. • Tags from different libraries can be called from the same JSP, but each must be declared separately with its own <%@taglib%> directive and must be uniquely identified with its own prefix. JSP Custom Tags

  11. What Can Tags Do? A custom tag can: • Know about its JSP environment. • Perform simple tasks. • Perform formatting or other customization of output. • Introduce variables usable from scriptlets later in the JSP. • Work together either as nested pairs or separately. JSP Custom Tags

  12. Why Are Tags Important? • Simplification of complex tasks across multiple JSPs in a web application. • Complex code hiding. • Design tool incorporation. • Portable to any JSP container. • Incorporation of functionality into JSPs not using Java as the scripting language. JSP Custom Tags

  13. Tag Interface All tag handler classes must implement the Tag interface. JSP Custom Tags

  14. Tag Interface public interface Tag { void setPageContext(PageContext pc) void setParent(Tag t) Tag getParent() int doStartTag() throws JspException int doEndTag() throws JspException void release() } JSP Custom Tags

  15. Tag Interface Constants The Tag interface also includes the following constants: public final static int SKIP_BODY = 0; public final static int EVAL_BODY_INCLUDE = 1; public final static int SKIP_PAGE = 5; public final static int EVAL_PAGE = 6; JSP Custom Tags

  16. Return Values int doStartTag() • SKIP_BODY • EVAL_BODY_INCLUDE int doEndTag() • SKIP_PAGE • EVAL_PAGE JSP Custom Tags

  17. TagSupport Class Additions In addition to those methods in Tag, the TagSupport class also provides the following: public static final Tag findAncestorWithClass(Tag from, Class klass) public void setId(String id) public String getId() public void setValue(String k, Object o) public Object getValue(String k) public void removeValue(String k) public Enumeration getValues() JSP Custom Tags

  18. Tag Library Descriptor Files • The JSP container generates code in the JSP implementation class for a tag according to information you set in the TLD. • The TLD is an XML document. • It contains a single <tag/> element for each tag in the library. JSP Custom Tags

  19. TLD Elements The TLD has the following elements: • The <taglib/> element contains all the other elements in the TLD. • <tlibversion/> • <uri/> • <jspversion/> • <shortname/> JSP Custom Tags

  20. TLD Elements, cont. The <taglib/> element contains a <tag/> element for each tag in the library. The possible sub-elements of <tag/> are as follows: • <name/> • <tagclass/> • <teiclass/> • <bodycontent/> • <info/> • <attribute/> JSP Custom Tags

  21. TLD Example <?xml version="1.0" encoding="ISO-8859-1" ?> <!DOCTYPE taglib PUBLIC "-//Sun Microsystems, Inc.//DTD JSP Tag Library 1.1//EN" "http://java.sun.com/j2ee/dtds/web-jsptaglibrary_1_1.dtd"> <taglib> <tlibversion>1.0</tlibversion> <jspversion>1.1</jspversion> <shortname>logtags</shortname> <uri></uri> <info>Log4J Wrapper Tags</info> <tag> <name>initLog</name> <tagclass> com.oreilly.jsp.taglibs.logtags.InitLogTag </tagclass> <info>Logging system initialization tag.</info> </tag> </taglib> JSP Custom Tags

  22. Example Background • Log4j is an open source project. • Allows developers to fully control logging of an application. • Fully runtime configurable. • Gentle learning curve. • See http://jakarta.apache.org/log4j/ JSP Custom Tags

  23. Example Background • A category is a named entity that categorizes a part of the logging space. • An appender is a logging output destination. • A layout is a formatter for output to an appender. • Priority is the minimum level of messages to be logged for a category. JSP Custom Tags

  24. Example Background root appender1 category1 appender2 category2 appender3 category 3 appender4 JSP Custom Tags

  25. Tag Attributes A tag can have zero or more attributes. Attribute values provide information to the tag’s tag handler class at execution time. JSP Custom Tags

  26. Tag Attributes • Each attribute has a corresponding setXxxx() method. • Each attribute value included in the tag triggers a call to its corresponding setter. No particular order. • The setter called can be determined by capitalizing the first letter of the attribute name and adding the word “set” as a prefix. JSP Custom Tags

  27. Tag Attribute Setters <log:logMessage message=“test msg”/> would call setMessage(String pMessage) with the String value “test msg” JSP Custom Tags

  28. Attribute Data Types • The signature of the attribute setter method in the tag handler class dictates the data type expected for that attribute. JSP Custom Tags

  29. Tag Attribute Setters <log:initLog override=“true”/> would call setOverride(boolean pOverride) with the boolean value true. JSP Custom Tags

  30. boolean or Boolean byte or Byte char or Character double or Double int or Integer float or Float long or Long Boolean.valueOf(String) Byte.valueOf(String) String.charAt(int) Double.valueOf(String) Integer.valueOf(String) Float.valueOf(String) Long.valueOf(String) Tag Attribute Value Conversion Property Type Conversion Method JSP Custom Tags

  31. JSP Container Call Sequence The JSP container call sequence goes as follows: • Instantiates tag handler. • Calls setup methods (setPageContext(), etc). • Calls each setXxxx() method depending on each attribute’s presence in the tag. • Calls doStartTag(). • Calls doEndTag(). JSP Custom Tags

  32. Call Sequence Diagram <%@ taglib …%> <log:category priority=“fatal” 1-> setPriority() > 2-> doStartTag() Body contents. </log:category>3-> doEndTag() JSP Custom Tags

  33. Runtime Attribute Values Attribute values can be the result of a runtime expression: <% Collection roomsCol = new ArrayList(); %> <log:logCollection collection="<%=roomsCol%>“/> NOTE: This is how to handle other data types. JSP Custom Tags

  34. <attribute/> Element There is an <attribute/> element in the TLD for each attribute for each tag: <attribute> <name>message</name> <required>true</required> <rtexprvalue>true</rtexprvalue> </attribute> JSP Custom Tags

  35. Example Background root appender1 category1 appender2 category2 appender3 category 3 appender4 JSP Custom Tags

  36. Example Background A log4j category’s “priority” setting specifies the “level” required for a message to be logged for a given category: • Debug • Info • Warn • Error • Fatal JSP Custom Tags

  37. Example Background Each category has methods corresponding to the various priorities. You use the appropriate method to log a message of a given priority for the category of interest: • myCat.debug(“Here’s a message.”); • myCat.info(“Here’s a message.”); • myCat.warn(“Here’s a message.”); • myCat.error(“Here’s a message.”); • myCat.fatal(“Here’s a message.”); JSP Custom Tags

  38. Tags and Scripting Variables Tags can introduce variables that can be used later in a JSP scriptlet or another custom tag. JSP Custom Tags

  39. Tag-Introduced Scripting Variables This mechanism allows your tag to introduce a variable that you can use in scriptlets later in the JSP: <log:category id=“myCat”/> <% myCat.debug(“Hello!”); %> JSP Custom Tags

  40. Introduction of Scripting Variables To have your tag introduce a scripting variable, you must perform the following steps: • Somewhere in your tag handler class you must add your variable to one of the JSP scopes. • Create a TagExtraInfo class for your tag handler class and reference it in your TLD. JSP Custom Tags

  41. Creating a Scripting Variable <log:category id=“myCat”/> From TagSupport base class: void setId(String id) { this.id = id; } From your tag’s doStartTag() method: Category rootCat = Category.getRoot(); pageContext.setAttribute(id, rootCat, Page.APPLICATION_SCOPE); JSP Custom Tags

  42. TagExtraInfo Class public class CategoryTagExtraInfo extends TagExtraInfo { public VariableInfo[] getVariableInfo(TagData data) { return new VariableInfo[] { new VariableInfo( data.getAttributeString("id"), "org.apache.log4j.Category", true, VariableInfo.AT_END) }; } } JSP Custom Tags

  43. Variable Scope Tags can introduce scripting variables with one of three scopes represented by the three VariableInfo constants, NESTED, AT_BEGIN, and AT_END. The following diagram explains these three scopes: NESTED AT_BEGIN AT_END doStartTag() doEndTag() JSP Custom Tags

  44. TLD Addition for TEI Class You must also add a <teiclass/> element to your TLD: <teiclass> com.foo.CategoryTagExtraInfo </teiclass> JSP Custom Tags

  45. Example Background SimpleAppenderTag CategoryTag LogMessageTag <log:simpleAppender id="myAppender" … /> <log:category id="testCat" appender="myAppender“ … /> <log:logMessage categoryName="testCat" … /> JSP Custom Tags

  46. Body Tags Body Tags process their tag body contents. The body contents can contain JSP content. JSP Custom Tags

  47. BodyTag Syntax An example call to a body tag: <log:logMessage priority=“fatal”> There was a fatal exception. Time: <%=Calendar.getInstance().getTime()%> </log:logMessage> JSP Custom Tags

  48. BodyTag Interface public interface BodyTag extends Tag { public final static int EVAL_BODY_TAG = 2; void setBodyContent( BodyContent b); void doInitBody() throws JspException; int doAfterBody() throws JspException; } JSP Custom Tags

  49. BodyContent Class • The tag accesses its body contents through a BodyContent object which is set by the JSP container. • BodyContent is a subclass of JspWriter which is used to write content to the response body. • BodyContent also has several methods to manipulate (flush, clear, return as a string) the contents of the body. JSP Custom Tags

  50. JSP Container Call Sequence The JSP container call sequence is different for a body tag than it is for a simple tag: • Instantiates tag handler, calls setup functions. • Calls setters. • JSP container calls doStartTag(). • Calls setBodyContent(). • Calls doInitBody(). • Calls doAfterBody(). • JSP container calls doEndTag(). JSP Custom Tags

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