3
JSP Markup Language Tags

This chapter documents the Oracle JSP Markup Language (JML) tag library, which provides a set of JSP tags to allow developers to script JSP pages without using Java statements. The JML library provides tags for variable declarations, control flow, conditional branches, iterative loops, parameter settings, and calls to objects.

The chapter consists of the following sections:

Note:

The library described here, which uses a standard runtime implementation, is also supported through an Oracle-specific compile-time implementation. The compile-time syntax and tags are documented in Appendix A, "JML Compile-Time Syntax and Tags". General considerations in using compile-time tags instead of runtime tags are discussed in the Oracle Application Server Containers for J2EE Support for JavaServer Pages Developer's Guide.

Overview of the JSP Markup Language (JML) Tag Library

OC4J supplies the JSP Markup Language (JML) tag library, developed according to JSP standards. JML tags are intended to simplify coding syntax for JSP developers who are not proficient with Java. There are two main categories of JML tags: logic/flow control and bean binding.

These topics are covered in the following sections:

Note the following requirements for using JML tags:

Verify that the file ojsputil.jar is installed and in your classpath. This file is provided with the OC4J installation, in the "well-known" tag library directory.
The tag library descriptor, jml.tld, must be available to the application, and any JSP page using the library must have an appropriate taglib directive. In an Oracle Application Server installation, the TLD is in ojsputil.jar. The uri value for jml.tld is the following:
```
http://xmlns.oracle.com/j2ee/jsp/tld/ojsp/jml.tld
```

You can refer to the Oracle Application Server Containers for J2EE Support for JavaServer Pages Developer's Guide for information about taglib directives, the well-known tag library directory, TLD files, and the meaning of uri values.

Note:

The custom JML tag library provided with OC4J pre-dates the JavaServer Pages Standard Tag Library (JSTL) and has areas of duplicate functionality. For standards compliance, it is now generally advisable to use JSTL instead. See "Support for the JavaServer Pages Standard Tag Library".

Oracle is not desupporting the existing library, however. For features in the custom library that are not yet available in JSTL, where there seems to be general usefulness, Oracle will try to have the features adopted into the JSTL standard as appropriate.

JML Tag Library Philosophy

JavaServer Pages technology is intended for two separate developer communities:

Those whose primary skill is Java programming
Those whose primary skill is in designing static content, particularly in HTML, and who may have limited scripting experience

The JML tag library is designed to allow most Web developers, with little or no knowledge of Java, to assemble JSP applications with a full complement of program flow-control features.

This model presumes that the business logic is contained in JavaBeans that are developed separately by a Java developer.

JML Tag Categories

The JML tag library covers a feature set split into two functional categories, as summarized in Table 3-1.

Table 3-1 JML Tag Functional Categories

Tag Categories	Functionality	Tags
Bean binding tags	The purpose of these tags is to declare or undeclare a JavaBean at a specified JSP scope. See "Bean Binding Tag Descriptions".	useVariable useForm useCookie remove
Logic/flow control tags	These tags offer simplified syntax to define code flow, such as for iterative loops or conditional branches. See "Logic and Flow Control Tag Descriptions".	if choose..when..[otherwise] foreach return flush

JSP Markup Language (JML) Tag Descriptions

The following sections document the JML tags that are supported in the current JSP runtime implementation:

Bean Binding Tag Descriptions

Logic and Flow Control Tag Descriptions

Notes:

The prefix "jml:" is used in the tag syntax here. This is by convention but is not required. You can specify any desired prefix in your taglib directive.
See "Tag Syntax Symbology and Notes" for general information about tag syntax conventions in this manual.

Bean Binding Tag Descriptions

The following sections document JML tags used for bean-binding operations:

JML useVariable Tag

This tag offers a convenient alternative to the jsp:useBean tag for declaring simple variables.

Syntax

<jml:useVariable id = "beanInstanceName" 
             [ scope = "page" | "request" | "session" | "application" ]
               type = "string" | "boolean" | "number" | "fpnumber" 
             [ value = "stringLiteral" ] />

Attributes

id (required): Specifies the variable being declared.
scope: Defines the duration or scope of the variable (as with a jsp:useBean tag). The default scope is page.
type (required): Specifies the type of the variable. Type specifications refer to JmlString, JmlBoolean, JmlNumber, or JmlFPNumber.
value: Use this to set the variable directly in the declaration, as either a string literal or a JSP expression enclosed in <%=... %> syntax. If this attribute is not specified, then the value remains the same as when it was last set (if it already exists) or is initialized with a default value. If it is specified, then the value is always set, regardless of whether this declaration instantiates the object or merely acquires it from the named scope.

Example

Consider the following example:

<jml:useVariable id = "isValidUser" type = "boolean" value = "<%= dbConn.isValid() %>" scope = "session" />

This is equivalent to the following:

<jsp:useBean id = "isValidUser" class = "oracle.jsp.jml.JmlBoolean" scope = "session" />
<jsp:setProperty name="isValidUser" property="value" value = "<%= dbConn.isValid() %>" />

JML useForm Tag

This tag provides a convenient syntax for declaring variables and setting them to values passed in from the request.

Syntax

<jml:useForm id = "beanInstanceName" 
           [ scope = "page" | "request" | "session" | "application" ]
           [ type = "string" | "boolean" | "number" | "fpnumber" ]
             param = "requestParameterName" />

Attributes

id (required): Specifies the variable being declared or referenced.
scope: Defines the duration or scope of the variable (as with a jsp:useBean tag). The default is "page".
type: Specifies the type of the variable. Type specifications refer to JmlString, JmlBoolean, JmlNumber, or JmlFPNumber. The default is "string".
param (required): Specifies the name of the request parameter whose value is used in setting the variable. If the request parameter exists, then the variable value is always updated, regardless of whether this declaration brings the variable into existence. If the request parameter does not exist, then the variable value remains unchanged.

Example

The following example sets a session variable named user of the type JmlString to the value of the request parameter named user.

<jml:useForm id = "user" type = "string" param = "user" scope = "session" />

This is equivalent to the following:

<jsp:useBean id = "user" class = "oracle.jsp.jml.JmlString" scope = "session" />
<jsp:setProperty name="user" property="value" param = "user" />

JML useCookie Tag

This tag offers a convenient syntax for declaring variables and setting them to values contained in cookies.

Syntax

<jml:useCookie id = "beanInstanceName" 
             [ scope = "page" | "request" | "session" | "application" ]
             [ type = "string" | "boolean" | "number" | "fpnumber" ]
               cookie = "cookieName" />

Attributes

id (required): Specifies the variable being declared or referenced.
scope: Defines the duration or scope of the variable. This attribute is optional; the default is "page".
type: Identifies the type of the variable. Type specifications refer to JmlString, JmlBoolean, JmlNumber, or JmlFPNumber. The default is "string".
cookie (required): Specifies the name of the cookie whose value is used in setting this variable. If the cookie exists, then the variable value is always updated, regardless of whether this declaration brings the variable into existence. If the cookie does not exist, then the variable value remains unchanged.

Example

The following example sets a request variable named user of the type JmlString to the value of the cookie named user.

<jml:useCookie id = "user" type = "string" cookie = "user" scope = "request" />

This is equivalent to the following:

<jsp:useBean id = "user" class = "oracle.jsp.jml.JmlString" scope = "request" /> 
<% 
     Cookies [] cookies = request.getCookies();
     for (int i = 0; i < cookies.length; i++) {
             if (cookies[i].getName().equals("user")) {
                     user.setValue(cookies[i].getValue());
                     break;
             }
     }
%>

JML remove Tag

This tag removes an object, typically a bean, from its scope.

Syntax

<jml:remove id = "beanInstanceName" 
          [ scope = "page" | "request" | "session" | "application" ] />

Attributes

id (required): Specifies the name of the bean being removed.
scope: Specifies the scope of the bean being removed. If not specified, then scopes are searched in the following order: 1) page, 2) request, 3) session, 4) application. The first object whose name matches id is removed.

Example

The following example removes the session user object:

<jml:remove id = "user" scope = "session" />

This is equivalent to the following:

<% session.removeValue("user"); %>

Logic and Flow Control Tag Descriptions

The following sections document JML tags that are used for logic and flow control:

These tags, which are intended for developers without extensive Java experience, can be used in place of Java logic and flow control syntax such as iterative loops and conditional branches.

JML if Tag

This tag evaluates a single conditional statement. If the condition is true, then the body of the if tag is executed.

Syntax

<jml:if condition = "<%= jspExpression %>" >
     ...body of jml:if tag (executed if the condition is true)...
</jml:if>

Attributes

condition (required): Specifies the conditional expression to be evaluated.

Example

The following e-commerce example displays information from a user's shopping cart. The code checks to see if the variable holding the current T-shirt order is empty. If not, then the size that the user has ordered is displayed. Assume currTS is of type JmlString.

<jml:if condition = "<%= !currTS.isEmpty() %>" >
     <S>(size: <%= currTS.getValue().toUpperCase() %>)</S>&nbsp
</jml:if>

JML choose...when...[otherwise] Tags

The choose tag, with associated when and otherwise tags, provides a multiple conditional statement.

The body of the choose tag contains one or more when tags, where each when tag represents a condition. For the first when condition that is true, the body of that when tag is executed. A maximum of one when body is executed.

If the when conditions are all false, and if the optional otherwise tag is specified, then the body of the otherwise tag is executed.

Syntax

<jml:choose>
    <jml:when condition = "<%= jspExpression %>" >
        ...body of 1st jml:when tag (executed if the condition is true)...
    </jml:when>
    ...
    [...optional additional when tags...]
    [ <jml:otherwise>
        ...body of jml:otherwise tag (executed if all when conditions false)...
    </jml:otherwise> ]
</jml:choose>

Attributes

The when tag uses the following attribute:

condition (required): Specifies the conditional expression to be evaluated.

The choose and otherwise tags have no attributes.

Example

The following e-commerce example displays information from a user's shopping cart. This code checks to see if anything has been ordered. If so, the current order is displayed; otherwise, the user is asked to shop again. (This example omits the code to display the current order.) Presume orderedItem is of the type JmlBoolean.

<jml:choose>
     <jml:when condition = "<%= orderedItem.getValue() %>"  >
          You have changed your order:
             -- output the current order --
     </jml:when>
     <jml:otherwise>
          Are you sure we can't interest you in something, cheapskate?
     </jml:otherwise>
</jml:choose>

JML for Tag

This tag provides the ability to iterate through a loop, as with a Java for loop.

The id attribute is a local loop variable of the type java.lang.Integer that contains the value of the current range element. The range starts at the value expressed in the from attribute and is incremented by one after each execution of the body of the loop, until it exceeds the value expressed in the to attribute.

Once the range has been traversed, control goes to the first statement following the for end-tag.

Note:

Descending ranges are not supported. The from value must be less than or equal to the to value.

Syntax

<jml:for id = "loopVariable"
         from = "<%= jspExpression %>" 
         to = "<%= jspExpression %>" >
     ...body of jml:for tag (executed once at each value of range, inclusive)...
</jml:for>

Attributes

id (required): This is the name of the loop variable, which holds the current value in the range. This is a java.lang.Integer value and can be used only within the body of the tag.
from (required): Specifies the start of the range. This is an expression that must evaluate to a Java int value.
to (required): Specifies the end of the range. This is an expression that must evaluate to a Java int value.

Example

The following example repeatedly prints "Hello World" in progressively smaller headings (H1, H2, H3, H4, H5).

<jml:for id="i" from="<%= 1 %>" to="<%= 5 %>" >
     <H<%=i%>>
            Hello World!
     </H<%=i%>>
</jml:for>

JML foreach Tag

This tag provides the ability to iterate over a homogeneous set of values. The body of the tag is executed once for each element in the set. If the set is empty, then the body is not executed.

The id attribute is a local loop variable containing the value of the current set element. Its type is specified in the type attribute. The specified type should match the type of the set elements, as applicable.

This tag currently supports iterations over the following types of data structures:

Java array
java.util.Enumeration
java.util.Vector

Syntax

<jml:foreach id = "loopVariable"
             in = "<%= jspExpression %>" 
             limit = "<%= jspExpression %>"
             type = "package.class" >
...body of jml:foreach tag (executes once for each element in data structure)...
</jml:foreach>

Attributes

id (required): This is the name of the loop variable, which holds the value of the current element at each step of the iteration. It can be used only within the body of the tag. Its type is the same as specified in the type attribute.
in (required): Specifies a JSP expression that evaluates to a Java array, Enumeration object, or Vector object containing the set of values over which to iterate.
limit (required): Specifies a JSP expression that evaluates to a Java int value defining the maximum number of iterations, regardless of the number of elements in the set.
type (required): Specifies the type of the loop variable. This should match the type of the set elements, as applicable.

Example

The following example iterates over the request parameters.

<jml:foreach id="name" in="<%= request.getParameterNames() %>" type="java.lang.String" >
    Parameter: <%= name %>
    Value: <%= request.getParameter(name) %> <br>
</jml:foreach>

Alternatively, if you want to handle parameters with multiple values:

<jml:foreach id="name" in="<%= request.getParameterNames() %>" type="java.lang.String" >
    Parameter: <%= name %>
    Value: <jml:foreach id="val" in="<%=request.getParameterValues(name)%>"
                       type="java.lang.String" >
                <%= val %> :
           </jml:foreach>
    <br>
</jml:foreach>

JML return Tag

When this tag is reached, execution returns from the page without further processing.

Syntax

<jml:return />

Attributes

None.

Example

The following example returns without processing the page if the timer has expired.

<jml:if condition="<%= timer.isExpired() %>" >
     You did not complete in time!
     <jml:return />
</jml:if>

JML flush Tag

This tag writes the current contents of the page buffer back to the client. This applies only if the page is buffered; otherwise, there is no effect.

Syntax

<jml:flush />

Attributes

None.

Example

The following example flushes the current page contents before performing an expensive operation.

<jml:flush />
<% myBean.expensiveOperation(out); %>

3JSP Markup Language Tags

Overview of the JSP Markup Language (JML) Tag Library

JML Tag Library Philosophy

JML Tag Categories

Table 3-1 JML Tag Functional Categories

JSP Markup Language (JML) Tag Descriptions

Bean Binding Tag Descriptions

JML useVariable Tag

Syntax

Attributes

Example

JML useForm Tag

Syntax

Attributes

Example

JML useCookie Tag

Syntax

Attributes

Example

JML remove Tag

Syntax

Attributes

Example

Logic and Flow Control Tag Descriptions

JML if Tag

Syntax

Attributes

Example

JML choose...when...[otherwise] Tags

Syntax

Attributes

Example

JML for Tag

Syntax

Attributes

Example

JML foreach Tag

Syntax

Attributes

Example

JML return Tag

Syntax

Attributes

Example

JML flush Tag

Syntax

Attributes

Example

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JSP Markup Language Tags