Aspectran JSP Tag Library Guide

The Aspectran JSP Tag Library provides a set of custom tags that simplify expression evaluation, multi-language message handling, URL generation, and environment-based content control within JSP views.

1. Tag Library Declaration

To use the Aspectran tags, include the following directive at the top of your JSP page:

<%@ taglib uri="http://aspectran.com/tags" prefix="aspectran" %>

2. Main Tags Overview

<aspectran:eval>

Evaluates an AsEL (Aspectran Expression Language) expression and either prints the result or assigns it to a variable. Since the AselExpressionPreprocessor is applied, you can use modern operators.

Preprocessor Supported Operators

1. Safe Navigation (?.): Checks if an object is null before accessing its properties.

   <%-- If user is null, returns null; otherwise, accesses the cart --%>
   <aspectran:eval expression="user?.cart?.numberOfItems"/>
   

2. Elvis Operator (?:): Provides a default value if the expression result is null.

   <%-- If the result is null, prints 0 --%>
   <aspectran:eval expression="user?.cart?.numberOfItems ?: 0"/>
   

3. Collection Selection (.?[...]): Filters elements in a collection that match a condition.

   <%-- List of products with price >= 100 --%>
   <aspectran:eval expression="products.?[price >= 100]"/>
   

4. Collection Projection (.![...]): Transforms elements in a collection into a new list.

   <%-- A list of only the names of all products --%>
   <aspectran:eval expression="products.![name]"/>
   

5. First/Last Selection (.^[...], .$[...]): Selects the first or last element matching a condition.

   <aspectran:eval expression="products.^[category == 'Electronics']"/>
   

6. Matches Operator (matches): Checks if a string matches a regular expression.

   <aspectran:eval expression="user.email matches '^[\\w-\\.]+@([\\w-]+\\.)+[\\w-]{2,4}$'"/>
   

7. Type Operator (T(...)): Accesses static class types to call constants or methods.

   <aspectran:eval expression="T(java.lang.Math).PI"/>
   <aspectran:eval expression="T(com.example.Constants).DEFAULT_PAGE_SIZE"/>
   

<aspectran:message>

Retrieves a message from the multi-language message source.

• Practical Examples:

  <%-- Simple message output --%>
  <aspectran:message code="label.username"/>
  
  <%-- Passing arguments --%>
  <aspectran:message code="msg.welcome" arguments="${user.name}"/>
  
  <%-- Default text if the code is not found --%>
  <aspectran:message code="non.existent.code" text="This is a default message."/>
  
  <%-- Store the result in a variable for reuse --%>
  <aspectran:message code="page.title" var="pTitle"/>
  <title>${pTitle}</title>
  

<aspectran:url>

Generates a URL with support for URL templates and parameter encoding.

• Practical Examples:

  <%-- Simple URL with context path --%>
  <a href="<aspectran:url value="/main"/>">Go Home</a>
  
  <%-- Adding query parameters --%>
  <aspectran:url value="/search" var="searchUrl">
      <aspectran:param name="keyword" value="Aspectran"/>
      <aspectran:param name="page" value="1"/>
  </aspectran:url>
  <a href="${searchUrl}">View Search Results</a>
  
  <%-- URL Template Parameter (PathVariable) --%>
  <aspectran:url value="/products/{id}">
      <aspectran:param name="id" value="${product.id}"/>
  </aspectran:url>
  

<aspectran:profile>

Controls UI components based on the active execution environment profiles.

• Practical Examples:

  <%-- Show debug tools only in the development environment --%>
  <aspectran:profile expression="dev">
      <div id="debug-toolbar">...</div>
  </aspectran:profile>
  
  <%-- Include specific scripts only if not in production --%>
  <aspectran:profile expression="!prod">
      <script src="/js/test-helper.js"></script>
  </aspectran:profile>
  
  <%-- Multiple profile conditions (OR operation) --%>
  <aspectran:profile expression="dev | staging">
      <p>This is a test server.</p>
  </aspectran:profile>
  

3. Resolving Conflicts with JSP EL

In JSP 2.1 and higher, #{...} is recognized as a Deferred Expression, which can cause server errors. When using Aspectran token syntax in the eval tag, follow these recommendations:

Recommended: Direct Access

The eval tag operates within the current Activity context, so you can access objects directly by name without using tokens like #{currentActivityData}.

<%-- AS-IS (Potential Conflict) --%>
<aspectran:eval expression="#{currentActivityData}.user.name"/>

<%-- TO-BE (Safe and Concise) --%>
<aspectran:eval expression="user.name"/>

Alternative: Escaping the Character

If you must use the #{...} syntax, add a backslash (\) before the # character to prevent the JSP engine from processing it.

<aspectran:eval expression="\#{currentActivityData}.user.name"/>