REL Standard Tag Library - Expression Language

The REL Standard Tag Library (RSTL) is the Tridion equivalent to JSTL (JSP Standard Tag Library) and aims to provide the same functionality for Tridion REL.

This post presents the Expression Language used in RSTL tag attributes. If you are interested in the RSTL itself, check out my post REL Standard Tag Library.

RSTL can be downloaded from my Google Code page, under REL Standard Tag Library. Always use the latest JAR.

Expression Language

The tag attributes support an expression language allowing it to reference existing context variables, to evaluate mathematical and conditional expressions. I implemented an expression parser to evaluate such expressions.

${expression} Notation

This notation is available inside tag attributes. The parser evaluates the expression inside (as mathematical expression) and returns its value.

Logical Expressions

Logical expressions produce a boolean value: true or false.

Syntax:
    [!|not] expression [logical_operator expression]

where expression is either:

• a mathematical expression;
• a variable;
• a bean with potential properties or index;

Examples:

• a + 2 == 3
• a || (b < 5 and !c)

A logical expression without a logical_operator and without a negation (e.g. a+1) is evaluated as mathematical expression and its value is evaluated to a boolean according to the following rules:

• if null, returns false;
• if numeric, returns false if value is zero;
• if string, returns false if empty string or if the value is the string "false";
• if collection, returns false if empty collection;
• if boolean, returns boolean value;
• anything else returns true;

Logical Operators

• and or &&: perform logical AND (shortcut operator);
• or or ||: perform logical OR (shortcut operator);
• not or !: perform logical negation;
• eq or ==: equals;
• neq or !=: not equals;
• gt or >: greater than;
• ge or >=: greater than or equal;
• lt or <: lower than;
• le or <=: lower than or equal;
• ( and ): parentheses used for grouping and precedence;

Mathematical Expressions

Mathematical expressions are used inside logical expression as operands. They produce a value.

Syntax:
    [-]term [mathematical_operator term]

where term is either:

• a (signed) number;
• a string;
• a variable;
• a function;
• a bean with potential properties or index;

Examples:

• 1 + 2
• 2 * 3 + a ^ 4
• sin(pi) + 3 * (4 - 2)
• 'Hello ' + 'world'

Mathematical Operators

• + : performs addition, if both operands are numeric; otherwise, string concatenation;
• - : subtraction;
• * : multiplication;
• / : division;
• ^ : exponentiation;
• ( and ): parentheses used for grouping and precedence;

Mathematical Functions

• sin : sine;
• cos : cosine;
• tan : tangent;
• asin : arc sine;
• acos : arc cosine;
• atan : arc tangent;
• sqrt : square root;
• exp : Euler's number e raised to a power;
• ln : natural logarithm;
• log : common logarithm;
• log2 : binary logarithm;

Dot (.) Notation

JavaBeans can be accessed as simple variables. However, if they contain properties, then they can be accessed using the following syntax:

    bean.property

Examples:

• componentPresentation.componentId
• componentPresentation.meta.contentType

The property is accessed on the bean by usage of reflection and the actual getter method is invoked using the JavaBeans naming standards.

Indexed Values

When the value of an expression evaluates to java.util.List, java.util.Map or an array of Object, it is possible to access an element in the collection by using the following syntax:

    collection[index]

Examples:

• pageMetas[0]
• dcps[1].componentId

For maps, the value of the index expression is used as key in order to return its mapped value.

For lists and arrays, the index expression must evaluate to an Integer, and the element on the corresponding position is returned.

It is possible to mix the dot (.) notation with the square brackets ([ ])  indexing syntax on both the bean itself as well as on the property.