Package org.apache.taglibs.standard.lang.jstl

Class ELEvaluator

  • public class ELEvaluator
extends Object

    This is the main class for evaluating expression Strings. An expression String is a String that may contain expressions of the form ${...}. Multiple expressions may appear in the same expression String. In such a case, the expression String's value is computed by concatenating the String values of those evaluated expressions and any intervening non-expression text, then converting the resulting String to the expected type using the PropertyEditor mechanism.

    In the special case where the expression String is a single expression, the value of the expression String is determined by evaluating the expression, without any intervening conversion to a String.

    The evaluator maintains a cache mapping expression Strings to their parsed results. For expression Strings containing no expression elements, it maintains a cache mapping ExpectedType/ExpressionString to parsed value, so that static expression Strings won't have to go through a conversion step every time they are used. All instances of the evaluator share the same cache. The cache may be bypassed by setting a flag on the evaluator's constructor.

    The evaluator must be passed a VariableResolver in its constructor. The VariableResolver is used to resolve variable names encountered in expressions, and can also be used to implement "implicit objects" that are always present in the namespace. Different applications will have different policies for variable lookups and implicit objects - these differences can be encapsulated in the VariableResolver passed to the evaluator's constructor.

    Most VariableResolvers will need to perform their resolution against some context. For example, a JSP environment needs a PageContext to resolve variables. The evaluate() method takes a generic Object context which is eventually passed to the VariableResolver - the VariableResolver is responsible for casting the context to the proper type.

    Once an evaluator instance has been constructed, it may be used multiple times, and may be used by multiple simultaneous Threads. In other words, an evaluator instance is well-suited for use as a singleton.

    Version:
    $Change: 181177 $$DateTime: 2001/06/26 08:45:09 $$Author: kchung $
    Author:
    Nathan Abramson - Art Technology Group, Shawn Bayern

    • Constructor Detail

      • ELEvaluator

        public ELEvaluator​(VariableResolver pResolver)
        Constructor
        Parameters:
        pResolver - the object that should be used to resolve variable names encountered in expressions. If null, all variable references will resolve to null.

      • ELEvaluator

        public ELEvaluator​(VariableResolver pResolver,
                   boolean pBypassCache)
        Constructor
        Parameters:
        pResolver - the object that should be used to resolve variable names encountered in expressions. If null, all variable references will resolve to null.
        pBypassCache - flag indicating if the cache should be bypassed

    • Method Detail

      • evaluate

        public Object evaluate​(String pExpressionString,
                       Object pContext,
                       Class pExpectedType,
                       Map functions,
                       String defaultPrefix)
                throws ELException
        Evaluates the given expression String
        Parameters:
        pExpressionString - the expression String to be evaluated
        pContext - the context passed to the VariableResolver for resolving variable names
        pExpectedType - the type to which the evaluated expression should be coerced
        Returns:
        the expression String evaluated to the given expected type
        Throws:
        ELException

      • parseExpressionString

        public Object parseExpressionString​(String pExpressionString)
                             throws ELException
        Gets the parsed form of the given expression string. If the parsed form is cached (and caching is not bypassed), return the cached form, otherwise parse and cache the value. Returns either a String, Expression, or ExpressionString.
        Throws:
        ELException

      • parseAndRender

        public String parseAndRender​(String pExpressionString)
                      throws ELException
        Parses the given expression string, then converts it back to a String in its canonical form. This is used to test parsing.
        Throws:
        ELException