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.
Constructor and Description |
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ELEvaluator(VariableResolver pResolver)
Constructor
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Modifier and Type | Method and Description |
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Object |
evaluate(String pExpressionString,
Object pContext,
Class pExpectedType,
Map functions,
String defaultPrefix)
Evaluates the given expression String
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String |
parseAndRender(String pExpressionString)
Parses the given expression string, then converts it back to a
String in its canonical form.
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Object |
parseExpressionString(String pExpressionString)
Gets the parsed form of the given expression string.
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void |
setBypassCache(boolean pBypassCache)
Enable cache bypass
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public ELEvaluator(VariableResolver pResolver)
pResolver
- the object that should be used to resolve
variable names encountered in expressions. If null, all variable
references will resolve to null.public void setBypassCache(boolean pBypassCache)
pBypassCache
- flag indicating cache should be bypassedpublic Object evaluate(String pExpressionString, Object pContext, Class pExpectedType, Map functions, String defaultPrefix) throws ELException
pExpressionString
- the expression String to be evaluatedpContext
- the context passed to the VariableResolver for
resolving variable namespExpectedType
- the type to which the evaluated expression
should be coercedELException
public Object parseExpressionString(String pExpressionString) throws ELException
ELException
public String parseAndRender(String pExpressionString) throws ELException
ELException
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