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package javax.el;
import java.util.
List;
import java.util.
Map;
import java.util.
HashMap;
import java.util.
ArrayList;
/**
* <p>Encapsulates a parameterized {@link ValueExpression}.</p>
*
* <p>A <code>LambdaExpression</code> is a representation of the EL Lambda
* expression syntax. It consists of a list of the formal parameters and a
* body, represented by a {@link ValueExpression}.
* The body can be any valid <code>Expression</code>, including another
* <code>LambdaExpression</code>.</p>
* A <code>LambdaExpression</code> is created when an EL expression containing
* a Lambda expression is evaluated.</p>
* <p>A <code>LambdaExpression</code> can be invoked by calling
* {@link LambdaExpression#invoke}, with
* an {@link javax.el.ELContext} and a list of the actual arguments.
* Alternately, a <code>LambdaExpression</code> can be invoked without passing
* a <code>ELContext</code>, in which case the <code>ELContext</code> previously
* set by calling {@link LambdaExpression#setELContext} will be used.
* The evaluation of the <code>ValueExpression</code> in the body uses the
* {@link ELContext} to resolve references to the parameters, and to evaluate
* the lambda expression.
* The result of the evaluation is returned.</p>
* @see ELContext#getLambdaArgument
* @see ELContext#enterLambdaScope
* @see ELContext#exitLambdaScope
*/
public class
LambdaExpression {
private
List<
String>
formalParameters = new
ArrayList<
String>();
private
ValueExpression expression;
private
ELContext context;
// Arguments from nesting lambdas, when the body is another lambda
private
Map<
String,
Object>
envirArgs = null;
/**
* Creates a new LambdaExpression.
* @param formalParameters The list of String representing the formal
* parameters.
* @param expression The <code>ValueExpression</code> representing the
* body.
*/
public
LambdaExpression (
List<
String>
formalParameters,
ValueExpression expression) {
this.
formalParameters =
formalParameters;
this.
expression =
expression;
this.
envirArgs = new
HashMap<
String,
Object>();
}
/**
* Set the ELContext to use in evaluating the LambdaExpression.
* The ELContext must to be set prior to the invocation of the LambdaExpression,
* unless it is supplied with {@link LambdaExpression#invoke}.
* @param context The ELContext to use in evaluating the LambdaExpression.
*/
public void
setELContext(
ELContext context) {
this.
context =
context;
}
/**
* Invoke the encapsulated Lambda expression.
* <p> The supplied arguments are matched, in
* the same order, to the formal parameters. If there are more arguments
* than the formal parameters, the extra arguments are ignored. If there
* are less arguments than the formal parameters, an
* <code>ELException</code> is thrown.</p>
*
* <p>The actual Lambda arguments are added to the ELContext and are
* available during the evaluation of the Lambda expression. They are
* removed after the evaluation.</p>
*
* @param elContext The ELContext used for the evaluation of the expression
* The ELContext set by {@link #setELContext} is ignored.
* @param args The arguments to invoke the Lambda expression. For calls with
* no arguments, an empty array must be provided. A Lambda argument
* can be <code>null</code>.
* @return The result of invoking the Lambda expression
* @throws ELException if not enough arguments are provided
* @throws NullPointerException is elContext is null
*/
public
Object invoke(
ELContext elContext,
Object...
args)
throws
ELException {
int
i = 0;
Map<
String,
Object>
lambdaArgs = new
HashMap<
String,
Object>();
// First get arguments injected from the outter lambda, if any
lambdaArgs.
putAll(
envirArgs);
for (
String fParam:
formalParameters) {
if (
i >=
args.length) {
throw new
ELException("Expected Argument " +
fParam +
" missing in Lambda Expression");
}
lambdaArgs.
put(
fParam,
args[
i++]);
}
elContext.
enterLambdaScope(
lambdaArgs);
Object ret =
expression.
getValue(
elContext);
// If the result of evaluating the body is another LambdaExpression,
// whose body has not been evaluated yet. (A LambdaExpression is
// evaluated iff when its invoke method is called.) The current lambda
// arguments may be needed in that body when it is evaluated later,
// after the current lambda exits. To make these arguments available
// then, they are injected into it.
if (
ret instanceof
LambdaExpression) {
((
LambdaExpression)
ret).
envirArgs.
putAll(
lambdaArgs);
}
elContext.
exitLambdaScope();
return
ret;
}
/**
* Invoke the encapsulated Lambda expression.
* <p> The supplied arguments are matched, in
* the same order, to the formal parameters. If there are more arguments
* than the formal parameters, the extra arguments are ignored. If there
* are less arguments than the formal parameters, an
* <code>ELException</code> is thrown.</p>
*
* <p>The actual Lambda arguments are added to the ELContext and are
* available during the evaluation of the Lambda expression. They are
* removed after the evaluation.</p>
*
* The ELContext set by {@link LambdaExpression#setELContext} is used in
* the evaluation of the lambda Expression.
*
* @param args The arguments to invoke the Lambda expression. For calls with
* no arguments, an empty array must be provided. A Lambda argument
* can be <code>null</code>.
* @return The result of invoking the Lambda expression
* @throws ELException if not enough arguments are provided
*/
public
Object invoke(
Object...
args) {
return
invoke(this.
context,
args);
}
}