javax.el.LambdaExpression Maven / Gradle / Ivy
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package javax.el;
import java.util.List;
import java.util.Map;
import java.util.HashMap;
import java.util.ArrayList;
/**
* Encapsulates a parameterized {@link ValueExpression}.
*
* A LambdaExpression 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 Expression, including another
* LambdaExpression.
* A LambdaExpression is created when an EL expression containing
* a Lambda expression is evaluated.
* A LambdaExpression can be invoked by calling
* {@link LambdaExpression#invoke}, with
* an {@link javax.el.ELContext} and a list of the actual arguments.
* Alternately, a LambdaExpression can be invoked without passing
* a ELContext, in which case the ELContext previously
* set by calling {@link LambdaExpression#setELContext} will be used.
* The evaluation of the ValueExpression 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.
* @see ELContext#getLambdaArgument
* @see ELContext#enterLambdaScope
* @see ELContext#exitLambdaScope
*/
public class LambdaExpression {
private List formalParameters = new ArrayList();
private ValueExpression expression;
private ELContext context;
// Arguments from nesting lambdas, when the body is another lambda
private Map envirArgs = null;
/**
* Creates a new LambdaExpression.
* @param formalParameters The list of String representing the formal
* parameters.
* @param expression The ValueExpression representing the
* body.
*/
public LambdaExpression (List formalParameters,
ValueExpression expression) {
this.formalParameters = formalParameters;
this.expression = expression;
this.envirArgs = new HashMap();
}
/**
* 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.
* 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
* ELException is thrown.
*
* 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.
*
* @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 null.
* @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 lambdaArgs = new HashMap();
// 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.
* 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
* ELException is thrown.
*
* 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.
*
* 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 null.
* @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);
}
}