SpEL parsing parameters with "Collectors.toList()" in stream won't work as expressionString? - spring

Question

This will work
parser.parseExpression("#configList.stream().toArray()").getValue(context)
but the following won't
parser.parseExpression("#configList.stream().map(o -> o.ruleId).collect(Collectors.toList())").getValue(context)
F.Y.I the context is constructed as follows:
Object[] args = joinPoint.getArgs();
Method method = ((MethodSignature) joinPoint.getSignature()).getMethod();
String[] params = discoverer.getParameterNames(method);
EvaluationContext context = new StandardEvaluationContext();
for (int i = 0; i < params.length; i++) {
context.setVariable(params[i], args[i]);
}

Answer 1

Although Java can be used in SPeL expressions, SPeL itself is a separate language and does not fully support the Java language. From the documentation:
SpEL is based on a technology agnostic API allowing other expression language implementations to be integrated should the need arise.
To perform filtering and mapping operations on a List in a SPeL expression, use collection selection and collection projection respectively:
Collection selection example
// Java:
configList.stream().filter(o -> o.getRuleId() > 2).collect(Collectors.toList())
// SPeL (notice the question mark) :
"#configList.?[ruleId>2]"
Collection projection example
// Java:
configList.stream().map(o -> o.getRuleId()).collect(Collectors.toList())
// SPeL (notice the exclamation mark) :
"#configList.![ruleId]"
I have set up a small example to demonstrate it:
public class So64738543ExpressionTest {
public static class RuleItem {
private int ruleId;
public RuleItem(int ruleId) {
this.ruleId = ruleId;
}
public int getRuleId() {
return ruleId;
}
}
#Test
public void collectionProjection() {
List<RuleItem> ruleItems = Arrays.asList(new RuleItem(1), new RuleItem(2), new RuleItem(3));
EvaluationContext context = new StandardEvaluationContext(ruleItems);
Expression expression = new SpelExpressionParser().parseExpression("#root.![ruleId]");
Assert.assertEquals(Arrays.asList(1,2,3), expression.getValue(context));
}
}
[edit]
Furthermore, if a SPeL expression becomes increasingly complex, I highly recommend to move the expression to a static method and invoke it using a T operator. Don't forget to include the fully qualified package name when referring to the static method.