what is shouldFail in groovy? - groovy

Question

How does the code within the 'shouldFail?' block work? I understand that it is a closure, but the code gets called regardless of whether or not I call is using its signature. Besides, what's the deal with the "ReadOnlyPropertyException" showing up in the parenthesis? If it is a parameter, then it is not setup as listed in the official documentation!!
Questions: What is 'shouldFail'? How should it be invoked? How to handle the exception purportedly thrown by this method/function/closure?
void test02_ReadOnlyFieldInGroovyBean() {
// You've probably noticed how Groovy automatically generates getters/setters for you. But what if you don't
// want to generate a setter because it's a read-only field? Just mark it with 'final'. Groovy will understand.
// Try to modify Ken's ssn. You should get a ReadOnlyPropertyException.
def person = new GroovyPerson('Ken', 'Kousen', '7878')
def failed = false
shouldFail (ReadOnlyPropertyException) {
// ------------ START EDITING HERE ----------------------
System.out.println(" i am in should fail")
person.ssn='8332';
// ------------ STOP EDITING HERE ----------------------
failed = false
System.out.println(" exiting should fail")
}
//def foobar=shouldFail("hjh");
//def foobar=true;
failed=shouldFail('abc');
//System.out.println("Failed: "+failed);
assert failed
// The code wrapping your additions verifies that the ReadOnlyProperty exception has been thrown.
// The curly brackets ({}) represent a closure. We'll get into what that means very soon.
}

Answer 1

shouldFail() (in this variant) takes a class and a closure. It runs the closure and reports a test failure if the closure does NOT exit by throwing an exception of that type. As to catching the exception, you don't - shouldFail() does that for you.
See: http://docs.groovy-lang.org/latest/html/gapi/groovy/test/GroovyAssert.html#shouldFail%28java.lang.Class,%20groovy.lang.Closure%29
(Reading the comments and the code around them, it looks like this unit test should pass because setting the .ssn property of GroovyPerson will fail as it is a read-only property, causing a ReadOnlyPropertyException.)