In Groovy/Spock assert call methods are not executed - groovy

Question

In Groovy Unit Test with Spock the following task is quite common:
assert myResult == calculateExpectedResult() (With or without the assert keyword.)
The groovy assert prints out lots of infomation on what is going on here and why my assertion failed. But when the compared objects are very complex and deep it can be tricky go get the concrete property that failed the test.
For this I found the Javers Framework that does an excellent Job comparing the objects and producing an exact diff. I created a trait to do this:
trait DiffTrait {
Javers javers = JaversBuilder.javers().build()
String diff(result, expected) {
Diff diff = javers.compare(result, expected);
def valueChanges = diff.getChangesByType(ValueChange)
String message = ""
valueChanges.each { message += "\n$it.propertyName = $it.left instead of expected: $it.right" }
return message
}
}
Now I can use it in my Unit Tests like this:
def expected = calculateExpectedResult()
assert myResult == expected, diff(myResult, expected)
This way I get a nicely printed list of differences.
But this is kind of verbose because I have to specify the values two times.
So I have changed the trait like this:
trait DiffTrait {
Javers javers = JaversBuilder.javers().build()
def result
def expected
String diff(result, expected) {
Diff diff = javers.compare(result, expected);
def valueChanges = diff.getChangesByType(ValueChange)
String message = ""
valueChanges.each { message += "\n$it.propertyName = $it.left instead of expected: $it.right" }
return message
}
String diff() {
diff(result, expected)
}
def result(result) {
this.result = result
return result
}
def expected(expected) {
this.expected = expected
return expected
}
}
The idea was to use it like this:
def result = callTheSystemToProduceTheRealResult()
def expected = calculateExpectedResult()
assert result(myResult) == expected(expected), diff()
But surprisingly this does not work! The two attributes are null and the diff Method fails with a NotNull-Exception. If I debug this code the expected/result methods are never called!
If I rewrite the code like this
def result = result(callTheSystemToProduceTheRealResult())
def expected = expected(calculateExpectedResult())
assert myResult == expected, diff()
everything works as expected. The methods get called correctly and the attributes are set.
My question is: Why can't I call these methods in the assert statement? What is the difference from the Groovy/Spock perspective of these two code fragements?
Here is a gist containing all the code as running example.

Answer 1

It is quite easy to explain. Assertion message is evaluated before the assertion itself. The following piece of code works perfectly, however it displays static diff message:
import org.javers.core.Javers
import org.javers.core.JaversBuilder
import org.javers.core.diff.Diff
import org.javers.core.diff.changetype.ValueChange
import spock.lang.Specification
class LolSpec extends Specification implements DiffTrait {
def 'lol'() {
expect:
def whatIGot = new Lol(l: 'a')
def whatIExpected = new Lol(l: 'b')
assert result(whatIGot) == expected(whatIExpected), 'diff'
}
}
trait DiffTrait {
Javers javers = JaversBuilder.javers().build()
def result
def expected
String diff() {
diff(result, expected)
}
String diff(result, expected) {
Diff diff = javers.compare(result, expected);
def valueChanges = diff.getChangesByType(ValueChange)
String message = ""
valueChanges.each { message += "\n$it.propertyName = $it.left instead of expected: $it.right" }
return message
}
def result(result) {
this.result = result
return result
}
def expected(expected) {
this.expected = expected
return expected
}
}
class Lol {
String l
}
You need to pass the arguments twice or change the implementation, e.g.:
import groovy.transform.EqualsAndHashCode
import org.javers.core.Javers
import org.javers.core.JaversBuilder
import org.javers.core.diff.changetype.ValueChange
import spock.lang.Specification
class LolSpec extends Specification {
def 'lol'() {
expect:
def whatIGot = new Lol(l: 'a')
def whatIExpected = new Lol(l: 'b')
def diff = new Diff(result: whatIGot, expected: whatIExpected)
assert diff.check(), diff.message()
}
}
class Diff {
Javers javers = JaversBuilder.javers().build()
def result
def expected
String message() {
def diff = javers.compare(result, expected);
def valueChanges = diff.getChangesByType(ValueChange)
String message = ""
valueChanges.each { message += "\n$it.propertyName = $it.left instead of expected: $it.right" }
return message
}
boolean check() {
result.equals(expected)
}
}
#EqualsAndHashCode
class Lol {
String l
}