8. The assert statement

The assert statement is perhaps out of order being described here but it will be relied on in many code examples.

The assert statement evaluates a boolean expression (one that is true or false). If the result is false then the assertion has failed, the program is halted and an error is reported. The following example provides an obviously incorrect statement:

assert 1 == 2

An expression can be appended to the assert statement after a colon (:):

assert true == false : 'true cannot be false'

The second expression can be anything Groovy can evaluate and the result is used in the error message. The following example will (unhelpfully) place the number “8.0” in the error message:

assert true == false : Math.sqrt(64)

Handling failed assertions

For the purposes of our tutorial scripts, using asserts is a handy way to demonstrate a result for a problem. However, it’s not good practice to have a program suddenly just quit when an assertion fails. When you start writing large programs, your code should aim to “fail gracefully” unless it’s really in a position where bailing out is the only option.

Groovy (unlike Java) does not provide a mechanism for turning off assertions so be careful about where you use the assert statement in larger systems. Remember that a failed assert raises an Error (which signals a critical problem) rather than an Exception (from which a program is more likely to recover). Arguably, in running (production) systems, assertions are best suited to dark places in code that should never be reached - they flag when the extremely unlikely condition has happened.

The error raised by a failed assertion can be caught within a try-catch and handled but this isn’t how errors are usually treated (we normally just let them happen). The following example illustrates a class handling a failed assertion by logging the problem - don’t be concerned if you don’t follow the code as it utilises a number of concepts not yet visited:

import groovy.util.logging.*

@Log
class AssertionTest {
    static void runTest() {
        try {
            assert true == false : 'true cannot be false'
        } catch(AssertionError err) {
            log.severe "An assertion failed ${err}"
        }
    }
}

AssertionTest.runTest()

The section on Exceptions will explain this syntax in more depth.

Although it’s Java-focussed, check out the Programming with Assertions guide for more information.