Hamcrest
Encyclopedia
Hamcrest is a framework for creating matchers ('Hamcrest' is an anagram of 'matchers'), allowing match rules to be defined declaratively
. These matchers have uses in unit testing frameworks such as JUnit
and jMock.
Hamcrest has been ported to Java
, C++
, Objective-C
, Python
, ActionScript 3 and PHP
.
assert(x y);
But this syntax fails to produce a sufficiently good error message if 'x' and 'y' are not equal.
It would be better if the error message displayed the value of 'x' and 'y'.
To solve this problem, "second generation" unit test frameworks provide a family of assertion statements, which produce better error messages. For example,
assert_equal(x, y);
assert_not_equal(x, y);
But this leads to an explosion in the number of assertion macros, as the above set is expanded to support comparisons different from simple equality. So "third generation" unit test frameworks use a library such as Hamcrest to support an 'assert_that' operator that can be combined with 'matcher' objects, leading to syntax like this:
assert_that(x, equal_to(y))
assert_that(x, is_not(equal_to(y)))
The benefit is that you still get fluent error messages when the assertion fails, but now you have greater extensibility. It is now possible to define operations that take matchers as arguments and return them as results, leading to a grammar that can generate a huge number of possible matcher expressions from a small number of primitive matchers.
These higher-order matcher operations include logical connectives (and, or and not), and operations for iterating over collections.
This results in a rich matcher language which allows complex assertions over collections to be written in a declarative, rather than a procedural, programming style.
Declarative programming
In computer science, declarative programming is a programming paradigm that expresses the logic of a computation without describing its control flow. Many languages applying this style attempt to minimize or eliminate side effects by describing what the program should accomplish, rather than...
. These matchers have uses in unit testing frameworks such as JUnit
JUnit
JUnit is a unit testing framework for the Java programming language. JUnit has been important in the development of test-driven development, and is one of a family of unit testing frameworks collectively known as xUnit that originated with SUnit....
and jMock.
Hamcrest has been ported to Java
Java (programming language)
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
, C++
C++
C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
, Objective-C
Objective-C
Objective-C is a reflective, object-oriented programming language that adds Smalltalk-style messaging to the C programming language.Today, it is used primarily on Apple's Mac OS X and iOS: two environments derived from the OpenStep standard, though not compliant with it...
, Python
Python (programming language)
Python is a general-purpose, high-level programming language whose design philosophy emphasizes code readability. Python claims to "[combine] remarkable power with very clear syntax", and its standard library is large and comprehensive...
, ActionScript 3 and PHP
PHP
PHP is a general-purpose server-side scripting language originally designed for web development to produce dynamic web pages. For this purpose, PHP code is embedded into the HTML source document and interpreted by a web server with a PHP processor module, which generates the web page document...
.
Rationale
"First generation" unit test frameworks provide an 'assert' statement, allowing one to assert during a test that a particular condition must be true. If the condition is false, the test fails. For example:assert(x y);
But this syntax fails to produce a sufficiently good error message if 'x' and 'y' are not equal.
It would be better if the error message displayed the value of 'x' and 'y'.
To solve this problem, "second generation" unit test frameworks provide a family of assertion statements, which produce better error messages. For example,
assert_equal(x, y);
assert_not_equal(x, y);
But this leads to an explosion in the number of assertion macros, as the above set is expanded to support comparisons different from simple equality. So "third generation" unit test frameworks use a library such as Hamcrest to support an 'assert_that' operator that can be combined with 'matcher' objects, leading to syntax like this:
assert_that(x, equal_to(y))
assert_that(x, is_not(equal_to(y)))
The benefit is that you still get fluent error messages when the assertion fails, but now you have greater extensibility. It is now possible to define operations that take matchers as arguments and return them as results, leading to a grammar that can generate a huge number of possible matcher expressions from a small number of primitive matchers.
These higher-order matcher operations include logical connectives (and, or and not), and operations for iterating over collections.
This results in a rich matcher language which allows complex assertions over collections to be written in a declarative, rather than a procedural, programming style.