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Hamcrest writing custom matchers diary

Hamcrest writing custom matchers diary to find an

jMock and Hamcrest 1 provide many Matcher classes and factory functions that allow you to specify the appropriate parameter values of the method invocation 2. However, sometimes the predefined constraints don’t let you specify an expectation precisely enough to share that which you mean or to maintain your tests flexible. In such instances, it is simple to define new matchers that seamlessly extend the present set based on jMock.

A Matcher is definitely an object that implements the org.hamcrest.Matcher 3 interface. It will a couple of things:

  • reports whether a parameter value meets the constraint (the matches method).
  • generates a readable description to become incorporated in test failure messages (the describeTo method inherited in the SelfDescribing 4 interface).

To produce a new matcher:

Write a category that extends Hamcrest’s BaseMatcher or TypeSafeMatcher 5 base classes. The next matcher class tests whether a string begins with confirmed prefix.

Write a properly-named factory way in which creates a clear case of your brand-new matcher.

The purpose of the factory technique is to help make the test code read clearly, so consider the way it will appear when utilized in an expectation.

Make use of your factory approach to create matchers inside your tests. The next expectation specifies the error approach to the logger object should be known as once by having an argument that’s a string beginning with “FATAL”.

An Essential Rule

Matcher objects should be stateless.

When dispatching 6 each invocation, jMock uses the matchers to locate an expectation that suits the invocation’s arguments. Which means that it’ll call the matchers many occasions throughout the test, possibly even following the expectation was already matched and invoked. Actually, jMock gives no guarantees of when and the number of occasions it’ll call the matchers.

Hamcrest writing custom matchers diary many times during the

It has no impact on stateless matchers but implies that the part of stateful matchers can’t be predicted.

If you wish to maintain condition as a result of invocations, make use of an Action 7. not really a Matcher.

More Details

More documentation on how to write Matchers can be obtained in the Hamcrest project 8 .

1. Hamcrest: code.google.com/p/hamcrest

2. specify the appropriate parameter values of the method invocation: world wide web.jmock.org/matchers.html

3. org.hamcrest.Matcher: code.google.com/p/hamcrest/source/browse/trunk/hamcrest-java/hamcrest-core/src/primary/java/org/hamcrest/Matcher.java

4. SelfDescribing: code.google.com/p/hamcrest/source/browse/trunk/hamcrest-java/hamcrest-core/src/primary/java/org/hamcrest/SelfDescribing.java

5. TypeSafeMatcher: world wide web.jmock.org/

6. dispatching: world wide web.jmock.org/dispatch.html

7. Action: world wide web.jmock.org/custom-actions.html

8. Hamcrest project: code.google.com/p/hamcrest/

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