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Approval Testing

Installation (Gradle)

compile group: "org.http4k", name: "http4k-testing-approval", version: "3.137.0"


Approval testing is a form of testing which allows the expected output of a test to be specified in a non-code but still source-controlled format, such as a text file. This is a powerful alternative to traditional assertion-based tests for a number of reasons:

  1. It is often inconvenient and/or error prone to attempt to write assertions to cover the entirety of test output - examples of this include JSON, HTML or XML documents.
  2. Output may not always be in a format that can be created easily in a test.
  3. In case of a mismatch, output can be more efficiently diagnosed by the human eye.
  4. The output of a test may change significantly in a short period (this is especially true for HTML content), but we also want to tightly control the contract.

The general idea for implementing this style of testing in http4k is based on the excellent okeydoke library, and is centered around the idea of comparing the output of an HTTP operation - this is generally the Response content, but it can also be the Request if we are interested in testing construction of request content.

For each test-case, a named <test name>.approved file is committed (under the src/test/resources folder), against which the test output can be compared by an Approver object injected into the test method. In case of a mismatch, an equivalent <test name>.actual file is written. This file can then be verified and if ok, renamed to become the approved file. To make this operation easier in the IDE, we recommend the usage of the IntelliJ OkeyDoke plugin which adds a mouse and keyboard shortcut to rename the file.

The http4k-testing-approval module implements this functionality as a JUnit5 extension that will inject the Approver automatically into test methods.

Standard Approval tests

By using the ApprovalTest extension, an instance of an Approver is injected into each test.


Content-type specific Approval tests

Because so many APIs are based around messages with a particular content type, the module also provides Junit 5 extensions that will:

  1. Check for the presence of the a particular content-type on the HttpMessage under test and fail if it is not valid.
  2. Validate that the HttpMessage actually contains valid content for the content type.
  3. Format and compare the approval output as pretty-printed version. Note that by default the http4k format modules use compact printing to conserve message space.

The module also provides the following built-in extensions:

  • HtmlApprovalTest
  • JsonApprovalTest
  • XmlApprovalTest


Implementing custom JUnit Extensions

As with the rest of http4k, a base implementation, BaseApprovalTest of the Junit5 Extension is provided, allowing API users to implement custom approval schemes or non-FS based approaches for storing the approval files.