A test fixture is something used to consistently test some item, device, or piece of software.Test fixtures can be found when testing electronics, software and physical devices.

In testing electronic equipment such as circuit boards, electronic components, and chips, a test fixture is a device or setup designed to hold the device under test in place and allow it to be tested by being subjected to controlled electronic test signals.

Examples are a bed of nails tester or SmartFixture.

A software test fixture sets up the system for the testing process by providing it with all the necessary code to initialize it, thereby satisfying whatever preconditions there maybe.^[1] An example could be loading up a database with known parameters from a customer site before running your test. The Ruby on Rails web framework uses YAML to initialize a database before running a test.^[2] This allows for tests to be repeatable, which is one of the key features of an effective test framework.^[1]

Test fixtures can be set up in three ways: in-line setup, delegate setup, and implicit setup. In-line setup is where the developer creates the test fixture in the same method as the rest of the test and is the simplest test fixture to create but can lead to duplication if multiple tests need to same initial data. Delegate setup is where the developer places the test fixture in a separate standalone helper method that can be accessed by multiple test methods. Implicit setup is where the developer places the test fixture in a setup method which is used to setup multiple test methods. This is different from delegate setup in that the overall setup of multiple tests are in a single setup method where the test fixture gets created instead of each test method having its own setup procedures and linking to an external test fixture.^[3]

The advantages of test fixture are that it allows for tests to be repeatable since you are always starting with the same setup every time. Test fixtures also eases test code design by allowing the developer to separate methods into different functions and reuse each function for other tests. It also preconfigures tests into a known state at start instead of working with whatever was left from a previous test run. A disadvantage would be that it could lead to duplication of test fixtures if using in-line setup.^[1][3]

It is considered bad practice when test fixtures are too general in implicit setup, or when the test method sets up a test fixture and does not use it during the test. A more subtle issue is if the test methods ignore certain fields within the test fixture. Another bad practice would be a test setup that contains more steps than what is needed for the test, this is a problem seen in in-line setup.^[3]

A test case is considered unsafe when it modifies the fixtures during the setup phase. This is more pronounced in the fixture setups other than the in-line setup. An unsafe test case is a serious problem, one that could render subsequent tests useless by leaving the fixture in an unexpected state. Also, the order of tests is important, so that you know whether a fixture setup needs to be rerun or not. If a safe test is run after an unsafe test then the fixture setup is rerun if the fixture is common for both but the same is not true for vice versa.^[1]

Examples of fixtures include, loading a database with a specific, known set of data, erasing a hard disk and installing a known clean operating system installation, copying a specific known set of files or the preparation of input data and set-up/creation of mock objects.

Software used to systematically run reproducible tests on a piece of software under test is known as a test harness; part of its job is to set up suitable test fixtures.

In generic xUnit, a test fixture is all the things that must be in place in order to run a test and expect a particular outcome.^[4]

Frequently fixtures are created by handling setUp() and tearDown() events of the unit testing framework. In setUp() one would create the expected state for the test, and in tearDown() it would clean up what had been set up.

Four phases of a test:

1. Set-up
2. Exercise, interacting with the system under test
3. Verify, determining whether the expected outcome has been obtained
4. Tear down, to return to the original state

In physical testing, a fixture is a device or apparatus to hold or support the test specimen during the test. The influence of test fixtures on test results is important and is an ongoing subject of research.^[5]

Many test methods detail the requirements of test fixtures in the text of the document.^[6][7]

• 
  Test fixture on universal testing machine for three-point flex test
• 
  Hydraulic system testing on fixture
• 
  jet engine fixtures for operational testing

Some fixtures employ clamps, wedge grips and pincer grips.

• 
  pincer clamps max. 50 kN spring-biased
• 
  offset compensated wedge grip max.50 kN
• 
  different vice and screw grips of a German manufacturer

Further types of construction are eccentric roller fixtures, thread grips and button head grips as well as rope grips.

• 
  symmetric roller grip, self-closing and self-adjusting
• 
  multiple button head grip for speedy tests on series
• 
  small rope grip 200N to test fine wires
• 
  very compact wedge grip for temperature chambers providing extreme temperatures

Mechanical holding apparatus provide the clamping force via arms, wedges or eccentric wheel to the jaws. Additional there are pneumatic and hydraulic fixtures for tensile testing that do allow very fast clamping procedures and very high clamping forces

• 
  pneumatic grip, symmetrical, clamping force 2.4 kN
• 
  heavy duty hydraulic clamps, clamping force 700 kN
• 
  Bending device for tensile testing machines
• 
  Equipment to test peeling forces up to 10 kN

• Unit testing

• Unit Testing with JUnit, by Yoonsik Cheon.
• The Low-Down on fixtures, from A Guide to Testing Rails Applications.