[[testcontext-parallel-test-execution]]
= Parallel Test Execution

Spring Framework 5.0 introduced basic support for executing tests in parallel within a
single JVM when using the Spring TestContext Framework. In general, this means that most
test classes or test methods can be run in parallel without any changes to test code
or configuration.

TIP: For details on how to set up parallel test execution, see the documentation for your
testing framework, build tool, or IDE.

Keep in mind that the introduction of concurrency into your test suite can result in
unexpected side effects, strange runtime behavior, and tests that fail intermittently or
seemingly randomly. The Spring Team therefore provides the following general guidelines
for when not to run tests in parallel.

Do not run tests in parallel if the tests:

* Use Spring Framework's `@DirtiesContext` support.
* Use Spring Boot's `@MockBean` or `@SpyBean` support.
* Use JUnit 4's `@FixMethodOrder` support or any testing framework feature
  that is designed to ensure that test methods run in a particular order. Note,
  however, that this does not apply if entire test classes are run in parallel.
* Change the state of shared services or systems such as a database, message broker,
  filesystem, and others. This applies to both embedded and external systems.

[TIP]
====
If parallel test execution fails with an exception stating that the `ApplicationContext`
for the current test is no longer active, this typically means that the
`ApplicationContext` was removed from the `ContextCache` in a different thread.

This may be due to the use of `@DirtiesContext` or due to automatic eviction from the
`ContextCache`. If `@DirtiesContext` is the culprit, you either need to find a way to
avoid using `@DirtiesContext` or exclude such tests from parallel execution. If the
maximum size of the `ContextCache` has been exceeded, you can increase the maximum size
of the cache. See the discussion on xref:testing/testcontext-framework/ctx-management/caching.adoc[context caching]
for details.
====

WARNING: Parallel test execution in the Spring TestContext Framework is only possible if
the underlying `TestContext` implementation provides a copy constructor, as explained in
the javadoc for {spring-framework-api}/test/context/TestContext.html[`TestContext`]. The
`DefaultTestContext` used in Spring provides such a constructor. However, if you use a
third-party library that provides a custom `TestContext` implementation, you need to
verify that it is suitable for parallel test execution.


