[[migrating-from-junit4]]
== Migrating from JUnit 4

Although the JUnit Jupiter programming model and extension model do not support JUnit 4
features such as `Rules` and `Runners` natively, it is not expected that source code
maintainers will need to update all of their existing tests, test extensions, and custom
build test infrastructure to migrate to JUnit Jupiter.

Instead, JUnit provides a gentle migration path via a _JUnit Vintage test engine_ which
allows existing tests based on JUnit 3 and JUnit 4 to be executed using the JUnit Platform
infrastructure. Since all classes and annotations specific to JUnit Jupiter reside under
the `org.junit.jupiter` base package, having both JUnit 4 and JUnit Jupiter in the
classpath does not lead to any conflicts. It is therefore safe to maintain existing JUnit
4 tests alongside JUnit Jupiter tests. Furthermore, since the JUnit team will continue to
provide maintenance and bug fix releases for the JUnit 4.x baseline, developers have
plenty of time to migrate to JUnit Jupiter on their own schedule.


[[migrating-from-junit4-running]]
=== Running JUnit 4 Tests on the JUnit Platform

Make sure that the `junit-vintage-engine` artifact is in your test runtime path. In that
case JUnit 3 and JUnit 4 tests will automatically be picked up by the JUnit Platform
launcher.

See the example projects in the {junit5-samples-repo}[`junit5-samples`] repository to
find out how this is done with Gradle and Maven.

[[migrating-from-junit4-categories-support]]
==== Categories Support

For test classes or methods that are annotated with `@Category`, the _JUnit Vintage test
engine_ exposes the category's fully qualified class name as a <<running-tests-tags, tag>>
for the corresponding test class or test method. For example, if a test method is
annotated with `@Category(Example.class)`, it will be tagged with `"com.acme.Example"`.
Similar to the `Categories` runner in JUnit 4, this information can be used to filter the
discovered tests before executing them (see <<running-tests>> for details).


[[migrating-from-junit4-tips]]
=== Migration Tips

The following are topics that you should be aware of when migrating existing JUnit 4
tests to JUnit Jupiter.

* Annotations reside in the `org.junit.jupiter.api` package.
* Assertions reside in `org.junit.jupiter.api.Assertions`.
  - Note that you may continue to use assertion methods from `org.junit.Assert` or any
    other assertion library such as {AssertJ}, {Hamcrest}, {Truth}, etc.
* Assumptions reside in `org.junit.jupiter.api.Assumptions`.
  - Note that JUnit Jupiter 5.4 and later versions support methods from JUnit 4's
    `org.junit.Assume` class for assumptions. Specifically, JUnit Jupiter supports JUnit
    4's `AssumptionViolatedException` to signal that a test should be aborted instead of
    marked as a failure.
* `@Before` and `@After` no longer exist; use `@BeforeEach` and `@AfterEach` instead.
* `@BeforeClass` and `@AfterClass` no longer exist; use `@BeforeAll` and `@AfterAll`
  instead.
* `@Ignore` no longer exists: use `@Disabled` or one of the other built-in
  <<writing-tests-conditional-execution, execution conditions>> instead
  - See also <<migrating-from-junit4-ignore-annotation-support>>.
* `@Category` no longer exists; use `@Tag` instead.
* `@RunWith` no longer exists; superseded by `@ExtendWith`.
* `@Rule` and `@ClassRule` no longer exist; superseded by `@ExtendWith` and
  `@RegisterExtension`.
  - See also <<migrating-from-junit4-rule-support>>.
* Assertions and assumptions in JUnit Jupiter accept the failure message as their last
  argument instead of the first one.
  - See <<migrating-from-junit4-failure-message-arguments>> for details.


[[migrating-from-junit4-rule-support]]
=== Limited JUnit 4 Rule Support

As stated above, JUnit Jupiter does not and will not support JUnit 4 rules natively. The
JUnit team realizes, however, that many organizations, especially large ones, are likely
to have large JUnit 4 code bases that make use of custom rules. To serve these
organizations and enable a gradual migration path the JUnit team has decided to support a
selection of JUnit 4 rules verbatim within JUnit Jupiter. This support is based on
adapters and is limited to those rules that are semantically compatible to the JUnit
Jupiter extension model, i.e. those that do not completely change the overall execution
flow of the test.

The `junit-jupiter-migrationsupport` module from JUnit Jupiter currently supports the
following three `Rule` types including subclasses of these types:

* `org.junit.rules.ExternalResource` (including `org.junit.rules.TemporaryFolder`)
* `org.junit.rules.Verifier` (including `org.junit.rules.ErrorCollector`)
* `org.junit.rules.ExpectedException`

As in JUnit 4, Rule-annotated fields as well as methods are supported. By using these
class-level extensions on a test class such `Rule` implementations in legacy code bases
can be _left unchanged_ including the JUnit 4 rule import statements.

This limited form of `Rule` support can be switched on by the class-level annotation
`{EnableRuleMigrationSupport}`. This annotation is a _composed annotation_ which enables
all rule migration support extensions: `VerifierSupport`, `ExternalResourceSupport`, and
`ExpectedExceptionSupport`. You may alternatively choose to annotate your test class with
`@EnableJUnit4MigrationSupport` which registers migration support for rules _and_ JUnit
4's `@Ignore` annotation (see <<migrating-from-junit4-ignore-annotation-support>>).

However, if you intend to develop a new extension for JUnit Jupiter please use the new
extension model of JUnit Jupiter instead of the rule-based model of JUnit 4.


[[migrating-from-junit4-ignore-annotation-support]]
=== JUnit 4 @Ignore Support

In order to provide a smooth migration path from JUnit 4 to JUnit Jupiter, the
`junit-jupiter-migrationsupport` module provides support for JUnit 4's `@Ignore`
annotation analogous to Jupiter's `{Disabled}` annotation.

To use `@Ignore` with JUnit Jupiter based tests, configure a _test_ dependency on the
`junit-jupiter-migrationsupport` module in your build and then annotate your test class
with `@ExtendWith(IgnoreCondition.class)` or `{EnableJUnit4MigrationSupport}` (which
automatically registers the `IgnoreCondition` along with
<<migrating-from-junit4-rule-support>>). The `IgnoreCondition` is an
`{ExecutionCondition}` that disables test classes or test methods that are annotated with
`@Ignore`.

[source,java,indent=0]
----
include::{testDir}/example/IgnoredTestsDemo.java[tags=user_guide]
----


[[migrating-from-junit4-failure-message-arguments]]
=== Failure Message Arguments

The `Assumptions` and `Assertions` classes in JUnit Jupiter declare arguments in a
different order than in JUnit 4. In JUnit 4 assertion and assumption methods accept the
failure message as the first argument; whereas, in JUnit Jupiter assertion and assumption
methods accept the failure message as the last argument.

For instance, the method `assertEquals` in JUnit 4 is declared as `assertEquals(String
message, Object expected, Object actual)`, but in JUnit Jupiter it is declared as
`assertEquals(Object expected, Object actual, String message)`. The rationale for this is
that a failure message is _optional_, and optional arguments should be declared after
required arguments in a method signature.

The methods affected by this change are the following:

- Assertions
  * `assertTrue`
  * `assertFalse`
  * `assertNull`
  * `assertNotNull`
  * `assertEquals`
  * `assertNotEquals`
  * `assertArrayEquals`
  * `assertSame`
  * `assertNotSame`
  * `assertThrows`
- Assumptions
  * `assumeTrue`
  * `assumeFalse`
