[[extensions]]
== Extension Model

[[extensions-overview]]
=== Overview

In contrast to the competing `Runner`, `TestRule`, and `MethodRule` extension points in
JUnit 4, the JUnit Jupiter extension model consists of a single, coherent concept: the
`Extension` API. Note, however, that `Extension` itself is just a marker interface.

[[extensions-registration]]
=== Registering Extensions

Extensions can be registered _declaratively_ via
<<extensions-registration-declarative,`@ExtendWith`>>, _programmatically_ via
<<extensions-registration-programmatic,`@RegisterExtension`>>, or _automatically_ via
Java's <<extensions-registration-automatic,`ServiceLoader`>> mechanism.

[[extensions-registration-declarative]]
==== Declarative Extension Registration

Developers can register one or more extensions _declaratively_ by annotating a test
interface, test class, test method, or custom _<<writing-tests-meta-annotations,composed
annotation>>_ with `@ExtendWith(...)` and supplying class references for the extensions to
register. As of JUnit Jupiter 5.8, `@ExtendWith` may also be declared on fields or on
parameters in test class constructors, in test methods, and in `@BeforeAll`, `@AfterAll`,
`@BeforeEach`, and `@AfterEach` lifecycle methods.

For example, to register a `WebServerExtension` for a particular test method, you would
annotate the test method as follows. We assume the `WebServerExtension` starts a local web
server and injects the server's URL into parameters annotated with `@WebServerUrl`.

[source,java,indent=0]
----
@Test
@ExtendWith(WebServerExtension.class)
void getProductList(@WebServerUrl String serverUrl) {
	WebClient webClient = new WebClient();
	// Use WebClient to connect to web server using serverUrl and verify response
	assertEquals(200, webClient.get(serverUrl + "/products").getResponseStatus());
}
----

To register the `WebServerExtension` for all tests in a particular class and its
subclasses, you would annotate the test class as follows.

[source,java,indent=0]
----
@ExtendWith(WebServerExtension.class)
class MyTests {
	// ...
}
----

Multiple extensions can be registered together like this:

[source,java,indent=0]
----
@ExtendWith({ DatabaseExtension.class, WebServerExtension.class })
class MyFirstTests {
	// ...
}
----

As an alternative, multiple extensions can be registered separately like this:

[source,java,indent=0]
----
@ExtendWith(DatabaseExtension.class)
@ExtendWith(WebServerExtension.class)
class MySecondTests {
	// ...
}
----

[TIP]
.Extension Registration Order
====
Extensions registered declaratively via `@ExtendWith` at the class level, method level, or
parameter level will be executed in the order in which they are declared in the source
code. For example, the execution of tests in both `MyFirstTests` and `MySecondTests` will
be extended by the `DatabaseExtension` and `WebServerExtension`, **in exactly that order**.
====

If you wish to combine multiple extensions in a reusable way, you can define a custom
_<<writing-tests-meta-annotations,composed annotation>>_ and use `@ExtendWith` as a
_meta-annotation_ as in the following code listing. Then `@DatabaseAndWebServerExtension`
can be used in place of `@ExtendWith({ DatabaseExtension.class, WebServerExtension.class })`.

[source,java,indent=0]
----
@Target({ ElementType.TYPE, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@ExtendWith({ DatabaseExtension.class, WebServerExtension.class })
public @interface DatabaseAndWebServerExtension {
}
----

The above examples demonstrate how `@ExtendWith` can be applied at the class level or at
the method level; however, for certain use cases it makes sense for an extension to be
registered declaratively at the field or parameter level. Consider a
`RandomNumberExtension` which generates random numbers that can be injected into a field or
via a parameter in a constructor, test method, or lifecycle method. If the extension
provides a `@Random` annotation that is meta-annotated with
`@ExtendWith(RandomNumberExtension.class)` (see listing below), the extension can be used
transparently as in the following `RandomNumberDemo` example.

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

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

[[extensions-RandomNumberExtension]]
The following code listing provides an example of how one might choose to implement such a
`RandomNumberExtension`. This implementation works for the use cases in
`RandomNumberDemo`; however, it may not prove robust enough to cover all use cases -- for
example, the random number generation support is limited to integers; it uses
`java.util.Random` instead of `java.security.SecureRandom`; etc. In any case, it is
important to note which extension APIs are implemented and for what reasons.

Specifically, `RandomNumberExtension` implements the following extension APIs:

- `BeforeAllCallback`: to support static field injection
- `BeforeEachCallback`: to support non-static field injection
- `ParameterResolver`: to support constructor and method injection

[NOTE]
====
Ideally, the `RandomNumberExtension` would implement `TestInstancePostProcessor` instead
of `BeforeEachCallback` in order to support non-static field injection immediately after
the test class has been instantiated.

However, JUnit Jupiter currently does not allow a `TestInstancePostProcessor` to be
registered via `@ExtendWith` on a non-static field (see
link:{junit5-repo}/issues/3437[issue 3437]). In light of that, the `RandomNumberExtension`
implements `BeforeEachCallback` as an alternative approach.
====

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

[TIP]
.Extension Registration Order for `@ExtendWith` on Fields
====
Extensions registered declaratively via `@ExtendWith` on fields will be ordered relative
to `@RegisterExtension` fields and other `@ExtendWith` fields using an algorithm that is
deterministic but intentionally nonobvious. However, `@ExtendWith` fields can be ordered
using the `@Order` annotation. See the <<extensions-registration-programmatic-order,
Extension Registration Order>> tip for `@RegisterExtension` fields for details.
====

NOTE: `@ExtendWith` fields may be either `static` or non-static. The documentation on
<<extensions-registration-programmatic-static-fields, Static Fields>> and
<<extensions-registration-programmatic-instance-fields, Instance Fields>> for
`@RegisterExtension` fields also applies to `@ExtendWith` fields.

[[extensions-registration-programmatic]]
==== Programmatic Extension Registration

Developers can register extensions _programmatically_ by annotating fields in test classes
with `{RegisterExtension}`.

When an extension is registered _declaratively_ via
<<extensions-registration-declarative,`@ExtendWith`>>, it can typically only be configured
via annotations. In contrast, when an extension is registered via `@RegisterExtension`, it
can be configured _programmatically_ -- for example, in order to pass arguments to the
extension's constructor, a static factory method, or a builder API.

[[extensions-registration-programmatic-order]]
[TIP]
.Extension Registration Order
====
By default, extensions registered programmatically via `@RegisterExtension` or
declaratively via `@ExtendWith` on fields will be ordered using an algorithm that is
deterministic but intentionally nonobvious. This ensures that subsequent runs of a test
suite execute extensions in the same order, thereby allowing for repeatable builds.
However, there are times when extensions need to be registered in an explicit order. To
achieve that, annotate `@RegisterExtension` fields or `@ExtendWith` fields with `{Order}`.

Any `@RegisterExtension` field or `@ExtendWith` field not annotated with `@Order` will be
ordered using the _default_ order which has a value of `Integer.MAX_VALUE / 2`. This
allows `@Order` annotated extension fields to be explicitly ordered before or after
non-annotated extension fields. Extensions with an explicit order value less than the
default order value will be registered before non-annotated extensions. Similarly,
extensions with an explicit order value greater than the default order value will be
registered after non-annotated extensions. For example, assigning an extension an explicit
order value that is greater than the default order value allows _before_ callback
extensions to be registered last and _after_ callback extensions to be registered first,
relative to other programmatically registered extensions.
====

NOTE: `@RegisterExtension` fields must not be `null` (at evaluation time) but may be
either `static` or non-static.

[[extensions-registration-programmatic-static-fields]]
===== Static Fields

If a `@RegisterExtension` field is `static`, the extension will be registered after
extensions that are registered at the class level via `@ExtendWith`. Such _static
extensions_ are not limited in which extension APIs they can implement. Extensions
registered via static fields may therefore implement class-level and instance-level
extension APIs such as `BeforeAllCallback`, `AfterAllCallback`,
`TestInstancePostProcessor`, and `TestInstancePreDestroyCallback` as well as method-level
extension APIs such as `BeforeEachCallback`, etc.

In the following example, the `server` field in the test class is initialized
programmatically by using a builder pattern supported by the `WebServerExtension`. The
configured `WebServerExtension` will be automatically registered as an extension at the
class level -- for example, in order to start the server before all tests in the class
and then stop the server after all tests in the class have completed. In addition, static
lifecycle methods annotated with `@BeforeAll` or `@AfterAll` as well as `@BeforeEach`,
`@AfterEach`, and `@Test` methods can access the instance of the extension via the
`server` field if necessary.

[source,java,indent=0]
.Registering an extension via a static field in Java
----
include::{testDir}/example/registration/WebServerDemo.java[tags=user_guide]
----

[[extensions-registration-programmatic-static-fields-kotlin]]
====== Static Fields in Kotlin

The Kotlin programming language does not have the concept of a `static` field. However,
the compiler can be instructed to generate a `private static` field using the `@JvmStatic`
annotation in Kotlin. If you want the Kotlin compiler to generate a `public static` field,
you can use the `@JvmField` annotation instead.

The following example is a version of the `WebServerDemo` from the previous section that
has been ported to Kotlin.

[source,kotlin,indent=0]
.Registering an extension via a static field in Kotlin
----
include::{kotlinTestDir}/example/registration/KotlinWebServerDemo.kt[tags=user_guide]
----

[[extensions-registration-programmatic-instance-fields]]
===== Instance Fields

If a `@RegisterExtension` field is non-static (i.e., an instance field), the extension
will be registered after the test class has been instantiated and after each registered
`TestInstancePostProcessor` has been given a chance to post-process the test instance
(potentially injecting the instance of the extension to be used into the annotated
field). Thus, if such an _instance extension_ implements class-level or instance-level
extension APIs such as `BeforeAllCallback`, `AfterAllCallback`, or
`TestInstancePostProcessor`, those APIs will not be honored. By default, an instance
extension will be registered _after_ extensions that are registered at the method level
via `@ExtendWith`; however, if the test class is configured with
`@TestInstance(Lifecycle.PER_CLASS)` semantics, an instance extension will be registered
_before_ extensions that are registered at the method level via `@ExtendWith`.

In the following example, the `docs` field in the test class is initialized
programmatically by invoking a custom `lookUpDocsDir()` method and supplying the result
to the static `forPath()` factory method in the `DocumentationExtension`. The configured
`DocumentationExtension` will be automatically registered as an extension at the method
level. In addition, `@BeforeEach`, `@AfterEach`, and `@Test` methods can access the
instance of the extension via the `docs` field if necessary.

[source,java,indent=0]
.An extension registered via an instance field
----
include::{testDir}/example/registration/DocumentationDemo.java[tags=user_guide]
----

[[extensions-registration-automatic]]
==== Automatic Extension Registration

In addition to <<extensions-registration-declarative,declarative extension registration>>
and <<extensions-registration-programmatic,programmatic extension registration>> support
using annotations, JUnit Jupiter also supports _global extension registration_ via Java's
`{ServiceLoader}` mechanism, allowing third-party extensions to be auto-detected and
automatically registered based on what is available in the classpath.

Specifically, a custom extension can be registered by supplying its fully qualified class
name in a file named `org.junit.jupiter.api.extension.Extension` within the
`/META-INF/services` folder in its enclosing JAR file.

[[extensions-registration-automatic-enabling]]
===== Enabling Automatic Extension Detection

Auto-detection is an advanced feature and is therefore not enabled by default. To enable
it, set the `junit.jupiter.extensions.autodetection.enabled` _configuration parameter_ to
`true`. This can be supplied as a JVM system property, as a _configuration parameter_ in
the `LauncherDiscoveryRequest` that is passed to the `Launcher`, or via the JUnit Platform
configuration file (see <<running-tests-config-params>> for details).

For example, to enable auto-detection of extensions, you can start your JVM with the
following system property.

`-Djunit.jupiter.extensions.autodetection.enabled=true`

When auto-detection is enabled, extensions discovered via the `{ServiceLoader}` mechanism
will be added to the extension registry after JUnit Jupiter's global extensions (e.g.,
support for `TestInfo`, `TestReporter`, etc.).

[[extensions-registration-inheritance]]
==== Extension Inheritance

Registered extensions are inherited within test class hierarchies with top-down
semantics. Similarly, extensions registered at the class-level are inherited at the
method-level. Furthermore, a specific extension implementation can only be registered
once for a given extension context and its parent contexts. Consequently, any attempt to
register a duplicate extension implementation will be ignored.

[[extensions-conditions]]
=== Conditional Test Execution

`{ExecutionCondition}` defines the `Extension` API for programmatic, _conditional test
execution_.

An `ExecutionCondition` is _evaluated_ for each container (e.g., a test class) to
determine if all the tests it contains should be executed based on the supplied
`ExtensionContext`. Similarly, an `ExecutionCondition` is _evaluated_ for each test to
determine if a given test method should be executed based on the supplied
`ExtensionContext`.

When multiple `ExecutionCondition` extensions are registered, a container or test is
disabled as soon as one of the conditions returns _disabled_. Thus, there is no guarantee
that a condition is evaluated because another extension might have already caused a
container or test to be disabled. In other words, the evaluation works like the
short-circuiting boolean OR operator.

See the source code of `{DisabledCondition}` and `{Disabled}` for concrete examples.

[[extensions-conditions-deactivation]]
==== Deactivating Conditions

Sometimes it can be useful to run a test suite _without_ certain conditions being active.
For example, you may wish to run tests even if they are annotated with `@Disabled` in
order to see if they are still _broken_. To do this, provide a pattern for the
`junit.jupiter.conditions.deactivate` _configuration parameter_ to specify which
conditions should be deactivated (i.e., not evaluated) for the current test run. The
pattern can be supplied as a JVM system property, as a _configuration parameter_ in the
`LauncherDiscoveryRequest` that is passed to the `Launcher`, or via the JUnit Platform
configuration file (see <<running-tests-config-params>> for details).

For example, to deactivate JUnit's `@Disabled` condition, you can start your JVM with the
following system property.

`-Djunit.jupiter.conditions.deactivate=org.junit.*DisabledCondition`

[[extensions-conditions-deactivation-patterns]]
===== Pattern Matching Syntax

Refer to <<running-tests-config-params-deactivation-pattern>> for details.

[[extensions-test-instance-pre-construct-callback]]
=== Test Instance Pre-construct Callback

`{TestInstancePreConstructCallback}` defines the API for `Extensions` that wish to be invoked
_prior_ to test instances being constructed (by a constructor call or via
`{TestInstanceFactory}`).

This extension provides a symmetric call to `{TestInstancePreDestroyCallback}` and is useful
in combination with other extensions to prepare constructor parameters or keeping track of test
instances and their lifecycle.

[[extensions-test-instance-factories]]
=== Test Instance Factories

`{TestInstanceFactory}` defines the API for `Extensions` that wish to _create_ test class
instances.

Common use cases include acquiring the test instance from a dependency injection
framework or invoking a static factory method to create the test class instance.

If no `TestInstanceFactory` is registered, the framework will invoke the _sole_
constructor for the test class to instantiate it, potentially resolving constructor
arguments via registered `ParameterResolver` extensions.

Extensions that implement `TestInstanceFactory` can be registered on test interfaces,
top-level test classes, or `@Nested` test classes.

[WARNING]
====
Registering multiple extensions that implement `TestInstanceFactory` for any single class
will result in an exception being thrown for all tests in that class, in any subclass,
and in any nested class. Note that any `TestInstanceFactory` registered in a superclass
or _enclosing_ class (i.e., in the case of a `@Nested` test class) is _inherited_. It is
the user's responsibility to ensure that only a single `TestInstanceFactory` is
registered for any specific test class.
====

[[extensions-test-instance-post-processing]]
=== Test Instance Post-processing

`{TestInstancePostProcessor}` defines the API for `Extensions` that wish to _post
process_ test instances.

Common use cases include injecting dependencies into the test instance, invoking custom
initialization methods on the test instance, etc.

For a concrete example, consult the source code for the `{MockitoExtension}` and the
`{SpringExtension}`.

[[extensions-test-instance-pre-destroy-callback]]
=== Test Instance Pre-destroy Callback

`{TestInstancePreDestroyCallback}` defines the API for `Extensions` that wish to process
test instances _after_ they have been used in tests and _before_ they are destroyed.

Common use cases include cleaning dependencies that have been injected into the
test instance, invoking custom de-initialization methods on the test instance, etc.

[[extensions-parameter-resolution]]
=== Parameter Resolution

`{ParameterResolver}` defines the `Extension` API for dynamically resolving parameters at
runtime.

If a _test class_ constructor, _test method_, or _lifecycle method_ (see
<<writing-tests-definitions>>) declares a parameter, the parameter must be _resolved_ at
runtime by a `ParameterResolver`. A `ParameterResolver` can either be built-in (see
`{TestInfoParameterResolver}`) or <<extensions-registration,registered by the user>>.
Generally speaking, parameters may be resolved by _name_, _type_, _annotation_, or any
combination thereof.

If you wish to implement a custom `{ParameterResolver}` that resolves parameters based
solely on the type of the parameter, you may find it convenient to extend the
`{TypeBasedParameterResolver}` which serves as a generic adapter for such use cases.

For concrete examples, consult the source code for `{CustomTypeParameterResolver}`,
`{CustomAnnotationParameterResolver}`, and `{MapOfListsTypeBasedParameterResolver}`.

[WARNING]
====
Due to a bug in the byte code generated by `javac` on JDK versions prior to JDK 9,
looking up annotations on parameters directly via the core `java.lang.reflect.Parameter`
API will always fail for _inner class_ constructors (e.g., a constructor in a `@Nested`
test class).

The `{ParameterContext}` API supplied to `ParameterResolver` implementations therefore
includes the following convenience methods for correctly looking up annotations on
parameters. Extension authors are strongly encouraged to use these methods instead of
those provided in `java.lang.reflect.Parameter` in order to avoid this bug in the JDK.

* `boolean isAnnotated(Class<? extends Annotation> annotationType)`
* `Optional<A> findAnnotation(Class<A> annotationType)`
* `List<A> findRepeatableAnnotations(Class<A> annotationType)`
====

[NOTE]
====
Other extensions can also leverage registered `ParameterResolvers` for method and
constructor invocations, using the `{ExecutableInvoker}` available via the
`getExecutableInvoker()` method in the `ExtensionContext`.
====

[[extensions-test-result-processing]]
=== Test Result Processing

`{TestWatcher}` defines the API for extensions that wish to process the results of _test
method_ executions. Specifically, a `TestWatcher` will be invoked with contextual
information for the following events.

* `testDisabled`: invoked after a disabled _test method_ has been skipped
* `testSuccessful`: invoked after a _test method_ has completed successfully
* `testAborted`: invoked after a _test method_ has been aborted
* `testFailed`: invoked after a _test method_ has failed

NOTE: In contrast to the definition of "test method" presented in
<<writing-tests-definitions>>, in this context _test method_ refers to any `@Test` method
or `@TestTemplate` method (for example, a `@RepeatedTest` or `@ParameterizedTest`).

Extensions implementing this interface can be registered at the class level, instance
level, or method level. When registered at the class level, a `TestWatcher` will be
invoked for any contained _test method_ including those in `@Nested` classes. When
registered at the method level, a `TestWatcher` will only be invoked for the _test method_
for which it was registered.

[WARNING]
====
If a `TestWatcher` is registered via a non-static (instance) field – for example, using
`@RegisterExtension` – and the test class is configured with
`@TestInstance(Lifecycle.PER_METHOD)` semantics (which is the default lifecycle mode), the
`TestWatcher` will **not** be invoked with events for `@TestTemplate` methods (for
example, `@RepeatedTest` or `@ParameterizedTest`).

To ensure that a `TestWatcher` is invoked for all _test methods_ in a given class, it is
therefore recommended that the `TestWatcher` be registered at the class level with
`@ExtendWith` or via a `static` field with `@RegisterExtension` or `@ExtendWith`.
====

If there is a failure at the class level — for example, an exception thrown by a
`@BeforeAll` method — no test results will be reported. Similarly, if the test class is
disabled via an `ExecutionCondition` — for example, `@Disabled` — no test results will be
reported.

In contrast to other Extension APIs, a `TestWatcher` is not permitted to adversely
influence the execution of tests. Consequently, any exception thrown by a method in the
`TestWatcher` API will be logged at `WARNING` level and will not be allowed to propagate
or fail test execution.

[WARNING]
====
Any instances of `ExtensionContext.Store.CloseableResource` stored in the `Store` of the
provided `{ExtensionContext}` will be closed _before_ methods in the `TestWatcher` API are
invoked (see <<extensions-keeping-state>>). You can use the parent context's `Store` to
work with such resources.
====

[[extensions-lifecycle-callbacks]]
=== Test Lifecycle Callbacks

The following interfaces define the APIs for extending tests at various points in the
test execution lifecycle. Consult the following sections for examples and the Javadoc for
each of these interfaces in the `{extension-api-package}` package for further details.

* `{BeforeAllCallback}`
** `{BeforeEachCallback}`
*** `{BeforeTestExecutionCallback}`
*** `{AfterTestExecutionCallback}`
** `{AfterEachCallback}`
* `{AfterAllCallback}`

.Implementing Multiple Extension APIs
NOTE: Extension developers may choose to implement any number of these interfaces
within a single extension. Consult the source code of the `{SpringExtension}` for a
concrete example.

[[extensions-lifecycle-callbacks-before-after-execution]]
==== Before and After Test Execution Callbacks

`{BeforeTestExecutionCallback}` and `{AfterTestExecutionCallback}` define the APIs for
`Extensions` that wish to add behavior that will be executed _immediately before_ and
_immediately after_ a test method is executed, respectively. As such, these callbacks are
well suited for timing, tracing, and similar use cases. If you need to implement
callbacks that are invoked _around_ `@BeforeEach` and `@AfterEach` methods, implement
`BeforeEachCallback` and `AfterEachCallback` instead.

The following example shows how to use these callbacks to calculate and log the execution
time of a test method. `TimingExtension` implements both `BeforeTestExecutionCallback`
and `AfterTestExecutionCallback` in order to time and log the test execution.

[[extensions-lifecycle-callbacks-timing-extension]]
[source,java,indent=0]
.An extension that times and logs the execution of test methods
----
include::{testDir}/example/timing/TimingExtension.java[tags=user_guide]
----

Since the `TimingExtensionTests` class registers the `TimingExtension` via `@ExtendWith`,
its tests will have this timing applied when they execute.

[source,java,indent=0]
.A test class that uses the example TimingExtension
----
include::{testDir}/example/timing/TimingExtensionTests.java[tags=user_guide]
----

The following is an example of the logging produced when `TimingExtensionTests` is run.

....
INFO: Method [sleep20ms] took 24 ms.
INFO: Method [sleep50ms] took 53 ms.
....

[[extensions-exception-handling]]
=== Exception Handling

Exceptions thrown during the test execution may be intercepted and handled accordingly
before propagating further, so that certain actions like error logging or resource releasing
may be defined in specialized `Extensions`. JUnit Jupiter offers API for `Extensions` that
wish to handle exceptions thrown during `@Test` methods via `{TestExecutionExceptionHandler}`
and for those thrown during one of test lifecycle methods (`@BeforeAll`, `@BeforeEach`,
`@AfterEach` and `@AfterAll`) via `{LifecycleMethodExecutionExceptionHandler}`.

The following example shows an extension which will swallow all instances of `IOException`
but rethrow any other type of exception.

[source,java,indent=0]
.An exception handling extension that filters IOExceptions in test execution
----
include::{testDir}/example/exception/IgnoreIOExceptionExtension.java[tags=user_guide]
----

Another example shows how to record the state of an application under test exactly at
the point of unexpected exception being thrown during setup and cleanup. Note that unlike
relying on lifecycle callbacks, which may or may not be executed depending on the test
status, this solution guarantees execution immediately after failing `@BeforeAll`,
`@BeforeEach`, `@AfterEach` or `@AfterAll`.

[source,java,indent=0]
.An exception handling extension that records application state on error
----
include::{testDir}/example/exception/RecordStateOnErrorExtension.java[tags=user_guide]
----

Multiple execution exception handlers may be invoked for the same lifecycle method in
order of declaration. If one of the handlers swallows the handled exception, subsequent
ones will not be executed, and no failure will be propagated to JUnit engine, as if the
exception was never thrown. Handlers may also choose to rethrow the exception or throw
a different one, potentially wrapping the original.

Extensions implementing `{LifecycleMethodExecutionExceptionHandler}` that wish to handle
exceptions thrown during `@BeforeAll` or `@AfterAll` need to be registered on a class level,
while handlers for `BeforeEach` and `AfterEach` may be also registered for individual
test methods.

[source,java,indent=0]
.Registering multiple exception handling extensions
----
include::{testDir}/example/exception/MultipleHandlersTestCase.java[tags=user_guide]
----

[[extensions-intercepting-invocations]]
=== Intercepting Invocations

`{InvocationInterceptor}` defines the API for `Extensions` that wish to intercept calls to
test code.

The following example shows an extension that executes all test methods in Swing's Event
Dispatch Thread.

[source,java,indent=0]
.An extension that executes tests in a user-defined thread
----
include::{testDir}/example/interceptor/SwingEdtInterceptor.java[tags=user_guide]
----

[[extensions-test-templates]]
=== Providing Invocation Contexts for Test Templates

A `{TestTemplate}` method can only be executed when at least one
`{TestTemplateInvocationContextProvider}` is registered. Each such provider is responsible
for providing a `Stream` of `{TestTemplateInvocationContext}` instances. Each context may
specify a custom display name and a list of additional extensions that will only be used
for the next invocation of the `{TestTemplate}` method.

The following example shows how to write a test template as well as how to register and
implement a `{TestTemplateInvocationContextProvider}`.

[source,java,indent=0]
.A test template with accompanying extension
----
include::{testDir}/example/TestTemplateDemo.java[tags=user_guide]
----

In this example, the test template will be invoked twice. The display names of the
invocations will be `apple` and `banana` as specified by the invocation context. Each
invocation registers a custom `{ParameterResolver}` which is used to resolve the method
parameter. The output when using the `ConsoleLauncher` is as follows.

....
└─ testTemplate(String) ✔
   ├─ apple ✔
   └─ banana ✔
....

The `{TestTemplateInvocationContextProvider}` extension API is primarily intended for
implementing different kinds of tests that rely on repetitive invocation of a test-like
method albeit in different contexts — for example, with different parameters, by preparing
the test class instance differently, or multiple times without modifying the context.
Please refer to the implementations of <<writing-tests-repeated-tests>> or
<<writing-tests-parameterized-tests>> which use this extension point to provide their
functionality.


[[extensions-keeping-state]]
=== Keeping State in Extensions

Usually, an extension is instantiated only once. So the question becomes relevant: How do
you keep the state from one invocation of an extension to the next? The
`ExtensionContext` API provides a `Store` exactly for this purpose. Extensions may put
values into a store for later retrieval. See the
`<<extensions-lifecycle-callbacks-timing-extension, TimingExtension>>` for an example of
using the `Store` with a method-level scope. It is important to remember that values
stored in an `ExtensionContext` during test execution will not be available in the
surrounding `ExtensionContext`. Since `ExtensionContexts` may be nested, the scope of
inner contexts may also be limited. Consult the corresponding Javadoc for details on the
methods available for storing and retrieving values via the `{ExtensionContext_Store}`.

.`ExtensionContext.Store.CloseableResource`
NOTE: An extension context store is bound to its extension context lifecycle. When an
extension context lifecycle ends it closes its associated store. All stored values
that are instances of `CloseableResource` are notified by an invocation of their `close()`
method in the inverse order they were added in.

[[extensions-supported-utilities]]
=== Supported Utilities in Extensions

The `junit-platform-commons` artifact exposes a package named
`{junit-platform-support-package}` that contains _maintained_ utility methods for working
with annotations, classes, reflection, and classpath scanning tasks. `TestEngine` and
`Extension` authors are encouraged to use these supported methods in order to align with
the behavior of the JUnit Platform.

[[extensions-supported-utilities-annotations]]
==== Annotation Support

`AnnotationSupport` provides static utility methods that operate on annotated elements
(e.g., packages, annotations, classes, interfaces, constructors, methods, and fields).
These include methods to check whether an element is annotated or meta-annotated with a
particular annotation, to search for specific annotations, and to find annotated methods
and fields in a class or interface. Some of these methods search on implemented
interfaces and within class hierarchies to find annotations. Consult the Javadoc for
`{AnnotationSupport}` for further details.

[[extensions-supported-utilities-classes]]
==== Class Support

`ClassSupport` provides static utility methods for working with classes (i.e., instances
of `java.lang.Class`). Consult the Javadoc for `{ClassSupport}` for further details.

[[extensions-supported-utilities-reflection]]
==== Reflection Support

`ReflectionSupport` provides static utility methods that augment the standard JDK
reflection and class-loading mechanisms. These include methods to scan the classpath in
search of classes matching specified predicates, to load and create new instances of a
class, and to find and invoke methods. Some of these methods traverse class hierarchies
to locate matching methods. Consult the Javadoc for `{ReflectionSupport}` for further
details.

[[extensions-supported-utilities-modifier]]
==== Modifier Support

`ModifierSupport` provides static utility methods for working with member and class
modifiers -- for example, to determine if a member is declared as `public`, `private`,
`abstract`, `static`, etc. Consult the Javadoc for `{ModifierSupport}` for further
details.


[[extensions-execution-order]]
=== Relative Execution Order of User Code and Extensions

When executing a test class that contains one or more test methods, a number of extension
callbacks are called in addition to the user-supplied test and lifecycle methods.

NOTE: See also: <<writing-tests-test-execution-order>>

[[extensions-execution-order-overview]]
==== User and Extension Code

The following diagram illustrates the relative order of user-supplied code and extension
code. User-supplied test and lifecycle methods are shown in orange, with callback code
implemented by extensions shown in blue. The grey box denotes the execution of a single
test method and will be repeated for every test method in the test class.

:figure-caption: User code and extension code

[#extensions-execution-order-diagram,reftext='{figure-caption}']
image::extensions_lifecycle.png[caption='',title='{figure-caption}']

The following table further explains the sixteen steps in the
<<extensions-execution-order-diagram>> diagram.

[cols="5,15,80"]
|===
| Step | Interface/Annotation | Description

| 1
| interface `org.junit.jupiter.api.extension.BeforeAllCallback`
| extension code executed before all tests of the container are executed

| 2
| annotation `org.junit.jupiter.api.BeforeAll`
| user code executed before all tests of the container are executed

| 3
| interface `org.junit.jupiter.api.extension.LifecycleMethodExecutionExceptionHandler
#handleBeforeAllMethodExecutionException`
| extension code for handling exceptions thrown from `@BeforeAll` methods

| 4
| interface `org.junit.jupiter.api.extension.BeforeEachCallback`
| extension code executed before each test is executed

| 5
| annotation `org.junit.jupiter.api.BeforeEach`
| user code executed before each test is executed

| 6
| interface `org.junit.jupiter.api.extension.LifecycleMethodExecutionExceptionHandler
#handleBeforeEachMethodExecutionException`
| extension code for handling exceptions thrown from `@BeforeEach` methods

| 7
| interface `org.junit.jupiter.api.extension.BeforeTestExecutionCallback`
| extension code executed immediately before a test is executed

| 8
| annotation `org.junit.jupiter.api.Test`
| user code of the actual test method

| 9
| interface `org.junit.jupiter.api.extension.TestExecutionExceptionHandler`
| extension code for handling exceptions thrown during a test

| 10
| interface `org.junit.jupiter.api.extension.AfterTestExecutionCallback`
| extension code executed immediately after test execution and its corresponding exception handlers

| 11
| annotation `org.junit.jupiter.api.AfterEach`
| user code executed after each test is executed

| 12
| interface `org.junit.jupiter.api.extension.LifecycleMethodExecutionExceptionHandler
#handleAfterEachMethodExecutionException`
| extension code for handling exceptions thrown from `@AfterEach` methods

| 13
| interface `org.junit.jupiter.api.extension.AfterEachCallback`
| extension code executed after each test is executed

| 14
| annotation `org.junit.jupiter.api.AfterAll`
| user code executed after all tests of the container are executed

| 15
| interface `org.junit.jupiter.api.extension.LifecycleMethodExecutionExceptionHandler
#handleAfterAllMethodExecutionException`
| extension code for handling exceptions thrown from `@AfterAll` methods

| 16
| interface `org.junit.jupiter.api.extension.AfterAllCallback`
| extension code executed after all tests of the container are executed

|===

In the simplest case only the actual test method will be executed (step 8); all other
steps are optional depending on the presence of user code or extension support for the
corresponding lifecycle callback. For further details on the various lifecycle callbacks
please consult the respective Javadoc for each annotation and extension.

All invocations of user code methods in the above table can additionally be intercepted
by implementing <<extensions-intercepting-invocations, `InvocationInterceptor`>>.

[[extensions-execution-order-wrapping-behavior]]
==== Wrapping Behavior of Callbacks

JUnit Jupiter always guarantees _wrapping_ behavior for multiple registered extensions
that implement lifecycle callbacks such as `BeforeAllCallback`, `AfterAllCallback`,
`BeforeEachCallback`, `AfterEachCallback`, `BeforeTestExecutionCallback`, and
`AfterTestExecutionCallback`.

That means that, given two extensions `Extension1` and `Extension2` with `Extension1`
registered before `Extension2`, any "before" callbacks implemented by `Extension1` are
guaranteed to execute **before** any "before" callbacks implemented by `Extension2`.
Similarly, given the two same two extensions registered in the same order, any "after"
callbacks implemented by `Extension1` are guaranteed to execute **after** any "after"
callbacks implemented by `Extension2`. `Extension1` is therefore said to _wrap_
`Extension2`.

JUnit Jupiter also guarantees _wrapping_ behavior within class and interface hierarchies
for user-supplied _lifecycle methods_ (see <<writing-tests-definitions>>).

* `@BeforeAll` methods are inherited from superclasses as long as they are not _hidden_,
  _overridden_, or _superseded_ (i.e., replaced based on signature only, irrespective of
  Java's visibility rules). Furthermore, `@BeforeAll` methods from superclasses will be
  executed **before** `@BeforeAll` methods in subclasses.
** Similarly, `@BeforeAll` methods declared in an interface are inherited as long as they
   are not _hidden_ or _overridden_, and `@BeforeAll` methods from an interface will be
   executed **before** `@BeforeAll` methods in the class that implements the interface.
* `@AfterAll` methods are inherited from superclasses as long as they are not _hidden_,
  _overridden_, or _superseded_ (i.e., replaced based on signature only, irrespective of
  Java's visibility rules). Furthermore, `@AfterAll` methods from superclasses will be
  executed **after** `@AfterAll` methods in subclasses.
** Similarly, `@AfterAll` methods declared in an interface are inherited as long as they
   are not _hidden_ or _overridden_, and `@AfterAll` methods from an interface will be
   executed **after** `@AfterAll` methods in the class that implements the interface.
* `@BeforeEach` methods are inherited from superclasses as long as they are not
  _overridden_ or _superseded_ (i.e., replaced based on signature only, irrespective of
  Java's visibility rules). Furthermore, `@BeforeEach` methods from superclasses will be
  executed **before** `@BeforeEach` methods in subclasses.
** Similarly, `@BeforeEach` methods declared as interface default methods are inherited as
   long as they are not _overridden_, and `@BeforeEach` default methods will be executed
   **before** `@BeforeEach` methods in the class that implements the interface.
* `@AfterEach` methods are inherited from superclasses as long as they are not
  _overridden_ or _superseded_ (i.e., replaced based on signature only, irrespective of
  Java's visibility rules). Furthermore, `@AfterEach` methods from superclasses will be
  executed **after** `@AfterEach` methods in subclasses.
** Similarly, `@AfterEach` methods declared as interface default methods are inherited as
   long as they are not _overridden_, and `@AfterEach` default methods will be executed
   **after** `@AfterEach` methods in the class that implements the interface.

The following examples demonstrate this behavior. Please note that the examples do not
actually do anything realistic. Instead, they mimic common scenarios for testing
interactions with the database. All methods imported statically from the `Logger` class
log contextual information in order to help us better understand the execution order of
user-supplied callback methods and callback methods in extensions.

[source,java,indent=0]
.Extension1
----
include::{testDir}/example/callbacks/Extension1.java[tags=user_guide]
----

[source,java,indent=0]
.Extension2
----
include::{testDir}/example/callbacks/Extension2.java[tags=user_guide]
----

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

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

When the `DatabaseTestsDemo` test class is executed, the following is logged.

----
@BeforeAll AbstractDatabaseTests.createDatabase()
@BeforeAll DatabaseTestsDemo.beforeAll()
  Extension1.beforeEach()
  Extension2.beforeEach()
    @BeforeEach AbstractDatabaseTests.connectToDatabase()
    @BeforeEach DatabaseTestsDemo.insertTestDataIntoDatabase()
      @Test DatabaseTestsDemo.testDatabaseFunctionality()
    @AfterEach DatabaseTestsDemo.deleteTestDataFromDatabase()
    @AfterEach AbstractDatabaseTests.disconnectFromDatabase()
  Extension2.afterEach()
  Extension1.afterEach()
@BeforeAll DatabaseTestsDemo.afterAll()
@AfterAll AbstractDatabaseTests.destroyDatabase()
----

The following sequence diagram helps to shed further light on what actually goes on within
the `JupiterTestEngine` when the `DatabaseTestsDemo` test class is executed.

////
PNG generated using ZenUML: https://app.zenuml.com

See corresponding *.txt file in images folder for the source.
////
image::extensions_DatabaseTestsDemo.png[caption='',title='DatabaseTestsDemo']

JUnit Jupiter does **not** guarantee the execution order of multiple lifecycle methods
that are declared within a _single_ test class or test interface. It may at times appear
that JUnit Jupiter invokes such methods in alphabetical order. However, that is not
precisely true. The ordering is analogous to the ordering for `@Test` methods within a
single test class.

[NOTE]
====
Lifecycle methods that are declared within a _single_ test class or test interface will be
ordered using an algorithm that is deterministic but intentionally non-obvious. This
ensures that subsequent runs of a test suite execute lifecycle methods in the same order,
thereby allowing for repeatable builds.
====

In addition, JUnit Jupiter does **not** support _wrapping_ behavior for multiple lifecycle
methods declared within a single test class or test interface.

The following example demonstrates this behavior. Specifically, the lifecycle method
configuration is _broken_ due to the order in which the locally declared lifecycle methods
are executed.

* Test data is inserted _before_ the database connection has been opened, which results in
  a failure to connect to the database.
* The database connection is closed _before_ deleting the test data, which results in a
  failure to connect to the database.

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

When the `BrokenLifecycleMethodConfigDemo` test class is executed, the following is logged.

----
Extension1.beforeEach()
Extension2.beforeEach()
  @BeforeEach BrokenLifecycleMethodConfigDemo.insertTestDataIntoDatabase()
  @BeforeEach BrokenLifecycleMethodConfigDemo.connectToDatabase()
    @Test BrokenLifecycleMethodConfigDemo.testDatabaseFunctionality()
  @AfterEach BrokenLifecycleMethodConfigDemo.disconnectFromDatabase()
  @AfterEach BrokenLifecycleMethodConfigDemo.deleteTestDataFromDatabase()
Extension2.afterEach()
Extension1.afterEach()
----

The following sequence diagram helps to shed further light on what actually goes on within
the `JupiterTestEngine` when the `BrokenLifecycleMethodConfigDemo` test class is executed.

////
PNG generated using ZenUML: https://app.zenuml.com

See corresponding *.txt file in images folder for the source.
////
image::extensions_BrokenLifecycleMethodConfigDemo.png[caption='',title='BrokenLifecycleMethodConfigDemo']

[TIP]
====
Due to the aforementioned behavior, the JUnit Team recommends that developers declare at
most one of each type of _lifecycle method_ (see <<writing-tests-definitions>>) per test
class or test interface unless there are no dependencies between such lifecycle methods.
====
