I examined the life cycle of the extension of JUnit Jupiter

What is the life cycle of an Extension instance trying to write an Extension for JUnit 5 (JUnit Jupiter)? Since it became, it is a memorandum.

environment

• AdoptOpenJDK 11
• JUnit 5.5.0

TL;TR

--The instance registered with @Extension is shared for each test class, and if the test class is different, the Extension instance will be different. --Instance registered with @RegisterExtension --In the case of static field, it is shared for each test class. --For non-static fields, it depends on the life cycle of the test class.

Review of Extension Model

The JUnit 5 Extension Model is a mechanism for standardizing pre-processing, post-processing, exception handling, etc. for each test. Compared to Junit 4's Rule etc., the degree of freedom is lower, but you can easily create an extension just by implementing an interface corresponding to the extension point.

There are three ways to use Extension.

1. Register the extension with @Extension.
2. Register the extension programmatically with @RegisterExtension.
3. Register automatically with Service Loader.

This time, use methods 1 and 2 to check when the Extension instance is created.

The extensions used for confirmation are as follows.

FooExtension.java

import org.junit.jupiter.api.extension.*;

public class FooExtension implements BeforeEachCallback, AfterEachCallback, BeforeAllCallback, AfterAllCallback {

    {
        System.out.println(String.format("[%s] created", this.toString()));
    }

    @Override
    public void beforeAll(ExtensionContext extensionContext) throws Exception {
        System.out.println(String.format("[%s] before all", this.toString()));
    }

    @Override
    public void beforeEach(ExtensionContext context) throws Exception {
        System.out.println(String.format("[%s] before each", this.toString()));
    }

    @Override
    public void afterEach(ExtensionContext extensionContext) throws Exception {
        System.out.println(String.format("[%s] after each", this.toString()));
    }

    @Override
    public void afterAll(ExtensionContext extensionContext) throws Exception {
        System.out.println(String.format("[%s] after all", this.toString()));
    }
}

Register Extension with @Extension

Source

@ExtendWith(FooExtension.class)
public class ExtensionTest {

    @Test
    void test1() { System.out.println("test 1"); }

    @Test
    void test2() { System.out.println("test 2"); }

    @Test
    void test3() { System.out.println("test 3"); }
}

result

[example.FooExtension@70325e14] created
[example.FooExtension@70325e14] before all
[example.FooExtension@70325e14] before each
test 1
[example.FooExtension@70325e14] after each
[example.FooExtension@70325e14] before each
test 2
[example.FooExtension@70325e14] after each
[example.FooExtension@70325e14] before each
test 3
[example.FooExtension@70325e14] after each
[example.FooExtension@70325e14] after all

You can see that each test shares one Extension instance. An instance of ExtensionTest is created for each test, but the same result was obtained when changing to share the instance of ExtensionTest with @TestInstance (Lifecycle.PER_CLASS).

@RegisterExtension + Register Extension with static field

Source

SecondExtensionTest.java

public class SecondExtensionTest {

    @RegisterExtension
    static FooExtension fooExtension = new FooExtension();

    @Test
    void test1() { System.out.println("test 1"); }

    @Test
    void test2() { System.out.println("test 2"); }

    @Test
    void test3() { System.out.println("test 3"); }
}

result

[example.FooExtension@55183b20] created
[example.FooExtension@55183b20] before all
[example.FooExtension@55183b20] before each
test 1
[example.FooExtension@55183b20] after each
[example.FooExtension@55183b20] before each
test 2
[example.FooExtension@55183b20] after each
[example.FooExtension@55183b20] before each
test 3
[example.FooExtension@55183b20] after each
[example.FooExtension@55183b20] after all

Of course, since it is a static field, the same instance is shared.

@RegisterExtension + Register Extension in non-static field

Let's remove the static of the fooExtension of the SecondExtensionTest earlier.

For Lifecycle.PER_METHOD

Source

SecondExtensionTest.java

public class SecondExtensionTest {

    @RegisterExtension
-   static FooExtension fooExtension = new FooExtension();
+   FooExtension fooExtension = new FooExtension();

    //Omission
}

result

[example.FooExtension@40a4337a] created
[example.FooExtension@40a4337a] before each
test 1
[example.FooExtension@40a4337a] after each
[example.FooExtension@fa4c865] created
[example.FooExtension@fa4c865] before each
test 2
[example.FooExtension@fa4c865] after each
[example.FooExtension@3bd82cf5] created
[example.FooExtension@3bd82cf5] before each
test 3
[example.FooExtension@3bd82cf5] after each

You can see that each test has a different instance of Extension. This is because an instance of SecondExtensionTest itself is created for each test. In addition, BeforeAll and AfterAll are not working, but in the case of Lifecycle.PER_METHOD as well as when adding @BeforeAll to the method, it must be static.

For Lifecycle.PER_CLASS

So, let's make the life cycle of Second Extension Test a form in which an instance is created only once.

Source

SecondExtensionTest.java

+ @TestInstance(Lifecycle.PER_CLASS)
public class SecondExtensionTest {

    @RegisterExtension
    FooExtension fooExtension = new FooExtension();

    //Omission
}

result

[example.FooExtension@c730b35] created
[example.FooExtension@c730b35] before all
[example.FooExtension@c730b35] before each
test 1
[example.FooExtension@c730b35] after each
[example.FooExtension@c730b35] before each
test 2
[example.FooExtension@c730b35] after each
[example.FooExtension@c730b35] before each
test 3
[example.FooExtension@c730b35] after each
[example.FooExtension@c730b35] after all

Extension instance is shared. Also, Before All and After All are now working. If you register an extension in a non-static field in this way, it naturally depends on the life cycle of the original test class.

If you read it properly, it will be written

https://junit.org/junit5/docs/current/user-guide/#extensions