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JUnit5 Inner Test Classes Should Be Annotated with @Nested

Medium
best practicestestingstructure

What is it?​

This practice is triggered when inner test classes in JUnit5 are not annotated with @Nested. Without this annotation, tests within inner classes will not be executed during the build process.

Why apply it?​

If an inner test class is not annotated with @Nested, its tests will not be executed during the build, potentially leading to untested code. On the other hand, static nested test classes should be left without @Nested annotations because they do not share setup and state with the enclosing class.

How to fix it?​

Annotate inner test classes with @Nested to ensure they are executed during testing. Do not annotate static nested classes with @Nested to ensure proper test behavior.

Examples​

Example 1:​

Negative​

In this negative example, the inner class is missing the @Nested annotation, meaning its tests will not be executed during the build.

import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.Nested;

class MyJunit5Test {
  
  @Test
  void test() { /* ... */ }
  
  class InnerClassTest { // Noncompliant, missing @Nested annotation
    @Test
    void testInner() { /* ... */ }
  }
  
  @Nested
  static class StaticNestedClassTest { // Noncompliant, static class should not use @Nested
    @Test
    void testStatic() { /* ... */ }
  }
}

Example 2:​

Positive​

In this positive example, the inner class is correctly annotated with @Nested, ensuring its test methods are executed during the build.

import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.Nested;

class MyJunit5Test {
  
  @Test
  void test() { /* ... */ }
  
  @Nested
  class InnerClassTest {
    @Test
    void testInner() { /* ... */ }
  }
  
  static class StaticNestedClassTest {
    @Test
    void testStatic() { /* ... */ }
  }
}

Negative​

This negative example shows an inner class without @Nested, meaning tests in the inner class won't run, and a static nested class incorrectly using @Nested.

import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.Nested;

class OuterTest {

  @Test
  void testOuter() { /* ... */ }

  class NonStaticNested { // Noncompliant, missing @Nested annotation
    @Test
    void testNonStaticNested() { /* ... */ }
  }

  @Nested
  static class FullyStaticNested { // Noncompliant, static nested class erroneously annotated
    @Test
    void testFullyStatic() { /* ... */ }
  }
}

Example 3:​

Positive​

In the positive example, a @Nested annotation is used correctly on an inner class, and the static nested class is left without @Nested to maintain clear setup separation.

import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.Nested;

class OuterTest {

  @Test
  void testOuter() { /* ... */ }

  @Nested
  class NonStaticNested {
    @Test
    void testNonStaticNested() { /* ... */ }
  }

  static class FullyStaticNested {
    @Test
    void testFullyStatic() { /* ... */ }
  }
}