Achieving 100% Branch Code Coverage for Lombok @Data with JUnit

When using Lombok to generate boilerplate code in your Java model classes, you might encounter a challenge: ensuring 100% branch code coverage with JUnit tests. In this post, we’ll explore why some branches in Lombok-generated code might be missed and how you can write tests to cover every branch in your model classes.

Understanding the Issue

Lombok’s @Data annotation automatically generates getters, setters, equals(), hashCode(), and toString() methods. The generated equals() method, for example, has several branches:

• Checking if the two objects are identical.
• Verifying that the other object is not null.
• Confirming that the other object is of the same type.
• Comparing each field for equality (which itself can include branches for null and non-null values).

Because these methods are generated at compile time, writing JUnit tests that exercise every possible branch isn’t always straightforward. Tools like JaCoCo might report less than 100% branch coverage, even if you’ve written tests for every behavior.

Best Practices for Testing Lombok Models

To achieve complete branch coverage, follow these guidelines:

1. Test the No-Args Constructor:
   Ensure that an object is properly created using Lombok’s @NoArgsConstructor.
2. Test All Getters and Setters:
   Manually set each field and retrieve it using the generated getter to ensure all setter/getter logic is executed.
3. Cover the Equals Method:
   Write tests that:
   • Compare the same instance to itself.
   • Compare to null.
   • Compare with an object of a different class.
   • Compare two objects where one or more fields are null or have differing values.
4. Verify HashCode Consistency:
   Ensure that objects with identical field values return the same hash code and that hash codes remain consistent when fields are null.
5. Examine toString Output:
   Although toString() is mainly used for debugging, verifying its output helps ensure all fields are represented as expected.

Sample Code: Testing a Lombok Model

Consider the following model class using Lombok:

import lombok.Data;
import lombok.NoArgsConstructor;

@NoArgsConstructor
@Data
public class MyDetailsEntity {
    private String myDetailsIdentifier;
    private String approvedDetailsIdentifier;
}

Below is a comprehensive JUnit test that covers all branches:

import static org.junit.Assert.*;
import org.junit.Test;

public class MyDetailsEntityTest {

    @Test
    public void testNoArgsConstructor() {
        // Test that the no-args constructor creates a non-null object.
        MyDetailsEntity entity = new MyDetailsEntity();
        assertNotNull(entity);
    }

    @Test
    public void testLombokGeneratedMethods() {
        // Create first instance with non-null fields.
        MyDetailsEntity entity1 = new MyDetailsEntity();
        entity1.setMyDetailsIdentifier("UL123");
        entity1.setApprovedDetailsIdentifier("APP123");

        // Validate getters.
        assertEquals("UL123", entity1.getMyDetailsIdentifier());
        assertEquals("APP123", entity1.getApprovedDetailsIdentifier());

        // Create a second instance with the same values.
        MyDetailsEntity entity2 = new MyDetailsEntity();
        entity2.setMyDetailsIdentifier("UL123");
        entity2.setApprovedDetailsIdentifier("APP123");

        // Test equals: same object, equal objects, and not equal to null or other classes.
        assertEquals(entity1, entity2);
        assertEquals(entity1.hashCode(), entity2.hashCode());
        assertEquals(entity1, entity1); // Identity check.
        assertNotEquals(entity1, null);
        assertNotEquals(entity1, new Object());

        // Create a third instance with default (null) fields.
        MyDetailsEntity entity3 = new MyDetailsEntity();

        // Test equals with null fields.
        assertNotEquals(entity1, entity3);
        // Consistent hashCode when fields are null.
        assertEquals(entity3.hashCode(), new MyDetailsEntity().hashCode());

        // Validate toString output.
        String toStringOutput1 = entity1.toString();
        String toStringOutput3 = entity3.toString();
        assertNotNull(toStringOutput1);
        assertNotNull(toStringOutput3);
        assertTrue(toStringOutput1.contains("UL123"));
        assertFalse(toStringOutput3.contains("UL123"));
    }
}

Explanation

• No-Args Constructor:
  The testNoArgsConstructor() method ensures that the Lombok-generated constructor creates a valid instance.
• Getters and Setters:
  The test sets the values for myDetailsIdentifier and approvedDetailsIdentifier and then retrieves them to confirm that the Lombok-generated methods work as expected.
• Equals and HashCode:
  Multiple scenarios are tested:
  • Two instances with the same non-null values are considered equal.
  • An instance is compared to itself.
  • Comparisons against null and an instance of a different class are correctly handled.
  • Branches in the generated code that check for null field values are covered by testing an instance (entity3) with default null values.
• ToString Method:
  Verifies that toString() includes the expected field values for non-null fields while excluding them when they are null.

Conclusion

Achieving 100% branch coverage for Lombok-generated code requires you to think about all possible execution paths in the generated methods. By writing comprehensive JUnit tests that cover the no-args constructor, getters/setters, equals(), hashCode(), and toString(), you can ensure that every branch is executed. This not only improves your test coverage reports but also provides confidence that your model behaves correctly in all scenarios.

For more insights on unit testing and code coverage best practices with Lombok, stay tuned to our blog.

Keywords: JUnit, code coverage, branch coverage, Lombok, Lombok @Data, Lombok unit tests, Java unit testing, test coverage, @NoArgsConstructor, Java model class testing

By following these best practices and writing thorough tests, you can achieve full branch coverage in your Lombok-enhanced model classes without compromising on code quality or readability.