JaCoCo setup and configuration in JUnit - Build an Automation Script

Verify JaCoCo code coverage report generation after running JUnit tests

Preconditions (3)

Step 1: Run 'mvn clean test' command in the project root directory

Step 2: Locate the generated JaCoCo report in 'target/site/jacoco/index.html'

Step 3: Open the 'index.html' report in a browser

Step 4: Verify that the report shows coverage metrics for the tested classes

✅ Expected Result: JaCoCo generates a coverage report showing coverage percentages for classes tested by JUnit tests

Automation Requirements - JUnit 5 with Maven

Assertions Needed:

Verify that the JaCoCo report file 'target/site/jacoco/index.html' exists after tests run

Verify that the coverage report contains coverage data for at least one class

Best Practices:

Use Maven lifecycle to run tests and generate reports

Check file existence and content programmatically

Keep test code and coverage configuration separate

Use assertions to validate report generation

Automated Solution

JUnit

import static org.junit.jupiter.api.Assertions.*;

import java.io.File;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;

import org.junit.jupiter.api.Test;

public class JaCoCoReportTest {

    @Test
    public void testJaCoCoReportExistsAndHasContent() throws Exception {
        // Path to the JaCoCo report
        Path reportPath = Paths.get("target/site/jacoco/index.html");

        // Assert the report file exists
        assertTrue(Files.exists(reportPath), "JaCoCo report file should exist");

        // Read report content
        String content = Files.readString(reportPath);

        // Assert the report contains coverage data (check for a known marker like 'Coverage Summary')
        assertTrue(content.contains("Coverage Summary") || content.contains("class coverage"),
                "JaCoCo report should contain coverage summary");
    }
}

This JUnit 5 test checks if the JaCoCo coverage report is generated after running tests.

First, it verifies the report file target/site/jacoco/index.html exists. This confirms the report was created.

Then, it reads the report content as a string and checks for a known phrase like Coverage Summary to ensure the report contains coverage data.

This approach uses standard Java file APIs and JUnit assertions to validate the presence and content of the coverage report, following best practices for automation.

Common Mistakes - 3 Pitfalls

{'mistake': "Not running 'mvn test' before checking the report", 'why_bad': 'The JaCoCo report is generated only after tests run, so checking before running tests will fail.', 'correct_approach': "Always run 'mvn clean test' or equivalent to generate the report before verifying it."}

{'mistake': 'Hardcoding absolute file paths for the report', 'why_bad': 'Absolute paths reduce portability and cause failures on different machines or environments.', 'correct_approach': "Use relative paths like 'target/site/jacoco/index.html' relative to the project root."}

Checking only file existence without validating content

Bonus Challenge

Now add data-driven testing to verify JaCoCo reports for multiple Maven modules with different paths

Show Hint

Practice

1. What is the primary purpose of JaCoCo in a JUnit testing environment?

easy

A. To replace JUnit as a testing framework

B. To run JUnit tests faster

C. To generate test data automatically

D. To measure how much of your code is tested by your unit tests

Solution

1. Step 1: Understand JaCoCo's role

   JaCoCo is a code coverage tool that tracks which parts of your code are executed during tests.

2. Step 2: Compare options

   Only To measure how much of your code is tested by your unit tests correctly describes JaCoCo's purpose; others describe unrelated functions.

3. Final Answer:

   To measure how much of your code is tested by your unit tests -> Option D

4. Quick Check:

   JaCoCo = Code coverage tool [OK]

Hint: JaCoCo measures tested code coverage, not test speed [OK]

Common Mistakes:

• Confusing JaCoCo with test runners
• Thinking JaCoCo generates tests
• Assuming JaCoCo replaces JUnit

2. Which of the following is the correct way to add JaCoCo plugin in a Maven pom.xml file?

easy

A.
org.jacoco
jacoco-maven-plugin
0.8.8

B.
org.junit.jupiter
junit-jupiter-engine
5.9.2

C.
org.jacoco
jacoco-core
0.8.8

D.
org.apache.maven.plugins
maven-compiler-plugin
3.8.1

Solution

1. Step 1: Identify JaCoCo plugin syntax

   JaCoCo is added as a Maven plugin with groupId 'org.jacoco' and artifactId 'jacoco-maven-plugin'.

2. Step 2: Eliminate incorrect options

   Options B and D are unrelated plugins; A is a dependency, not a plugin.

3. Final Answer:

   Correct JaCoCo plugin configuration in Maven -> Option A

4. Quick Check:

   JaCoCo plugin = org.jacoco:jacoco-maven-plugin [OK]

Hint: JaCoCo is a plugin, not a dependency in Maven [OK]

Common Mistakes:

• Adding JaCoCo as a dependency instead of plugin
• Confusing JUnit plugin with JaCoCo plugin
• Using unrelated Maven plugins

3. Given the following JaCoCo configuration snippet in a Maven pom.xml:

<plugin>
  <groupId>org.jacoco</groupId>
  <artifactId>jacoco-maven-plugin</artifactId>
  <version>0.8.8</version>
  <executions>
    <execution>
      <goals>
        <goal>prepare-agent</goal>
      </goals>
    </execution>
    <execution>
      <id>report</id>
      <phase>test</phase>
      <goals>
        <goal>report</goal>
      </goals>
    </execution>
  </executions>
</plugin>

What will happen when you run mvn test?

medium

A. JaCoCo agent prepares coverage data before tests, then generates a coverage report after tests

B. Tests will run without coverage data collection

C. The build will fail due to missing report goal

D. JaCoCo will only generate a report without running tests

Solution

1. Step 1: Understand prepare-agent goal

   The 'prepare-agent' goal sets up JaCoCo to collect coverage data during test execution.

2. Step 2: Understand report goal in test phase

   The 'report' goal runs after tests to generate the coverage report.

3. Final Answer:

   JaCoCo collects coverage during tests and generates report after -> Option A

4. Quick Check:

   prepare-agent + report = coverage data + report [OK]

Hint: prepare-agent collects data; report generates coverage output [OK]

Common Mistakes:

• Assuming coverage is collected without prepare-agent
• Thinking report runs before tests
• Confusing goals causing build failure

4. You added JaCoCo plugin to your Maven project but no coverage report is generated after running mvn test. What is the most likely cause?

medium

A. JUnit tests are not written correctly

B. You used an incorrect JaCoCo version number

C. You forgot to bind the report goal to the test phase

D. Maven compiler plugin is missing

Solution

1. Step 1: Check JaCoCo execution bindings

   Without binding the 'report' goal to the 'test' phase, JaCoCo won't generate reports automatically.

2. Step 2: Eliminate other causes

   Incorrect version usually causes build errors; test correctness or compiler plugin absence won't stop report generation if configured.

3. Final Answer:

   Missing binding of report goal to test phase -> Option C

4. Quick Check:

   Report goal must be bound to test phase [OK]

Hint: Bind report goal to test phase to generate coverage report [OK]

Common Mistakes:

• Ignoring execution phase bindings
• Blaming test code for missing reports
• Assuming version mismatch causes silent failure

5. You want to configure JaCoCo in a Gradle project to generate an XML coverage report after tests run. Which snippet correctly configures this in build.gradle?

hard

A. jacocoTestReport {
reports {
xml.required = true
html.required = false
}
}

B. jacocoTestReport {
reports {
xml.enabled = true
html.enabled = true
}
}

C. test {
useJUnitPlatform()
jacoco {
xml.enabled = true
}
}

D. jacoco {
toolVersion = '0.8.8'
reports {
xml.enabled = true
}
}

Solution

1. Step 1: Identify correct JaCoCo Gradle report syntax

   The 'jacocoTestReport' task configures reports with 'xml.enabled' and 'html.enabled' properties.

2. Step 2: Compare options for XML report generation

   jacocoTestReport { reports { xml.enabled = true html.enabled = true } } correctly enables XML report; the snippet using 'required' properties is outdated; others configure in wrong blocks.

3. Final Answer:

   jacocoTestReport with reports { xml.enabled = true } -> Option B

4. Quick Check:

   Use xml.enabled = true in jacocoTestReport [OK]

Hint: Use xml.enabled = true inside jacocoTestReport for XML reports [OK]

Common Mistakes:

• Using xml.required instead of xml.enabled
• Configuring jacoco block instead of jacocoTestReport
• Missing enabling of reports