Why Code coverage reports in Jenkins? - Purpose & Use Cases

The Big Idea

What if you could instantly see which parts of your code are risky and untested every time you save your work?

The Scenario

Imagine you have a big project with hundreds of tests. You want to know which parts of your code are tested and which are not. Without code coverage reports, you have to guess or check files one by one.

The Problem

Manually checking test coverage is slow and confusing. You might miss untested code, leading to bugs in production. It's easy to overlook important parts, and fixing problems later costs more time and effort.

The Solution

Code coverage reports automatically show which lines of code are tested and which are not. They give clear visuals and numbers, so you quickly see gaps in testing. This helps you improve tests and keep your code reliable.

Before vs After

✗ Before

Run tests and guess coverage by reading logs

✓ After

Use Jenkins plugins to generate and display coverage reports automatically

What It Enables

Code coverage reports let teams confidently improve test quality and catch bugs early, making software safer and development faster.

Real Life Example

A team uses Jenkins to run tests on every code change. The coverage report shows that some new features have no tests, so they add tests before merging, preventing bugs from reaching users.

Key Takeaways

Manual test coverage checking is slow and error-prone.

Code coverage reports give clear, automatic insights into tested code.

They help teams write better tests and deliver reliable software faster.

Practice

1. What is the main purpose of code coverage reports in Jenkins?

easy

A. To show which parts of the code are tested

B. To deploy the application automatically

C. To monitor server CPU usage

D. To manage user permissions

Solution

1. Step 1: Understand code coverage reports

   Code coverage reports indicate which parts of the source code have been executed during tests.

2. Step 2: Identify Jenkins role

   Jenkins uses these reports to help developers see test completeness and improve code quality.

3. Final Answer:

   To show which parts of the code are tested -> Option A

4. Quick Check:

   Code coverage = tested code parts [OK]

Hint: Coverage reports show tested code areas, not deployment [OK]

Common Mistakes:

• Confusing coverage with deployment
• Thinking coverage monitors server stats
• Mixing coverage with user management

2. Which Jenkins plugin is commonly used to publish Java code coverage reports?

easy

A. Git Plugin

B. JaCoCo Plugin

C. Docker Plugin

D. Slack Notification Plugin

Solution

1. Step 1: Identify coverage plugins for Java

   JaCoCo is a popular plugin for Java projects to generate code coverage reports in Jenkins.

2. Step 2: Exclude unrelated plugins

   Git manages source code, Docker manages containers, Slack sends messages, so they don't handle coverage.

3. Final Answer:

   JaCoCo Plugin -> Option B

4. Quick Check:

   Java coverage = JaCoCo [OK]

Hint: JaCoCo is the standard for Java coverage in Jenkins [OK]

Common Mistakes:

• Choosing Git for coverage
• Confusing Docker with coverage tools
• Selecting notification plugins

3. Given this Jenkins pipeline snippet, what will be the result of the jacoco step?

pipeline {
  agent any
  stages {
    stage('Test') {
      steps {
        junit '**/test-results.xml'
        jacoco execPattern: '**/jacoco.exec'
      }
    }
  }
}

medium

A. Publishes JaCoCo coverage report using the specified exec file

B. Runs unit tests and skips coverage report

C. Fails because jacoco is not a valid step

D. Publishes test results but ignores coverage data

Solution

1. Step 1: Understand the jacoco step usage

   The jacoco step in Jenkins pipeline publishes coverage reports using the exec file pattern provided.

2. Step 2: Analyze the snippet

   The pipeline runs tests with junit and then publishes coverage with jacoco execPattern: '**/jacoco.exec'.

3. Final Answer:

   Publishes JaCoCo coverage report using the specified exec file -> Option A

4. Quick Check:

   jacoco step publishes coverage report [OK]

Hint: jacoco step publishes coverage from exec files [OK]

Common Mistakes:

• Thinking jacoco runs tests
• Assuming jacoco step is invalid
• Ignoring coverage data in pipeline

4. You added the JaCoCo plugin to Jenkins but the coverage report does not appear after the build. What is the most likely cause?

medium

A. The Jenkins server is offline

B. The Jenkins user lacks admin rights

C. The build script does not run any tests

D. The jacoco.exec file path is incorrect or missing

Solution

1. Step 1: Check coverage report prerequisites

   JaCoCo needs the jacoco.exec file generated by tests to publish coverage.

2. Step 2: Identify common error

   If the exec file path is wrong or the file is missing, Jenkins cannot show the coverage report.

3. Final Answer:

   The jacoco.exec file path is incorrect or missing -> Option D

4. Quick Check:

   Missing exec file = no coverage report [OK]

Hint: Check jacoco.exec file path if coverage missing [OK]

Common Mistakes:

• Blaming Jenkins offline status
• Ignoring test execution for coverage
• Assuming user rights block coverage display

5. You want Jenkins to fail the build if code coverage falls below 80%. Which configuration should you add to the JaCoCo plugin in your pipeline?

hard

A. jacoco(execPattern: '**/jacoco.exec', failBuildIfCoverageLow: 80)

B. jacoco(execPattern: '**/jacoco.exec', changeBuildStatus: true, minimumCoverage: 80)

C. jacoco(execPattern: '**/jacoco.exec', changeBuildStatus: true, minimumInstructionCoverage: 80)

D. jacoco(execPattern: '**/jacoco.exec', coverageThreshold: 80)

Solution

1. Step 1: Understand JaCoCo coverage thresholds

   The JaCoCo plugin uses minimumInstructionCoverage to set the minimum coverage percentage for instructions.

2. Step 2: Identify correct syntax to fail build

   Setting changeBuildStatus: true with minimumInstructionCoverage: 80 makes Jenkins fail if coverage is below 80%.

3. Final Answer:

   jacoco(execPattern: '**/jacoco.exec', changeBuildStatus: true, minimumInstructionCoverage: 80) -> Option C

4. Quick Check:

   minimumInstructionCoverage + changeBuildStatus = fail below threshold [OK]

Hint: Use minimumInstructionCoverage with changeBuildStatus true [OK]

Common Mistakes:

• Using non-existent parameters like coverageThreshold
• Omitting changeBuildStatus to fail build
• Confusing minimumCoverage with minimumInstructionCoverage