For most Java projects, use JaCoCo to collect coverage during Maven or Gradle test runs, then inspect its report for missed lines and decision branches. IntelliJ IDEA’s coverage runner is useful for interactive, local inspection. Coverage shows what code ran—not whether tests would catch defects—so use the results to find risk-relevant behavior that still needs tests, rather than chasing a universal percentage.
Choose a Java coverage tool for your workflow
| Need | Practical option | What to know |
|---|---|---|
| Repeatable Gradle reports and build checks | Gradle JaCoCo plugin | Integrates with test tasks, provides the jacocoTestReport task, and supports rule-based verification. The report task does not run tests for you. Gradle JaCoCo Plugin documentation. |
| Maven test and report workflow | JaCoCo Maven plugin | Attaches the coverage agent and can create reports. Surefire or Failsafe must run tests in a fork that permits the agent; the documented setup cannot collect coverage with forkCount=0 or forkMode=never. Source-line mapping requires debug information. JaCoCo Maven documentation. |
| Interactive local inspection | IntelliJ IDEA coverage runner | Can show project, class, method, line, and branch information depending on runner and settings. Branch coverage is available with JaCoCo or with the IDEA runner when branch coverage is enabled. IntelliJ IDEA code coverage documentation. |
| One HTML report across Gradle subprojects | Gradle JaCoCo report aggregation plugin | Aggregates coverage data from multiple Gradle projects into a combined report. Gradle JaCoCo report aggregation documentation. |
JaCoCo is a practical default when you need build-integrated reports or CI checks; choose an IDE runner when the immediate goal is to explore execution while working locally. For a multi-module Gradle build, aggregation can provide a combined view. Compare tools by build integration, metric detail, local visualization, aggregation needs, and whether instrumentation works with your test execution setup—not by assuming one tool is best for every project.
Measure coverage with JaCoCo
1. Decide what the report should include
Choose the production code scope first: which modules or source sets count, and whether unit and integration tests should be reported separately or together. Keep the scope consistent when comparing reports over time. Generated code or legacy areas may need separate treatment when setting verification rules.
2. Enable JaCoCo in the build
For Gradle, apply the JaCoCo plugin alongside the Java plugin as appropriate for the project. Gradle’s documentation summarizes it this way: “The JaCoCo plugin provides code coverage metrics for Java code via integration with JaCoCo.” For Maven, configure the JaCoCo Maven plugin for the test workflow. Use the official Gradle setup documentation or JaCoCo Maven documentation to match the project’s build and plugin versions.
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3. Run tests with instrumentation active
Execute the test task or Maven test lifecycle that has the coverage agent enabled. In Maven’s documented Surefire/Failsafe setup, tests need a forked process for agent collection; disabling forking with forkCount=0 or forkMode=never prevents collection. If the execution-data file is missing, first confirm that tests actually ran in the expected task and that the agent was attached to that process.
4. Generate and open the report
In Gradle, run tests before jacocoTestReport: that report task does not automatically depend on the test task. The default HTML report location is build/reports/jacoco/test/html. Maven report examples place output under target/site/jacoco. Keep XML output in CI when downstream tools need machine-readable coverage data.
5. Investigate missed behavior
Open the report and examine missed lines and branches in context. Add tests for meaningful boundaries, error conditions, state transitions, and important decision outcomes. A green line is not a goal by itself: a test can execute code without checking whether the result is correct.
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6. Add project-specific verification when useful
JaCoCo verification rules can make a build fail when configured coverage rules are violated. Set a threshold that suits the code scope and the cost of useful tests; the tool documentation does not prescribe one universal target. Apply rules deliberately around generated code, legacy components, and modules with different risk profiles.
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Understand the coverage numbers
JaCoCo measures Java bytecode instructions as its smallest unit, then reports additional counters that summarize execution at other levels. The metric definitions matter because a single percentage can conceal untested outcomes.
- Instruction coverage: counts bytecode instructions executed. Compiler-generated instructions mean this is not simply a source-line count.
- Line coverage: maps execution to source lines when debug line information is available. A line counts as covered if at least one instruction assigned to it executes.
- Branch coverage: records outcomes of branches associated with
ifandswitch. Exception handling is not counted as branch coverage in JaCoCo’s counter definition. - Method and class counters: provide broader summaries of whether methods or classes had execution.
- Complexity counters: estimate complexity and missed complexity. Missed complexity can point to code worth inspecting, but it remains a metric rather than proof of test quality.
Line and branch coverage answer different questions. A conditional line may execute while only one outcome has been exercised; branch coverage can reveal that gap. IntelliJ IDEA can mark partial conditional execution and show covered versus total branches, subject to the selected runner and settings. See the JaCoCo counters documentation and IntelliJ coverage documentation.
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Use reports in CI and multi-module builds
Make data collection and report generation explicit in the CI workflow: run the instrumented tests, generate the report, then publish the report or preserve its XML output for later processing. For Gradle multi-project builds, the JaCoCo report aggregation plugin can combine subproject coverage into one HTML report. Confirm that the aggregated inputs represent the test runs and code scope you intend to measure; a combined number is only useful when its included modules are understood.
Troubleshoot missing or misleading coverage
- No execution-data file: tests may not have run, or the agent may not have attached to the process. Check the task and test execution configuration; in the documented Maven setup, ensure Surefire/Failsafe forks tests rather than using
forkCount=0orforkMode=never. - Report is empty or stale: run the relevant tests first and verify the report consumes their execution data. In Gradle, invoking
jacocoTestReportalone does not run tests. - Lines do not map to source: ensure compilation includes debug line information, which JaCoCo needs for source-line reporting.
- Line coverage looks good but decisions are untested: inspect branch coverage and add cases for alternate outcomes of
ifandswitchlogic. - Aggregate total seems unexpected: verify that the intended subprojects and test data are included and that the report scope is consistent.
- A threshold causes an unexpected build failure: inspect the configured verification rule and its scope, including any generated or legacy code, then decide whether the rule reflects a meaningful project requirement.
Interpret percentages without mistaking them for quality
A coverage percentage describes execution against a selected counter and code scope. It does not establish that assertions are strong or that defects would be detected. Two suites with the same line percentage can exercise different branches and behaviors; tests that execute a line but never check its result can inflate coverage without providing the protection the number may suggest. Use reports to identify unexamined behavior and make tests assert the outcomes that matter.
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Java coverage itself is collected by JaCoCo or an IDE runner, not by a screenshot service. If you need a screenshot of a generated coverage report for a ticket, review, or documentation, ScreenshotNeo can capture its page by API; its options also include PDF output and an MCP server for AI agents. For example, save a rendered report page as WebP:
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Can I measure coverage for only selected Gradle modules?
Yes. Define the report and verification scope around the modules or source sets relevant to the question you want the metric to answer.
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Does branch coverage count exceptions in JaCoCo?
No. JaCoCo’s documented branch counter covers outcomes associated with if and switch, not exception handling.
Can IntelliJ IDEA show which branches a test covered?
Yes, branch details are available with JaCoCo or with the IDEA runner when branch coverage is enabled, subject to runner and settings.
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