Code coverage with JaCoCo offline instrumentation with Maven

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Post summary: Tutorial how to do code coverage with offline instrumentation with JaCoCo and Maven.

Offline instrumentation

Code coverage of manual or automated tests with JaCoCo post describes how to do code coverage with JaCoCo. This is applicable in many of the cases. Still there could be a case where application under test does not support Java agents. In this case JaCoCo offers offline instrumentation of the code. Once instrumented code can be run and coverage to be measured. This post describes how to do it.

JaCoCo for Maven

JaCoCo Maven plugin is discussed in Automated code coverage of unit tests with JaCoCo and Maven post. In order to get code instrumented “instrument” task should be added to Maven:


In current example this task will be executed when “mvn test” is called. It can be configured to be called on “package” or “install” by changing the “” element.

Make it configurable

Instrumentation MUST not be done on every build. You do not want to release instrumented code, first because this is bad practice and second code will not run unless jacocoagent.jar is in classpath. This is why instrumentation should be disabled by default with jacoco.skip.instrument=true pom.xml property, which can be overridden when needed with “mvn clean test -Djacoco.skip.instrument=false” command. Other option is separate pom-offline.xml file and build with it when needed.

Get sample application

Current post uses sample application first introduced in Build a RESTful stub server with Dropwizard post. It can be found in GitHub sample-dropwizard-rest-stub repository. For current tutorial it has been downloaded to C:\sample-dropwizard-rest-stub. This application gets packaged to a single JAR file by “mvn clean package”. If instrumentation gets put in “package” phase then it is not working, as packaging happens before instrumentation. This is why “test” phase is the correct for current example, as “package” includes test by default.

Instrument the code

Once application is downloaded it can be built with instrumentation with “mvn clean package -Djacoco.skip.instrument=false” command. It can be easily check if given class has been instrumented by opening it with some kind of decompiler. Image bellow shows instrumented class on the right hand side vs non-instrumented in the left hand side.


Run it with JaCoCo agent in the class path

If not instrumented sample application is started with “java -jar target/sample-dropwizard-rest-stub-1.0-SNAPSHOT.jar server config.yml” command. In case of instrumented code this command will give exception:

Exception in thread “main” java.lang.NoClassDefFoundError: org/jacoco/agent/rt/internal_773e439/Offline

This is because jacocoagent.jar is not in the classpath.

Adding JaCoCo agent in class path varies from case to case. In this particular tutorial application is a single JAR run by “java -jar” command. In order to add something in classpath “java -cp” command should be used. Problem is both “-jar” and “-cp” are mutually exclusive. Only way to do it is with following command:

java -Djacoco-agent.output=tcpserver -cp C:\JaCoCo\jacocoagent.jar;target/sample-dropwizard-rest-stub-1.0-SNAPSHOT.jar com.automationrhapsody.reststub.RestStubApp server config.yml

Where -Djacoco-agent.output=tcpserver is configuration to make JaCoCo agent report on TCP port. More about JaCoCo Offline settings here. “C:\JaCoCo\jacocoagent.jar” is the location of the JaCoCo agent JAR file. “com.automationrhapsody.reststub.RestStubApp” is main method to be run from target/sample-dropwizard-rest-stub-1.0-SNAPSHOT.jar file.


Now when we have the application running is time to run all tests we have both automation and manual. For manual it is important to be documented scenario which is reproducible on each regression testing, not just some random clicking. The idea is that we want to measure what coverage our tests do.

Import coverage results

In order to import results Eclipse with installed JaCoCo plugin from market place is needed. See Code coverage of manual or automated tests with JaCoCo post for more details how to install plugin.

Open Eclipse and import C:\sample-dropwizard-rest-stub project as Maven one.

Import the results into Eclipse. This is done from File -> Import -> Coverage Session -> select Agent address radio button but leave defaults -> enter some name and select code under test.


Once imported results can be seen and code gets highlighted.

In case no results are imported delete “target\classes” folder and rebuild with “mvn compile”.

Export to HTML and analyse

See Code coverage of manual or automated tests with JaCoCo how to export and analyse.


Although very rare to happen offline instrumentation is a way to measure code coverage with JaCoCo.

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  • miaomiao

    i downloaded the project sample-dropwizard-rest-stub ,
    and used command “mvn clean package -Djacoco.skip.instrument=false” to instrument the code .
    But when I used “java -Djacoco-agent.output=tcpserver -cp C:JaCoCojacocoagent.jar;target/sample-dropwizard-rest-stub-1.0-SNAPSHOT.jar com.automationrhapsody.reststub.RestStubApp server config.yml” to run Jar File,
    the command still give exception:
    Exception in thread “main” java.lang.NoClassDefFoundError: org/jacoco/agent/rt/i

    my command is :
    java -Djacoco-agent.output=tcpserver -cp D:JaCoCojacocoagent.jar;target/sample-dropwizard-rest-stub-1.0-SNAPSHOT.jar com.automationrhapsody.reststub.RestStubApp server config.yml