Code coverage is a metric used in software testing to measure the percentage of code that has been executed during testing. Java code coverage tools analyze the Java bytecode generated by the Java compiler to determine which parts of the code have been executed during a test run.

One popular Java code coverage tool is JaCoCo, which can be integrated into a Java project using a plugin for the build tool, such as Maven or Gradle. Once the tool is integrated, you can run the tests and generate a report that shows the code coverage.

Here are the steps to set up JaCoCo and generate a code coverage report:

1. Add the JaCoCo plugin to your build tool configuration file. For example, in Maven, add the following to the <build><plugins> section:

<plugin>
    <groupId>org.jacoco</groupId>
    <artifactId>jacoco-maven-plugin</artifactId>
    <version>0.8.7</version>
    <executions>
        <execution>
            <id>prepare-agent</id>
            <goals>
                <goal>prepare-agent</goal>
            </goals>
        </execution>
        <execution>
            <id>report</id>
            <phase>test</phase>
            <goals>
                <goal>report</goal>
            </goals>
        </execution>
    </executions>
</plugin>

This configuration adds the JaCoCo plugin to the build process, which will prepare the code for analysis and generate a report after running the tests.

2. Run the tests with the code coverage plugin enabled. For example, in Maven, run the following command:

mvn test

This command will execute the tests and generate a code coverage report using JaCoCo.

3. View the code coverage report. The report will be generated in the target/site/jacoco directory. Open the index.html file in a web browser to view the report.

The report will show the percentage of code that has been executed, broken down by package, class, and method. It will also show which lines of code have been executed and which have not.

Code coverage is an important metric in software testing because it helps identify which parts of the code have not been tested and may contain bugs. By using a code coverage tool like JaCoCo, you can ensure that your Java code is thoroughly tested and free of bugs.