@Override
    public void beforeAll(ExtensionContext context) {
        if(extent==null){
            ExtentSparkReporter spark = new ExtentSparkReporter("build/reports/ScreenshotDiffReport.html");
            spark.config().setTheme(Theme.STANDARD);
            spark.config().setDocumentTitle("Screenshot Test Report");
            spark.config().setReportName("Screenshot Test Report");
            extent = new ExtentReports();
            extent.attachReporter(spark);
        }
    }
This creates the Extent report and manages the test lifecycle

@author Richard Tingle (aka richtea)
jme3-screenshot-tests/src/main/java/org/jmonkeyengine/screenshottests/testframework/ExtentReportExtension.java