private static void assertAvailable() throws AssertionError {
        if (INITIALIZATION_EXCEPTION != null) {
            throw new AssertionError("TestPlugins did not initialize completely",
                INITIALIZATION_EXCEPTION);
        }
        if (PLUGIN_JARS.isEmpty()) {
            throw new AssertionError("No test plugins loaded");
        }
    }
A ServiceLoader discovered plugin which subclasses another plugin which is present on the classpath
connect/runtime/src/test/java/org/apache/kafka/connect/runtime/isolation/TestPlugins.java