public EventLoopGroup group() {
        final EventLoopGroup group = this.group;
        if (group == null) {
            throw new IllegalStateException(EventLoopGroup.class.getSimpleName() + " not initialized");
        }
        return group;
    }
A delegate that has common testing methods of {@link EventLoopGroup}.
junit5/src/main/java/com/linecorp/armeria/internal/testing/EventLoopGroupRuleDelegate.java