public void after() {
        final EventLoopGroup group = this.group;
        if (group != null) {
            this.group = null;
            group.shutdownGracefully();
        }
    }
A delegate that has common testing methods of {@link EventLoopGroup}.
junit5/src/main/java/com/linecorp/armeria/internal/testing/EventLoopGroupRuleDelegate.java