`alice` 没有调用中间件

Question

为什么这里只调用了 的ServeHTTP方法，H1而H2和 的方法H3似乎被忽略了？alice似乎是一个不错的中间件链接，在这里我尝试将它与httprouter一起使用，但只有外部/最后一个中间件被调用：package mainimport (    "fmt"    "github.com/julienschmidt/httprouter"    "github.com/justinas/alice"    "net/http"    "log"    "time")func main() {    fmt.Println("started ", time.Now())    c := alice.New(S1, S2, S3).Then(nil)    router := httprouter.New()    router.Handler("GET", "/app", c)    http.ListenAndServe(":27007", router)}func S1(h http.Handler) http.Handler {    var x H1    return &x}func S2(h http.Handler) http.Handler {    var x H2    return &x}func S3(h http.Handler) http.Handler {    var x H3    return &x}type H1 struct{}func (h *H1) ServeHTTP(rw http.ResponseWriter, req *http.Request) {    log.Println("H1", time.Now())}type H2 struct{}func (h *H2) ServeHTTP(rw http.ResponseWriter, req *http.Request) {    log.Println("H2")}type H3 struct{}func (h *H3) ServeHTTP(rw http.ResponseWriter, req *http.Request) {    log.Println("H3")}