为什么在每个URL请求上都提供此服务？

package main

import "fmt"
import "net/http"

func home(w http.ResponseWriter, r *http.Request) {
  fmt.Fprintf(w, "What!")
}

func bar(w http.ResponseWriter, r *http.Request) {
  fmt.Fprintf(w, "Bar!")
}

func main() {
  http.HandleFunc("/", home)
  http.HandleFunc("/foo", bar)
  http.ListenAndServe(":5678", nil)
}

If I visit /foo, bar will run.

If I visit / or /any/other/path, home will run.

Any idea why this happens? How can I handle 404's?

This is a behaviour by design - handler defined for path ending with / will also handle any subpath.

Note that since a pattern ending in a slash names a rooted subtree, the pattern "/" matches all paths not matched by other registered patterns, not just the URL with Path == "/".

http://golang.org/pkg/net/http/#ServeMux

You have to implement you own logic for 404. Consider the following example from the golang doc:

mux := http.NewServeMux()
mux.Handle("/api/", apiHandler{})
mux.HandleFunc("/", func(w http.ResponseWriter, req *http.Request) {
        // The "/" pattern matches everything, so we need to check
        // that we're at the root here.
        if req.URL.Path != "/" {
                http.NotFound(w, req)
                return
        }
        fmt.Fprintf(w, "Welcome to the home page!")
})

http://golang.org/pkg/net/http/#ServeMux.Handle

You have to handle your own 404s in home.