为什么http.Header中的切片长度返回0？

From the source code of net/http. The definition of http.Header is map[string][]string. Right?

But why go run below code, I get the result:

0

2

func main() {
    var header = make(http.Header)
    header.Add("hello", "world")
    header.Add("hello", "anotherworld")
    var t = []string {"a", "b"}
    fmt.Printf("%d
", len(header["hello"]))
    fmt.Print(len(t))
}

if you try

fmt.Println(header)

you'll notice that the key has been capitalized. This is actually noted in the documentation of net/http.

// HTTP defines that header names are case-insensitive.
// The request parser implements this by canonicalizing the
// name, making the first character and any characters
// following a hyphen uppercase and the rest lowercase.

This can be found in the comment on the field Header of type Request.

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

Comment should probably be moved though..

Take a look at the reference of http.Header and the code of Get:

Get gets the first value associated with the given key. If there are no values associated with the key, Get returns "". To access multiple values of a key, access the map directly with CanonicalHeaderKey.

So it helps to use http.CanonicalHeaderKey instead of strings for keys.

package main

import (
    "net/http"
    "fmt"
)

func main() {
    header := make(http.Header)
    var key = http.CanonicalHeaderKey("hello")

    header.Add(key, "world")
    header.Add(key, "anotherworld")

    fmt.Printf("%#v
", header)
    fmt.Printf("%#v
", header.Get(key))
    fmt.Printf("%#v
", header[key])
}

The output:

http.Header{"Hello":[]string{"world", "anotherworld"}}
"world"
[]string{"world", "anotherworld"}