前言
在对接Alexa Smart Home时，有的请求Payload中需要传入Access Token，但是这个Token是由OAuth2 Client管理的，封装Payload时并不知道Access Token。
所以使用自定义RoundTripper，在请求前取出Header里的token，修改body，实现动态修改payload。
原理
go中可以使用http.DefaultClient进行http请求，也可以自己创建http.Client，传入自定义Transport可以实现对request的处理。
http.Client
// A Client is an HTTP client. Its zero value (DefaultClient) is a
// usable client that uses DefaultTransport.
//
// The Client's Transport typically has internal state (cached TCP
// connections), so Clients should be reused instead of created as
// needed. Clients are safe for concurrent use by multiple goroutines.
//
// A Client is higher-level than a RoundTripper (such as Transport)
// and additionally handles HTTP details such as cookies and
// redirects.
//
// When following redirects, the Client will forward all headers set on the
// initial Request except:
//
// • when forwarding sensitive headers like "Authorization",
// "WWW-Authenticate", and "Cookie" to untrusted targets.
// These headers will be ignored when following a redirect to a domain
// that is not a subdomain match or exact match of the initial domain.
// For example, a redirect from "foo.com" to either "foo.com" or "sub.foo.com"
// will forward the sensitive headers, but a redirect to "bar.com" will not.
//
// • when forwarding the "Cookie" header with a non-nil cookie Jar.
// Since each redirect may mutate the state of the cookie jar,
// a redirect may possibly alter a cookie set in the initial request.
// When forwarding the "Cookie" header, any mutated cookies will be omitted,
// with the expectation that the Jar will insert those mutated cookies
// with the updated values (assuming the origin matches).
// If Jar is nil, the initial cookies are forwarded without change.
//
type Client struct {
// Transport specifies the mechanism by which individual
// HTTP requests are made.
// If nil, DefaultTransport is used.
Transport RoundTripper
// CheckRedirect specifies the policy for handling redirects.
// If CheckRedirect is not nil, the client calls it before
// following an HTTP redirect. The arguments req and via are
// the upcoming request and the requests made already, oldest
// first. If CheckRedirect returns an error, the Client's Get
// method returns both the previous Response (with its Body
// closed) and CheckRedirect's error (wrapped in a url.Error)
// instead of issuing the Request req.
// As a special case, if CheckRedirect returns ErrUseLastResponse,
// then the most recent response is returned with its body
// unclosed, along with a nil error.
//
// If CheckRedirect is nil, the Client uses its default policy,
// which is to stop after 10 consecutive requests.
CheckRedirect func(req *Request, via []*Request) error
// Jar specifies the cookie jar.
//
// The Jar is used to insert relevant cookies into every
// outbound Request and is updated with the cookie values
// of every inbound Response. The Jar is consulted for every
// redirect that the Client follows.
//
// If Jar is nil, cookies are only sent if they are explicitly
// set on the Request.
Jar CookieJar
// Timeout specifies a time limit for requests made by this
// Client. The timeout includes connection time, any
// redirects, and reading the response body. The timer remains
// running after Get, Head, Post, or Do return and will
// interrupt reading of the Response.Body.
//
// A Timeout of zero means no timeout.
//
// The Client cancels requests to the underlying Transport
// as if the Request's Context ended.
//
// For compatibility, the Client will also use the deprecated
// CancelRequest method on Transport if found. New
// RoundTripper implementations should use the Request's Context
// for cancellation instead of implementing CancelRequest.
Timeout time.Duration
}
http.RoundTripper
// RoundTripper is an interface representing the ability to execute a
// single HTTP transaction, obtaining the Response for a given Request.
//
// A RoundTripper must be safe for concurrent use by multiple
// goroutines.
type RoundTripper interface {
// RoundTrip executes a single HTTP transaction, returning
// a Response for the provided Request.
//
// RoundTrip should not attempt to interpret the response. In
// particular, RoundTrip must return err == nil if it obtained
// a response, regardless of the response's HTTP status code.
// A non-nil err should be reserved for failure to obtain a
// response. Similarly, RoundTrip should not attempt to
// handle higher-level protocol details such as redirects,
// authentication, or cookies.
//
// RoundTrip should not modify the request, except for
// consuming and closing the Request's Body. RoundTrip may
// read fields of the request in a separate goroutine. Callers
// should not mutate or reuse the request until the Response's
// Body has been closed.
//
// RoundTrip must always close the body, including on errors,
// but depending on the implementation may do so in a separate
// goroutine even after RoundTrip returns. This means that
// callers wanting to reuse the body for subsequent requests
// must arrange to wait for the Close call before doing so.
//
// The Request's URL and Header fields must be initialized.
RoundTrip(*Request) (*Response, error)
}
实现
我们先写一个server，打印出访问的payload信息。
package main
import (
"fmt"
"io/ioutil"
"net/http"
)
func main() {
mux := http.NewServeMux()
mux.HandleFunc("/", func(rw http.ResponseWriter, r *http.Request) {
req, err := ioutil.ReadAll(r.Body)
if err != nil {
rw.WriteHeader(500)
rw.Write([]byte(err.Error()))
return
}
fmt.Println(string(req))
})
if err := http.ListenAndServe(":8000", mux); err != nil {
panic(err)
}
}
如果使用默认的DefaultClient，只会打印出我们传入的payload。
package main
import (
"fmt"
"io/ioutil"
"net/http"
"strings"
"github.com/google/uuid"
)
func main() {
id := uuid.NewString()
req, _ := http.NewRequest("GET", "http://localhost:8000", strings.NewReader(fmt.Sprintf(`{"id":"%s"}`, id)))
req.Header.Add("Authorization", fmt.Sprintf("Bearer token%s", id))
resp, err := http.DefaultClient.Do(req)
if err != nil {
panic(err)
}
fmt.Println(resp)
}()
结果：
{"id":"912733ce-4e17-4209-ad9e-71159fd37845"}
&{200 OK 200 HTTP/1.1 1 1 map[Content-Length:[0] Date:[Sun, 28 Nov 2021 06:48:50 GMT]] {} 0 [] false false map[] 0xc000194000 }
使用自定义Transport
package main
import (
"bytes"
"encoding/json"
"io/ioutil"
"net/http"
"strings"
)
type customTransport struct {
}
func (t *customTransport) RoundTrip(req *http.Request) (*http.Response, error) {
token := req.Header.Get("Authorization")
if len(token) != 0 && strings.HasPrefix(token, "Bearer ") {
token = token[7:]
var bodyBytes []byte
if req.Body != nil {
bodyBytes, _ = ioutil.ReadAll(req.Body)
}
var payload map[string]interface{}
if err := json.Unmarshal(bodyBytes, &payload); err != nil {
return nil, err
} else {
payload["token"] = token
if bodyBytes, err := json.Marshal(payload); err != nil {
return nil, err
} else {
req.ContentLength = int64(len(bodyBytes))
req.Body = ioutil.NopCloser(bytes.NewBuffer(bodyBytes))
}
}
}
resp, err := http.DefaultTransport.RoundTrip(req)
if err != nil {
return nil, err
}
return resp, nil
}
使用自定义Client
package main
import (
"fmt"
"io/ioutil"
"net/http"
"strings"
"github.com/google/uuid"
)
func main() {
id := uuid.NewString()
req, _ := http.NewRequest("GET", "http://localhost:8000", strings.NewReader(fmt.Sprintf(`{"id":"%s"}`, id)))
req.Header.Add("Authorization", fmt.Sprintf("Bearer token%s", id))
client := &http.Client{
Transport: &customTransport{},
}
resp, err := client.Do(req)
if err != nil {
panic(err)
}
fmt.Println(resp)
}()
最终结果：
{"id":"ebcceb4b-1979-457b-bf49-9255ceb77322","token":"tokenebcceb4b-1979-457b-bf49-9255ceb77322"}
&{200 OK 200 HTTP/1.1 1 1 map[Content-Length:[0] Date:[Sun, 28 Nov 2021 06:49:25 GMT]] {} 0 [] false false map[] 0xc000140000 }
总结
我们可以使用http.DefaultClient完成大部分http请求，但是如果我们需要实现一些自定义逻辑时，可以传入http.Client中对应自定义的部分，实现自定义逻辑。
本文中通过修改Transport，读取请求Header，并修改请求Body，动态修改请求Payload。