Gin: how do i get response body in after router middleware?

[privationel]

rt

r := gin.Default()
r.GET("/test",handler,middleware())

func middleware() gin.HandlerFunc{
// get response body in here;
...
}

[appleboy]

what is your scenario?

[privationel]

I want to cache response body.
an request flow like this ：
frist:before router middleware(query in cache,no found) -> controller -> after router middleware(save in cache) -> return
second: before router middleware(query in cache,founded) ->return

[Ramzec]

I think you need to replace ctx.Writer by a wrapped version, that will implement buffered write-operation

r := gin.Default()
r.GET("/test",handler,middleware())

func middleware(ctx *gin.Context) gin.HandlerFunc {
    // here you need to replace ctx.Writer by your wrapper

    ctx.Next()

    // here you can access the data, that your wrapper saved into a buffer
}

[kerbalwzy]

I think you need to replace ctx.Writer by a wrapped version, that will implement buffered write-operation

r := gin.Default()
r.GET("/test",handler,middleware())

func middleware(ctx *gin.Context) gin.HandlerFunc {
    // here you need to replace ctx.Writer by your wrapper

    ctx.Next()

    // here you can access the data, that your wrapper saved into a buffer
}

The problem is how to get data from ctx, after ctx.Next() infact ;
Do you agree? @privationel

[montanaflynn]

Here's a full example of using a custom response writer to capture the response body:

```go
package main

import (
"bytes"
"fmt"

"github.com/gin-gonic/gin"

)

type responseBodyWriter struct {
gin.ResponseWriter
body *bytes.Buffer
}

func (r responseBodyWriter) Write(b []byte) (int, error) {
r.body.Write(b)
return r.ResponseWriter.Write(b)
}

func logResponseBody(c *gin.Context) {
w := &responseBodyWriter{body: &bytes.Buffer{}, ResponseWriter: c.Writer}
c.Writer = w
c.Next()
fmt.Println("Response body: " + w.body.String())
}

func sayHello(c *gin.Context) {
c.JSON(200, gin.H{
"hello": "privationel",
})
}

func main() {
r := gin.Default()
r.Use(logResponseBody)
r.GET("/", sayHello)
r.Run()
}

[GaoJuqian]

👍

[kougazhang]

Here's a full example of using a custom response writer to capture the response body:

package main

import (
  "bytes"
  "fmt"

  "github.com/gin-gonic/gin"
)

type responseBodyWriter struct {
  gin.ResponseWriter
  body *bytes.Buffer
}

func (r responseBodyWriter) Write(b []byte) (int, error) {
  r.body.Write(b)
  return r.ResponseWriter.Write(b)
}

func logResponseBody(c *gin.Context) {
  w := &responseBodyWriter{body: &bytes.Buffer{}, ResponseWriter: c.Writer}
  c.Writer = w
  c.Next()
  fmt.Println("Response body: " + w.body.String())
}

func sayHello(c *gin.Context) {
  c.JSON(200, gin.H{
      "hello": "privationel",
  })
}

func main() {
  r := gin.Default()
  r.Use(logResponseBody)
  r.GET("/", sayHello)
  r.Run()
}

If gin response like c.String(200, "hello world"), the code does not work.
Because text implements the Render interface by io.WriteString, see the picture in the blow.

And WriteString calls sw.WriteString

So, i think it should supplement these code:

func (r responseBodyWriter) WriteString(s string) (n int, err error)  {
    r.body.WriteString(s)
    return r.ResponseWriter.WriteString(s)
}