How to log response body in gin

Question

I need to log the response body in a middleware of gin, but I don't find how to get the response body. Can anyone help?

I am using a middleware like this:

func Logger() gin.HandlerFunc {

    return func(c *gin.Context) {

        c.Next()

        statusCode := c.Writer.Status()
        if statusCode >= 400 {
            //ok this is an request with error, let's make a record for it
            //log body here
        }
    }
}

My question is, how to get response body from Context in middleware?

Answer 1

You need to intercept writing of response and store it somewhere first. Then you can log it. And to do that you need to implement your own Writer intercepting Write() calls.

For example, as follows:

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

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

func ginBodyLogMiddleware(c *gin.Context) {
    blw := &bodyLogWriter{body: bytes.NewBufferString(""), ResponseWriter: c.Writer}
    c.Writer = blw
    c.Next()
    statusCode := c.Writer.Status()
    if statusCode >= 400 {
        //ok this is an request with error, let's make a record for it
        // now print body (or log in your preferred way)
        fmt.Println("Response body: " + blw.body.String())
    }
}

Then use this middleware like this:

router.Use(ginBodyLogMiddleware)

Note that this sill won't work for static files as gin does not seem to use c.Writer for them. But in most cases, that's what you want anyway.

If you want to intercept all files, you need to use a slightly more complicated approach. Instead of Middleware, you'll need to implement a wrapper http.Handler that will wrap gin.Engine and will use same approach as shown above to intercept and log whatever is written to http.ResponseWriter. Then run gin server like this:

ginRouter := gin.New()
// configure your middleware and routes as required

// Run http server as follows, where bodyLogHandler is your wrapper handler
http.ListenAndServe(bindAddress, &bodyLogHandler{wrappedHandler: ginRouter}

Answer 2

FYI

Note: implement WriteString() if using c.String() for writing response body

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

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

func (w bodyLogWriter) WriteString(s string) (int, error) {
    w.body.WriteString(s)
    return w.ResponseWriter.WriteString(s)
}

func ginBodyLogMiddleware() gin.HandlerFunc {
    return func(c *gin.Context) {
        blw := &bodyLogWriter{body: bytes.NewBufferString(""), ResponseWriter: c.Writer}
        c.Writer = blw
        c.Next()

        fmt.Println("Response body: " + blw.body.String())
    }
}

...

// register
router := r.Group("/", ginBodyLogMiddleware())