英文:
Why response code is set firstly in method *Context.Render(code int, r render.Render) of gin?
问题
Gin封装了一些用于构建响应的方法，例如*Context.JSON(code int, obj interface{})和*Context.String(code int, format string, values ...interface{})。这些方法都会调用*Context.Render(code int, r render.Render)方法。
// Render写入响应头并调用render.Render来渲染数据。
func (c *Context) Render(code int, r render.Render) {
c.Status(code)
if !bodyAllowedForStatus(code) {
r.WriteContentType(c.Writer)
c.Writer.WriteHeaderNow()
return
}
if err := r.Render(c.Writer); err != nil {
panic(err)
}
}
我想知道为什么Render方法会调用Status方法，而Status方法会首先调用ResponseWriter.WriterHeader(statusCode int)方法来设置HTTP响应代码。
r.Render(c.Writer)将会向响应中写入相应的Content-Type。显然，这发生在设置状态码之后（在调用WriterHeader方法之后）。根据ResponseWriter.Header()方法的注释，在调用WriteHeader（或Write）之后修改头部映射是没有效果的，除非修改的头部是尾部。但是在Gin中设置Content-Type是有效的。
func writeContentType(w http.ResponseWriter, value []string) {
header := w.Header()
if val := header["Content-Type"]; len(val) == 0 {
header["Content-Type"] = value
}
}
英文:
Gin has encapsulated some methods for constructing response, such as methods *Context.JSON(code int, obj interface{}) and *Context.String(code int, format string, values ...interface{}). These methods all call the method *Context.Render(code int, r render.Render).
// Render writes the response headers and calls render.Render to render data.
func (c *Context) Render(code int, r render.Render) {
c.Status(code)
if !bodyAllowedForStatus(code) {
r.WriteContentType(c.Writer)
c.Writer.WriteHeaderNow()
return
}
if err := r.Render(c.Writer); err != nil {
panic(err)
}
}
I wonder why the Render method call the method Status which will set the HTTP response code by calling the method ResponseWriter.WriterHeader(statusCode int) firstly.
r.Render(c.Writer) will write Corresponding Content-Type to the response. Apparently it occurs after setting the status code (after calling method WriterHeader). According to the comment on method ResponseWriter.Header(), changing the header map after a call to WriteHeader (or Write) has no effect unless the modified headers are trailers. But setting Content-Type works in Gin.
func writeContentType(w http.ResponseWriter, value []string) {
header := w.Header()
if val := header["Content-Type"]; len(val) == 0 {
header["Content-Type"] = value
}
}
答案1
得分: 2
c.Writer 是一个 gin.ResponseWriter（可能是具体类型 gin.responseWriter），而不是 http.ResponseWriter。虽然它实现了相同的接口，但它的实现方式并不完全相同。Gin 的 WriteHeader 并不会立即发送头部信息，它只是将 code 内部存储在 writer 中，而 WriteHeaderNow 方法会使用存储的 code 调用 net/http 中的真正的 WriteHeader 方法。
在你引用的函数中，如果没有响应体，则直接调用 WriteHeaderNow 方法；如果有响应体，则在第一次对响应体进行写入时调用 WriteHeaderNow。
英文:
c.Writer is a gin.ResponseWriter (probably the concrete type gin.responseWriter), not an http.ResponseWriter. While it implements the same interface, it doesn't do it in an identical way. Gin's WriteHeader doesn't send the headers immediately; it just stores the code internally in the writer, and WriteHeaderNow calls the "real" WriteHeader from net/http with the stored code.
WriteHeaderNow is called directly by the function you quoted in the case where there's no body; if there is a body, then WriteHeaderNow gets called on the first Write to the body.
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