为什么我不能在http重定向中添加正文？

Question

Here is what I've tried:

w.WriteHeader(301)
w.Write([]byte("Redirecting..."))
w.Header().Set("Location", "/myredirecturl")
w.Header().Set("Content-Length", contentLength) // I thought this might help

It won't add the Location header for some odd reason.
Why isn't it possible to add a body and redirect golang's http package?

Answer 1

This is documented in the net/http package:

type ResponseWriter

type ResponseWriter interface {
    // Header returns the header map that will be sent by WriteHeader.
    // Changing the header after a call to WriteHeader (or Write) has
    // no effect.
    Header() Header

    // Write writes the data to the connection as part of an HTTP reply.
    // If WriteHeader has not yet been called, Write calls WriteHeader(http.StatusOK)
    // before writing the data.  If the Header does not contain a
    // Content-Type line, Write adds a Content-Type set to the result of passing
    // the initial 512 bytes of written data to DetectContentType.
    Write([]byte) (int, error)

    // WriteHeader sends an HTTP response header with status code.
    // If WriteHeader is not called explicitly, the first call to Write
    // will trigger an implicit WriteHeader(http.StatusOK).
    // Thus explicit calls to WriteHeader are mainly used to
    // send error codes.
    WriteHeader(int)
}

The above states that you cannot change Header() after a call to Write() or WriteHeader(). You should change your code to the following:

w.Header().Set("Location", "/myredirecturl")
w.WriteHeader(301)
w.Write('Redirecting...')