Interface ResponseWriter

Header returns the header map that will be sent by [ResponseWriter.WriteHeader]. The [Header] map also is the mechanism with which [Handler] implementations can set HTTP trailers.

Changing the header map after a call to [ResponseWriter.WriteHeader] (or [ResponseWriter.Write]) has no effect unless the HTTP status code was of the 1xx class or the modified headers are trailers.

There are two ways to set Trailers. The preferred way is to predeclare in the headers which trailers you will later send by setting the "Trailer" header to the names of the trailer keys which will come later. In this case, those keys of the Header map are treated as if they were trailers. See the example. The second way, for trailer keys not known to the [Handler] until after the first [ResponseWriter.Write], is to prefix the [Header] map keys with the [TrailerPrefix] constant value.

To suppress automatic response headers (such as "Date"), set their value to nil.

Write writes the data to the connection as part of an HTTP reply.

If [ResponseWriter.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]. Additionally, if the total size of all written data is under a few KB and there are no Flush calls, the Content-Length header is added automatically.

Depending on the HTTP protocol version and the client, calling Write or WriteHeader may prevent future reads on the Request.Body. For HTTP/1.x requests, handlers should read any needed request body data before writing the response. Once the headers have been flushed (due to either an explicit Flusher.Flush call or writing enough data to trigger a flush), the request body may be unavailable. For HTTP/2 requests, the Go HTTP server permits handlers to continue to read the request body while concurrently writing the response. However, such behavior may not be supported by all HTTP/2 clients. Handlers should read before writing if possible to maximize compatibility.

WriteHeader sends an HTTP response header with the provided 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 or 1xx informational responses.

The provided code must be a valid HTTP 1xx-5xx status code. Any number of 1xx headers may be written, followed by at most one 2xx-5xx header. 1xx headers are sent immediately, but 2xx-5xx headers may be buffered. Use the Flusher interface to send buffered data. The header map is cleared when 2xx-5xx headers are sent, but not with 1xx headers.

The server will automatically send a 100 (Continue) header on the first read from the request body if the request has an "Expect: 100-continue" header.