How to set multiple headers at once in Spring WebClient?

If those headers change on a per request basis, you can use:

webClient.get().uri("/resource").headers(httpHeaders -> {
    httpHeaders.setX("");
    httpHeaders.setY("");
});

This doesn't save much typing; so for the headers that don't change from one request to another, you can set those as default headers while building the client:

WebClient webClient = WebClient.builder().defaultHeader("...", "...").build();
WebClient webClient = WebClient.builder().defaultHeaders(httpHeaders -> {
    httpHeaders.setX("");
    httpHeaders.setY("");
}).build();

The consumer is correct, though it's hard to visualize, esp. in that you can continue with additional fluent-composition method calls in the webclient construction, after you've done your work with the headers.

....suppose you have a HttpHeaders (or MutliValue map) holding your headers in scope. here's an example, using an exchange object from spring cloud gateway:

final HttpHeaders headersFromExchangeRequest = exchange.getRequest().headers();
webclient.get().uri("blah-blah")
    .headers( httpHeadersOnWebClientBeingBuilt -> { 
         httpHeadersOnWebClientBeingBuilt.addAll( headersFromExchangeRequest );
    }
)...

the addAll can take a multivalued map. if that makes sense. if not, let your IDE be your guide.

to make the consumer clearer, let's rewrite the above as follows:

private Consumer<HttpHeaders> getHttpHeadersFromExchange(ServerWebExchange exchange) {
    return httpHeaders -> {
        httpHeaders.addAll(exchange.getRequest().getHeaders());
    };
}
.
.
.
webclient.get().uri("blah-blah")
    .headers(getHttpHeadersFromExchange(exchange))
    ...