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Using Webflux filter, I am trying to intercept the requests and check if the request is coming from certain URI then add a new Authorization header

The filter code is simple and straightforward

@Component
@Order(Ordered.HIGHEST_PRECEDENCE)
public class AuthorizationFilter implements WebFilter {
    @Override
    public Mono<Void> filter(ServerWebExchange exchange, WebFilterChain chain) {
        return chain.filter(Optional.of(exchange)
                .filter(serverWebExchange -> serverWebExchange.getRequest().getURI().getPath().endsWith("/callback"))
                .map(serverWebExchange -> addNewHeader(serverWebExchange))
                .orElse(exchange));
    private ServerWebExchange addNewHeader(ServerWebExchange serverWebExchange) {
        String authHeader=serverWebExchange.getRequest().getQueryParams().get("state").get(0);
        if (authHeader == null) {
            throw new BadRequestException("State not complete (access_token missing) for //callback");
        try {
            serverWebExchange.getRequest().getHeaders().setBearerAuth(authHeader);
        }catch (Throwable t){
            t.printStackTrace();
        return serverWebExchange;
But it throws an exception
java.lang.UnsupportedOperationException
    at org.springframework.http.ReadOnlyHttpHeaders.set(ReadOnlyHttpHeaders.java:99)
    at org.springframework.http.HttpHeaders.setBearerAuth(HttpHeaders.java:774)
It seems the header map is read only.How can i overcome this issue and add the new Header ?
You can mutate the ServerWebExchange and its ServerHttpRequest with their mutate() methods which returns a 'Builder' for each of them. 
Example Java:
@Component
public class AuthorizationFilter implements WebFilter {
    @Override
    public Mono<Void> filter(ServerWebExchange exchange, WebFilterChain chain) {
        ServerHttpRequest mutatedRequest = exchange.getRequest().mutate().header(HttpHeaders.AUTHORIZATION, "Bearer " + authHeader).build();
        ServerWebExchange mutatedExchange = exchange.mutate().request(mutatedRequest).build();
        return chain.filter(mutatedExchange);
Example Kotlin:
@Component
class AuthorizationFilter : WebFilter {
    override fun filter(exchange: ServerWebExchange, chain: WebFilterChain): Mono<Void> {
        val mutatedRequest = exchange.request.mutate().header(HttpHeaders.AUTHORIZATION, "Bearer $authHeader").build()
        val mutatedExchange = exchange.mutate().request(mutatedRequest).build()
        return chain.filter(mutatedExchange)
                The weird thing is.. we don't need to call mutate for modifying response. Just exchange.response.headers.add("x-test-response", "test-value")  will works.
– aldok
                Dec 17, 2020 at 2:03
                Thanks a lot for this solution! I used it to add header to responses but it worked only for non-error responses. I resolved this problem by adding highest order for the filter bean: @Order(Ordered.HIGHEST_PRECEDENCE)
– Rib47
                Jul 20, 2021 at 12:00
                I am doing the same way. But sometimes, I got mentioned error. Not sure what I am missing. out 10-15 times, I use to get once or twice this error
– prasingh
                Feb 23, 2021 at 10:20
                I had a logical error. Pretty new in Reactive programming. But Your answer makes sense to me. Got the answer to my error.
– prasingh
                Feb 24, 2021 at 9:29
        
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