How to customise "host" header in Java http client

Question

Here's my code:

HttpClient client = HttpClient.newHttpClient();
HttpRequest request = HttpRequest.newBuilder()
    .uri(URI.create("http://127.0.0.1:8081/"))
    .header("Host", "test.example.com")
    .build();
client.send(request, HttpResponse.BodyHandlers.ofString());

As a result I see that the above code sends:

GET / HTTP/1.1
Connection: Upgrade, HTTP2-Settings
Content-Length: 0
Host: 127.0.0.1:8081
HTTP2-Settings: AAEAAEAAAAIAAAABAAMAAABkAAQBAAAAAAUAAEAA
Upgrade: h2c
User-Agent: Java-http-client/10
Host: test.example.com

As you can see it sends two Host headers (the one from URI and the one I specified), but I would like it to send the Host header that I specified, not the one from the URI. Is it possible with this client?

EDIT: In Java 11, it gets even worse (you need to change the client.send line to: client.send(request, HttpResponse.BodyHandlers.ofString());):

java.lang.IllegalArgumentException: restricted header name: "Host"

How can I customize that header (needed for testing of virtual hosts)?

I also tried the setHeader and get exactly the same problem (either double Host headers, or the exception).

EDIT: I reported a JDK bug.

Answer 1

As of Java 12 (EA build 22) it has been solved by additional property jdk.httpclient.allowRestrictedHeaders (see https://bugs.openjdk.java.net/browse/JDK-8213696).

So now one can override Host (or any other restricted header) by executing the code with:

java -Djdk.httpclient.allowRestrictedHeaders=host ...

Allowing multiple restricted headers are as follows:

java -Djdk.httpclient.allowRestrictedHeaders=connection,content-length,host

and you can set it up in the eclipse menu > Run > Run Cunfigurations

Answer 2

The behavior from the Java11 client code seems correct. The Host section elaborates on the details. By the way, from the documentation of HttpRequest builder header(String name, String value) :

*    @throws IllegalArgumentException if the header name or value is not
*    valid, see <a href="https://www.rfc-editor.org/rfc/rfc7230#section-3.2">
*    RFC 7230 section-3.2</a>, or the header name or value is restricted
*    by the implementation.

Update: See this, for answer pertaining to JDK/12.