$ unzip -oq spring-test-7.0.9.jar "org/springframework/test/web/servlet/client/RestTestClient*.class"
$ javap -p org/springframework/test/web/servlet/client/RestTestClient.class
public interface org.springframework.test.web.servlet.client.RestTestClient {
  public static final java.lang.String RESTTESTCLIENT_REQUEST_ID;
  public abstract org.springframework.test.web.servlet.client.RestTestClient$RequestHeadersUriSpec<?> get();
  public abstract org.springframework.test.web.servlet.client.RestTestClient$RequestHeadersUriSpec<?> head();
  public abstract org.springframework.test.web.servlet.client.RestTestClient$RequestBodyUriSpec post();
  public abstract org.springframework.test.web.servlet.client.RestTestClient$RequestBodyUriSpec put();
  public abstract org.springframework.test.web.servlet.client.RestTestClient$RequestBodyUriSpec patch();
  public abstract org.springframework.test.web.servlet.client.RestTestClient$RequestHeadersUriSpec<?> delete();
  public abstract org.springframework.test.web.servlet.client.RestTestClient$RequestHeadersUriSpec<?> options();
  public abstract org.springframework.test.web.servlet.client.RestTestClient$RequestBodyUriSpec method(org.springframework.http.HttpMethod);
  public abstract <B extends org.springframework.test.web.servlet.client.RestTestClient$Builder<B>> org.springframework.test.web.servlet.client.RestTestClient$Builder<B> mutate();
  public static org.springframework.test.web.servlet.client.RestTestClient$StandaloneSetupBuilder bindToController(java.lang.Object...);
  public static org.springframework.test.web.servlet.client.RestTestClient$RouterFunctionSetupBuilder bindToRouterFunction(org.springframework.web.servlet.function.RouterFunction<?>...);
  public static org.springframework.test.web.servlet.client.RestTestClient$WebAppContextSetupBuilder bindToApplicationContext(org.springframework.web.context.WebApplicationContext);
  public static org.springframework.test.web.servlet.client.RestTestClient$Builder<?> bindTo(org.springframework.test.web.servlet.MockMvc);
  public static org.springframework.test.web.servlet.client.RestTestClient$Builder<?> bindToServer();
  public static org.springframework.test.web.servlet.client.RestTestClient$Builder<?> bindToServer(org.springframework.http.client.ClientHttpRequestFactory);
}

$ javap -p 'org/springframework/test/web/servlet/client/RestTestClient$RequestHeadersSpec.class'
public interface org.springframework.test.web.servlet.client.RestTestClient$RequestHeadersSpec<S extends org.springframework.test.web.servlet.client.RestTestClient$RequestHeadersSpec<S>> {
  public abstract S accept(org.springframework.http.MediaType...);
  public abstract S acceptCharset(java.nio.charset.Charset...);
  public abstract S cookie(java.lang.String, java.lang.String);
  public abstract S cookies(java.util.function.Consumer<org.springframework.util.MultiValueMap<java.lang.String, java.lang.String>>);
  public abstract S ifModifiedSince(java.time.ZonedDateTime);
  public abstract S ifNoneMatch(java.lang.String...);
  public abstract S header(java.lang.String, java.lang.String...);
  public abstract S headers(java.util.function.Consumer<org.springframework.http.HttpHeaders>);
  public abstract S apiVersion(java.lang.Object);
  public abstract S attribute(java.lang.String, java.lang.Object);
  public abstract S attributes(java.util.function.Consumer<java.util.Map<java.lang.String, java.lang.Object>>);
  public abstract org.springframework.test.web.servlet.client.RestTestClient$ResponseSpec exchange();
  public abstract org.springframework.test.web.servlet.client.RestTestClient$ResponseSpec exchangeSuccessfully();
}

$ javap -p 'org/springframework/test/web/servlet/client/RestTestClient$RequestBodySpec.class'
public interface org.springframework.test.web.servlet.client.RestTestClient$RequestBodySpec extends org.springframework.test.web.servlet.client.RestTestClient$RequestHeadersSpec<org.springframework.test.web.servlet.client.RestTestClient$RequestBodySpec> {
  public abstract org.springframework.test.web.servlet.client.RestTestClient$RequestBodySpec contentLength(long);
  public abstract org.springframework.test.web.servlet.client.RestTestClient$RequestBodySpec contentType(org.springframework.http.MediaType);
  public abstract org.springframework.test.web.servlet.client.RestTestClient$RequestHeadersSpec<?> body(java.lang.Object);
}

# No multipart() method and no MultiValueMap<String, Object> body builder anywhere in the chain --
# body(Object) is the only way to set a request body. Confirms the multipart limitation directly
# against the real class files instead of trusting a blog post's claim.