8252304: Seed an HttpRequest.Builder from an existing HttpRequest

Co-authored-by: Chris Hegarty <chegar@openjdk.org>
Reviewed-by: chegar, dfuchs, michaelm

Author: pconcannon

src/java.net.http/share/classes/java/net/http/HttpRequest.java

@@ -39,6 +39,7 @@
 import java.util.Objects;
 import java.util.Optional;
 import java.util.concurrent.Flow;
+import java.util.function.BiPredicate;
 import java.util.function.Supplier;
 
 import jdk.internal.net.http.HttpRequestBuilderImpl;
@@ -90,8 +91,9 @@ protected HttpRequest() {}
     /**
      * A builder of {@linkplain HttpRequest HTTP requests}.
      *
-     * <p> Instances of {@code HttpRequest.Builder} are created by calling {@link
-     * HttpRequest#newBuilder(URI)} or {@link HttpRequest#newBuilder()}.
+     * <p> Instances of {@code HttpRequest.Builder} are created by calling
+     * {@link HttpRequest#newBuilder()}, {@link HttpRequest#newBuilder(URI)},
+     * or {@link HttpRequest#newBuilder(HttpRequest, BiPredicate)}.
      *
      * <p> The builder can be used to configure per-request state, such as: the
      * request URI, the request method (default is GET unless explicitly set),
@@ -303,6 +305,74 @@ public static HttpRequest.Builder newBuilder(URI uri) {
         return new HttpRequestBuilderImpl(uri);
     }
 
+    /**
+     * Creates a {@code Builder} whose initial state is copied from an existing
+     * {@code HttpRequest}.
+     *
+     * <p> This builder can be used to build an {@code HttpRequest}, equivalent
+     * to the original, while allowing amendment of the request state prior to
+     * construction - for example, adding additional headers.
+     *
+     * <p> The {@code filter} is applied to each header name value pair as they
+     * are copied from the given request. When completed, only headers that
+     * satisfy the condition as laid out by the {@code filter} will be present
+     * in the {@code Builder} returned from this method.
+     *
+     * @apiNote
+     * The following scenarios demonstrate typical use-cases of the filter.
+     * Given an {@code HttpRequest} <em>request</em>:
+     * <br><br>
+     * <ul>
+     *  <li> Retain all headers:
+     *  <pre>{@code HttpRequest.newBuilder(request, (n, v) -> true)}</pre>
+     *
+     *  <li> Remove all headers:
+     *  <pre>{@code HttpRequest.newBuilder(request, (n, v) -> false)}</pre>
+     *
+     *  <li> Remove a particular header (e.g. Foo-Bar):
+     *  <pre>{@code HttpRequest.newBuilder(request, (name, value) -> !name.equalsIgnoreCase("Foo-Bar"))}</pre>
+     * </ul>
+     *
+     * @param request the original request
+     * @param filter a header filter
+     * @return a new request builder
+     * @throws IllegalArgumentException if a new builder cannot be seeded from
+     *         the given request (for instance, if the request contains illegal
+     *         parameters)
+     * @since 16
+     */
+    public static Builder newBuilder(HttpRequest request, BiPredicate<String, String> filter) {
+        Objects.requireNonNull(request);
+        Objects.requireNonNull(filter);
+
+        final HttpRequest.Builder builder = HttpRequest.newBuilder();
+        builder.uri(request.uri());
+        builder.expectContinue(request.expectContinue());
+
+        // Filter unwanted headers
+        HttpHeaders headers = HttpHeaders.of(request.headers().map(), filter);
+        headers.map().forEach((name, values) ->
+                values.forEach(value -> builder.header(name, value)));
+
+        request.version().ifPresent(builder::version);
+        request.timeout().ifPresent(builder::timeout);
+        var method = request.method();
+        request.bodyPublisher().ifPresentOrElse(
+                // if body is present, set it
+                bodyPublisher -> builder.method(method, bodyPublisher),
+                // otherwise, the body is absent, special case for GET/DELETE,
+                // or else use empty body
+                () -> {
+                    switch (method) {
+                        case "GET" -> builder.GET();
+                        case "DELETE" -> builder.DELETE();
+                        default -> builder.method(method, HttpRequest.BodyPublishers.noBody());
+                    }
+                }
+        );
+        return builder;
+    }
+
     /**
      * Creates an {@code HttpRequest} builder.
      *