Send "Accept" request header to indicate RFC 9457 compliance

maven-resolver-spi/src/main/java/org/eclipse/aether/spi/connector/transport/http/HttpConstants.java

@@ -36,6 +36,8 @@ private HttpConstants() {}
 
     public static final int PRECONDITION_FAILED = 412;
 
+    public static final String ACCEPT = "Accept";
+
     public static final String ACCEPT_ENCODING = "Accept-Encoding";
 
     public static final String CACHE_CONTROL = "Cache-Control";

maven-resolver-spi/src/main/java/org/eclipse/aether/spi/connector/transport/http/RFC9457/RFC9457Parser.java

@@ -30,12 +30,12 @@
 import com.google.gson.JsonParseException;
 
 public class RFC9457Parser {
-    private static Gson gson = new GsonBuilder()
+    private static final Gson GSON = new GsonBuilder()
             .registerTypeAdapter(RFC9457Payload.class, new RFC9457PayloadAdapter())
             .create();
 
     public static RFC9457Payload parse(String data) {
-        return gson.fromJson(data, RFC9457Payload.class);
+        return GSON.fromJson(data, RFC9457Payload.class);
     }
 
     private static class RFC9457PayloadAdapter implements JsonDeserializer<RFC9457Payload> {

maven-resolver-spi/src/main/java/org/eclipse/aether/spi/connector/transport/http/RFC9457/RFC9457Payload.java

@@ -68,11 +68,24 @@ public URI getInstance() {
 
     @Override
     public String toString() {
-        return "RFC9457Payload {" + "type="
-                + type + ", status="
-                + status + ", title='"
-                + title + ", detail='"
-                + detail + ", instance="
-                + instance + '}';
+        StringBuilder builder = new StringBuilder();
+        builder.append("RFC9457Payload [");
+        if (type != null) {
+            builder.append("type=").append(type).append(", ");
+        }
+        if (status != null) {
+            builder.append("status=").append(status).append(", ");
+        }
+        if (title != null) {
+            builder.append("title=").append(title).append(", ");
+        }
+        if (detail != null) {
+            builder.append("detail=").append(detail).append(", ");
+        }
+        if (instance != null) {
+            builder.append("instance=").append(instance);
+        }
+        builder.append("]");
+        return builder.toString();
     }
 }

maven-resolver-spi/src/main/java/org/eclipse/aether/spi/connector/transport/http/RFC9457/RFC9457Reporter.java

@@ -24,15 +24,19 @@
  * A reporter for RFC 9457 messages.
  * RFC 9457 is a standard for reporting problems in HTTP responses as a JSON object.
  * There are members specified in the RFC but none of those appear to be required,
- * @see <a href=https://www.rfc-editor.org/rfc/rfc9457#section-3-7>rfc9457 section 3.7</a>
+ * see <a href=https://www.rfc-editor.org/rfc/rfc9457#section-3-7>rfc9457 section 3.7</a>
  * Given the JSON fields are not mandatory, this reporter simply extracts the body of the
  * response without validation.
- * A RFC 9457 message is detected by the content type "application/problem+json".
+ * A RFC 9457 message is detected by the content type {@value #CONTENT_TYPE_PROBLEM_DETAILS_JSON} in the response header.
  *
  * @param <T> The type of the response.
  * @param <E> The base exception type to throw if the response is not a RFC9457 message.
+ * @param <R> The type of the request or request builder (which allows to modify headers)
+ * @see <a href=https://www.rfc-editor.org/rfc/rfc9457#section-3-7>RFC 9457</a>
  */
-public abstract class RFC9457Reporter<T, E extends Exception> {
+public abstract class RFC9457Reporter<T, E extends Exception, R> {
+    public static final String CONTENT_TYPE_PROBLEM_DETAILS_JSON = "application/problem+json";
+
     protected abstract boolean isRFC9457Message(T response);
 
     protected abstract int getStatusCode(T response);
@@ -41,8 +45,24 @@ public abstract class RFC9457Reporter<T, E extends Exception> {
 
     protected abstract String getBody(T response) throws IOException;
 
+    /**
+     * Prepares the request to accept RFC 9457 responses.
+     * This involves setting/updating the "Accept" header to include "application/problem+json".
+     * @param request The request or request builder to prepare
+     * @see <a href=https://www.rfc-editor.org/rfc/rfc9457#section-3-2>RFC 9457 section 3.2</a>
+     */
+    public abstract void prepareRequest(R request);
+
     protected boolean hasRFC9457ContentType(String contentType) {
-        return "application/problem+json".equals(contentType);
+        if (contentType == null) {
+            return false;
+        }
+        // strip off parameters
+        int idx = contentType.indexOf(';');
+        if (idx > -1) {
+            contentType = contentType.substring(0, idx);
+        }
+        return CONTENT_TYPE_PROBLEM_DETAILS_JSON.equals(contentType);
     }
 
     /**

maven-resolver-spi/src/test/java/org/eclipse/aether/spi/connector/transport/http/RFC9457/RFC9457ReporterTest.java

@@ -0,0 +1,56 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.eclipse.aether.spi.connector.transport.http.RFC9457;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class RFC9457ReporterTest {
+
+    @Test
+    void hasRFC9457ContentType() {
+        RFC9457Reporter<Object, Exception, Object> reporter = new RFC9457Reporter<Object, Exception, Object>() {
+            @Override
+            protected boolean isRFC9457Message(Object response) {
+                return false;
+            }
+
+            @Override
+            protected int getStatusCode(Object response) {
+                return 0;
+            }
+
+            @Override
+            protected String getReasonPhrase(Object response) {
+                return null;
+            }
+
+            @Override
+            protected String getBody(Object response) {
+                return null;
+            }
+
+            @Override
+            public void prepareRequest(Object request) {}
+        };
+        assertTrue(reporter.hasRFC9457ContentType("application/problem+json"));
+        assertTrue(reporter.hasRFC9457ContentType("application/problem+json; charset=utf-8"));
+    }
+}