8357728: Avoid caching synthesized names in synthesized parameters

src/java.base/share/classes/java/lang/reflect/Executable.java

@@ -427,7 +427,7 @@ private Parameter[] synthesizeAllParams() {
             // modifiers?  Probably not in the general case, since
             // we'd have no way of knowing about them, but there
             // may be specific cases.
-            out[i] = new Parameter("arg" + i, 0, this, i);
+            out[i] = new Parameter(null, 0, this, i);
         return out;
     }
 

src/java.base/share/classes/java/lang/reflect/Parameter.java

@@ -55,7 +55,7 @@ public final class Parameter implements AnnotatedElement {
      * absent, however, then {@code Executable} uses this constructor
      * to synthesize them.
      *
-     * @param name The name of the parameter.
+     * @param name The name of the parameter, or {@code null} if absent
      * @param modifiers The modifier flags for the parameter.
      * @param executable The executable which defines this parameter.
      * @param index The index of the parameter.
@@ -104,7 +104,7 @@ public int hashCode() {
      * to the class file.
      */
     public boolean isNamePresent() {
-        return executable.hasRealParameterData() && name != null;
+        return name != null;
     }
 
     /**

test/jdk/java/lang/reflect/Parameter/SyntheticNameRetention.java

@@ -0,0 +1,81 @@
+/*
+ * Copyright (c) 2025, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+import java.lang.invoke.MethodHandleProxies;
+import java.lang.invoke.MethodHandles;
+import java.lang.reflect.Constructor;
+import java.lang.reflect.Executable;
+import java.lang.reflect.Parameter;
+import java.util.stream.Stream;
+
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.MethodSource;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+/*
+ * @test
+ * @bug 8357728
+ * @summary Synthetic parameter names should not be retained.
+ * @modules java.base/java.lang.reflect:+open
+ * @run junit SyntheticNameRetention
+ */
+public class SyntheticNameRetention {
+
+    class Inner {
+        Inner() {}
+    }
+
+    public interface NameGetter {
+        String getRawName(Parameter parameter);
+    }
+    static final NameGetter GETTER;
+
+    static {
+        try {
+            var lookup = MethodHandles.privateLookupIn(Parameter.class, MethodHandles.lookup());
+            GETTER = MethodHandleProxies.asInterfaceInstance(NameGetter.class, lookup.findGetter(Parameter.class, "name", String.class));
+        } catch (ReflectiveOperationException ex) {
+            throw new ExceptionInInitializerError(ex);
+        }
+    }
+
+    static Stream<Executable> methods() throws Throwable {
+        return Stream.of(Inner.class.getDeclaredConstructor(SyntheticNameRetention.class), // Has MethodParameters with flags
+                         SyntheticNameRetention.class.getDeclaredMethod("test", Executable.class)); // No MethodParameters
+    }
+
+    @ParameterizedTest
+    @MethodSource("methods")
+    public void test(Executable exec) {
+        var params = exec.getParameters();
+        for (int i = 0; i < params.length; i++) {
+            var param = params[i];
+            assertEquals("arg" + i, param.getName(), "name " + i);
+            assertFalse(param.isNamePresent(), "name present " + i);
+            assertNull(GETTER.getRawName(param), "raw name " + i);
+            boolean mandated = exec instanceof Constructor<?> && i == 0;
+            assertEquals(mandated, param.isImplicit(), "mandated " + i);
+        }
+    }
+}