8265130: Make ConstantDesc class hierarchy sealed

Reviewed-by: mchung, jvernee, vromero

src/java.base/share/classes/java/lang/constant/ClassDesc.java

@@ -49,18 +49,15 @@ import static java.util.stream.Collectors.joining;
  * {@linkplain ClassDesc} for the component type and then call the {@link #arrayType()}
  * or {@link #arrayType(int)} methods.
  *
- * @apiNote In the future, if the Java language permits, {@linkplain ClassDesc}
- * may become a {@code sealed} interface, which would prohibit subclassing except
- * by explicitly permitted types.  Non-platform classes should not implement
- * {@linkplain ClassDesc} directly.
- *
  * @see ConstantDescs
  *
  * @since 12
  */
-public interface ClassDesc
+public sealed interface ClassDesc
         extends ConstantDesc,
-                TypeDescriptor.OfField<ClassDesc> {
+                TypeDescriptor.OfField<ClassDesc>
+        permits PrimitiveClassDescImpl,
+                ReferenceClassDescImpl {
 
     /**
      * Returns a {@linkplain ClassDesc} for a class or interface type,

src/java.base/share/classes/java/lang/constant/ConstantDesc.java

@@ -68,15 +68,6 @@ import java.lang.invoke.VarHandle.VarHandleDesc;
  * {@link Object#equals(Object)} method. There is no guarantee that any
  * particular entity will always be represented by the same descriptor instance.
  *
- * @apiNote In the future, if the Java language permits, {@linkplain ConstantDesc}
- * may become a {@code sealed} interface, which would prohibit subclassing except by
- * explicitly permitted types.  Clients can assume that the following
- * set of subtypes is exhaustive: {@link String}, {@link Integer},
- * {@link Long}, {@link Float}, {@link Double}, {@link ClassDesc},
- * {@link MethodTypeDesc}, {@link MethodHandleDesc}, and
- * {@link DynamicConstantDesc}; this list may be extended to reflect future
- * changes to the constant pool format as defined in JVMS 4.4.
- *
  * @see Constable
  * @see ConstantDescs
  *
@@ -84,7 +75,16 @@ import java.lang.invoke.VarHandle.VarHandleDesc;
  *
  * @since 12
  */
-public interface ConstantDesc {
+public sealed interface ConstantDesc
+        permits ClassDesc,
+                MethodHandleDesc,
+                MethodTypeDesc,
+                Double,
+                DynamicConstantDesc,
+                Float,
+                Integer,
+                Long,
+                String {
     /**
      * Resolves this descriptor reflectively, emulating the resolution behavior
      * of JVMS 5.4.3 and the access control behavior of JVMS 5.4.4.  The resolution

src/java.base/share/classes/java/lang/constant/DirectMethodHandleDesc.java

@@ -46,14 +46,11 @@ import static java.lang.invoke.MethodHandleInfo.REF_putStatic;
  * {@link MethodHandle}.  A {@linkplain DirectMethodHandleDesc} corresponds to
  * a {@code Constant_MethodHandle_info} entry in the constant pool of a classfile.
  *
- * @apiNote In the future, if the Java language permits, {@linkplain DirectMethodHandleDesc}
- * may become a {@code sealed} interface, which would prohibit subclassing except
- * by explicitly permitted types.  Non-platform classes should not implement
- * {@linkplain DirectMethodHandleDesc} directly.
- *
  * @since 12
  */
-public interface DirectMethodHandleDesc extends MethodHandleDesc {
+public sealed interface DirectMethodHandleDesc
+        extends MethodHandleDesc
+        permits DirectMethodHandleDescImpl {
     /**
      * Kinds of method handles that can be described with {@linkplain DirectMethodHandleDesc}.
      *

src/java.base/share/classes/java/lang/constant/DynamicConstantDesc.java

@@ -56,7 +56,7 @@ import static java.util.stream.Collectors.joining;
  *
  * @since 12
  */
-public abstract class DynamicConstantDesc<T>
+public abstract non-sealed class DynamicConstantDesc<T>
         implements ConstantDesc {
 
     private final DirectMethodHandleDesc bootstrapMethod;

src/java.base/share/classes/java/lang/constant/MethodHandleDesc.java

@@ -35,15 +35,12 @@ import static java.lang.constant.DirectMethodHandleDesc.Kind.CONSTRUCTOR;
  * A <a href="package-summary.html#nominal">nominal descriptor</a> for a
  * {@link MethodHandle} constant.
  *
- * @apiNote In the future, if the Java language permits, {@linkplain MethodHandleDesc}
- * may become a {@code sealed} interface, which would prohibit subclassing except
- * by explicitly permitted types.  Non-platform classes should not implement
- * {@linkplain MethodHandleDesc} directly.
- *
  * @since 12
  */
-public interface MethodHandleDesc
-        extends ConstantDesc {
+public sealed interface MethodHandleDesc
+        extends ConstantDesc
+        permits AsTypeMethodHandleDesc,
+                DirectMethodHandleDesc {
 
     /**
      * Creates a {@linkplain MethodHandleDesc} corresponding to an invocation of a

src/java.base/share/classes/java/lang/constant/MethodTypeDesc.java

@@ -34,16 +34,12 @@ import java.util.stream.Stream;
  * A <a href="package-summary.html#nominal">nominal descriptor</a> for a
  * {@linkplain MethodType} constant.
  *
- * @apiNote In the future, if the Java language permits, {@linkplain MethodTypeDesc}
- * may become a {@code sealed} interface, which would prohibit subclassing except
- * by explicitly permitted types.  Non-platform classes should not implement
- * {@linkplain MethodTypeDesc} directly.
- *
  * @since 12
  */
-public interface MethodTypeDesc
+public sealed interface MethodTypeDesc
         extends ConstantDesc,
-                TypeDescriptor.OfMethod<ClassDesc, MethodTypeDesc> {
+                TypeDescriptor.OfMethod<ClassDesc, MethodTypeDesc>
+        permits MethodTypeDescImpl {
     /**
      * Creates a {@linkplain MethodTypeDesc} given a method descriptor string.
      *