Add isClosed() to EnumDescriptor in Java.

PiperOrigin-RevId: 507489971

java/core/src/main/java/com/google/protobuf/Descriptors.java

@@ -1772,6 +1772,21 @@ public final class Descriptors {
       return file;
     }
 
+    /**
+     * Determines if the given enum is closed.
+     *
+     * <p>Closed enum means that it:
+     *
+     * <ul>
+     *   <li>Has a fixed set of named values. *
+     *   <li>Encountering values not in this set causes them to be treated as unknown fields.
+     *   <li>The first value (i.e., the default) may be nonzero.
+     * </ul>
+     */
+    public boolean isClosed() {
+      return getFile().getSyntax() != Syntax.PROTO3;
+    }
+
     /** If this is a nested type, get the outer descriptor, otherwise null. */
     public Descriptor getContainingType() {
       return containingType;

java/core/src/test/java/com/google/protobuf/DescriptorsTest.java

@@ -297,6 +297,7 @@ public class DescriptorsTest {
     assertThat(enumType.getName()).isEqualTo("ForeignEnum");
     assertThat(enumType.getFullName()).isEqualTo("protobuf_unittest.ForeignEnum");
     assertThat(enumType.getFile()).isEqualTo(UnittestProto.getDescriptor());
+    assertThat(enumType.isClosed()).isTrue();
     assertThat(enumType.getContainingType()).isNull();
     assertThat(enumType.getOptions()).isEqualTo(DescriptorProtos.EnumOptions.getDefaultInstance());
 

java/core/src/test/java/com/google/protobuf/UnknownEnumValueTest.java

@@ -56,6 +56,7 @@ public class UnknownEnumValueTest {
   @Test
   @SuppressWarnings("ProtoNewBuilderMergeFrom")
   public void testUnknownEnumValues() throws Exception {
+    assertThat(TestAllTypes.NestedEnum.getDescriptor().isClosed()).isFalse();
     TestAllTypes.Builder builder = TestAllTypes.newBuilder();
     builder.setOptionalNestedEnumValue(4321);
     builder.addRepeatedNestedEnumValue(5432);
@@ -270,6 +271,7 @@ public class UnknownEnumValueTest {
 
   @Test
   public void testUnknownEnumValuesInProto2() throws Exception {
+    assertThat(Proto2TestEnum.getDescriptor().isClosed()).isTrue();
     Proto2EnumMessage.Builder sourceMessage = Proto2EnumMessage.newBuilder();
     sourceMessage
         .addRepeatedPackedEnum(Proto2TestEnum.ZERO)