Add isPlaceholder() accessors to file, message, and enum descriptors

This aligns with C++ and Go.

PiperOrigin-RevId: 803010915

Author: protobuf-github-bot

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

@@ -177,6 +177,10 @@ GenericDescriptor getParent() {
       return null;
     }
 
+    public boolean isPlaceholder() {
+      return placeholder;
+    }
+
     /** Returns the same as getName(). */
     @Override
     public String getFullName() {
@@ -574,6 +578,7 @@ public interface InternalDescriptorAssigner {
     private final FileDescriptor[] dependencies;
     private final FileDescriptor[] publicDependencies;
     private final FileDescriptorTables tables;
+    private final boolean placeholder;
     private volatile boolean featuresResolved;
 
     private FileDescriptor(
@@ -610,6 +615,8 @@ private FileDescriptor(
       this.publicDependencies = new FileDescriptor[publicDependencies.size()];
       publicDependencies.toArray(this.publicDependencies);
 
+      placeholder = false;
+
       tables.addPackage(getPackage(), this);
 
       messageTypes =
@@ -663,6 +670,8 @@ private FileDescriptor(
       services = EMPTY_SERVICE_DESCRIPTORS;
       extensions = EMPTY_FIELD_DESCRIPTORS;
 
+      placeholder = true;
+
       tables.addPackage(packageName, this);
       tables.addSymbol(message);
     }
@@ -855,6 +864,10 @@ GenericDescriptor getParent() {
       return parent;
     }
 
+    public boolean isPlaceholder() {
+      return placeholder;
+    }
+
     /** If this is a nested type, get the outer descriptor, otherwise null. */
     public Descriptor getContainingType() {
       if (parent instanceof Descriptor) {
@@ -1091,6 +1104,8 @@ public EnumDescriptor findEnumTypeByName(final String name) {
     private final int[] extensionRangeLowerBounds;
     private final int[] extensionRangeUpperBounds;
 
+    private final boolean placeholder;
+
     // Used to create a placeholder when the type cannot be found.
     Descriptor(final String fullname) throws DescriptorValidationException {
       String name = fullname;
@@ -1122,6 +1137,8 @@ public EnumDescriptor findEnumTypeByName(final String name) {
 
       extensionRangeLowerBounds = new int[] {1};
       extensionRangeUpperBounds = new int[] {536870912};
+
+      placeholder = true;
     }
 
     private Descriptor(
@@ -1204,6 +1221,8 @@ private Descriptor(
       }
       this.realOneofCount = this.oneofs.length - syntheticOneofCount;
 
+      placeholder = false;
+
       file.tables.addSymbol(this);
 
       // NOTE: The defined extension ranges are guaranteed to be disjoint.
@@ -2306,6 +2325,10 @@ GenericDescriptor getParent() {
       return parent;
     }
 
+    public boolean isPlaceholder() {
+      return false;
+    }
+
     /**
      * Determines if the given enum is closed.
      *

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

@@ -92,6 +92,7 @@ public void testFieldTypeEnumMapping() throws Exception {
     public void testFileDescriptor() throws Exception {
       FileDescriptor file = UnittestProto.getDescriptor();
 
+      assertThat(file.isPlaceholder()).isFalse();
       assertThat(file.getName()).isEqualTo("google/protobuf/unittest.proto");
       assertThat(file.getPackage()).isEqualTo("proto2_unittest");
       assertThat(file.getOptions().getJavaOuterClassname()).isEqualTo("UnittestProto");
@@ -209,7 +210,10 @@ public void testFileDescriptorCopyHeadingTo() throws Exception {
     @Test
     public void testDescriptor() throws Exception {
       Descriptor messageType = TestAllTypes.getDescriptor();
+      assertThat(messageType.isPlaceholder()).isFalse();
+
       Descriptor nestedType = TestAllTypes.NestedMessage.getDescriptor();
+      assertThat(nestedType.isPlaceholder()).isFalse();
 
       assertThat(messageType.getName()).isEqualTo("TestAllTypes");
       assertThat(messageType.getFullName()).isEqualTo("proto2_unittest.TestAllTypes");
@@ -967,8 +971,21 @@ public void testUnknownFieldsAllowed() throws Exception {
                               .setName("bar")
                               .setNumber(1)))
               .build();
-      FileDescriptor unused =
+      FileDescriptor foo =
           Descriptors.FileDescriptor.buildFrom(fooProto, new FileDescriptor[0], true);
+      assertThat(
+              foo.findMessageTypeByName("Foo")
+                  .findFieldByName("bar")
+                  .getMessageType()
+                  .isPlaceholder())
+          .isTrue();
+      assertThat(
+              foo.findMessageTypeByName("Foo")
+                  .findFieldByName("bar")
+                  .getMessageType()
+                  .getFile()
+                  .isPlaceholder())
+          .isTrue();
     }
 
     @Test