Allow builders to return default instance in Java

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

@@ -476,6 +476,9 @@ public abstract static class Builder<BuilderT extends Builder<BuilderT>>
     // to dispatch dirty invalidations. See GeneratedMessage.BuilderListener.
     private boolean isClean;
 
+    // Indicates that the default instance can be used
+    private boolean isDefault = true;
+
     /**
      * This field holds either an {@link UnknownFieldSet} or {@link UnknownFieldSet.Builder}.
      *
@@ -527,6 +530,15 @@ protected boolean isClean() {
       return isClean;
     }
 
+    /**
+     * Gets whether the default instance can be used
+     *
+     * @return whether the default instance can be used
+     */
+    protected boolean isDefault() {
+      return isDefault;
+    }
+
     @Override
     public BuilderT clone() {
       BuilderT builder = (BuilderT) getDefaultInstanceForType().newBuilderForType();
@@ -834,6 +846,7 @@ protected final void onChanged() {
         // Don't keep dispatching invalidations until build is called again.
         isClean = false;
       }
+      isDefault = false;
     }
 
     /**

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

@@ -10,6 +10,7 @@
 final class NewInstanceSchemaFull implements NewInstanceSchema {
   @Override
   public Object newInstance(Object defaultInstance) {
-    return ((Message) defaultInstance).toBuilder().buildPartial();
+    return ((GeneratedMessage) defaultInstance)
+        .newInstance(GeneratedMessage.UnusedPrivateParameter.INSTANCE);
   }
 }

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

@@ -156,6 +156,20 @@ public void testMerge() {
     assertThat(vehicle1.getWheelList()).isSameInstanceAs(vehicle3.getWheelList());
   }
 
+  @Test
+  public void testUsesDefaultInstance() {
+    Vehicle vehicle1 = Vehicle.newBuilder().build();
+    // Should use the default instance -- no allocation
+    assertThat(vehicle1).isSameInstanceAs(Vehicle.getDefaultInstance());
+
+    Vehicle vehicle2 =
+        Vehicle.newBuilder()
+            .addWheel(Wheel.newBuilder().build())
+            .build();
+    // Should use the default instance -- no allocation
+    assertThat(vehicle2.getWheel(0)).isSameInstanceAs(Wheel.getDefaultInstance());
+  }
+
   @Test
   public void testGettingBuilderMarksFieldAsHaving() {
     Vehicle.Builder vehicleBuilder = Vehicle.newBuilder();

src/google/protobuf/compiler/java/immutable/message.cc

@@ -363,6 +363,15 @@ void ImmutableMessageGenerator::Generate(io::Printer* printer) {
       "}\n"
       "\n");
 
+  printer->Print(variables,
+                 "@java.lang.Override\n"
+                 "@SuppressWarnings({\"unused\"})\n"
+                 "protected java.lang.Object newInstance(\n"
+                 "    UnusedPrivateParameter unused) {\n"
+                 "  return new $classname$();\n"
+                 "}\n"
+                 "\n");
+
   GenerateDescriptorMethods(printer);
 
   // Nested types

src/google/protobuf/compiler/java/immutable/message_builder.cc

@@ -475,6 +475,7 @@ void MessageBuilderGenerator::GenerateBuildPartial(io::Printer* printer) {
   printer->Print(
       "@java.lang.Override\n"
       "public $classname$ buildPartial() {\n"
+      "  if (isDefault()) { onBuilt(); return $classname$.getDefaultInstance(); }\n"
       "  $classname$ result = new $classname$(this);\n",
       "classname", name_resolver_->GetImmutableClassName(descriptor_));