1 files changed, 24 insertions, 18 deletions

diff --git a/java/core/src/main/java/com/google/protobuf/GeneratedMessageLite.java b/java/core/src/main/java/com/google/protobuf/GeneratedMessageLite.java
index 09084646..8b8d9fc6 100644
--- a/java/core/src/main/java/com/google/protobuf/GeneratedMessageLite.java
+++ b/java/core/src/main/java/com/google/protobuf/GeneratedMessageLite.java
@@ -504,11 +504,8 @@ public abstract class GeneratedMessageLite<
             extends GeneratedMessageLite<MessageType, BuilderType>
             implements ExtendableMessageOrBuilder<MessageType, BuilderType> {
 
-    /**
-     * Represents the set of extensions on this message. For use by generated
-     * code only.
-     */
-    protected FieldSet<ExtensionDescriptor> extensions = FieldSet.newFieldSet();
+    /** Represents the set of extensions on this message. For use by generated code only. */
+    protected FieldSet<ExtensionDescriptor> extensions = FieldSet.emptySet();
 
     @SuppressWarnings("unchecked")
     protected final void mergeExtensionFields(final MessageType other) {
@@ -578,7 +575,11 @@ public abstract class GeneratedMessageLite<
       if (unknown) {  // Unknown field or wrong wire type.  Skip.
         return parseUnknownField(tag, input);
       }
-      
+
+      if (extensions.isImmutable()) {
+        extensions = extensions.clone();
+      }
+
       if (packed) {
         int length = input.readRawVarint32();
         int limit = input.pushLimit(length);
@@ -942,12 +943,6 @@ public abstract class GeneratedMessageLite<
       implements ExtendableMessageOrBuilder<MessageType, BuilderType> {
     protected ExtendableBuilder(MessageType defaultInstance) {
       super(defaultInstance);
-
-      // TODO(dweis): This is kind of an unnecessary clone since we construct a
-      //     new instance in the parent constructor which makes the extensions
-      //     immutable. This extra allocation shouldn't matter in practice
-      //     though.
-      instance.extensions = instance.extensions.clone();
     }
 
     // For immutable message conversion.
@@ -966,6 +961,15 @@ public abstract class GeneratedMessageLite<
       instance.extensions = instance.extensions.clone();
     }
 
+    private FieldSet<ExtensionDescriptor> ensureExtensionsAreMutable() {
+      FieldSet<ExtensionDescriptor> extensions = instance.extensions;
+      if (extensions.isImmutable()) {
+        extensions = extensions.clone();
+        instance.extensions = extensions;
+      }
+      return extensions;
+    }
+
     @Override
     public final MessageType buildPartial() {
       if (isBuilt) {
@@ -1024,7 +1028,8 @@ public abstract class GeneratedMessageLite<
 
       verifyExtensionContainingType(extensionLite);
       copyOnWrite();
-      instance.extensions.setField(extensionLite.descriptor, extensionLite.toFieldSetType(value));
+      ensureExtensionsAreMutable()
+          .setField(extensionLite.descriptor, extensionLite.toFieldSetType(value));
       return (BuilderType) this;
     }
 
@@ -1037,8 +1042,9 @@ public abstract class GeneratedMessageLite<
 
       verifyExtensionContainingType(extensionLite);
       copyOnWrite();
-      instance.extensions.setRepeatedField(
-          extensionLite.descriptor, index, extensionLite.singularToFieldSetType(value));
+      ensureExtensionsAreMutable()
+          .setRepeatedField(
+              extensionLite.descriptor, index, extensionLite.singularToFieldSetType(value));
       return (BuilderType) this;
     }
 
@@ -1051,8 +1057,8 @@ public abstract class GeneratedMessageLite<
 
       verifyExtensionContainingType(extensionLite);
       copyOnWrite();
-      instance.extensions.addRepeatedField(
-          extensionLite.descriptor, extensionLite.singularToFieldSetType(value));
+      ensureExtensionsAreMutable()
+          .addRepeatedField(extensionLite.descriptor, extensionLite.singularToFieldSetType(value));
       return (BuilderType) this;
     }
 
@@ -1063,7 +1069,7 @@ public abstract class GeneratedMessageLite<
 
       verifyExtensionContainingType(extensionLite);
       copyOnWrite();
-      instance.extensions.clearField(extensionLite.descriptor);
+      ensureExtensionsAreMutable().clearField(extensionLite.descriptor);
       return (BuilderType) this;
     }
   }