diff --git a/java/README.txt b/java/README.txt
index 96b042f19e..c6933138a7 100644
--- a/java/README.txt
+++ b/java/README.txt
@@ -437,6 +437,15 @@ and the runtime overhead. An overview of Nano features:
   MessageNano.
 - The 'bytes' type translates to the Java type byte[].
 
+The generated messages are not thread-safe for writes, but may be
+used simultaneously from multiple threads in a read-only manner.
+In other words, an appropriate synchronization mechanism (such as
+a ReadWriteLock) must be used to ensure that a message, its
+ancestors, and descendants are not accessed by any other threads
+while the message is being modified. Field reads, getter methods,
+toByteArray(...), writeTo(...), getCachedSize(), and
+getSerializedSize() are all considered read-only operations.
+
 IMPORTANT: If you have fields with defaults and opt out of accessors
 
 How fields with defaults are serialized has changed. Because we don't
diff --git a/java/src/main/java/com/google/protobuf/nano/ExtendableMessageNano.java b/java/src/main/java/com/google/protobuf/nano/ExtendableMessageNano.java
index 839f21c981..63c8afcbf5 100644
--- a/java/src/main/java/com/google/protobuf/nano/ExtendableMessageNano.java
+++ b/java/src/main/java/com/google/protobuf/nano/ExtendableMessageNano.java
@@ -47,7 +47,7 @@ public abstract class ExtendableMessageNano<M extends ExtendableMessageNano<M>>
     protected List<UnknownFieldData> unknownFieldData;
 
     @Override
-    public int getSerializedSize() {
+    protected int computeSerializedSize() {
         int size = 0;
         int unknownFieldCount = unknownFieldData == null ? 0 : unknownFieldData.size();
         for (int i = 0; i < unknownFieldCount; i++) {
@@ -55,7 +55,6 @@ public abstract class ExtendableMessageNano<M extends ExtendableMessageNano<M>>
             size += CodedOutputByteBufferNano.computeRawVarint32Size(unknownField.tag);
             size += unknownField.bytes.length;
         }
-        cachedSize = size;
         return size;
     }
 
diff --git a/java/src/main/java/com/google/protobuf/nano/MessageNano.java b/java/src/main/java/com/google/protobuf/nano/MessageNano.java
index 82dc6cca56..d6288c9280 100644
--- a/java/src/main/java/com/google/protobuf/nano/MessageNano.java
+++ b/java/src/main/java/com/google/protobuf/nano/MessageNano.java
@@ -38,7 +38,7 @@ import java.io.IOException;
  * @author wink@google.com Wink Saville
  */
 public abstract class MessageNano {
-    protected int cachedSize = -1;
+    protected volatile int cachedSize = -1;
 
     /**
      * Get the number of bytes required to encode this message.
@@ -61,9 +61,18 @@ public abstract class MessageNano {
      * using getCachedSize().
      */
     public int getSerializedSize() {
-        // This is overridden if the generated message has serialized fields.
-        cachedSize = 0;
-        return 0;
+        int size = computeSerializedSize();
+        cachedSize = size;
+        return size;
+    }
+
+    /**
+     * Computes the number of bytes required to encode this message. This does not update the
+     * cached size.
+     */
+    protected int computeSerializedSize() {
+      // This is overridden if the generated message has serialized fields.
+      return 0;
     }
 
     /**
diff --git a/java/src/test/java/com/google/protobuf/NanoTest.java b/java/src/test/java/com/google/protobuf/NanoTest.java
index 9987cacede..00e2597edd 100644
--- a/java/src/test/java/com/google/protobuf/NanoTest.java
+++ b/java/src/test/java/com/google/protobuf/NanoTest.java
@@ -105,6 +105,12 @@ public class NanoTest extends TestCase {
     assertEquals(456, newMsg.d);
     assertEquals(2, msg.nestedMsg.bb);
     assertEquals(SimpleMessageNano.BAR, msg.defaultNestedEnum);
+
+    msg.nestedMsg = null;
+    assertTrue(msgSerializedSize != msg.getSerializedSize());
+
+    msg.clear();
+    assertEquals(0, msg.getSerializedSize());
   }
 
   public void testRecursiveMessageNano() throws Exception {
@@ -143,6 +149,12 @@ public class NanoTest extends TestCase {
     assertEquals(3, newMsg.repeatedRecursiveMessageNano[0].id);
   }
 
+  public void testMessageNoFields() {
+    SingleMessageNano msg = new SingleMessageNano();
+    assertEquals(0, msg.getSerializedSize());
+    assertEquals(0, MessageNano.toByteArray(msg).length);
+  }
+
   public void testNanoRequiredInt32() throws Exception {
     TestAllTypesNano msg = new TestAllTypesNano();
     msg.id = 123;
@@ -2898,6 +2910,20 @@ public class NanoTest extends TestCase {
     assertTrue(Arrays.equals(enums, message.getExtension(RepeatedExtensions.repeatedEnum)));
   }
 
+  public void testNullExtensions() throws Exception {
+    // Check that clearing the extension on an empty message is a no-op.
+    Extensions.ExtendableMessage message = new Extensions.ExtendableMessage();
+    message.setExtension(SingularExtensions.someMessage, null);
+    assertEquals(0, MessageNano.toByteArray(message).length);
+
+    // Check that the message is empty after setting and clearing an extension.
+    AnotherMessage another = new AnotherMessage();
+    message.setExtension(SingularExtensions.someMessage, another);
+    assertTrue(MessageNano.toByteArray(message).length > 0);
+    message.setExtension(SingularExtensions.someMessage, null);
+    assertEquals(0, MessageNano.toByteArray(message).length);
+  }
+
   public void testUnknownFields() throws Exception {
     // Check that we roundtrip (serialize and deserialize) unrecognized fields.
     AnotherMessage message = new AnotherMessage();
diff --git a/src/google/protobuf/compiler/javanano/javanano_message.cc b/src/google/protobuf/compiler/javanano/javanano_message.cc
index e0b24b5d36..0cf9f97102 100644
--- a/src/google/protobuf/compiler/javanano/javanano_message.cc
+++ b/src/google/protobuf/compiler/javanano/javanano_message.cc
@@ -311,8 +311,8 @@ GenerateMessageSerializationMethods(io::Printer* printer) {
   printer->Print(
     "\n"
     "@Override\n"
-    "public int getSerializedSize() {\n"
-    "  int size = super.getSerializedSize();\n");
+    "protected int computeSerializedSize() {\n"
+    "  int size = super.computeSerializedSize();\n");
   printer->Indent();
 
   for (int i = 0; i < descriptor_->field_count(); i++) {
@@ -321,7 +321,6 @@ GenerateMessageSerializationMethods(io::Printer* printer) {
 
   printer->Outdent();
   printer->Print(
-    "  cachedSize = size;\n"
     "  return size;\n"
     "}\n");
 }