Get the map factory per mergeFrom() call for JavaNano.

javanano/src/main/java/com/google/protobuf/nano/InternalNano.java

@@ -30,6 +30,8 @@
 
 package com.google.protobuf.nano;
 
+import com.google.protobuf.nano.MapFactories.MapFactory;
+
 import java.io.IOException;
 import java.io.UnsupportedEncodingException;
 import java.util.Arrays;
@@ -406,12 +408,13 @@ public final class InternalNano {
   public static final <K, V> Map<K, V> mergeMapEntry(
       CodedInputByteBufferNano input,
       Map<K, V> map,
+      MapFactory mapFactory,
       int keyType,
       int valueType,
       V value,
       int keyTag,
       int valueTag) throws IOException {
-    map = MapFactories.getMapFactory().forMap(map);
+    map = mapFactory.forMap(map);
     final int length = input.readRawVarint32();
     final int oldLimit = input.pushLimit(length);
     K key = null;

src/google/protobuf/compiler/javanano/javanano_helpers.cc

@@ -560,6 +560,17 @@ void SetBitOperationVariables(const string name,
   (*variables)["different_" + name] = GenerateDifferentBit(bitIndex);
 }
 
+bool HasMapField(const Descriptor* descriptor) {
+  for (int i = 0; i < descriptor->field_count(); ++i) {
+    const FieldDescriptor* field = descriptor->field(i);
+    if (field->type() == FieldDescriptor::TYPE_MESSAGE &&
+        IsMapEntry(field->message_type())) {
+      return true;
+    }
+  }
+  return false;
+}
+
 }  // namespace javanano
 }  // namespace compiler
 }  // namespace protobuf

src/google/protobuf/compiler/javanano/javanano_helpers.h

@@ -187,6 +187,8 @@ inline bool IsMapEntry(const Descriptor* descriptor) {
       descriptor->file()->syntax() == FileDescriptor::SYNTAX_PROTO3;
 }
 
+bool HasMapField(const Descriptor* descriptor);
+
 }  // namespace javanano
 }  // namespace compiler
 }  // namespace protobuf

src/google/protobuf/compiler/javanano/javanano_map_field.cc

@@ -134,7 +134,7 @@ void MapFieldGenerator::
 GenerateMergingCode(io::Printer* printer) const {
   printer->Print(variables_,
     "this.$name$ = com.google.protobuf.nano.InternalNano.mergeMapEntry(\n"
-    "  input, this.$name$,\n"
+    "  input, this.$name$, mapFactory,\n"
     "  com.google.protobuf.nano.InternalNano.$key_desc_type$,\n"
     "  com.google.protobuf.nano.InternalNano.$value_desc_type$,\n"
     "  $value_default$,\n"

src/google/protobuf/compiler/javanano/javanano_message.cc

@@ -345,6 +345,11 @@ void MessageGenerator::GenerateMergeFromMethods(io::Printer* printer) {
     "classname", descriptor_->name());
 
   printer->Indent();
+  if (HasMapField(descriptor_)) {
+    printer->Print(
+      "com.google.protobuf.nano.MapFactories.MapFactory mapFactory =\n"
+      "  com.google.protobuf.nano.MapFactories.getMapFactory();\n");
+  }
 
   printer->Print(
     "while (true) {\n");