Address code review feedback.

Author: jtibshirani

server/src/main/java/org/elasticsearch/index/mapper/DocumentParser.java

@@ -469,7 +469,8 @@ private static void parseObjectOrField(ParseContext context, Mapper mapper) thro
         } else if (mapper instanceof FieldAliasMapper) {
             throw new IllegalArgumentException("Cannot write to a field alias [" + mapper.name() + "].");
         } else {
-            throw new IllegalStateException("The provided mapper [" + mapper.name() + "] has an unrecognized type.");
+            throw new IllegalStateException("The provided mapper [" + mapper.name() + "] has an unrecognized type [" +
+                mapper.getClass().getSimpleName() + "].");
         }
     }
 

server/src/main/java/org/elasticsearch/index/mapper/FieldAliasMapper.java

@@ -27,7 +27,13 @@
 import java.util.Iterator;
 import java.util.Map;
 
-public class FieldAliasMapper extends Mapper {
+/**
+ * A mapper for field aliases.
+ *
+ * A field alias has no concrete field mappings of its own, but instead points to another field by
+ * its path. Once defined, an alias can be used in place of the concrete field name in search requests.
+ */
+public final class FieldAliasMapper extends Mapper {
     public static final String CONTENT_TYPE = "alias";
 
     public static class Names {
@@ -89,7 +95,7 @@ public Mapper.Builder parse(String name, Map<String, Object> node, ParserContext
             Object pathField = node.remove(Names.PATH);
             String path = XContentMapValues.nodeStringValue(pathField, null);
             if (path == null) {
-                throw new MapperParsingException("The [path] property must be specified.");
+                throw new MapperParsingException("The [path] property must be specified for field [" + name + "].");
             }
             return builder.path(path);
         }

server/src/main/java/org/elasticsearch/index/mapper/FieldTypeLookup.java

@@ -105,10 +105,8 @@ private void checkCompatibility(MappedFieldType existingFieldType, MappedFieldTy
 
     /** Returns the field for the given field */
     public MappedFieldType get(String field) {
-        String resolvedField = aliasToConcreteName.get(field);
-        return resolvedField == null
-            ? fullNameToFieldType.get(field)
-            : fullNameToFieldType.get(resolvedField);
+        String resolvedField = aliasToConcreteName.getOrDefault(field, field);
+        return fullNameToFieldType.get(resolvedField);
     }
 
     /**

server/src/main/java/org/elasticsearch/index/mapper/MapperUtils.java

@@ -38,7 +38,12 @@ public static void collect(Mapper mapper, Collection<ObjectMapper> objectMappers
             fieldMappers.add((FieldMapper)mapper);
         } else if (mapper instanceof FieldAliasMapper) {
             fieldAliasMappers.add((FieldAliasMapper) mapper);
+        } else {
+            throw new IllegalStateException("Unrecognized mapper type [" +
+                mapper.getClass().getSimpleName() + "].");
         }
+
+
         for (Mapper child : mapper) {
             collect(child, objectMappers, fieldMappers, fieldAliasMappers);
         }

server/src/test/java/org/elasticsearch/index/mapper/FieldAliasMapperTests.java

@@ -72,7 +72,7 @@ public void testParsingWithMissingPath() throws IOException {
             .endObject());
         MapperParsingException exception = expectThrows(MapperParsingException.class,
             () -> parser.parse("type", new CompressedXContent(mapping)));
-        assertEquals("The [path] property must be specified.", exception.getMessage());
+        assertEquals("The [path] property must be specified for field [alias-field].", exception.getMessage());
     }
 
     public void testParsingWithExtraArgument() throws IOException {