diff --git a/core/src/main/java/org/elasticsearch/common/settings/Settings.java b/core/src/main/java/org/elasticsearch/common/settings/Settings.java
index 1f099f354ab..6b41c011591 100644
--- a/core/src/main/java/org/elasticsearch/common/settings/Settings.java
+++ b/core/src/main/java/org/elasticsearch/common/settings/Settings.java
@@ -21,7 +21,6 @@ package org.elasticsearch.common.settings;
 
 import com.google.common.base.Charsets;
 import com.google.common.collect.ImmutableMap;
-import com.google.common.collect.ImmutableSortedMap;
 import org.elasticsearch.Version;
 import org.elasticsearch.common.Booleans;
 import org.elasticsearch.common.Strings;
@@ -71,13 +70,12 @@ public final class Settings implements ToXContent {
         return settingsRequireUnits;
     }
 
-    private ImmutableMap<String, String> settings;
+    private SortedMap<String, String> settings;
     private final ImmutableMap<String, String> forcedUnderscoreSettings;
 
     Settings(Map<String, String> settings) {
         // we use a sorted map for consistent serialization when using getAsMap()
-        // TODO: use Collections.unmodifiableMap with a TreeMap
-        this.settings = ImmutableSortedMap.copyOf(settings);
+        this.settings = Collections.unmodifiableSortedMap(new TreeMap<>(settings));
         Map<String, String> forcedUnderscoreSettings = null;
         for (Map.Entry<String, String> entry : settings.entrySet()) {
             String toUnderscoreCase = Strings.toUnderscoreCase(entry.getKey());
diff --git a/core/src/main/java/org/elasticsearch/index/mapper/DocumentMapperParser.java b/core/src/main/java/org/elasticsearch/index/mapper/DocumentMapperParser.java
index 188dc8023c7..bb991ef2603 100644
--- a/core/src/main/java/org/elasticsearch/index/mapper/DocumentMapperParser.java
+++ b/core/src/main/java/org/elasticsearch/index/mapper/DocumentMapperParser.java
@@ -20,7 +20,6 @@
 package org.elasticsearch.index.mapper;
 
 import com.google.common.collect.ImmutableMap;
-import com.google.common.collect.ImmutableSortedMap;
 import org.elasticsearch.Version;
 import org.elasticsearch.common.Nullable;
 import org.elasticsearch.common.ParseFieldMatcher;
@@ -36,33 +35,10 @@ import org.elasticsearch.common.xcontent.XContentFactory;
 import org.elasticsearch.common.xcontent.XContentHelper;
 import org.elasticsearch.common.xcontent.XContentParser;
 import org.elasticsearch.index.analysis.AnalysisService;
-import org.elasticsearch.index.mapper.core.BinaryFieldMapper;
-import org.elasticsearch.index.mapper.core.BooleanFieldMapper;
-import org.elasticsearch.index.mapper.core.ByteFieldMapper;
-import org.elasticsearch.index.mapper.core.CompletionFieldMapper;
-import org.elasticsearch.index.mapper.core.DateFieldMapper;
-import org.elasticsearch.index.mapper.core.DoubleFieldMapper;
-import org.elasticsearch.index.mapper.core.FloatFieldMapper;
-import org.elasticsearch.index.mapper.core.IntegerFieldMapper;
-import org.elasticsearch.index.mapper.core.LongFieldMapper;
-import org.elasticsearch.index.mapper.core.ShortFieldMapper;
-import org.elasticsearch.index.mapper.core.StringFieldMapper;
-import org.elasticsearch.index.mapper.core.TokenCountFieldMapper;
-import org.elasticsearch.index.mapper.core.TypeParsers;
+import org.elasticsearch.index.mapper.core.*;
 import org.elasticsearch.index.mapper.geo.GeoPointFieldMapper;
 import org.elasticsearch.index.mapper.geo.GeoShapeFieldMapper;
-import org.elasticsearch.index.mapper.internal.AllFieldMapper;
-import org.elasticsearch.index.mapper.internal.FieldNamesFieldMapper;
-import org.elasticsearch.index.mapper.internal.IdFieldMapper;
-import org.elasticsearch.index.mapper.internal.IndexFieldMapper;
-import org.elasticsearch.index.mapper.internal.ParentFieldMapper;
-import org.elasticsearch.index.mapper.internal.RoutingFieldMapper;
-import org.elasticsearch.index.mapper.internal.SourceFieldMapper;
-import org.elasticsearch.index.mapper.internal.TTLFieldMapper;
-import org.elasticsearch.index.mapper.internal.TimestampFieldMapper;
-import org.elasticsearch.index.mapper.internal.TypeFieldMapper;
-import org.elasticsearch.index.mapper.internal.UidFieldMapper;
-import org.elasticsearch.index.mapper.internal.VersionFieldMapper;
+import org.elasticsearch.index.mapper.internal.*;
 import org.elasticsearch.index.mapper.ip.IpFieldMapper;
 import org.elasticsearch.index.mapper.object.ObjectMapper;
 import org.elasticsearch.index.mapper.object.RootObjectMapper;
@@ -71,11 +47,7 @@ import org.elasticsearch.index.similarity.SimilarityLookupService;
 import org.elasticsearch.script.Script;
 import org.elasticsearch.script.ScriptService;
 
-import java.util.Collections;
-import java.util.HashMap;
-import java.util.Iterator;
-import java.util.List;
-import java.util.Map;
+import java.util.*;
 
 import static org.elasticsearch.index.mapper.MapperBuilders.doc;
 
@@ -96,7 +68,7 @@ public class DocumentMapperParser {
 
     private volatile ImmutableMap<String, Mapper.TypeParser> typeParsers;
     private volatile ImmutableMap<String, Mapper.TypeParser> rootTypeParsers;
-    private volatile ImmutableMap<String, Mapper.TypeParser> additionalRootMappers;
+    private volatile SortedMap<String, Mapper.TypeParser> additionalRootMappers;
 
     public DocumentMapperParser(@IndexSettings Settings indexSettings, MapperService mapperService, AnalysisService analysisService,
                                 SimilarityLookupService similarityLookupService, ScriptService scriptService) {
@@ -145,7 +117,7 @@ public class DocumentMapperParser {
                 .put(IdFieldMapper.NAME, new IdFieldMapper.TypeParser())
                 .put(FieldNamesFieldMapper.NAME, new FieldNamesFieldMapper.TypeParser())
                 .immutableMap();
-        additionalRootMappers = ImmutableSortedMap.<String, Mapper.TypeParser>of();
+        additionalRootMappers = Collections.emptySortedMap();
         indexVersionCreated = Version.indexCreated(indexSettings);
     }
 
@@ -162,10 +134,10 @@ public class DocumentMapperParser {
             rootTypeParsers = new MapBuilder<>(rootTypeParsers)
                     .put(type, typeParser)
                     .immutableMap();
-            additionalRootMappers = ImmutableSortedMap.<String, Mapper.TypeParser>naturalOrder()
-                    .putAll(additionalRootMappers)
-                    .put(type, typeParser)
-                    .build();
+            SortedMap<String, Mapper.TypeParser> newAdditionalRootMappers = new TreeMap<>();
+            newAdditionalRootMappers.putAll(additionalRootMappers);
+            newAdditionalRootMappers.put(type, typeParser);
+            additionalRootMappers = Collections.unmodifiableSortedMap(newAdditionalRootMappers);
         }
     }
 
diff --git a/dev-tools/src/main/resources/forbidden/all-signatures.txt b/dev-tools/src/main/resources/forbidden/all-signatures.txt
index 03f59a6f599..e50f672850c 100644
--- a/dev-tools/src/main/resources/forbidden/all-signatures.txt
+++ b/dev-tools/src/main/resources/forbidden/all-signatures.txt
@@ -106,3 +106,4 @@ com.google.common.util.concurrent.ListenableFuture
 com.google.common.util.concurrent.SettableFuture
 com.google.common.util.concurrent.Futures
 com.google.common.util.concurrent.MoreExecutors
+com.google.common.collect.ImmutableSortedMap