Make the FieldQueryMapEncoder encoder thread safe

Change the caching map to be concurrent and only
insert items that are missing to the map.
Fixes #1257

core/src/main/java/feign/querymap/FieldQueryMapEncoder.java

@@ -1,5 +1,5 @@
 /**
- * Copyright 2012-2020 The Feign Authors
+ * Copyright 2012-2021 The Feign Authors
  *
  * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
  * in compliance with the License. You may obtain a copy of the License at
@@ -13,12 +13,17 @@
  */
 package feign.querymap;
 
+import java.lang.reflect.Field;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.stream.Collectors;
 import feign.Param;
 import feign.QueryMapEncoder;
 import feign.codec.EncodeException;
-import java.lang.reflect.Field;
-import java.util.*;
-import java.util.stream.Collectors;
 
 /**
  * the query map will be generated using member variable names as query parameter names.
@@ -31,7 +36,7 @@
 public class FieldQueryMapEncoder implements QueryMapEncoder {
 
   private final Map<Class<?>, ObjectParamMetadata> classToMetadata =
-      new HashMap<Class<?>, ObjectParamMetadata>();
+      new ConcurrentHashMap<>();
 
   @Override
   public Map<String, Object> encode(Object object) throws EncodeException {
@@ -56,7 +61,7 @@ private ObjectParamMetadata getMetadata(Class<?> objectType) {
     ObjectParamMetadata metadata = classToMetadata.get(objectType);
     if (metadata == null) {
       metadata = ObjectParamMetadata.parseObjectType(objectType);
-      classToMetadata.put(objectType, metadata);
+      classToMetadata.putIfAbsent(objectType, metadata);
     }
     return metadata;
   }