Make Jackson ObjectMapper initialization thread safe

Author: frantuma

modules/swagger-core/src/main/java/io/swagger/v3/core/util/Json31.java

@@ -13,23 +13,22 @@
 
 public class Json31 {
 
-    private static ObjectMapper mapper;
-    private static ObjectMapper converterMapper;
+    private static final class ObjectMapperHolder {
+        private static final ObjectMapper MAPPER = ObjectMapperFactory.createJson31();
+    }
+
+    private static final class ConverterMapperHolder {
+        private static final ObjectMapper MAPPER = ObjectMapperFactory.createJsonConverter();
+    }
 
     static Logger LOGGER = LoggerFactory.getLogger(Json31.class);
 
     public static ObjectMapper mapper() {
-        if (mapper == null) {
-            mapper = ObjectMapperFactory.createJson31();
-        }
-        return mapper;
+        return ObjectMapperHolder.MAPPER;
     }
 
     public static ObjectMapper converterMapper() {
-        if (converterMapper == null) {
-            converterMapper = ObjectMapperFactory.createJsonConverter();
-        }
-        return converterMapper;
+        return ConverterMapperHolder.MAPPER;
     }
 
     public static ObjectWriter pretty() {

modules/swagger-core/src/main/java/io/swagger/v3/core/util/Yaml.java

@@ -5,13 +5,13 @@
 import com.fasterxml.jackson.databind.ObjectWriter;
 
 public class Yaml {
-    static ObjectMapper mapper;
+
+    private static final class ObjectMapperHolder {
+        private static final ObjectMapper MAPPER = ObjectMapperFactory.createYaml();
+    }
 
     public static ObjectMapper mapper() {
-        if (mapper == null) {
-            mapper = ObjectMapperFactory.createYaml();
-        }
-        return mapper;
+        return ObjectMapperHolder.MAPPER;
     }
 
     public static ObjectWriter pretty() {

modules/swagger-core/src/main/java/io/swagger/v3/core/util/Yaml31.java

@@ -11,15 +11,16 @@
 import java.util.Map;
 
 public class Yaml31 {
-    static ObjectMapper mapper;
+
+    private static final class ObjectMapperHolder {
+        private static final ObjectMapper MAPPER = ObjectMapperFactory.createYaml31();
+    }
+
 
     static Logger LOGGER = LoggerFactory.getLogger(Yaml31.class);
 
     public static ObjectMapper mapper() {
-        if (mapper == null) {
-            mapper = ObjectMapperFactory.createYaml31();
-        }
-        return mapper;
+        return ObjectMapperHolder.MAPPER;
     }
 
     public static ObjectWriter pretty() {