Replace to ConcurrentMap from Map (#65)

Co-authored-by: heowc <[email protected]>

Author: heowc

src/main/java/graphql/validation/constraints/standard/PatternConstraint.java

@@ -7,17 +7,17 @@
 import graphql.validation.constraints.AbstractDirectiveConstraint;
 import graphql.validation.constraints.Documentation;
 import graphql.validation.rules.ValidationEnvironment;
-import java.util.HashMap;
 import java.util.List;
-import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ConcurrentMap;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 import static graphql.schema.GraphQLTypeUtil.isList;
 import static java.util.Collections.emptyList;
 
 public class PatternConstraint extends AbstractDirectiveConstraint {
 
-    private final static Map<String, Pattern> SEEN_PATTERNS = new HashMap<>();
+    private final static ConcurrentMap<String, Pattern> SEEN_PATTERNS = new ConcurrentHashMap<>();
 
     public PatternConstraint() {
         super("Pattern");