fix(GH-232): SimpleDateFormat is not threadsafe (#261)

* fix(GH-232):  SimpleDateFormat is not threadsafe

* fix: add dateCoercionThreadSafe unit test

Author: igdianov

graphql-jpa-query-schema/src/main/java/com/introproventures/graphql/jpa/query/schema/JavaScalars.java

@@ -290,14 +290,14 @@ public LocalTime parseLiteral(Object input) {
     }
 
     public static class GraphQLDateCoercing implements Coercing<Object, Object> {
-        final DateFormat df;
+        final String dateFormatString;
 
 
         /**
          * Default to pattern 'yyyy-MM-dd'
          */
         public GraphQLDateCoercing() {
-            df = new SimpleDateFormat("yyyy-MM-dd");
+        	dateFormatString = "yyyy-MM-dd";
         }
 
         /**
@@ -306,7 +306,7 @@ public GraphQLDateCoercing() {
          * @param dateFormatString e.g. "yyyy-MM-dd'T'HH:mm:ss.SSSXXX" for "2001-07-04T12:08:56.235-07:00"
          */
         public GraphQLDateCoercing(String dateFormatString) {
-            df = new SimpleDateFormat(dateFormatString);
+        	this.dateFormatString = dateFormatString;
         }
 
         @Override
@@ -340,6 +340,8 @@ public Object parseLiteral(Object input) {
         }
 
         private Date parseStringToDate(String input) {
+        	DateFormat df = new SimpleDateFormat(dateFormatString);
+        	
             try {
                 return df.parse(input);
             } catch (ParseException e) {
@@ -662,7 +664,7 @@ public Object parseValue(Object input) {
 
         @Override
         public Object parseLiteral(Object input) {
-            return parseLiteral((Value<?>) input, Collections.emptyMap());
+            return parseLiteral(input, Collections.emptyMap());
         }
 
         //recursively parse the input into a Map

graphql-jpa-query-schema/src/test/java/com/introproventures/graphql/jpa/query/schema/JavaScalarsTest.java

@@ -28,7 +28,11 @@
 import java.time.ZoneId;
 import java.time.ZoneOffset;
 import java.time.ZonedDateTime;
+import java.util.ArrayList;
+import java.util.List;
 import java.util.Map;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.ExecutionException;
 import java.util.concurrent.TimeUnit;
 
 import org.junit.Test;
@@ -289,4 +293,29 @@ public void string2Instant() {
         assert resultLDT.getMinute() == 15;
         assert resultLDT.getSecond() == 07;
     }
+    
+    @Test
+    public void dateCoercionThreadSafe() throws InterruptedException, ExecutionException {
+    	//given
+    	String dateLiteral = "2018-06-22T10:00:00";
+        Coercing<?, ?> subject = new JavaScalars.GraphQLDateCoercing();
+    	
+        List<CompletableFuture<Object>> dates = new ArrayList<>();
+
+        //when
+        for(int i = 0; i < 1000; i++) {
+            CompletableFuture<Object> task = CompletableFuture.supplyAsync(() -> {
+            	return subject.serialize(dateLiteral);
+            });
+        	dates.add(task);
+        }
+        
+        CompletableFuture<Void> result = CompletableFuture.allOf(dates.toArray(new CompletableFuture[] {}));
+        
+        result.join();
+        
+        //then
+        assertThat(result.isCompletedExceptionally()).isFalse();        
+    }
+    
 }