Fix default subscription execution strategy

Author: apottere

src/main/java/graphql/servlet/DefaultExecutionStrategyProvider.java

@@ -2,6 +2,9 @@
 
 import graphql.execution.AsyncExecutionStrategy;
 import graphql.execution.ExecutionStrategy;
+import graphql.execution.SubscriptionExecutionStrategy;
+
+import java.util.function.Supplier;
 
 /**
  * @author Andrew Potter
@@ -22,16 +25,16 @@ public DefaultExecutionStrategyProvider(ExecutionStrategy executionStrategy) {
 
     public DefaultExecutionStrategyProvider(ExecutionStrategy queryExecutionStrategy, ExecutionStrategy mutationExecutionStrategy, ExecutionStrategy subscriptionExecutionStrategy) {
         this.queryExecutionStrategy = defaultIfNull(queryExecutionStrategy);
-        this.mutationExecutionStrategy = defaultIfNull(mutationExecutionStrategy, this.queryExecutionStrategy);
-        this.subscriptionExecutionStrategy = defaultIfNull(subscriptionExecutionStrategy, this.queryExecutionStrategy);
+        this.mutationExecutionStrategy = defaultIfNull(mutationExecutionStrategy, () -> this.queryExecutionStrategy);
+        this.subscriptionExecutionStrategy = defaultIfNull(subscriptionExecutionStrategy, SubscriptionExecutionStrategy::new);
     }
 
     private ExecutionStrategy defaultIfNull(ExecutionStrategy executionStrategy) {
-        return defaultIfNull(executionStrategy, new AsyncExecutionStrategy());
+        return defaultIfNull(executionStrategy, AsyncExecutionStrategy::new);
     }
 
-    private ExecutionStrategy defaultIfNull(ExecutionStrategy executionStrategy, ExecutionStrategy defaultStrategy) {
-        return executionStrategy != null ? executionStrategy : defaultStrategy;
+    private ExecutionStrategy defaultIfNull(ExecutionStrategy executionStrategy, Supplier<ExecutionStrategy> defaultStrategy) {
+        return executionStrategy != null ? executionStrategy : defaultStrategy.get();
     }
 
     @Override