Add factory method for DataFetcherExceptionHandler

Closes gh-552

Author: rstoyanchev

spring-graphql/src/main/java/org/springframework/graphql/execution/AbstractGraphQlSourceBuilder.java

@@ -105,7 +105,8 @@ public GraphQlSource build() {
 		schema = applyTypeVisitors(schema);
 
 		GraphQL.Builder builder = GraphQL.newGraphQL(schema);
-		builder.defaultDataFetcherExceptionHandler(new ExceptionResolversExceptionHandler(this.exceptionResolvers));
+		builder.defaultDataFetcherExceptionHandler(
+				DataFetcherExceptionResolver.createExceptionHandler(this.exceptionResolvers));
 
 		if (!this.instrumentations.isEmpty()) {
 			builder = builder.instrumentation(new ChainedInstrumentation(this.instrumentations));

spring-graphql/src/main/java/org/springframework/graphql/execution/DataFetcherExceptionResolver.java

@@ -20,6 +20,7 @@
 import java.util.function.BiFunction;
 
 import graphql.GraphQLError;
+import graphql.execution.DataFetcherExceptionHandler;
 import graphql.schema.DataFetchingEnvironment;
 import reactor.core.publisher.Mono;
 
@@ -83,4 +84,24 @@ protected GraphQLError resolveToSingleError(Throwable ex, DataFetchingEnvironmen
 		};
 	}
 
+	/**
+	 * Factory method to create a {@link DataFetcherExceptionResolver} from a
+	 * list of resolvers. Spring for GraphQL uses this method to set
+	 * {@link graphql.GraphQL.Builder#defaultDataFetcherExceptionHandler(DataFetcherExceptionHandler)}
+	 * from resolvers found in Spring configuration, and that default handler
+	 * is used in turn to create each {@code ExecutionStrategy}. Applications
+	 * may also find this factory method useful when creating a custom
+	 * {@code ExecutionStrategy}.
+	 * <p>Resolvers are invoked in turn until one resolves the exception by
+	 * emitting a (possibly empty) {@code GraphQLError} list. If the exception
+	 * remains unresolved, the handler creates a {@code GraphQLError} with
+	 * {@link ErrorType#INTERNAL_ERROR} and a short message with the execution id.
+	 * @param resolvers the list of resolvers to use
+	 * @return the created {@link DataFetcherExceptionHandler} instance
+	 * @since 1.1.1
+	 */
+	static DataFetcherExceptionHandler createExceptionHandler(List<DataFetcherExceptionResolver> resolvers) {
+		return new ExceptionResolversExceptionHandler(resolvers);
+	}
+
 }