Refine null handling in ClientResponseField#toEntity

closes gh-525

Author: rstoyanchev

spring-graphql/src/main/java/org/springframework/graphql/client/ClientResponseField.java

@@ -20,7 +20,9 @@
 import java.util.List;
 
 import org.springframework.core.ParameterizedTypeReference;
+import org.springframework.graphql.GraphQlResponse;
 import org.springframework.graphql.ResponseField;
+import org.springframework.lang.Nullable;
 
 /**
  * Extends {@link ResponseField} to add options for decoding the field value.
@@ -33,27 +35,41 @@ public interface ClientResponseField extends ResponseField {
 	/**
 	 * Decode the field to an entity of the given type.
 	 * @param entityType the type to convert to
-	 * @return the decoded entity, never {@code null}
-	 * @throws FieldAccessException if the target field is {@code null} and has
+	 * @return the decoded entity, or {@code null} if the field is {@code null}
+	 * but otherwise there are no errors
+	 * @throws FieldAccessException if the target field is {@code null} and the
+	 * response is not {@link GraphQlResponse#isValid() valid} or the field has
 	 * {@link ResponseField#getErrors() errors}.
 	 */
+	@Nullable
 	<D> D toEntity(Class<D> entityType);
 
 	/**
 	 * Variant of {@link #toEntity(Class)} with a {@link ParameterizedTypeReference}.
 	 */
+	@Nullable
 	<D> D toEntity(ParameterizedTypeReference<D> entityType);
 
 	/**
 	 * Variant of {@link #toEntity(Class)} to decode to a list of entities.
 	 * @param elementType the type of elements in the list
+	 * @return the list of decoded entities, or an empty list if the field is
+	 * {@code null} but otherwise there are no errors
+	 * @throws FieldAccessException if the target field is {@code null} and the
+	 * response is not {@link GraphQlResponse#isValid() valid} or the field has
+	 * {@link ResponseField#getErrors() errors}.
 	 */
 	<D> List<D> toEntityList(Class<D> elementType);
 
-	/**
-	 * Variant of {@link #toEntity(Class)} to decode to a list of entities.
-	 * @param elementType the type of elements in the list
-	 */
+		/**
+		 * Variant of {@link #toEntity(Class)} to decode to a list of entities.
+		 * @param elementType the type of elements in the list
+		 * @return the list of decoded entities, or an empty list if the field is
+		 * {@code null} but otherwise there are no errors
+		 * @throws FieldAccessException if the target field is {@code null} and the
+		 * response is not {@link GraphQlResponse#isValid() valid} or the field has
+		 * {@link ResponseField#getErrors() errors}.
+		 */
 	<D> List<D> toEntityList(ParameterizedTypeReference<D> elementType);
 
 }

spring-graphql/src/main/java/org/springframework/graphql/client/DefaultClientGraphQlResponse.java

@@ -68,12 +68,28 @@ public ClientResponseField field(String path) {
 
 	@Override
 	public <D> D toEntity(Class<D> type) {
-		return field("").toEntity(type);
+		ClientResponseField field = field("");
+		D entity = field.toEntity(type);
+
+		// should never happen because toEntity checks response.isValid
+		if (entity == null) {
+			throw new FieldAccessException(getRequest(), this, field);
+		}
+
+		return entity;
 	}
 
 	@Override
 	public <D> D toEntity(ParameterizedTypeReference<D> type) {
-		return field("").toEntity(type);
+		ClientResponseField field = field("");
+		D entity = field.toEntity(type);
+
+		// should never happen because toEntity checks response.isValid
+		if (entity == null) {
+			throw new FieldAccessException(getRequest(), this, field);
+		}
+
+		return entity;
 	}
 
 }

spring-graphql/src/main/java/org/springframework/graphql/client/DefaultClientResponseField.java

@@ -29,6 +29,7 @@
 import org.springframework.core.io.buffer.DefaultDataBufferFactory;
 import org.springframework.graphql.ResponseError;
 import org.springframework.graphql.ResponseField;
+import org.springframework.lang.Nullable;
 import org.springframework.util.MimeType;
 import org.springframework.util.MimeTypeUtils;
 
@@ -98,17 +99,23 @@ public <D> D toEntity(ParameterizedTypeReference<D> entityType) {
 
 	@Override
 	public <D> List<D> toEntityList(Class<D> elementType) {
-		return toEntity(ResolvableType.forClassWithGenerics(List.class, elementType));
+		List<D> list = toEntity(ResolvableType.forClassWithGenerics(List.class, elementType));
+		return (list != null ? list : Collections.emptyList());
 	}
 
 	@Override
 	public <D> List<D> toEntityList(ParameterizedTypeReference<D> elementType) {
-		return toEntity(ResolvableType.forClassWithGenerics(List.class, ResolvableType.forType(elementType)));
+		List<D> list = toEntity(ResolvableType.forClassWithGenerics(List.class, ResolvableType.forType(elementType)));
+		return (list != null ? list : Collections.emptyList());
 	}
 
-	@SuppressWarnings({"unchecked", "ConstantConditions"})
+		@SuppressWarnings("unchecked")
+		@Nullable
 	private <T> T toEntity(ResolvableType targetType) {
 		if (getValue() == null) {
+			if (this.response.isValid() && getErrors().isEmpty()) {
+				return null;
+			}
 			throw new FieldAccessException(this.response.getRequest(), this.response, this);
 		}
 

spring-graphql/src/main/java/org/springframework/graphql/client/DefaultGraphQlClient.java

@@ -186,9 +186,8 @@ protected RetrieveSpecSupport(String path) {
 		}
 
 		/**
-		 * Return the field or {@code null}, but only if the response is valid
-		 * and there are no field errors, or raise {@link FieldAccessException}
-		 * otherwise.
+		 * Return the field or {@code null}, but only if the response is valid and
+		 * there are no field errors. Raise {@link FieldAccessException} otherwise.
 		 * @throws FieldAccessException in case of an invalid response or any
 		 * field error at, above or below the field path
 		 */
@@ -216,12 +215,12 @@ private static class DefaultRetrieveSpec extends RetrieveSpecSupport implements
 
 		@Override
 		public <D> Mono<D> toEntity(Class<D> entityType) {
-			return this.responseMono.mapNotNull(this::getValidField).map(field -> field.toEntity(entityType));
+			return this.responseMono.mapNotNull(this::getValidField).mapNotNull(field -> field.toEntity(entityType));
 		}
 
 		@Override
 		public <D> Mono<D> toEntity(ParameterizedTypeReference<D> entityType) {
-			return this.responseMono.mapNotNull(this::getValidField).map(field -> field.toEntity(entityType));
+			return this.responseMono.mapNotNull(this::getValidField).mapNotNull(field -> field.toEntity(entityType));
 		}
 
 		@Override
@@ -254,12 +253,12 @@ private static class DefaultRetrieveSubscriptionSpec extends RetrieveSpecSupport
 
 		@Override
 		public <D> Flux<D> toEntity(Class<D> entityType) {
-			return this.responseFlux.mapNotNull(this::getValidField).map(field -> field.toEntity(entityType));
+			return this.responseFlux.mapNotNull(this::getValidField).mapNotNull(field -> field.toEntity(entityType));
 		}
 
 		@Override
 		public <D> Flux<D> toEntity(ParameterizedTypeReference<D> entityType) {
-			return this.responseFlux.mapNotNull(this::getValidField).map(field -> field.toEntity(entityType));
+			return this.responseFlux.mapNotNull(this::getValidField).mapNotNull(field -> field.toEntity(entityType));
 		}
 
 		@Override