Inject Spring Bean in Jackson VirtualBeanPropertyWriter - spring

Question

tldr; I want to add virtual fields while serializing the JPA entity into JSON using Jackson #JsonAppend. The value of the virtual fields must be determined via service managed by Spring. How do I inject my spring-managed service inside a Jackson class?
Technologies: Spring Boot 1.5.10, Spring Data Rest, JPA 2.1, Jackson 2.8.10
Details:
I have a Spring Data managed JPA entity:
#Entity
public class Stream {
...
}
I created a Custom Jackson module with a Mixin to add #JsonAppend virtual field as below:
#Bean
public Module customModule() {
return new CustomModule();
}
#Component
class CustomModule extends SimpleModule {
CustomModule() {
setMixInAnnotation(Stream.class, StreamMixin.class);
}
#JsonAppend(
props = {
#JsonAppend.Prop(name = "canEdit", value = ABACInspector.class)
}
)
abstract class StreamMixin {}
}
The ABACInspector class extends Jackson's VirtualBeanPropertyWriter to determine the value of the virtual field canEdit. If this class does not use a Spring service (sets hard-coded value for example), it works fine and the field shows up in REST API JSON response. But autowiring a Spring bean doesn't work and the object remains null.
#Component
class ABACInspector extends VirtualBeanPropertyWriter {
#Autowired
private PermissionEvaluator permissionEvaluator;
public ABACInspector() {
}
public ABACInspector(BeanPropertyDefinition propDef, Annotations contextAnnotations, JavaType declaredType) {
super(propDef, contextAnnotations, declaredType);
}
#Override
protected Object value(Object bean, JsonGenerator gen, SerializerProvider prov) throws Exception {
Authentication authentication = SecurityContextHolder.getContext().getAuthentication();
boolean permission = permissionEvaluator.hasPermission(authentication, bean, Action.STREAM_VIEW);
System.out.println("evaluated permission is " + permission);
return permission;
}
#Override
public VirtualBeanPropertyWriter withConfig(MapperConfig<?> config, AnnotatedClass declaringClass, BeanPropertyDefinition propDef, JavaType type) {
return new ABACInspector(propDef, null, type);
}
}
Below is the NPE error (because permissionEvaluator is never injected):
{"status":"INTERNAL_SERVER_ERROR","message":"Could not write JSON:
(was java.lang.NullPointerException); nested exception is com.fasterxml.jackson.databind.JsonMappingException:
(was java.lang.NullPointerException) (through reference chain: org.springframework.data.rest.webmvc.json.PersistentEntityJackson2Module$PersistentEntityResourceSerializer$1[\"content\"]->com.example.streammanagement.Stream[\"canView\"])"
I am aware of Spring Data Rest's HalHandlerInstantiator that contains the AutowireCapableBeanFactory but I am not sure how/if that can help here. Refer DATAREST-840

Answer 1

Jackson internally calls withConfig function of your component to build VirtualBeanPropertyWriter.
So if you use breakpoints, you can see that first a component with injected bean is created, then withConfig function is called and new VirtualBeanPropertyWriter object is created which is used by jackson and of course does not have the injected bin (since you called the constructor manually).
So you can change it by this way:
#Component
class ABACInspector extends VirtualBeanPropertyWriter {
private PermissionEvaluator permissionEvaluator;
#Autowired
public ABACInspector(PermissionEvaluator permissionEvaluator) {
this.permissionEvaluator = permissionEvaluator;
}
public ABACInspector(BeanPropertyDefinition propDef, Annotations contextAnnotations, JavaType declaredType, PermissionEvaluator permissionEvaluator) {
super(propDef, contextAnnotations, declaredType);
this.permissionEvaluator = permissionEvaluator;
}
#Override
protected Object value(Object bean, JsonGenerator gen, SerializerProvider prov) throws Exception {
Authentication authentication = SecurityContextHolder.getContext().getAuthentication();
boolean permission = permissionEvaluator.hasPermission(authentication, bean, Action.STREAM_VIEW);
System.out.println("evaluated permission is " + permission);
return permission;
}
#Override
public VirtualBeanPropertyWriter withConfig(MapperConfig<?> config, AnnotatedClass declaringClass, BeanPropertyDefinition propDef, JavaType type) {
return new ABACInspector(propDef, null, type, permissionEvaluator);
}
}