Question

tldr; 我想在使用Jackson @JsonAppend将JPA实体序列化为JSON的同时添加虚拟字段。虚拟字段的值必须通过Spring管理的服务来确定。如何将我的Spring托管服务注入Jackson类中？

技术：Spring Boot 1.5.10，Spring Data Rest，JPA 2.1，Jackson 2.8.10

详细信息：

我有一个由Spring Data管理的JPA实体：

@Entity
public class Stream {
   ...
}

我创建了一个带有Mixin的Custom Jackson模块，以添加@JsonAppend虚拟字段，如下所示：

@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 {}

}

ABACInspector类扩展了Jackson的VirtualBeanPropertyWriter，以确定虚拟字段canEdit的值。如果此类不使用Spring服务（例如，设置硬编码值），则它将正常工作，并且该字段将显示在REST API JSON响应中。但是自动装配Spring bean不起作用，该对象仍为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);
    }
}

以下是NPE错误（因为从未注入permissionEvaluator）：

{"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\"])"

我知道包含HalHandlerInstantiator的Spring Data Rest的AutowireCapableBeanFactory，但不确定如何/是否有帮助。请参阅DATAREST-840

Answer 1

Jackson内部调用组件的withConfig函数以构建VirtualBeanPropertyWriter。因此，如果使用断点，则可以看到首先创建了一个带有注入bean的组件，然后调用了withConfig函数，并且创建了新的VirtualBeanPropertyWriter对象，该对象由jackson使用，当然也没有注入的bin（因为您调用了构造函数）手动）。

因此您可以通过以下方式进行更改：

@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);
    }
}

在Jackson VirtualBeanPropertyWriter中注入Spring Bean

1 个答案: