Spring filtering response fields with jackson

Question

I am trying to filter fields in a nested object:

class Response {

  // These objects themselves can have many fields within
  private final PropA a;
  private final PropB b;

  @JsonCreator
  public Response(PropA a, PropB b) { ... }
}

I'd like a generic 'filter helper' to achieve the above logic. Here is what I have so far (following a similar approach as this project)

public class FilterHelper {

    private final ObjectMapper objectMapper;

    public FilterHelper(ObjectMapper objectMapper) {
        this.objectMapper = objectMapper;
        this.objectMapper.addMixIn(Object.class, MyFilterMixin.class);
    }

    @JsonFilter("myfilter")
    public static class MyFilterMixin {
    }

    private static class MyFilter extends SimpleBeanPropertyFilter {
        private final Set<String> properties;

        public MyFilter(Set<String> properties) {
            super();
            this.properties = properties;
        }

        @Override
        public void serializeAsField(final Object pojo, final JsonGenerator jgen, final SerializerProvider provider,
                                     final PropertyWriter writer) throws Exception {

            System.out.println("************************** " + writer.getName());
            if (properties.contains(writer.getName())) {
                writer.serializeAsField(pojo, jgen, provider);
            } else if (!jgen.canOmitFields()) {
                writer.serializeAsOmittedField(pojo, jgen, provider);
            }
        }
    }

    public String filter(T obj, Set<String> fields) {

        FilterProvider filterProvider = new SimpleFilterProvider().addFilter("myfilter", new MyFilter(fields));

        return objectMapper.writer(filterProvider).writeValueAsString(obj);
    }
}

When I hit this endpoint with ?fields=one,two as query parameter I expect to see from a line printed to console for every field within that top level Response object as follows:

******************* a
******************* a1
******************* a2
******************* ..etc
******************* b
******************* b1
******************* b2
******************* ..etc

but I am only seeing output for the top level a and b fields followed by an error before getting a 500 status code from the endpoint:

com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot resolve PropertyFilter with id 'myfilter'; no FilterProvider configured (through reference chain: com.google.common.collect.SingletonImmutableList[0])

It is worth mentioning that I had this working somehow, but it was broken after some changes I don't recall.

Answer 1

Unless you need to provide custom serialization for different fields, you should not be hooking the serializeAsField and instead you should be overriding the #include variant methods:

• com.fasterxml.jackson.databind.ser.impl.SimpleBeanPropertyFilter#include(com.fasterxml.jackson.databind.ser.BeanPropertyWriter)
• com.fasterxml.jackson.databind.ser.impl.SimpleBeanPropertyFilter#include(com.fasterxml.jackson.databind.ser.PropertyWriter)

as follows:

private static class MyFilter extends SimpleBeanPropertyFilter {
    private final Set<String> properties;

    public MyFilter(Set<String> properties) {
        super();
        this.properties = properties;
    }

    @Override
    protected boolean include(BeanPropertyWriter writer) {
        return !this.properties.contains(writer.getName());
    }

    @Override
    protected boolean include(PropertyWriter writer) {
        return !this.properties.contains(writer.getName());
    }
}

There is even a static factory providing a com.fasterxml.jackson.databind.ser.PropertyFilter that filters out a specific set of fields:

com.fasterxml.jackson.databind.ser.impl.SimpleBeanPropertyFilter#serializeAllExcept(java.util.Set<java.lang.String>)

Extra issue

At the filter helper level, you are serializing the filtered object to JSON then deserializing it back (with filtered fields) to an object that you are handing back as the endpoint response.

Solution / Alternative

You can simply omit the intermediary step by just sterilizing the result Response with the filter fields predicate and returning the result JSON as ResponseEntity:

FilterHelper:

@Component
public class FilterHelper {

    private final ObjectMapper objectMapper;

    @Autowired
    public FilterHelper(ObjectMapper objectMapper) {
        this.objectMapper = objectMapper;
        this.objectMapper.addMixIn(Object.class, MyFilterMixin.class);
    }

    @JsonFilter("myfilter")
    public static class MyFilterMixin {
    }

    private static class MyFilter extends SimpleBeanPropertyFilter {
        private final Set<String> properties;

        public MyFilter(Set<String> properties) {
            super();
            this.properties = properties;
        }

        @Override
        protected boolean include(BeanPropertyWriter writer) {
            return !this.properties.contains(writer.getName());
        }

        @Override
        protected boolean include(PropertyWriter writer) {
            return !this.properties.contains(writer.getName());
        }
    }

    public String filter(Object dto, Set<String> fields) {
        if (fields == null || fields.isEmpty()) {
            return "";
        }

        FilterProvider filterProvider = new SimpleFilterProvider()
                .addFilter("myfilter", SimpleBeanPropertyFilter.serializeAllExcept(fields));
        try {
            return objectMapper.writer(filterProvider).writeValueAsString(dto);
        } catch (JsonProcessingException e) {
            return "";
        }
    }
}

Controller:

@GetMapping(value = "/", produces = MediaType.APPLICATION_JSON_VALUE)
@ResponseStatus(OK)
ReponseEntity<String> someEndpoint(@RequestParam(name = "fields") Set<String> fields) {
  Response response = getResponseFromSomewhere();
  return ResponseEntity.ok(filterHelper.filter(response, fields));
}