Asaf's technical blog

Conditionally serializing fields using Jackson

17 Apr 2015 | java

When interacting with some REST API, we often deal with serialization of Java objects to JSON strings. Lately, I came across a requirement to conditionally skip an object’s field, according to its value. Assume, for example, the following class:

public class MyType {
    private final double value;

    public MyType(final double value) {
        this.value = value;
    }

    @JsonProperty("value")
    public double getValue() {
        return value;
    }
}

Assume we’d like to include value in the JSON serialized string only if its value is not equal to 0. I’m using Jackson for JSON serialization, and the solution, as I found, was to implement and register a PropertyFilter:

public interface PropertyFilter {
    void serializeAsField(Object pojo, JsonGenerator jgen, SerializerProvider prov, PropertyWriter writer);

    void serializeAsElement(Object elementValue, JsonGenerator jgen, SerializerProvider prov, PropertyWriter writer) throws Exception;

    void depositSchemaProperty(PropertyWriter writer, JsonObjectFormatVisitor objectVisitor, SerializerProvider provider) throws JsonMappingException;

    @Deprecated 
    void depositSchemaProperty(PropertyWriter writer, ObjectNode propertiesNode, SerializerProvider provider) throws JsonMappingException;
}

In my case, only the first method required an special implementation:

public class ZeroValueFilter implements PropertyFilter {
    void serializeAsField(Object pojo, JsonGenerator jgen, SerializerProvider prov, PropertyWriter writer) {
        if (pojo instanceof MyType && isValueFieldZero((MyType) pojo)) {
            return; // skip this field
        }
        writer.serializeAsField(pojo, jgen, prov);
    }

    private isValueFieldZero(MyType myClass) {
        return Double.compare(myClass.getValue(), 0.0) == 0;
    }

    void serializeAsElement(Object elementValue, JsonGenerator jgen, SerializerProvider prov, PropertyWriter writer) throws Exception {
        writer.serializeAsField(elementValue, jgen, prov);
    }

    void depositSchemaProperty(PropertyWriter writer, JsonObjectFormatVisitor objectVisitor, SerializerProvider provider) throws JsonMappingException {
        writer.depositSchemaProperty(objectVisitor);
    }

    @Deprecated 
    void depositSchemaProperty(PropertyWriter writer, ObjectNode propertiesNode, SerializerProvider provider) throws JsonMappingException {
        writer.depositSchemaProperty(propertiesNode, provider);
    }
}

I also had to annotate the class with the required filter:

@JsonFilter("zeroValueFilter")
public class MyType {
    // as above
}

Lastly, upon creating an ObjectMapper to be used for JSON serialization, the filter should be registered:

final ObjectMapper mapper = new ObjectMapper();
final SimpleFilterProvider filterProvider = new SimpleFilterProvider()
        .addFilter("zeroValueFilter", new ZeroValueFilter());
mapper.setFilters(filterProvider);

We can now test the object mapper to see the filtering working:

System.out.println(mapper.writeValueAsString(new MyType(42.0))); // prints {value:42.0}
System.out.println(mapper.writeValueAsString(new MyType(0.0)));  // prints {}

Related Posts

Comments