Jackson @JsonUnwrapped 行为与自定义 JsonSerializer

我有两个这样的课程：

public class A {
    String aProp = "aProp";

    public String getAProp() {
        return aProp;
    }
}

public class B {
    String bProp = "bProp";
    A a = new A();

    @JsonProperty("bProp")
    public String getBProp() {
        return bProp;
    }

    @JsonSerialize(using = CustomSerializer.class)
    public A getA() {
        return a;
    }     
}

我期望得到这样的 JSON：

{
    "bProp": "bProp",         // just serizlised bProp
    "sProp1": "sProp1_aProp", // computed using aProp
    "sProp2": "sProp2_aProp"  // computed another way
}

所以我写了自定义JsonSerializer像这样：

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.SerializerProvider;

public class CustomSerializer extends JsonSerializer<A> {
    @Override
    public void serialize(A a, JsonGenerator json, SerializerProvider provider) throws IOException {
        json.writeStringField("sProp1", "sProp1_" + a.getAProp());
        json.writeStringField("sProp2", "sProp2_" + a.getAProp());
    }
}

但我不断收到错误：

com.fasterxml.jackson.core.JsonGenerationException: Can not write a field name, expecting a value

除非我把json.writeStartObject(); and json.writeEndObject();在序列化方法中（因此它会产生错误的 JSON）。

所以我正在寻找像这样的解决方案@JsonUnwrapped与自定义一起使用JsonSerializer.

我理解你的问题，你需要的是UnwrappingBeanSerializer。你可以看到另一个相关的SO帖子：在 Spring Data Rest 中使用自定义 json 序列化器时不同的 JSON 输出 https://stackoverflow.com/questions/22613143/different-json-output-when-using-custom-json-serializer-in-spring-data-rest

问题是你不能同时拥有两个注释@JacksonUnwrapped and @JsonSerialize在一个领域，因为当你有@JsonSerializer杰克逊总是会写字段名称。

这是完整的解决方案：

public class CustomSerializer  extends UnwrappingBeanSerializer {
    public CustomSerializer(BeanSerializerBase src, NameTransformer transformer) {
        super(src, transformer);
    }

    @Override
    public JsonSerializer<Object> unwrappingSerializer(NameTransformer transformer) {
        return new CustomSerializer(this, transformer);
    }

    @Override
    protected void serializeFields(Object bean, JsonGenerator jgen, SerializerProvider provider) throws IOException, JsonGenerationException {
        A a = (A) bean;
        jgen.writeStringField("custom", a.getAProp());
        jgen.writeStringField("custom3", a.getAProp());
    }

    @Override
    public boolean isUnwrappingSerializer() {
        return true;
    }

}

测试用例，您应该使用自定义配置或研究其他方法来重新定义对象映射器。

@RunWith(SpringJUnit4ClassRunner.class)
@WebAppConfiguration
@SpringApplicationConfiguration(classes = Application.class)
public class ColorsTest {

    ObjectMapper mapper = new ObjectMapper();

    @Before
    public void setUp(){
        mapper.registerModule(new Module() {
            @Override
            public String getModuleName() {
                return "my.module";
            }

            @Override
            public Version version() {
                return Version.unknownVersion();
            }

            @Override
            public void setupModule(SetupContext context) {

                context.addBeanSerializerModifier(new BeanSerializerModifier() {
                    @Override
                    public JsonSerializer<?> modifySerializer(SerializationConfig config, BeanDescription beanDesc, JsonSerializer<?> serializer) {
                        if(beanDesc.getBeanClass().equals(A.class)) {
                            return new CustomSerializer((BeanSerializerBase) serializer, NameTransformer.NOP);
                        }
                        return serializer;
                    }
                });

            }
        });
    }
    @Test
    public void testSerializer() throws JsonProcessingException {
        System.out.println(mapper.writeValueAsString(new B()));
    }
}

Class B:

public class B {

        @JsonProperty("bProp")
        public String getBProp() {
            return "bProp";
        }


    @JsonUnwrapped
        public A getA() {
            return new A();
        }
}