Jackson定制序列化和反序列化

Question

我无法找出使用杰克逊实现自定义序列化/反序列化的正确方法。我有很多类（〜50），它们带有应被序列化/反序列化而不是原始的原始字段。喜欢：

class User {
    int height // this field should be serialized as "height": "10 m"
}

class Food {
    int temperature // this field should be serialized as "temperature": "50 C"
}

class House {
    int width // this field should be serialized as "width": "10 m"
}

所有序列化和反序列化都非常相似，我只需要在整数之后添加一个后缀（C，页面，米等）。

一种简单的方法是在每个这样的字段中添加一对@JsonSerialize/ @JsonDeserialize注释并实现它们。但是我最终会得到100个
非常相似的 序列化器/反序列化器。

我想到了添加自定义注释的各个领域，说@Units("Degree")或@Units("Meters")，这样的整数字段和实施SerializationProvider，将在基于注释值，一个通用的方法创建序列化。但是我找不到一个可以获取有关属性注释的信息的地方。

Answer 1

带Unit注释的想法真的很好。我们只需要添加自定义com.fasterxml.jackson.databind.ser.BeanSerializerModifier和com.fasterxml.jackson.databind.ser.BeanPropertyWriter实现。首先创建我们的注释类：

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
@interface Unit {
    String value();
}

POJO 模型可能如下所示：

class Pojo {

    private User user = new User();
    private Food food = new Food();
    private House house = new House();

    // getters, setters, toString
}

class User {

    @Unit("m")
    private int height = 10;

    // getters, setters, toString
}

class Food {

    @Unit("C")
    private int temperature = 50;

    // getters, setters, toString
}

class House {

    @Unit("m")
    private int width = 10;

    // getters, setters, toString
}

具有所有这些之后，我们需要自定义属性序列化：

class UnitBeanSerializerModifier extends BeanSerializerModifier {

    @Override
    public List<BeanPropertyWriter> changeProperties(SerializationConfig config, BeanDescription beanDesc, List<BeanPropertyWriter> beanProperties) {
        for (int i = 0; i < beanProperties.size(); ++i) {
            final BeanPropertyWriter writer = beanProperties.get(i);
            AnnotatedMember member = writer.getMember();
            Unit units = member.getAnnotation(Unit.class);
            if (units != null) {
                beanProperties.set(i, new UnitBeanPropertyWriter(writer, units.value()));
            }
        }
        return beanProperties;
    }
}

class UnitBeanPropertyWriter extends BeanPropertyWriter {

    private final String unit;

    protected UnitBeanPropertyWriter(BeanPropertyWriter base, String unit) {
        super(base);
        this.unit = unit;
    }

    @Override
    public void serializeAsField(Object bean, JsonGenerator gen, SerializerProvider prov) throws Exception {
        gen.writeFieldName(_name);
        final Object value = (_accessorMethod == null) ? _field.get(bean) : _accessorMethod.invoke(bean, (Object[]) null);
        gen.writeString(value + " " + unit);
    }
}

使用SimpleModule我们可以注册它并与使用ObjectMapper：

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.BeanDescription;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationConfig;
import com.fasterxml.jackson.databind.SerializerProvider;
import com.fasterxml.jackson.databind.introspect.AnnotatedMember;
import com.fasterxml.jackson.databind.module.SimpleModule;
import com.fasterxml.jackson.databind.ser.BeanPropertyWriter;
import com.fasterxml.jackson.databind.ser.BeanSerializerModifier;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import java.util.List;

public class JsonApp {

    public static void main(String[] args) throws Exception {
        SimpleModule unitModule = new SimpleModule();
        unitModule.setSerializerModifier(new UnitBeanSerializerModifier());

        ObjectMapper mapper = new ObjectMapper();
        mapper.registerModule(unitModule);

        Pojo pojo = new Pojo();
        System.out.println(mapper.writeValueAsString(pojo));
    }
}

印刷品：

{
  "user" : {
    "height" : "10 m"
  },
  "food" : {
    "temperature" : "50 C"
  },
  "house" : {
    "width" : "10 m"
  }
}

当然，您需要对其进行测试并处理所有极端情况，但以上示例显示了总体思路。以类似的方式，我们可以处理反序列化。我们需要实现custom
BeanDeserializerModifier和一个custom UnitDeserialiser：

class UnitBeanDeserializerModifier extends BeanDeserializerModifier {

    @Override
    public JsonDeserializer<?> modifyDeserializer(DeserializationConfig config, BeanDescription beanDesc, JsonDeserializer<?> deserializer) {
        JsonDeserializer<?> jsonDeserializer = super.modifyDeserializer(config, beanDesc, deserializer);
        if (jsonDeserializer instanceof StdScalarDeserializer) {
            StdScalarDeserializer scalarDeserializer = (StdScalarDeserializer) jsonDeserializer;
            Class scalarClass = scalarDeserializer.handledType();
            if (int.class == scalarClass) {
                return new UnitIntStdScalarDeserializer(scalarDeserializer);
            }
        }
        return jsonDeserializer;
    }
}

反序列化器示例int：

class UnitIntStdScalarDeserializer extends StdScalarDeserializer<Integer> {

    private StdScalarDeserializer<Integer> src;

    public UnitIntStdScalarDeserializer(StdScalarDeserializer<Integer> src) {
        super(src);
        this.src = src;
    }

    @Override
    public Integer deserialize(JsonParser p, DeserializationContext ctxt) throws IOException {
        String value = p.getValueAsString();
        String[] parts = value.split("\\s+");
        if (parts.length == 2) {
            return Integer.valueOf(parts[0]);
        }
        return src.deserialize(p, ctxt);
    }
}

上面的实现只是一个例子，应该针对其他原始类型进行改进。我们可以使用简单的模块以相同的方式注册它。重复使用与序列化相同：

unitModule.setDeserializerModifier(new UnitBeanDeserializerModifier());