如何将枚举序列化为对象形状和默认字符串？答案

【问题标题】：How to serialise Enums as both Object Shape and default string?如何将枚举序列化为对象形状和默认字符串？
【发布时间】：2020-07-31 13:32:43
【问题描述】：

对于带有属性的枚举，例如：

public enum Thing {
  THING_A("a"),
  THING_B("b");

  private String thing;

  private Thing(String thing) {
    this.thing = thing;
  }

  // Getters...
}

Jackson 序列化为值的名称，例如：

mapper.writeValueAsString(Thing.THING_A)); // "THING_A"

如果我们添加注解将序列化视为对象：
@JsonFormat(shape = JsonFormat.Shape.OBJECT) 它将序列化属性：

mapper.writeValueAsString(Thing.THING_A)); // "{"thing":"a"}"

我希望能够决定，在序列化过程中，使用这些方法中的哪一个。因为这跨越了大量的枚举，我宁愿不编辑每一个。有什么好办法吗？

例如：这样的东西会很棒：

mapper.writeValueAsString(Thing.THING_A, JsonFormat.Shape.OBJECT); // "{"thing":"a"}"
mapper.writeValueAsString(Thing.THING_A, JsonFormat.Enum.DEFAULT); // "THING_A"

【问题讨论】：

标签： java json serialization enums jackson

【解决方案1】：

上面的问题类似，已经回答过了。 Jackson ObjectMapper set JsonFormat.Shape.ARRAY without annotation.

您可以使用特定于 Enum 的自定义对象映射器和其他类的不同对象映射器。

【讨论】：

【解决方案2】：

因为，com.fasterxml.jackson.annotation.JsonFormat 是一个注释，您可以实现自己的 com.fasterxml.jackson.databind.AnnotationIntrospector 并为所有枚举返回您想要的值。您可以在下面找到简单的示例：

import com.fasterxml.jackson.annotation.JsonFormat;
import com.fasterxml.jackson.core.Version;
import com.fasterxml.jackson.databind.AnnotationIntrospector;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.introspect.Annotated;
import com.fasterxml.jackson.databind.introspect.JacksonAnnotationIntrospector;

public class JsonPathApp {

    public static void main(String[] args) throws Exception {
        ObjectMapper mapper = new ObjectMapper();
        mapper.setAnnotationIntrospector(AnnotationIntrospector.pair(new DynamicEnumAnnotationIntrospector(), new JacksonAnnotationIntrospector()));

        System.out.println(mapper.writeValueAsString(Thing.THING_A));
    }
}

class DynamicEnumAnnotationIntrospector extends AnnotationIntrospector {

    @Override
    public Version version() {
        return new Version(1, 0, 0, "Dynamic enum object", "your.package", "jackson.dynamic.enum");
    }

    @Override
    public JsonFormat.Value findFormat(Annotated memberOrClass) {
        final Class<?> rawType = memberOrClass.getRawType();
        if (rawType.isEnum() && rawType.getPackage().getName().startsWith("your.package")) {
            return JsonFormat.Value.forShape(JsonFormat.Shape.OBJECT);
        }

        return super.findFormat(memberOrClass);
    }
}

上面的代码打印：

{"thing":"a"}

现在，您可以创建两个ObjectMapper 实例，其中一个配置您自己的注释内省器，第二个保留默认值。如果您真的想以动态方式使用它，您可以为每个可用的Shape 值创建一个ObjectMapper，并为给定形状选择所需的一个：

import com.fasterxml.jackson.annotation.JsonFormat;
import com.fasterxml.jackson.annotation.JsonFormat.Shape;
import com.fasterxml.jackson.core.Version;
import com.fasterxml.jackson.databind.AnnotationIntrospector;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.introspect.Annotated;
import com.fasterxml.jackson.databind.introspect.JacksonAnnotationIntrospector;

import java.util.Arrays;
import java.util.EnumMap;
import java.util.List;
import java.util.Objects;

public class JsonPathApp {

    public static void main(String[] args) throws Exception {
        JsonFactory factory = new JsonFactory();

        for (Shape shape : Shape.values()) {
            ObjectMapper mapper = factory.getWithEnumShapeSetTo(shape);
            System.out.println(shape + " => " + mapper.writeValueAsString(Thing.THING_A));
        }
    }
}

class JsonFactory {
    private final AnnotationIntrospector defaultIntrospector = new JacksonAnnotationIntrospector();
    private final EnumMap<Shape, ObjectMapper> instances = new EnumMap<>(Shape.class);

    public JsonFactory() {
        final List<Shape> notAllowed = Arrays.asList(Shape.BOOLEAN, Shape.BINARY);
        Arrays.stream(Shape.values())
                .filter(shape -> !notAllowed.contains(shape))
                .forEach(shape -> instances.put(shape, createNewWithEnumShape(shape)));
    }

    private ObjectMapper createNewWithEnumShape(Shape shape) {
        DynamicEnumAnnotationIntrospector enumIntrospector = new DynamicEnumAnnotationIntrospector(shape);

        ObjectMapper mapper = new ObjectMapper();
        mapper.setAnnotationIntrospector(AnnotationIntrospector.pair(enumIntrospector, defaultIntrospector));

        return mapper;
    }

    public ObjectMapper getWithEnumShapeSetTo(Shape shape) {
        Objects.requireNonNull(shape);

        final ObjectMapper mapper = instances.get(shape);
        if (mapper == null) {
            return new ObjectMapper();
        }

        return mapper;
    }
}

class DynamicEnumAnnotationIntrospector extends AnnotationIntrospector {

    private final Shape shape;

    public DynamicEnumAnnotationIntrospector(Shape shape) {
        this.shape = Objects.requireNonNull(shape);
    }

    @Override
    public Version version() {
        return new Version(1, 0, 0, "Dynamic enum shape", "your.package", "jackson.dynamic.enum");
    }

    @Override
    public JsonFormat.Value findFormat(Annotated memberOrClass) {
        final Class<?> rawType = memberOrClass.getRawType();
        if (rawType.isEnum() && rawType.getPackage().getName().startsWith("your.package")) {
            return JsonFormat.Value.forShape(shape);
        }

        return super.findFormat(memberOrClass);
    }
}

上面的代码打印：

ANY => "THING_A"
NATURAL => "THING_A"
SCALAR => "THING_A"
ARRAY => 0
OBJECT => {"thing":"a"}
NUMBER => 0
NUMBER_FLOAT => 0
NUMBER_INT => 0
STRING => "THING_A"
BOOLEAN => "THING_A"
BINARY => "THING_A"

上面的代码当然是多余的，但我想展示我们拥有的可能性。我们只有 3 个不同的输出，因此您可以将具有相同输出的值分组并创建最多 3 个不同的ObjectMappers。

【讨论】：