java - 如何使用 JsonTypeInfo 和反/序列化器来自定义处理多态子类型？

Question

我遇到了一种情况，我需要自定义某些 JSON 的序列化/反序列化。我已将其简化为一个可读的示例。我有一个 Container 类，其中包含实现 MyInterface 的对象。在我的示例中，ClassA、ClassB、IntegerHolder 和 StringHolder 实现了该接口(interface)。通过将 @JsonTypeInfo 注释添加到我的界面(和容器):

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, property = "type")

并为每个类注册类型名称，我可以成功地从该 JSON 读取/写入这些名称:

{"type":"Container","items":
    [   {"type":"classA","aValue":"AAA"},
        {"type":"classB","bValue":"BBB"},
        {"type":"intHolder","value":123},
        {"type":"stringHolder","value":"abc"} ] }

这一切都非常好:)我的问题是我想自定义 intHolder 和 stringHolder 的序列化，因为它们只是 native 类型的包装器。我的 JSON 将经常手动编辑，并且将大量使用原始类型。所以我想将 JSON 简化为:

{"type":"Container","items":
    [   {"type":"classA","aValue":"AAA"},
        {"type":"classB","bValue":"BBB"},
        123,
        "abc" ] }

我编写了一个序列化器和反序列化器(扩展 StdSeralizer 和 StdDeserializer)，将它们放入 SimpleModule 中并使用映射器注册它(如图 here on SO 所示)，并且单独运行，效果很好。我的意思是，如果 IntegerHolder 和 StringHolder 是容器中唯一的对象，并且只有从接口(interface)中删除 @JsonTypeInfo 注释，我就可以序列化/反序列化它们。如果我不这样做，那么我在写入 JSON 时会遇到此失败:

[main] ERROR MyTests - can't write the Container
com.fasterxml.jackson.databind.JsonMappingException: Type id handling not implemented for type MyInterface (by serializer of type MyTests$MyInterfaceSerializer) (through reference chain: Container["items"])
    at com.fasterxml.jackson.databind.SerializerProvider.mappingException(SerializerProvider.java:1047)
    at com.fasterxml.jackson.databind.JsonSerializer.serializeWithType(JsonSerializer.java:142)
    at com.fasterxml.jackson.databind.ser.std.ObjectArraySerializer.serializeTypedContents(ObjectArraySerializer.java:316)
    at com.fasterxml.jackson.databind.ser.std.ObjectArraySerializer.serializeContents(ObjectArraySerializer.java:217)
    at com.fasterxml.jackson.databind.ser.std.ObjectArraySerializer.serialize(ObjectArraySerializer.java:201)
    at com.fasterxml.jackson.databind.ser.std.ObjectArraySerializer.serialize(ObjectArraySerializer.java:25)
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:575)
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:666)
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeWithType(BeanSerializerBase.java:552)
    at com.fasterxml.jackson.databind.ser.impl.TypeWrappedSerializer.serialize(TypeWrappedSerializer.java:32)
    at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:129)
    at com.fasterxml.jackson.databind.ObjectMapper._configAndWriteValue(ObjectMapper.java:3387)
    at com.fasterxml.jackson.databind.ObjectMapper.writeValue(ObjectMapper.java:2747)
    at MyTests.testItemSerializationDeserializationEquality(MyTests.java:51)
    at MyTests.testSerialization(MyTests.java:41)

但是，当然，删除了 @JsonTypeInfo 后，Jackson 不知道如何反序列化 ClassA 和 ClassB...因此在读取 JSON 时会失败:

[main] INFO MyTests - {"type":"Container","items":[{"aValue":"AAA"},{"bValue":"BBB"},123,"abc"]}
[main] ERROR MyTests - can't read the Container
com.fasterxml.jackson.databind.JsonMappingException: Can not construct instance of MyInterface, problem: abstract types either need to be mapped to concrete types, have custom deserializer, or be instantiated with additional type information
 at [Source: java.io.ByteArrayInputStream@37883b97; line: 1, column: 45] (through reference chain: Container["items"]->Object[][0])
    at com.fasterxml.jackson.databind.JsonMappingException.from(JsonMappingException.java:148)
    at com.fasterxml.jackson.databind.DeserializationContext.instantiationException(DeserializationContext.java:857)
    at com.fasterxml.jackson.databind.deser.AbstractDeserializer.deserialize(AbstractDeserializer.java:139)
    at MyTests$MyInterfaceDeserializer.deserialize(MyTests.java:163)
    at MyTests$MyInterfaceDeserializer.deserialize(MyTests.java:139)

我觉得 Jackson 可以做到这一点，并且我即将将 Jackson 配置为序列化/反序列化两组类，但到目前为止，我的尝试尚未取得成果。

任何能让我朝着正确方向前进的指示将不胜感激......提前致谢!

以下是我的测试示例中的 7 个类:

MyInterface.java

import com.fasterxml.jackson.annotation.*;

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, property = "type")
public interface MyInterface
    {
    }

容器.java

import com.fasterxml.jackson.annotation.*;

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, property = "type")
public class Container
    {
    public Container()
        {
        }

    public Container(MyInterface... items)
        {
        this.items = items;
        }

    public MyInterface[] getItems()
        {
        return items;
        }

    public void setItems(MyInterface[] items)
        {
        this.items = items;
        }

    @Override
    public boolean equals(Object obj)
        {
        for (int i = 0; i < items.length; i++)
            if (!(items[i].equals(((Container)obj).items[i])))
                return false;
        return true;
        }

    private MyInterface[] items;
    }

MyTests.java

import com.fasterxml.jackson.core.*;
import com.fasterxml.jackson.databind.*;
import com.fasterxml.jackson.databind.deser.*;
import com.fasterxml.jackson.databind.deser.std.*;
import com.fasterxml.jackson.databind.jsontype.*;
import com.fasterxml.jackson.databind.module.*;
import com.fasterxml.jackson.databind.node.*;
import com.fasterxml.jackson.databind.ser.*;
import com.fasterxml.jackson.databind.ser.std.*;
import org.junit.*;
import org.slf4j.*;

import java.io.*;

public class MyTests
    {
    @Test
    public void testSerialization()
        {
        ClassA a = new ClassA();
        a.setaValue("AAA");

        ClassB b = new ClassB();
        b.setbValue("BBB");

        IntegerHolderClass int_holder = new IntegerHolderClass();
        int_holder.setValue(123);

        StringHolderClass string_holder = new StringHolderClass();
        string_holder.setValue("abc");

        // Testing with ONLY the non-customized classes works fine with the @JsonTypeInfo annotation on MyInterface
        // if the custom de/serializers are not registered via the module
//        testItemSerializationDeserializationEquality(new Container(a, b), Container.class);

        // Testing with ONLY the customized classes works fine with the custom de/serializers registered via the module
        // if the @JsonTypeInfo annotation on MyInterface is removed
//        testItemSerializationDeserializationEquality(new Container(int_holder, string_holder), Container.class);

        // This variation tests them all together...doesn't work under either scenario
        testItemSerializationDeserializationEquality(new Container(a, b, int_holder, string_holder), Container.class);
        }

    private void testItemSerializationDeserializationEquality(Object original, Class expected_super_type)
        {
        ObjectMapper mapper = createMapper();

        ByteArrayOutputStream outstream = new ByteArrayOutputStream();
        try
            {
            mapper.writeValue(outstream, original);
            outstream.flush();
            }
        catch (IOException e)
            {
            LOG.error("can't write the " + original.getClass().getSimpleName(), e);
            }

LOG.info(outstream.toString());

        Object copy = null;
        try
            {
            copy = mapper.readValue(new ByteArrayInputStream(outstream.toByteArray()), expected_super_type);
            }
        catch (Exception e)
            {
            LOG.error("can't read the " + original.getClass().getSimpleName(), e);
            }

        Assert.assertNotNull(copy);
        Assert.assertTrue(copy.equals(original));
        }

    private ObjectMapper createMapper()
        {
        ObjectMapper mapper = new ObjectMapper();
        mapper.registerSubtypes(new NamedType(ClassA.class, "classA"));
        mapper.registerSubtypes(new NamedType(ClassB.class, "classB"));
        mapper.registerSubtypes(new NamedType(IntegerHolderClass.class, "intHolder"));
        mapper.registerSubtypes(new NamedType(StringHolderClass.class, "stringHolder"));

        SimpleModule module = new SimpleModule();
        module.setDeserializerModifier(new BeanDeserializerModifier()
            {
            @Override
            public JsonDeserializer<?> modifyDeserializer(DeserializationConfig config, BeanDescription beanDesc, JsonDeserializer<?> deserializer)
                {
                if (MyInterface.class.isAssignableFrom(beanDesc.getBeanClass()))
                    return new MyInterfaceDeserializer(deserializer);
                return deserializer;
                }
            });
        module.setSerializerModifier(new BeanSerializerModifier()
            {
            @Override
            public JsonSerializer<?> modifySerializer(SerializationConfig config, BeanDescription beanDesc, JsonSerializer<?> serializer)
                {
                if (MyInterface.class.isAssignableFrom(beanDesc.getBeanClass()))
                    return new MyInterfaceSerializer(serializer);
                return serializer;
                }
            });

        mapper.registerModule(module);


        return mapper;
        }

    static class MyInterfaceSerializer extends StdSerializer<MyInterface> implements ResolvableSerializer
        {
        public MyInterfaceSerializer(JsonSerializer<?> def)
            {
            super(MyInterface.class);
            _default = (JsonSerializer<MyInterface>) def;
            }

        @Override
        public void serialize(MyInterface value, JsonGenerator jgen, SerializerProvider provider) throws IOException
            {
            if (value instanceof IntegerHolderClass)
                jgen.writeNumber(((IntegerHolderClass) value).getValue());
            else if (value instanceof StringHolderClass)
                jgen.writeString(((StringHolderClass) value).getValue());
            else
                _default.serialize(value, jgen, provider);
            }

        @Override
        public void resolve(SerializerProvider provider) throws JsonMappingException
            {

            }

        private final JsonSerializer<MyInterface> _default;
        }

    static class MyInterfaceDeserializer extends StdDeserializer<MyInterface> implements ResolvableDeserializer
        {
        public MyInterfaceDeserializer(JsonDeserializer<?> def)
            {
            super(MyInterface.class);
            _default = def;
            }

        @Override
        public MyInterface deserialize(JsonParser parser, DeserializationContext context) throws IOException
            {
            TreeNode node = parser.getCodec().readTree(parser);
            if (node instanceof TextNode)
                {
                StringHolderClass holder = new StringHolderClass();
                holder.setValue(((TextNode) node).textValue());
                return holder;
                }
            else if (node instanceof IntNode)
                {
                IntegerHolderClass holder = new IntegerHolderClass();
                holder.setValue(((IntNode) node).intValue());
                return holder;
                }
            return (MyInterface) _default.deserialize(parser, context);
            }

        @Override
        public void resolve(DeserializationContext context) throws JsonMappingException
            {
//            ((ResolvableDeserializer)_default).resolve(context);
            }

        private final JsonDeserializer<?> _default;
        }


    final static Logger LOG = LoggerFactory.getLogger(MyTests.class);
    }

ClassA.java

public class ClassA implements MyInterface
    {
    public String getaValue()
        {
        return _aValue;
        }

    public void setaValue(String aValue)
        {
        _aValue = aValue;
        }

    @Override
    public boolean equals(Object obj)
        {
        return obj instanceof ClassA && _aValue.equals(((ClassA)obj)._aValue);
        }

    private String _aValue;
    }

B类.java

public class ClassB implements MyInterface
    {
    public String getbValue()
        {
        return _bValue;
        }

    public void setbValue(String bValue)
        {
        _bValue = bValue;
        }

    @Override
    public boolean equals(Object obj)
        {
        return obj instanceof ClassB && _bValue.equals(((ClassB)obj)._bValue);
        }

    private String _bValue;
    }

StringHolderClass.java

public class StringHolderClass implements MyInterface
    {
    public String getValue()
        {
        return _value;
        }

    public void setValue(String value)
        {
        _value = value;
        }

    @Override
    public boolean equals(Object obj)
        {
        return obj instanceof StringHolderClass && _value.equals(((StringHolderClass)obj)._value);
        }

    private String _value;
    }

IntegerHolderClass.java

public class IntegerHolderClass implements MyInterface
    {
    public int getValue()
        {
        return _value;
        }

    public void setValue(int value)
        {
        _value = value;
        }

    @Override
    public boolean equals(Object obj)
        {
        return obj instanceof IntegerHolderClass && _value.equals(((IntegerHolderClass)obj)._value);
        }

    private Integer _value;
    }

Answer 1

两个选项:

1. 为 MyInterface 自定义反序列化器，然后您不需要 JsonTypeInfo - 所有逻辑都将在反序列化器中。
2. 您可以尝试让 IntegerHolder 和 StringHolder 实现另一个接口(interface)(例如 Holder)，并将 JsonTypeInfo 注释更改为: @JsonTypeInfo(use = JsonTypeInfo.Id.NAME, property = "type", defaultImpl=Holder.class) 并为 Holder.class 指定一个解串器。