JSon-嵌套地图的自定义键序列化 [英] JSon - custom key serialization of nested maps

问题描述

我有一个嵌套的Map<StructureNode, Map<String, String>>，为此我需要一个自定义键序列化器&反序列化器(StructureNode包含对其他对象的引用，这些对象需要用作此映射的键).我为此使用了以下方法:

I have a nested Map<StructureNode, Map<String, String>> for which I need a custom key serializer & deserializer (StructureNode contains references to other objects which are needed to function as key for this map). I used the following method for this:

用于地图序列化的杰克逊模块

给出以下结果.自定义序列化器:

Giving the following result. Custom Serializer:

public class StructureNodeKeySerializer extends JsonSerializer<StructureNode> {

    private static final ObjectMapper mapper = new ObjectMapper();

    @Override
    public void serialize(StructureNode value, JsonGenerator gen, SerializerProvider serializers) throws IOException {
        StringWriter writer = new StringWriter();
        mapper.writeValue(writer, value.copyUpwards());
        gen.writeFieldName(writer.toString());
    }
}

自定义反序列化器:

public class StructureNodeKeyDeserializer extends KeyDeserializer  {

    private static final ObjectMapper mapper = new ObjectMapper();

    @Override
    public Object deserializeKey(String key, DeserializationContext ctxt) throws IOException {
        return mapper.readValue(key, StructureNode.class);
    }
}

用法:

@JsonDeserialize(keyUsing = StructureNodeKeyDeserializer.class) @JsonSerialize(keyUsing = StructureNodeKeySerializer.class)
private Map<StructureNode, String> structureIds;
@JsonDeserialize(keyUsing = StructureNodeKeyDeserializer.class) @JsonSerialize(keyUsing = StructureNodeKeySerializer.class)
private Map<StructureNode, Map<String, String>> metadata;

这正确地序列化了Map<StructureNode, String>，但是将其应用于嵌套的Map<StructureNode, Map<String, String>>，则会出现以下错误:

This correctly serializes a Map<StructureNode, String>, but applied to a nested Map<StructureNode, Map<String, String>>, it gives the following error:

Exception in thread "main" com.fasterxml.jackson.databind.JsonMappingException: java.lang.String cannot be cast to structure.StructureNode

杰克逊似乎对子图"使用了相同的自定义序列化方法.是否有一种解决此问题的好方法，而无需用另一个自定义对象(非Map)替换子图"?

Jackson seems to be using the same custom serialization method for the "sub-map". Is there a good way to solve this problem, without replacing the "sub-map" with another custom (non-Map) object?

推荐答案

您可以使用

public static class Bean{
    @JsonSerialize(using = MapStructureNodeKeySerializer.class)
    public Map<StructureNode, Map<String, String>> metadata;
}

与实现序列化器的方式略有不同:

And implement your serializer a little bit differently:

public static class MapStructureNodeKeySerializer 
        extends JsonSerializer<Map<StructureNode, Object>> {

    private static final ObjectMapper mapper = new ObjectMapper();

    @Override
    public void serialize(Map<StructureNode, Object> value, JsonGenerator gen, 
                          SerializerProvider serializers) throws IOException {
        gen.writeStartObject();

        for(Map.Entry<StructureNode, Object> val: value.entrySet()){
            // your custom serialization code here
            StringWriter writer = new StringWriter();
            mapper.writeValue(writer, val.getKey().copyUpwards());

            gen.writeObjectField(writer.toString(), val.getValue());
        }

        gen.writeEndObject();
    }
}

---

或者如果您想保留keyUsing = StructureNodeKeySerializer.class

public static class Bean{
    @JsonSerialize(keyUsing = StructureNodeKeySerializer.class)
    public Map<StructureNode, Map<String, String>> metadata;
}

您可以像这样实现它:

public static class StructureNodeKeySerializer extends JsonSerializer {

    private static final ObjectMapper mapper = new ObjectMapper();

    @Override
    public void serialize(Object value, JsonGenerator gen,
                          SerializerProvider serializers) throws IOException {

        if (value instanceof StructureNode){ // <= type of 1-st level Map key
            // your custom serialization code here
            StringWriter writer = new StringWriter();
            mapper.writeValue(writer, ((StructureNode)value).copyUpwards());
            gen.writeFieldName(writer.toString());
        }else if(value instanceof String){   // <= type of 2-nd level Map key
            gen.writeFieldName((String) value);
        }
    }
}

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