JSON - 嵌套映射的自定义键序列化答案

【问题标题】：JSon - custom key serialization of nested mapsJSON - 嵌套映射的自定义键序列化
【发布时间】：2017-10-19 18:14:57
【问题描述】：

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

Jackson Modules for Map Serialization

给出以下结果。自定义序列化器：

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;

这会正确序列化Map<StructureNode, String>，但应用于嵌套的Map<StructureNode, Map<String, String>>，会出现以下错误：

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

Jackson 似乎对“子地图”使用了相同的自定义序列化方法。有没有解决这个问题的好方法，而不用另一个自定义（非Map）对象替换“子图”？

【问题讨论】：

你不应该在你的序列化器和反序列化器中使用私有的ObjectMapper。
@teppic 你为什么这么说？我正在按照我提到的另一篇文章中的示例进行操作，但很高兴对其进行改进。但无论哪种方式，这都不能解决我遇到的异常。
因为它与客户端代码正在使用（并且可能已检测）的代码是分开的。

标签： java json serialization jackson

【解决方案1】：

你可以用

解决这个问题

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

并以不同的方式实现您的序列化程序：

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();
    }
}

或者如果你想保留keyUsing = StructureNodeKeySerializer.class

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

你可以像这样实现它：

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);
        }
    }
}

【讨论】：

【解决方案2】：

如果你想更通用地序列化为keySerializer，你可以重写else子句如下

if (value instanceof StructureNode) {
  // ...
} else {
  serializers
    .findKeySerializer(value.class, null)
    .serialize(value, gen, serializers);
}

【讨论】：