java - 如何使用 jackson (com.fasterxml.jackson) 为相同的 key 使用不一致的 json 结构？

Question

在下面的 JSON 结构中，fileMetaData 参数有不同的类型。 Jackson 抛出异常 com.fasterxml.jackson.databind.JsonMappingException: Can not deserialize instance of com.XXX.XXX.XXX.FILENAME out of START_ARRAY token 。

示例 JSON 负载:

[
    {
        "fileName": "file_name_1",
        "fileMetaData": {
          "abcd": "valueabcd",
          "xyz": "valuexyz"
        }
    },
    {
        "fileName": "file_name_2",
        "fileMetaData": [
            {
                "123": "value123",
                "456": "value456"
            },
            {
                "123": "value123-1",
                "456": "value456-1"
            }
        ]
    },
    {
        "fileName": "file_name_3",
        "fileMetaData": {
            "key1": {
                "key11": "val11",
                "key12": "val22"
            },
            "key2": "val2"
        }
    },
    {
        "fileName": "abc.xyz",
        "fileMetaData": null
    }
]

如何反序列化？

Answer 1

一般Jackson将JSON Array转换为List，将JSON Object转换为Map .在这种情况下，我们可以使用一般的 Object 类型，它将被正确设置。我们只需要检查给定情况下的类型并正确转换为正确的 Java 类型。请参见以下示例:

import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.ObjectMapper;

import java.io.File;
import java.util.List;
import java.util.Map;

public class JsonApp {

    public static void main(String[] args) throws Exception {
        File jsonFile = new File("./resource/test.json").getAbsoluteFile();

        ObjectMapper mapper = new ObjectMapper();
        TypeReference<List<Item>> typeReference = new TypeReference<List<Item>>() {
        };
        List<Item> items = mapper.readValue(jsonFile, typeReference);

        items.forEach(item -> {
            if (item.isArray()) {
                System.out.println("List => " + item.getFileMetaDataAsList());
            } else if (item.isObject()) {
                System.out.println("Map => " + item.getFileMetaDataAsMap());
            }
        });

    }
}

class Item {

    private String fileName;
    private Object fileMetaData;

    public String getFileName() {
        return fileName;
    }

    public void setFileName(String fileName) {
        this.fileName = fileName;
    }

    public Object getFileMetaData() {
        return fileMetaData;
    }

    public void setFileMetaData(Object fileMetaData) {
        this.fileMetaData = fileMetaData;
    }

    public boolean isObject() {
        return this.fileMetaData instanceof Map;
    }

    public boolean isArray() {
        return this.fileMetaData instanceof List;
    }

    public Map<String, Object> getFileMetaDataAsMap() {
        return (Map<String, Object>) this.fileMetaData;
    }

    public List<Map<String, Object>> getFileMetaDataAsList() {
        return (List<Map<String, Object>>) this.fileMetaData;
    }

    @Override
    public String toString() {
        return "Item{" +
                "fileName='" + fileName + '\'' +
                ", fileMetaData=" + fileMetaData +
                '}';
    }
}

对于您的 JSON 负载打印:

Map => {abcd=valueabcd, xyz=valuexyz}
List => [{123=value123, 456=value456}, {123=value123-1, 456=value456-1}]
Map => {key1={key11=val11, key12=val22}, key2=val2}