java - 无法构造 java.util.LinkedHashMap : no String-argument constructor/factory 的实例

Question

在解析文件并转换为 POJO 时遇到问题。我得到以下异常。

com.fasterxml.jackson.databind.JsonMappingException: Can not construct instance of java.util.LinkedHashMap: no String-argument constructor/factory method to deserialize from String value ('{\"hosturl_path\":\"/images\"}')

示例 json 文件:

{"test": [{
     "a112a": "testhost", 
     "a112b": "{\"hosturl_path\":\"/images\"}"
}]}

POJO-

import java.io.Serializable;
import java.util.Map;

import com.fasterxml.jackson.annotation.JsonAutoDetect;
import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
import com.fasterxml.jackson.annotation.JsonInclude;
import com.fasterxml.jackson.annotation.JsonProperty;

@JsonInclude(JsonInclude.Include.NON_NULL)
@JsonIgnoreProperties(ignoreUnknown = true)
@JsonAutoDetect(fieldVisibility = JsonAutoDetect.Visibility.ANY, getterVisibility = JsonAutoDetect.Visibility.ANY,
        setterVisibility = JsonAutoDetect.Visibility.ANY)
public class TestPojo implements Serializable
{
    private static final long serialVersionUID = 638312139361412L;

    @JsonProperty("a112a")
    private String host;

    @JsonProperty("a112b")
    private Map<String,String> parameterMap;

    public TestPojo()
    {
    }

    public TestPojo(String host, Map<String, String> parameterMap)
    {
        this.host = host;
        this.parameterMap = parameterMap;
    }

    public String getHost()
    {
        return host;
    }

    public void setHost(String host)
    {
        this.host = host;
    }

    public Map<String, String> getParameterMap()
    {
        return parameterMap;
    }

    public void setParameterMap(Map<String, String> parameterMap)
    {
        this.parameterMap = parameterMap;
    }
}

主类

import java.io.IOException;
import java.net.URL;
import java.util.List;
import java.util.Map;

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


public class TestApp
{
    public static void main(String[] args) throws IOException
    {
        ObjectMapper mapper = new ObjectMapper();

        String fileName = "sampleFile.json";

        URL resource = TestApp.class.getResource(fileName);

        Map<String,List<TestPojo>> map = mapper.readValue(resource, new
                TypeReference<Map<String, List<TestPojo>>>()
                {
                });

        System.out.println(map);
    }
}

Answer 1

您的异常准确地告诉您这里发生了什么。

com.fasterxml.jackson.databind.JsonMappingException: Can not construct instance of java.util.LinkedHashMap: no String-argument constructor/factory method to deserialize from String value ('{\"hosturl_path\":\"/images\"}')

在您的 TestPojo.class 中，您试图将 String 反序列化为 Map。

@JsonProperty("a112b")
private Map<String,String> parameterMap;

如果您要将其调整为您的 TestPojo.class

@JsonProperty("a112b")
private String parameterMap;

您将成功运行。但是，我认为这不是期望的行为。如果您有能力更改您的 sampleFile，Jackson 应该能够正确反序列化它。

{"test": [{
 "a112a": "testhost", 
 "a112b": {"hosturl_path":"images"}
}]}

否则。您将需要指定自定义反序列化器。

@JsonDeserialize(CustomParameterDeserializer.class)
@JsonProperty("a112b")
private Map<String,String> parameterMap;