无法从字符串值 ('{') 中实例化 java.util.LinkedHashMap 类型的值；没有单字符串 constructor/factory 方法

Question

我有以下 2 个类:

@JsonIgnoreProperties(ignoreUnknown = true)
public class ChangesJSON {

    @JsonProperty("changes")
    List<ChangeJSON> changes;

    @JsonProperty("more")
    Boolean more;
}

public class ChangeJSON {

    @JsonProperty("epoch")
    Long epoch;

    @JsonProperty("payload")
    Map<String, Object> payload;
}

当我尝试使用此测试反序列化时：

String test = "{\"changes\":[{\"epoch\":1441556306522,\"payload\":\"{\"to\":1}\"},{\"epoch\":1441555481524,\"payload\":\"{\"to\":-1}\"}],\"more\":false}";

@Test
public void myTest() {
    ObjectMapper mapper = new ObjectMapper();
    ChangesJSON result = null;
    try {
        result = mapper.readValue(test, ChangesJSON.class);
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
    assertNotNull(result);
}

我得到以下异常：

com.fasterxml.jackson.databind.JsonMappingException: Can not instantiate value of type java.util.LinkedHashMap from String value ('{'); no single-String constructor/factory method at [Source: {"changes":[{"epoch":1441556306522,"payload":"{"to":1}"},{"epoch":1441555481524,"payload":"{"to":-1}"}],"more":false}; line: 1, column: 35] (through reference chain: demo.ChangesJSON["changes"]->java.util.ArrayList[0]->demo.ChangeJSON["payload"])

地图似乎有问题，但我认为 Jackson 应该能够处理地图。当我将地图更改为地图时，我也会遇到同样的问题。但我确实需要支持各种类作为地图的值。

Answer 1

您在 payload 对象周围加了引号。尝试更改此部分：

\"payload\":\"{\"to\":1}\"

进入这个：

\"payload\":{\"to\":1}

Answer 2

我认为是 JSON 本身有问题。它转义为：

{"changes":[{"epoch":1441556306522,"payload":"{"to":1}"},{"epoch":1441555481524,"payload":"{"to":-1}"}],"more":false}

大概应该是这样的：

{"changes":[{"epoch":1441556306522,"payload":{"to":1}},{"epoch":1441555481524,"payload":{"to":-1}}],"more":false}

所以：

String test = "{\"changes\":[{\"epoch\":1441556306522,\"payload\":{\"to\":1}},{\"epoch\":1441555481524,\"payload\":{\"to\":-1}}],\"more\":false}";

无法从字符串值 ('{') 中实例化 java.util.LinkedHashMap 类型的值；没有单字符串 constructor/factory 方法

Cannot instantiate value of type java.util.LinkedHashMap from String value ('{'); no single-String constructor/factory method

java

json

jackson

fasterxml