JSON解析错误:无法构造io.starter.topic.Topic的实例

问题描述

我正在学习Spring Boot，我做了一个演示，但是当我发布添加对象的请求时，它不起作用！

Im learning Spring Boot and I made a demo but when I POST a request to add a Object it didn't work!

错误消息是:

{
    "timestamp": 1516897619316,
    "status": 400,
    "error": "Bad Request",
    "exception": "org.springframework.http.converter.HttpMessageNotReadableException",
    "message": "JSON parse error: Can not construct instance of io.starter.topic.Topic: no suitable constructor found, can not deserialize from Object value (missing default constructor or creator, or perhaps need to add/enable type information?); nested exception is com.fasterxml.jackson.databind.JsonMappingException: Can not construct instance of io.starter.topic.Topic: no suitable constructor found, can not deserialize from Object value (missing default constructor or creator, or perhaps need to add/enable type information?)\n at [Source: java.io.PushbackInputStream@1ff3f09a; line: 2, column: 9]",
    "path": "/topics/"
}

我的实体:

public class Topic {
    private String id;
    private String name;
    private String author;
    private String desc;

    public Topic(String id, String name, String author, String desc) {

        this.id = id;
        this.name = name;
        this.author = author;
        this.desc = desc;
    }
    //getters and setters

我的控制器:

public class TopicController {

    @Autowired
    private TopicService topicService;


    @RequestMapping(value = "/topics", method = RequestMethod.POST)
    public void addTopic(@RequestBody Topic topic) {
        topicService.addTopic(topic);
    }

我的服务:

@Service
public class TopicService {
    private List<Topic> topics = new ArrayList<>(Arrays.asList(
            new Topic("1", "topic1", "Martin", "T1"),
            new Topic("2", "topic2", "Jessie", "T2")  
            ));



    public void addTopic(Topic topic) {

        topics.add(topic);
    }

}

我的json:

{
    "id": "3",
    "name": "topic3",
    "author": "Jessie3",
    "desc": "T3"
}

请帮助！

推荐答案

出于反序列化目的， Topic 必须具有零参数构造函数.

For deserialisation purposes Topic must have a zero-arg constructor.

例如:

public class Topic {
    private String id;
    private String name;
    private String author;
    private String desc;

    // for deserialisation
    public Topic() {}    

    public Topic(String id, String name, String author, String desc) {    
        this.id = id;
        this.name = name;
        this.author = author;
        this.desc = desc;
    }

    // getters and setters

}     

这是 Jackson 库的默认行为.

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