java - 在 Java Spring 中将 JSON 绑定(bind)到 Arraylist？

Question

我希望能够从 POST 请求传递数据并访问我的 API，其中我传递的 JSON 绑定(bind)到 POTD 类。在 POTD 中，我有一个 POTDResources 类型的 ArrayList，这是我在绑定(bind)数据时遇到问题的地方。

我最终只是打算使用整个 Problem 对象，但现在我只是测试类中的两个字段。

我需要 JSON 输入能够绑定(bind)到 POTDResources 类型的 ArrayList

我正在传递如下数据:

{
    "problemTitle": "Balancing Binary Tree",
    "resources": [{"name": "youtube", "link": "http://yotube.com"}]
}

API 调用首次命中:

Controller

@RequestMapping(value = "/generatepotd", method = RequestMethod.POST)
@ResponseBody
public void generatePOTD(@RequestBody POTD problem) throws IOException {

    POTD prob = new POTD();
    prob.setProblemTitle(problem.getProblemTitle());
    prob.setResources(problem.getResources());
    tempGen.generateProblemOfTheDay(prob);
}

POTD 类(class)

    package com.algoq.algoq.models;

    import org.springframework.context.annotation.Description;
    import java.util.ArrayList;

    @Description("Handles the fields required for processing problem of the day")
    public class POTD {
        private String subject;
        private String problemTitle;
        private String problemDescription;
        private ArrayList<POTDResources> resources;

    //    POTD(String subject, String problemTitle, String problemDescription, ArrayList<POTDResources> resources) {
        POTD(String problemTitle, ArrayList<POTDResources> resources) {
    //        this.subject = subject;
            this.problemTitle = problemTitle;
    //        this.problemDescription = problemDescription;
            this.resources = resources;
        }

        public POTD() {

        }

        public String getSubject() {
            return subject;
        }

        public void setSubject(String subject) {
            this.subject = subject;
        }
        public String getProblemTitle() {
            return problemTitle;
        }

        public void setProblemTitle(String problemTitle) {
            this.problemTitle = problemTitle;
        }

        public String getProblemDescription() {
            return problemDescription;
        }

        public void setProblemDescription(String problemDescription) {
            this.problemDescription = problemDescription;
        }

        public ArrayList<POTDResources> getResources() {
            return resources;
        }

        public void setResources(ArrayList<POTDResources> resources) {
            this.resources = resources;
        }
    }

POTD资源类

    package com.algoq.algoq.models;

    public class POTDResources {
        private String name;
        private String link;

        public POTDResources(String name, String link) {
            this.name = name;
            this.link = link;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public String getLink() {
            return link;
        }

        public void setLink(String link) {
            this.link = link;
        }
    }

错误信息

{
    "timestamp": 1513192593064,
    "status": 400,
    "error": "Bad Request",
    "exception": "org.springframework.http.converter.HttpMessageNotReadableException",
    "message": "JSON parse error: Can not construct instance of com.algoq.algoq.models.POTDResources: 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 com.algoq.algoq.models.POTDResources: 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@1710624f; line: 3, column: 17] (through reference chain: com.algoq.algoq.models.POTD[\"resources\"]->java.util.ArrayList[0])",
    "path": "/generatepotd"
}

Answer 1

您的问题在于错误消息:

no suitable constructor found

您正在覆盖默认构造函数，并且 ObjectMapper 无法创建您的 Model 类的实例。

尝试为 POTDResources 添加默认构造函数:

    public POTDResources() {

    }

Spring 用于 JSON 和 XML 处理的 Jackson 可以通过 2 种方式与 Object 一起工作。 基于构造函数和 Setter。 如果它能找到具有相同字段名称和类型的构造函数，它将尝试使用该构造函数。如果没有合适的构造函数，那么它将尝试创建对象的实例，并使用setters。您没有合适的构造函数，它无法创建实例。