反序列化json从Moodle Web服务与杰克逊（java）

Question

我需要解析从调用Moodle REST api获得的JSON对象。我得到的JSON响应是：

{"users":[{"id":2,"username":"admin","firstname":"Amministratore","lastname":"Utente","fullname":"Amministratore Utente","email":"[e-mail]","department":"","firstaccess":1475831148,"lastaccess":1477493148,"description":"","descriptionformat":1,"profileimageurlsmall":"http:\/\/vmoodle.local\/theme\/image.php\/clean\/core\/1475846316\/u\/f2","profileimageurl":"http:\/\/vmoodle.local\/theme\/image.php\/clean\/core\/1475846316\/u\/f1","preferences":[{"name":"auth_manual_passwordupdatetime","value":"1475831184"},{"name":"email_bounce_count","value":"1"},{"name":"email_send_count","value":"1"},{"name":"_lastloaded","value":1477561370}]}],"warnings":[]}

我想将这个json映射到POJO，所以我编写了两个类：

public class MoodleUser {

        public MoodleUser() {
        }

        private int id;
        private String username;

        /**
         * @return the id
         */
        public int getId() {
            return id;
        }

        /**
         * @param id the id to set
         */
        public void setId(int id) {
            this.id = id;
        }

        /**
         * @return the username
         */
        public String getUsername() {
            return username;
        }

        /**
         * @param username the username to set
         */
        public void setUsername(String username) {
            this.username = username;
        }

    }

    public class MoodleUsers {

        public MoodleUsers() {

        }

        private List<MoodleUser> users;

        /**
         * @return the users
         */
        public List<MoodleUser> getUsers() {
            return users;
        }

        /**
         * @param users the users to set
         */
        public void setUsers(List<MoodleUser> users) {
            this.users = users;
        }

    }

然后我打电话：

MoodleUsers response = oMapper.readValue(result, MoodleUsers.class);

我得到的是报告以下错误的异常：

com.fasterxml.jackson.databind.JsonMappingException: Can not construct instance of mypackage.lms.MoodleClientRest$MoodleUsers: no suitable constructor found, can not deserialize from Object value (missing default constructor or creator, or perhaps need to add/enable type information?)
 at [Source: {"users":[{"id":12,"username":"userok","firstname":"Nome test","lastname":"Cognome test","fullname":"Nome test Cognome test","email":"[e-mail]","department":"","idnumber":"cftest","firstaccess":0,"lastaccess":0,"description":"","descriptionformat":1,"country":"IT","profileimageurlsmall":"http:\/\/vmoodle.local\/theme\/image.php\/clean\/core\/1475846316\/u\/f2","profileimageurl":"http:\/\/vmoodle.local\/theme\/image.php\/clean\/core\/1475846316\/u\/f1"}],"warnings":[]}
; line: 1, column: 2]
    at com.fasterxml.jackson.databind.JsonMappingException.from(JsonMappingException.java:261)
    at com.fasterxml.jackson.databind.DeserializationContext.instantiationException(DeserializationContext.java:1456)
    at com.fasterxml.jackson.databind.DeserializationContext.handleMissingInstantiator(DeserializationContext.java:1012)
    at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.deserializeFromObjectUsingNonDefault(BeanDeserializerBase.java:1203)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:314)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:148)
    at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:3789)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2833)

我的pojo中已经有了默认构造函数，所以我无法理解问题所在。有什么建议吗？非常感谢。

Answer 1

expetion:mypackage中类的名称。lms。MoodleClientRest$MoodleUsers表明MoodleUsers是MoodleClientRest的一个内部类，在这种情况下，它必须是公共静态的，以便jackson成功实例化对象。