MappingJackson2HttpMessageConverter Can not find a (Map) Key deserializer for type

Question

Here are the entity classes for my project

import java.util.Iterator;
    import java.util.Map;
    import java.util.Set;

    @Entity
    @Table(name="training")
    public class Training {

        @Id
        @GeneratedValue
        private long id;

        private String topic;


        @OneToMany(mappedBy="training")     
        private Set sessions = new HashSet();

        public Training(){

        }

        public Training(String topic, TransitionLevel level, Set sessions) {
            this.topic = topic;
            this.level = level;
            this.sessions = sessions;
        }


        public long getId() {
            return id;
        }


        public void setId(long id) {
            this.id = id;
        }


        public String getTopic() {
            return topic;
        }


        public void setTopic(String topic) {
            this.topic = topic;
        }


        public Set getSessions() {
            return sessions;
        }


        public void setSessions(Set sessions) {
            this.sessions = sessions;
        }

    }

Here is the Session Table

@Entity
        @Table(name="session")
        public class Session {

            @Id
            @GeneratedValue
            private long id;

            private String location;

            @ManyToOne  
            @JoinColumn(name="training_id") 
            @JsonIgnore
            private Training training;

            private Date start;

            private Date end;


            @JoinTable(name="session_user",
                    joinColumns = @JoinColumn(name="session_id"),
                    inverseJoinColumns = @JoinColumn(name="trainingRole_id"))
            @MapKeyJoinColumn(name="user_id")
            @ElementCollection  
            @JsonIgnore


    private Map<User, TrainingRole> users = new HashMap<User, TrainingRole>();

            public long getId() {
                return id;
            }


            public void setId(long id) {
                this.id = id;
            }


            public String getLocation() {
                return location;
            }


            public void setLocation(String location) {
                this.location = location;
            }


            public Training getTraining() {
                return training;
            }


            public void setTraining(Training training) {
                this.training = training;
            }


            public Date getStart() {
                return start;
            }


            public void setStart(Date start) {
                this.start = start;
            }


            public Date getEnd() {
                return end;
            }


            public void setEnd(Date end) {
                this.end = end;
            }


            public Map <User, TrainingRole> getUsers() {
                return users;
            }


            public void setUsers(Map<User, TrainingRole> users) {
                this.users = users;
            }

        }

Here is the User Entity

@Entity
    @Table(name="user")
    public class User {

        @Id
        @GeneratedValue(strategy=GenerationType.AUTO)
        @Column(name="id")
        private long id;

        @Column(name="csl",unique=true)
        private String csl;

        @Column(name="fullName")
        private String fullName;


        @Column(name="email")
        private String email;       

        public User() {

        }

        public long getId() {
            return id;
        }

        public void setId(long id) {
            this.id = id;
        }


        public String getEmail() {
            return email;
        }

        public void setEmail(String email) {
            this.email = email;
        }


        public String getCsl() {
            return csl;
        }


        public void setCsl(String csl) {
            this.csl = csl;
        }


        public String getFullName() {
            return fullName;
        }


        public void setFullName(String fullName) {
            this.fullName = fullName;
        }






    }

I am using JPARepository to save training and session objects in my mysql database

But Whenever I am saving the training object or session object

I am getting the error

c.j.MappingJackson2HttpMessageConverter : Failed to evaluate Jackson deserialization for type [simple type, class Session]: com.fasterxml.jackson.databind.JsonMappingException: Can not find a (Map) Key deserializer for type [simple type, class User]

I googled it and found I need to serialize and deserialize manually.. but I have no idea how to do that .. Please help me ..

Answer 1

To use your own classes as keys for maps you need a serializer and deserializer, like you indicate. Something like:

public class CustomKeyDeserializer extends KeyDeserializer {

    private static ObjectMapper mapper = new ObjectMapper();

    @Override
    public Object deserializeKey(String key, DeserializationContext deserializationContext) throws IOException, JsonProcessingException {
        return mapper.readValue(key, User.class);
    }
}

public class CustomKeySerializer extends JsonSerializer<User> {

    private static ObjectMapper mapper = new ObjectMapper();

    @Override
    public void serialize(User value, JsonGenerator gen, SerializerProvider serializers) throws IOException, JsonProcessingException {
        gen.writeFieldName(mapper.writeValueAsString(value));
    }
}

And also annotate the field

    @JsonDeserialize(keyUsing = CustomKeyDeserializer.class)
    @JsonSerialize(keyUsing = CustomKeySerializer.class)
    private Map<User, TrainingRole> users = new HashMap<User, TrainingRole>();