java - com.fasterxml.jackson.databind.exc.InvalidDefinitionException : No serializer found

Question

出现错误，

com.fasterxml.jackson.databind.exc.InvalidDefinitionException: No serializer found for class com.app.server.data.MyUser and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS) (through reference chain: java.util.ArrayList[0])
    at com.fasterxml.jackson.databind.exc.InvalidDefinitionException.from(InvalidDefinitionException.java:77) ~[jackson-databind-2.10.2.jar:2.10.2]
    at com.fasterxml.jackson.databind.SerializerProvider.reportBadDefinition(SerializerProvider.java:1191) ~[jackson-databind-2.10.2.jar:2.10.2]
    at com.fasterxml.jackson.databind.DatabindContext.reportBadDefinition(DatabindContext.java:404) ~[jackson-databind-2.10.2.jar:2.10.2]
    at com.fasterxml.jackson.databind.ser.impl.UnknownSerializer.failForEmpty(UnknownSerializer.java:71) ~[jackson-databind-2.10.2.jar:2.10.2]
    at com.fasterxml.jackson.databind.ser.impl.UnknownSerializer.serialize(UnknownSerializer.java:33) ~[jackson-databind-2.10.2.jar:2.10.2]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serializeContents(CollectionSerializer.java:145) ~[jackson-databind-2.10.2.jar:2.10.2]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serialize(CollectionSerializer.java:107) ~[jackson-databind-2.10.2.jar:2.10.2]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serialize(CollectionSerializer.java:25) ~[jackson-databind-2.10.2.jar:2.10.2]
    at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider._serialize(DefaultSerializerProvider.java:480) ~[jackson-databind-2.10.2.jar:2.10.2]
    at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:400) ~[jackson-databind-2.10.2.jar:2.10.2]
    at com.fasterxml.jackson.databind.ObjectWriter$Prefetch.serialize(ObjectWriter.java:1429) ~[jackson-databind-2.10.2.jar:2.10.2]
    at com.fasterxml.jackson.databind.ObjectWriter.writeValue(ObjectWriter.java:921) ~[jackson-databind-2.10.2.jar:2.10.2]

当从 Spring boot Controller 调用以下函数时，

import java.util.List;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.jdbc.core.JdbcTemplate;
import org.springframework.stereotype.Repository;

@Repository
public class JdbcUserRepository implements UserRepository {

    @Autowired
    private JdbcTemplate jdbcTemplate;

    @Override
    public List<MyUser> findAll() {

        return jdbcTemplate.query( "select * from users", (rs, rowNum) ->
            new MyUser(
                    rs.getLong("user_id"),
                    rs.getString("user_nid"),
                    rs.getString("user_name"),
                    rs.getString("user_duid")
            )
        );
    }
}

型号:

public class MyUser {

    Long userId;
    String userNid;
    String userName;
    String userDuid;

    public MyUser(Long userId, String userNid, String userName, String userDuid) {
        super();
        this.userId = userId;
        this.userNid = userNid;
        this.userName = userName;
        this.userDuid = userDuid;
    }
}

浏览器出现错误，

There was an unexpected error (type=Internal Server Error, status=500).
Type definition error: [simple type, class com.app.server.data.MyUser]; nested exception is com.fasterxml.jackson.databind.exc.InvalidDefinitionException: No serializer found for class com.app.server.data.MyUser and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS) (through reference chain: java.util.ArrayList[0])

如何解决？

Answer 1

您需要添加 Jackson 可以访问对象状态的 getter 和 setter。

public class MyUser {

    Long userId;
    String userNid;
    String userName;
    String userDuid;

    public MyUser(Long userId, String userNid, String userName, String userDuid) {
        super();
        this.userId = userId;
        this.userNid = userNid;
        this.userName = userName;
        this.userDuid = userDuid;
    }

    // public getters and setters
}