java - 无法在Spring中初始化bean类

我正在尝试使用spring开发rest代码。

@RestController
public class MongoController {

    @Autowired
    private MongoDAO mongoDAO ;

    @RequestMapping(value = "/mongotest", method = RequestMethod.GET)
    public String creatTest() {

        return " YO Mongo";
    }


}

我的MongoDAO课程如下

@Service("mongoDAO")
@Component
public class MongoDAO {


    @Resource
    private DBCollection user ;
    @Resource
    private MongoDatabase userDatabase;
    @Resource
    private MongoCollection<Document> usersCollection;
    @Resource
    private MongoClient mongoClient;

/*  public MongoDAO(final MongoDatabase userDatabase) {
        usersCollection = userDatabase.getCollection("user");
        MongoClient mongoClient = new MongoClient("localhost", 27017);
        DB db = mongoClient.getDB("ITAU");
        user = db.getCollection("user");
    }*/

    public MongoDAO(MongoDatabase userDatabase1) {
        usersCollection = userDatabase1.getCollection("user");

    }

    public MongoDAO() {
        usersCollection = userDatabase.getCollection("user");

    }

但是我得到的错误是..


  引起原因：org.springframework.beans.factory.BeanCreationException：
  在文件中定义名称为“ mongoDAO”的bean时出错
  [C：\ Users \ user \ Desktop \ ITAU \ 77 \ ABC-iCOE-WAL \ ItauBankingTransac
  s \ rest-client \ target \ classes \ com \ ABC \ WAL \ itaudemo \ mongo \ dao \ MongoDAO.class]：实例化Bean失败；嵌套异常为
  org.springframework.beans.BeanInstantiationException：是否可以
  实例化bean类[com.ABC.WAL.itaudemo.mongo.dao.MongoDAO]：
  构造函数抛出异常；嵌套异常为
  java.lang.NullPointerException
          在org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateBean（AbstractAutowireCapableBeanFactory.java:1076）

最佳答案

创建userDatabase bean之后，将自动连接MongoDao字段。如果要在构造动作后执行一些操作，请尝试以下操作：

@Service("mongoDAO")
public class MongoDAO {


    @Resource
    private DBCollection user ;
    @Resource
    private MongoDatabase userDatabase;
    @Resource
    private MongoCollection<Document> usersCollection;
    @Resource
    private MongoClient mongoClient;

    public MongoDAO() {

    }

    @PostConstruct
    public postConstruct() {
        usersCollection = userDatabase.getCollection("user");

    }

构造函数可以省略。通常，最好不要为组件类提供自定义构造函数。