无法在Spring中初始化bean类

Question

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类如下所示

@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");

    }

但我得到的错误是..

原因:org . spring framework . beans . factory . beancreationexception:创建在文件[C:\ Users \ user \ Desktop \ ITAU \ 77 \ ABC-iCOE-WAL \ ItauBankingTransac s \ rest-client \ target \ classes \ com \ ABC \ WAL \ itaudemo \ mongo \ Dao \ mongoDAO . class]中定义的名为“mongoDAO”的bean时出错:bean的实例化失败；嵌套异常为org . spring framework . beans . bean instantiation异常:无法实例化bean类[com。ABC . wal . itaudemo . mongo . Dao . mongodao]:构造函数抛出异常；嵌套异常是org . spring framework . beans . factory . support . abstractautowirecapablebeanfactory . instantiate bean中的Java . lang . nullpointerexception(abstractautowirecapablebeanfactory . Java:1076)

Answer 1

只需使用@Service（“蒙古道”），无需@Component

@Service("mongoDAO")
public class MongoDAO {

您可以使用这两种注释中的任何一种，为了可读性，在服务层代码中使用了@Service。@Component更像是一个通用的注释。

Answer 2

userDatabase字段在创建MongoDaaobean后自动装配。如果您想在构造操作后执行一些操作，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");

    }

可以省略构造函数。通常，最好不要为组件类使用自定义构造函数。