java - 在玩java mongodb时注入(inject)构造函数时出错

Question

我正在尝试将我的 dao 对象注入(inject) Controller 中。我已经这样做了:我有一个:
1.MongoDBHelper
2.MerchantDAO
3.商户服务
4. MerchantController

这是 MongoDBHelper 类:

import javax.inject.Singleton;

@Singleton
public class MongoDBHelper {
    private DB db;
    private Datastore datastore;
    private Configuration config = Play.application().configuration();

    private final String SERVER_URL = config.getString("server_url");

    private final String USERNAME = config.getString("database.userName");
    private final String PASSWORD = config.getString("database.password");
    private final String DATABASE_NAME = config.getString("database.name");

    public MongoDBHelper() {

        try {
            MongoClient mongoClient = new MongoClient();
            this.db = mongoClient.getDB(DATABASE_NAME);
            this.db.authenticate(USERNAME, PASSWORD.toCharArray());
            Morphia morphia = new Morphia();
            this.datastore = morphia.createDatastore(mongoClient, DATABASE_NAME);
            morphia.mapPackage("models");
        } catch (UnknownHostException e) {
            e.printStackTrace();
        }
    }

    public DB getDB() {
        return this.db;
    }

    public Datastore getDatastore() {
        return this.datastore;
    }
}

这是 MerchantDAO 类

public class MerchantDAO {

    @Inject MongoDBHelper mongoDBHelper;
    private Datastore datastore = mongoDBHelper.getDatastore();
    private DB db = mongoDBHelper.getDB();

    private static final String AUTH_TOKEN = "authToken";

    private static final Config config = ConfigFactory.load(Play.application().configuration().getString("property.file.name"));

    public void updateMerchantWithAuthToken(Merchant merchant){

        Query<Merchant> query = datastore.createQuery(Merchant.class).field(config.getString("string.email")).equal(merchant.getEmail());
        UpdateOperations<Merchant> ops = datastore.createUpdateOperations(Merchant.class).set(AUTH_TOKEN, merchant.getAuthToken()).set("lastRequestTime",merchant.getLastRequestTime());
        UpdateResults res = datastore.update(query, ops);
    }
 }
}

这是 MerchantService 类:

public class MerchantService {

    static final Config config = ConfigFactory.load(Play.application().configuration().getString("property.file.name"));

    @Inject
    MerchantDAO merchantDAO;

    // Creating unique authToken for already logged in merchant
    public String createToken(Merchant merchant) {
        merchantDAO.updateMerchantWithAuthToken(merchant);
        return authToken;
    }
}

这是 MerchantController

import javax.inject.Inject;

public class MerchantController extends Controller {

    @Inject MerchantService merchantService;

    public final static String AUTH_TOKEN_HEADER = "X-AUTH-TOKEN";
    public static final String AUTH_TOKEN = "authToken";
    public static final Config config = ConfigFactory.load(Play.application().configuration().getString("property.file.name")); 

    public static Merchant getMerchant() {
        return (Merchant)Http.Context.current().args.get("merchant");
    }

    public Result login() throws Exception {
        // code to perform login
        return ok();  // status success / failure
    }
}

我收到以下错误:

ProvisionException: Unable to provision, see the following errors:

1) Error injecting constructor, java.lang.NullPointerException
  at daos.MerchantDAO.<init>(MerchantDAO.java:22)
  while locating daos.MerchantDAO
    for field at services.MerchantService.merchantDAO(MerchantService.java:26)
  while locating services.MerchantService
    for field at controllers.MerchantController.merchantService(MerchantController.java:21)
  while locating controllers.MerchantController
    for parameter 2 at router.Routes.<init>(Routes.scala:36)
  while locating router.Routes
  while locating play.api.inject.RoutesProvider
  while locating play.api.routing.Router

1 error

我可能做错了什么？为什么DI不能正常工作？

提前致谢。

Answer 1

我认为问题出在这些行上:

private Datastore datastore = mongoDBHelper.getDatastore();
private DB db = mongoDBHelper.getDB();

这些是在对象实例的构造期间评估的。我相信在对象实例完成构造之后才会发生注入(inject)。因此，在进行上述分配时，mongoDBHelper 为 null。

解决此问题的一种方法是在 updateMerchantWithAuthToken 方法中设置数据存储和数据库。