Dagger 2 - 在实现类中注入接口 - “尝试在空对象引用上调用接口方法”

Question

当我尝试在实现类中注入接口时，总是出现空指针异常。这是我的代码：

获取：

LoginManagerImpl 类中的空指针错误：
@Override
公共字符串 getLoginResponse(字符串请求) {
return networkManager.getLoginResponse(request);
}

这里networkManager总是为空，你能看看代码吗。

下面是我的源代码：

public class LoginActivity extends Activity {

    @Inject
    NetworkManager networkManager;

    @Inject
    LoginManager loginManager;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        getApplicationComponent().inject(this);

        loginManager.getLoginResponse("valid Login Request");
    }
}

public interface NetworkManager
{
    String getLoginResponse(String request);
    String getUserDetailResponse(String request);
    String getABCResponse(String request);
}

public interface LoginManager
{
    String getLoginResponse(String request);
}

public class LoginManagerImpl implements LoginManager {
    @Inject
    NetworkManager networkManager;

    public LoginManagerImpl(Context context)
    {

    }

    @Override
    public String getLoginResponse(String request) {
        return networkManager.getLoginResponse(request);
    }
}

public class NetworkManagerImpl implements NetworkManager {

    public NetworkManagerImpl(Context context)
    {

    }
    @Override
    public String getLoginResponse(String request) {
        return "valid login response fetched from server";
    }

    @Override
    public String getUserDetailResponse(String request) {
        return "valid user deails";
    }

    @Override
    public String getABCResponse(String request) {
        return "valid ABC request response";
    }
}

@Module
public class AppModule {

    DaggerApplication application;

    public AppModule(DaggerApplication application) {
        this.application = application;
    }

    @Provides
    @Singleton
    DaggerApplication provideDaggerApplication() {
        return application;
    }

    @Provides
    @Singleton
    public Context provideApplicationContext()
    {
        return application.getApplicationContext();
    }

    @Provides
    @Singleton
    public LoginManager providesLoginManager(Context context)
    {
        return new LoginManagerImpl();
    }

    @Provides
    @Singleton
    public LoyaltyCardManager providesLoyaltyCardManager(Context context)
    {
        return new LoyaltyCardManagerImpl(context);
    }

    @Provides
    @Singleton
    public NetworkManager providesNetworkManager(Context context)
    {
        return new NetworkManagerImpl(context);
    }
}

@Singleton
@Component(
        modules = {
                com.dagger.component.AppModule.class
        })
public interface ApplicationComponent {
    void inject (LoginActivity activity);

    NetworkManager getNetworkManager();
    LoginManager getLoginManager();
}

Answer 1

有两种方法可以让 Dagger 将 networkManager 字段等依赖项注入 LoginManagerImpl。

1. 用@Inject注释其构造函数并将providesLoginManager(Context)更改为

   @Provides @Singleton
   LoginManager providesLoginManager(LoginManagerImpl manager) {
     return manager;
   }
   

   这意味着 Dagger 将提供LoginManagerImpl，并且您的@Provides 方法将LoginManager 绑定到它。

2. 如果你想在providesLoginManager(Context) 中自己实例化LoginManagerImpl，那么也请求一个MembersInjector<LoginManagerImpl> 并用它来注入像networkManager 这样的字段：

   @Provides @Singleton
   LoginManager providesLoginManager(Context context, MembersInjector<LoginManagerImpl> membersInjector) {
     LoginManagerImpl manager = new LoginManagerImpl(context);
     membersInjector.injectMembers(manager);
     return manager;
   }
   

对于您的示例，我推荐选项 #1。使用 MembersInjector 对于不能让 Dagger 实例化的类非常有用，例如 Android 活动。

Answer 2

字段注入不是自动的。您需要通过构造函数提供这些依赖项，或者您需要通过组件在类上调用成员注入。

public class LoginActivity extends Activity {    
    @Inject
    NetworkManager networkManager;

    @Inject
    LoginManager loginManager;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        getApplicationComponent().inject(this);

        loginManager.getLoginResponse("valid Login Request");
    }
}

public interface NetworkManager {
    String getLoginResponse(String request);
    String getUserDetailResponse(String request);
    String getABCResponse(String request);
}

public interface LoginManager {
    String getLoginResponse(String request);
}

@Singleton
public class LoginManagerImpl implements LoginManager {
    private final NetworkManager networkManager;

    @Inject
    public LoginManagerImpl(NetworkManager networkManager) {    
        this.networkManager = networkManager;
    }

    @Override
    public String getLoginResponse(String request) {
        return networkManager.getLoginResponse(request);
    }
}

@Singleton
public class NetworkManagerImpl implements NetworkManager {
    @Inject
    NetworkManagerImpl(){
    }

    @Override
    public String getLoginResponse(String request) {
        return "valid login response fetched from server";
    }

    @Override
    public String getUserDetailResponse(String request) {
        return "valid user deails";
    }

    @Override
    public String getABCResponse(String request) {
        return "valid ABC request response";
    }
}

@Module
public class AppModule {
    DaggerApplication application;

    public AppModule(DaggerApplication application) {
        this.application = application;
    }

    @Provides
    DaggerApplication provideDaggerApplication() {
        return application;
    }

    @Provides
    Context provideApplicationContext() {
        return application.getApplicationContext();
    }
}

@Module
public abstract class ManagerModule {    
    @Binds
    abstract LoginManager loginManager(LoginManagerImpl impl);

    @Binds
    abstract LoyaltyCardManager loyaltyCardManager(LoyaltyCardManagerImpl impl);

    @Binds
    abstract NetworkManager networkManager(NetworkManagerImpl impl); 
}

@Singleton
@Component(
        modules = {
                com.acme.di.component.AppModule.class,
                com.acme.di.component.ManagerModule.class
        })
public interface ApplicationComponent {
    void inject (LoginActivity activity);

    NetworkManager getNetworkManager();
    LoginManager getLoginManager();
}

Answer 3

我可以通过将 android:name="my_application_class_name" 添加到 AndroidManifest 文件中的应用程序标签来解决错误。