Dagger2 注入未发生 - 注入字段结果为空

Question

我正在努力完成我的第一个 Dagger2 项目。

我想给单例注入一个上下文class。这是一个最小的例子（我已经删除了包名、导入并且没有包括布局、值或清单文件）。理想情况下，这段代码应该选择字符串 <string name="app_name">My Application</string>.

MainApplication.java:

public class MainApplication extends Application {

    private MainComponent mComponent;

    @Override
    public void onCreate() {

        super.onCreate();

        mComponent = DaggerMainComponent.builder()
                .mainModule(new MainModule(this))
                .build();

        getComponent().inject(this);
    }

    public MainComponent getComponent() {

        return mComponent;
    }
}

MainModule.java:

@Module
public class MainModule {

    private final MainApplication mApplication;

    public MainModule(MainApplication application) {

        this.mApplication = application;
    }

    @Provides
    @Singleton
    public Context getApplicationContext() {

        return mApplication.getApplicationContext();
    }
}

MainComponent.java:

@Singleton
@Component(modules = MainModule.class)
public interface MainComponent {

    void inject(MainApplication application);
}

MainActivity.java:

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        TextView tvAppName = findViewById(R.id.main_app_name);
        tvAppName.setText(MainHelper.getInstance().getApplicationName());
    }
}

MainHelper.java:

public class MainHelper {

    private static final MainHelper INSTANCE = new MainHelper();

    @Inject
    Context mContext;

    public static MainHelper getInstance() {

        return INSTANCE;
    }

    public String getApplicationName() {

        return mContext.getString(R.string.app_name);
    }
}

实际发生的是我在应用程序本身启动时遇到此异常：

2019-11-14 18:46:47.561 9116-9116/com.example.myapplication E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.myapplication, PID: 9116
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.myapplication/com.example.myapplication.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getString(int)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2830)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2909)
        at android.app.ActivityThread.-wrap11(Unknown Source:0)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1606)
        at android.os.Handler.dispatchMessage(Handler.java:105)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6592)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:769)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getString(int)' on a null object reference
        at com.example.myapplication.MainHelper.getApplicationName(MainHelper.java:21)
        at com.example.myapplication.MainActivity.onCreate(MainActivity.java:16)
        at android.app.Activity.performCreate(Activity.java:6984)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1235)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2783)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2909) 
        at android.app.ActivityThread.-wrap11(Unknown Source:0) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1606) 
        at android.os.Handler.dispatchMessage(Handler.java:105) 
        at android.os.Looper.loop(Looper.java:164) 
        at android.app.ActivityThread.main(ActivityThread.java:6592) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:769)

基本上，MainHelper 中的 mContext 对象为 null，这对我来说意味着注入根本不起作用。出了什么问题？这段代码似乎遵循了几乎所有的教程。

Answer 1

您没有将 Context 注入您的 MainHelper。我建议你重构你的 MainHelper class 以通过构造函数注入获取上下文（你应该为你拥有的所有 class 执行此操作）：

@Singleton
public class MainHelper {

   private final Context context;

   @Inject
   public MainHelper(Context context) {
      this.context = context;
   }

}

构造函数上的 @Inject 注释应该足以让 Dagger 知道 instantiate/create MainHelper 为您服务。然后为了将此 MainHelper class 注入到您的 MainActivity，您首先需要告诉 Dagger 您需要将依赖项注入到您的 MainActivity。您已经为 MainApplication class:

做了类似的事情

@Singleton
@Component(modules = MainModule.class)
public interface MainComponent {
    void inject(MainApplication application);
    void inject(MainActivity activity);
}

然后在你的 MainActivity 上，你通过调用 Dagger 为你生成的 inject 方法注入 MainHelper：

public class MainActivity extends AppCompatActivity {

     @Inject
     MainHelper helper;

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

        TextView tvAppName = findViewById(R.id.main_app_name);
        tvAppName.setText(helper.getApplicationName());
    }
}

Dagger2 注入未发生 - 注入字段结果为空

Dagger2 injection not happening - injected field is turning out to be null

android

dependency-injection

dagger-2