Dagger2 Field Injection无法正常工作

Question

我熟悉Spring依赖注入,但Dagger2对我来说是新手.我试图让一个简单的Java应用程序工作,但字段注入不起作用.

我有一个课程Main,这取决于一个课程,而这个课程MainBusiness又取决于课程MainDao.

MainBusiness被注入Main罚款,但MainDao没有被注入MainBusiness并抛出一个NullPointerException.

这是MainDao实现的interface IMainDao

public class MainDao implements IMainDao
{       
    @Inject
    public MainDao()
    {
    }

    public String getResource()
    {
        return "This is the resource you are looking for";
    }
}

这是MainBusiness实现的IMainBusiness

public class MainBusiness implements IMainBusiness
{
    // package friendly field.  
    @Inject
    IMainDao mainDao;

    @Inject
    public MainBusiness()
    {
    }

    public String getResource()
    {       
        return mainDao.getResource(); // throws NPE here.
    }

}

这里是 AppModule

@Module
public class AppModule
{   
    /**
     * Provides a singleton instance of the MainDao 
     * 
     * @return
     */
    @Provides
    @Singleton
    public IMainDao providesMainDao()
    {
        return new MainDao();
    }

    /**
     * Provides a singleton instance of the MainBusiness Class
     * 
     * @return
     */
    @Provides
    @Singleton
    public IMainBusiness providesMainBusiness()
    {
        return new MainBusiness();
    }
}

该 AppComponent

@Singleton
@Component(modules = {AppModule.class})
public interface AppComponent
{

    /**
     * @param main
     */
    void inject(Main main); 
}

和Main班级

public class Main
{   
    @Inject
    IMainBusiness mainBusiness;

    public Main()
    {
        AppComponent dac = DaggerAppComponent.builder().build();        
        dac.inject(this);       
    }

    public static void main(String args[])
    {
        Main main = new Main();     
        System.out.println(main.mainBusiness.getResource());
    }
}

它抛出一个NPE MainBusiness.getResource(MainBusiness.java:14)

我在这里错过了什么？感谢我能得到的任何帮助.

PS这是一个简单的Java应用程序.它不是一个Android应用程序.

Answer 1

字段注入将注入您传入的对象的字段.someComponent.inject(someObject)然后将注入字段someObject,但它不会传递依赖注入.因此MainBusiness仍然会有一个未初始化的DAO.

如果你打电话给new MainBusiness();自己,你不能指望Dagger照顾它.你应该使用构造函数注入,而不是new自己调用.模块用于绑定或附加设置,并且应该减少自己调用所有构造函数的样板.

我冒昧地切换到构造函数注入并完全删除了现场注入,因为您的用例不需要它.(也许有一些错字,我没有编译)

@Singleton
public class MainDao implements IMainDao
{       
    @Inject
    public MainDao()
    {
    }
}

@Singleton
public class MainBusiness implements IMainBusiness
{
    private IMainDao mainDao;

    @Inject
    public MainBusiness(IMainDao mainDao)
    {
        this.mainDao = mainDao;
    }
}

@Module
interface AppModule {

    @Binds IMainBusiness bindBusiness(MainBusiness impl);

    @Binds IMainDao bindDao(MainDao impl);

}

@Singleton
@Component(modules = {AppModule.class})
public interface AppComponent
{
    //  -> no reason not to use constructor injection here as well
    Main getMain(); // provision method
}

public class Main
{   

    private IMainBusiness mainBusiness;

    // favor constructor injection
    @Inject
    public Main(IMainBusiness mainBusiness)
    {
        this.mainBusiness = mainBusiness;
    }

    public static void main(String args[])
    {
        AppComponent component = DaggerAppComponent.builder().build()
        Main main = component.getMain();
        System.out.println(main.mainBusiness.getResource());
    }
}

我建议你再看一些指南,以确保你理解这些概念,或者你可能为自己创造了比必要更多的工作.我最近还写了一篇文章,试图澄清一些关于Dagger的误解.