作者:mobiledu2502856013 | 来源:互联网 | 2023-01-09 12:16
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.
1> David Medenj..:
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.