Dagger教程编译报错：main.Database.Account无法提供的解决求助

完成Dagger官方教程至Withdraw Command章节时，编译程序出现如下错误：

java: [Dagger/MissingBinding] main.Database.Account cannot be provided without an @Inject constructor or an @Provides-annotated method.
main.Database.Account is injected at
main.DepositCommand(account, …)
main.DepositCommand is injected at
main.UserCommandsModule.depositCommand(command)
java.util.Map<java.lang.String,main.Command> is injected at
main.CommandRouter(commands)
main.CommandRouter is injected at
main.CommandProcessor(firstCommandRouter)
main.CommandProcessor is requested at
main.CommandProcessorFactory.commandProcessor() It is also requested at:
main.WithdrawCommand(…, account, …) The following other entry points also depend on it:
main.UserCommandsRouter.router() [main.CommandProcessorFactory → main.UserCommandsRouter]

问题原因 #

Dagger无法在根组件CommandProcessorFactory中提供Account实例——Account是用户登录后通过UserCommandsRouter子组件绑定的上下文对象，而你错误地将包含deposit/withdraw命令的UserCommandsModule添加到了根组件中，导致根组件试图为这些命令注入Account，但根组件中并没有Account的绑定逻辑。

解决步骤 #

1. 移除根组件对UserCommandsModule的依赖
   修改CommandProcessorFactory的@Component注解，删除UserCommandsModule.class——deposit/withdraw命令属于登录后的子路由，不应出现在根组件的全局命令集合中：

   @Singleton
   @Component(modules = {
           LoginCommandModule.class,
           HelloWorldModule.class,
           SystemOutModule.class,
           UserCommandsRouter.InstallationModule.class,
           AmountsModule.class
          })
   public interface CommandProcessorFactory {
       CommandProcessor commandProcessor();
   }
   

2. 确认子组件配置正确
   你的UserCommandsRouter子组件已经正确包含UserCommandsModule，这部分无需修改，它会在用户登录时通过@BindsInstance注入当前用户的Account，为deposit/withdraw命令提供依赖：

   @Subcomponent(modules = { AmountsModule.class, UserCommandsModule.class})
   interface UserCommandsRouter {
       CommandRouter router();
   
       @Subcomponent.Factory
       interface Factory {
           UserCommandsRouter create(@BindsInstance Account account);
       }
   
       @Module(subcomponents = UserCommandsRouter.class)
       interface InstallationModule {}
   }
   

完成上述修改后，根组件的命令集合仅包含hello和login命令，deposit/withdraw命令仅在用户登录后由子组件提供，Dagger就能正确解析Account的依赖，编译错误即可解决。

内容的提问来源于stack exchange，提问作者Mohammad AttarZoghy