golang 的facebookgo/inject是不是只能注入指针类型

Question

github.com/facebookgo/inject
类似这个:

type UserDB struct {
    Db *DBEngine `inject:""`
}

我发现如果类型把指针去掉

type UserDB struct {
    Db DBEngine `inject:""`
}

变成这样的，就注入不了， 而且只能注入首字母大写的

下面的是能跑的例子

package main

import (
    "fmt"
    "github.com/facebookgo/inject"
)

type DBEngine struct {
    Name string
}

type UserDB struct {
    Db *DBEngine `inject:""`
}

type UserService struct {
    Db *UserDB `inject:""`
}

type App struct {
    Name string
    User *UserService `inject:""`
}

func (a *App) Create() string {
    return "create app, in db name:" + a.User.Db.Db.Name+" app name :"+ a.Name
}

type Object struct {
    App *App
}

func Init() *Object {
    db := DBEngine{Name: "db1"}
    var g inject.Graph
    app := App{Name: "go-app"}

    _ = g.Provide(
        &inject.Object{Value: &app},
        &inject.Object{Value: &db},
    )
    _ = g.Populate()
    return &Object{
        App: &app,
    }

}

func main() {
    obj := Init()
    fmt.Println(obj.App.Create())
}