I'm a newbie on Golang, and I'm trying to use [Gin](https://github.com/gin-gonic/gin) to develop a web server on Ubuntu 16.04.

After executing `go get -u github.com/gin-gonic/gin`, many folders appear at `~/go/pkg/mod/github.com/`.

Then I try to make an example:

```
package main

import "github.com/gin-gonic/gin"

func main() {
    r := gin.Default()
    r.GET("/ping", func(c *gin.Context) {
        c.JSON(200, gin.H{
            "message": "pong",
        })
    })
    r.Run() // listen and serve on 0.0.0.0:8080 (for windows "localhost:8080")
}

```

However, `go run example.go` made the error:

```
example.go:3:8: cannot find package "github.com/gin-gonic/gin" in any of:
        /usr/local/go/src/github.com/gin-gonic/gin (from $GOROOT)
        /home/zyh/go/src/github.com/gin-gonic/gin (from $GOPATH)

```

In my system, `$GOROOT` is `/usr/local/go/` and `$GOPATH` is `~/go/`.

How could I solve this problem?