Golang Function Call in Map - json

Question

I am a newbie at GO Programming. Here is scenario :-
There exists a JSON file that looks like this :-
{
"template": "linuxbase1",
"checkname": ["check_disk"],
"checkmethod": ["check_disk"]
}
I am Unmarshalling this data into a structure :-
package func1
import (
"io/ioutil"
"os"
"encoding/json"
"fmt"
)
type pluginfunc func() string
type Plugindata struct {
Template string `json:"template"`
Checkname []string `json:"checkname"`
Checkmethod []pluginfunc `json:"checkmethod"`
}
var (
Templatepath = "json_sample1.json"
Templateitems Plugindata
)
func Gettemplatedata() {
tdata, err := ioutil.ReadFile(Templatepath)
if err != nil {
fmt.Printf("Unable to read file %s. Error - %v\n",Templatepath, err.Error())
os.Exit(3)
}
json.Unmarshal(tdata, &Templateitems)
}
The "check_disk" function is here :-
package func1
func check_disk() string {
return "Called check_disk"
}
This is the program with main() :-
package main
import (
"fmt"
"checksexpt/func1"
)
func main() {
func1.Gettemplatedata()
fmt.Printf("Templateitems in Main() => %v\n",func1.Templateitems)
for index,funcname := range func1.Templateitems.Checkmethod {
fmt.Printf("%d = %s\n",index,funcname())
}
}
As expected, when I run main(); I see the error :-
Templateitems in Main() => {linuxbase1 [check_cpu check_disk] [<nil> <nil>]}
panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x0 pc=0x40115e]
goroutine 1 [running]:
panic(0x50e980, 0xc82000a100)
/opt/go/src/runtime/panic.go:481 +0x3e6
So, I am trying to grab a string from the JSON file and treat it as a function call. That obviously fails ! But, the primary constraint here is that the function names have to be picked from the JSON file. How can I do this ? I know that I can create a static map as follows :-
type checkfunc func() string
var (
Templateitems = map[string]map[string]checkfunc {
"linuxbase1": {
"check_disk": check_disk,
},
}
)
So, A call like - Templateitems["linuxbase1"]["check_disk"]() would work just fine. But, I dont want to create any such static map as the elements in that map needs to keep growing. Any ideas on this?

Answer 1

There is no direct way to parse a function directly from a JSON value. Also, you cannot use string values to refer to variables. So a string check_cpu would not be able to refer to the function with the same name directly.
What you can do instead is parse the json string as is and have a global map for functions. That way, you can call your functions like so:
var funcMap = map[string]pluginfunc{
"check_disk": check_disk,
"check_cpu": check_cpu
}
In your main loop:
for index, funcname := range func1.Templateitems.Checkmethod {
fmt.Printf("%d = %s\n", index, funcMap[funcname]())
}
If however, you really need to put the value in your structure, you can try implementing UnmarshalJSON from the json.Unmarshaler interface. A simple example would be:
type pf map[string]pluginfunc
type Plugindata struct {
Template string `json:"template"`
Checkname []string `json:"checkname"`
Checkmethod pf `json:"checkmethod"`
}
func (p *pf) UnmarshalJSON(data []byte) error {
d := []string{}
if err := json.Unmarshal(data, &d); err != nil {
return err
}
*p = make(pf)
for _, s := range d {
(*p)[s] = funcMap[s]
}
return nil
}
var funcMap = pf{
"check_disk": check_disk,
"check_cpu": check_cpu
}
func main() {
json.Unmarshal(tdata, &Templateitems)
for k, f := range Templateitems.Checkmethod {
fmt.Printf("%s -- %s\n", k, f())
}
}
Working code
Note that this way is not as readable or simple as the first method and it still relies on a function map.
You can read more about json.Unmarshaler here.