What input will cause golang's json.Marshal to return an error?
Solution 1
Just to complement Jonathan's answer, the json.Marshal function can return two types of errors: UnsupportedTypeError
or UnsupportedValueError
The first one can be caused, as Jonathan said by trying to Marshal an invalid type:
_, err := json.Marshal(make(chan int))
_, ok := err.(*json.UnsupportedTypeError) // ok == true
On the other hand you can also have the Marshal function return an error by passing an invalid value:
_, err := json.Marshal(math.Inf(1))
_, ok := err.(*json.UnsupportedValueError) // ok == true
Solution 2
Update: now using a channel instead of a map[int]int to elicit the error
Go-specific structures,e.g. func
or chan
refuse to serialize:
package main
import (
"encoding/json"
"fmt"
)
func main() {
value := make(chan int)
_, err := json.Marshal(value)
fmt.Println(err)
}
Solution 3
Read the source code you can found such a function to judge a encoder if not exist will return marshal error: https://github.com/golang/go/blob/master/src/encoding/json/encode.go
func newTypeEncoder(t reflect.Type, allowAddr bool) encoderFunc {
// ignored
switch t.Kind() {
case reflect.Bool:
return boolEncoder
case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
return intEncoder
case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
return uintEncoder
case reflect.Float32:
return float32Encoder
case reflect.Float64:
return float64Encoder
case reflect.String:
return stringEncoder
case reflect.Interface:
return interfaceEncoder
case reflect.Struct:
return newStructEncoder(t)
case reflect.Map:
return newMapEncoder(t)
case reflect.Slice:
return newSliceEncoder(t)
case reflect.Array:
return newArrayEncoder(t)
case reflect.Ptr:
return newPtrEncoder(t)
default:
return unsupportedTypeEncoder
}
}
We can find all kinds enum at https://github.com/golang/go/blob/master/src/reflect/type.go
So it's not hard to see that kinds not in above function are unable to marshal:
UnsafePointer,Complex64,Complex128,Chan,Func
Examples:
json.Marshal(unsafe.Pointer(nil)) // UnsafePointer
json.Marshal(complex64(1)) // Complex64
json.Marshal(complex128(1)) // Complex128
json.Marshal(make(chan struct{})) // Chan
json.Marshal(func() {}) // Func
mdwhatcott
I'm a software developer at Clean Coders Studio (https://cleancoders.com/studio).
Updated on January 17, 2021Comments
-
mdwhatcott over 3 years
From the docs:
JSON cannot represent cyclic data structures and Marshal does not handle them. Passing cyclic structures to Marshal will result in an infinite recursion.
I've experienced this situation, which results in a runtime panic.
What I'm wondering is if anyone can provide a working program that demonstrates a non-panic situation where json.Marshal returns a non-nil error. The best answers would clearly include the inputs used.