Go Academy · Lesson

Practical Reflection: Serializers

Building a simple JSON-like serializer with reflect

Lesson 4 of 413 steps

Practical Reflection: Serializers is a free Go Academy lesson on CoddyKit. This is lesson 4 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the Go Academy learning path, and your progress syncs across the web and the CoddyKit app. The Go Academy course includes 4 lessons in total.

Custom JSON serializer overview

Building a custom serializer with reflection demonstrates practical reflection: iterate struct fields, read tags, access values, and build output — the same approach used by encoding/json internally.

Simple struct-to-map

Convert a struct to a map[string]any using reflection:

func StructToMap(v any) map[string]any {
    t := reflect.TypeOf(v)
    val := reflect.ValueOf(v)
    result := make(map[string]any, t.NumField())
    for i := 0; i < t.NumField(); i++ {
        f := t.Field(i)
        if !f.IsExported() { continue }
        key := f.Tag.Get("json")
        if key == "" { key = f.Name }
        result[key] = val.Field(i).Interface()
    }
    return result
}

Handling pointers

Dereference pointer fields before reading their values:

fv := val.Field(i)
if fv.Kind() == reflect.Ptr {
    if fv.IsNil() { continue }
    fv = fv.Elem()
}

Handling nested structs

Recursively process nested struct fields:

if fv.Kind() == reflect.Struct {
    result[key] = StructToMap(fv.Interface())
    continue
}

omitempty support

Skip zero-value fields if the tag includes "omitempty":

if strings.Contains(tag, "omitempty") && fv.IsZero() { continue }

Map-to-struct (deserializer)

Populate a struct from a map by matching map keys to field tags:

func MapToStruct(m map[string]any, dest any) {
    t := reflect.TypeOf(dest).Elem()
    v := reflect.ValueOf(dest).Elem()
    for i := 0; i < t.NumField(); i++ {
        key := t.Field(i).Tag.Get("json")
        if val, ok := m[key]; ok {
            v.Field(i).Set(reflect.ValueOf(val))
        }
    }
}

Type coercion

Map values from JSON are often float64 for numbers. Coerce them to the target field type when setting:

if f.Type.Kind() == reflect.Int && reflect.TypeOf(val).Kind() == reflect.Float64 {
    v.Field(i).SetInt(int64(val.(float64)))
}

Caching field metadata

Cache the field-to-tag mapping in a sync.Map keyed by reflect.Type to avoid repeated reflection on every serialization call.

Code generation alternative

For performance-critical paths, generate type-specific serialisers with tools like easyjson or ffjson. They avoid runtime reflection entirely.

When to build custom serializers

Custom serializers are useful for: non-JSON formats (BSON, Avro, CSV), custom encoding rules, or tight performance requirements. For most use cases, encoding/json is sufficient.

Testing the serializer

Round-trip test: serialize a struct to map, deserialize back, assert the values match the original:

u := User{Name: "Alice", Age: 30}
m := StructToMap(u)
var u2 User
MapToStruct(m, &u2)
assert.Equal(t, u, u2)

Quick Check

Why is field metadata caching important when building a reflection-based serializer?

Answer it in the interactive lesson

Recap: Reflection-Based Serializers

Key points:

Iterate fields with NumField/Field; read tags with Tag.Get
Handle pointers, nested structs, and zero values
Cache field metadata in sync.Map for performance
Use code generation for highest throughput

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Frequently Asked Questions

Is the “Practical Reflection: Serializers” lesson free?

Yes — any 3 lessons of the Go Academy learning path, including “Practical Reflection: Serializers”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The Go Academy course includes 4 lessons in total.

What will I learn in “Practical Reflection: Serializers”?

Building a simple JSON-like serializer with reflect You practise Go Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Go Academy?

No prior experience is required. Go Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 4 of 4.

How long does the “Practical Reflection: Serializers” lesson take?

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