Go Academy · Lesson

Practical Reflection: Serializers

Building a simple JSON-like serializer with reflect

Practical Reflection: Serializers is a free Go Academy lesson on CoddyKit — lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Go Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

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?

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

Frequently asked questions

Is the “Practical Reflection: Serializers” lesson free?

Yes — the full text of “Practical Reflection: Serializers” is free to read here on the web, and the Go Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Go Academy course, upgrade to CoddyKit PRO.

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; this is — lesson 4 of 4, so you can start here or from the beginning and move at your own pace.

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

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Go Academy lesson?

Yes. Every Go Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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