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Tutorial: Working with JSON

JSON (JavaScript Object Notation) is a simple data interchange format. Syntactically it resembles the objects and lists of JavaScript. It is commonly used for communication with networked API services, but it is used in many other places, too. Its home page, json.org, provides a wonderfully clear and concise definition of the standard.

With the encoding/json/v2 package it’s a snap to read and write JSON data from your Go programs. This package provides a cleaner API and better defaults than the older encoding/json package.

Encoding

To encode JSON data we use the Marshal function.

func Marshal(in any, opts ...Options) (out []byte, err error)

Given the Go data structure, Message,

type Message struct {
    Name string
    Body string
    Time time.Time
}

and an instance of Message

m := Message{"Alice", "Hello", time.Date(2011, 1, 25, 0, 0, 0, 0, time.UTC)}

we can marshal a JSON-encoded version of m using json.Marshal:

b, err := json.Marshal(m)

If all is well, err will be nil and b will be a []byte containing this JSON data:

b == []byte(`{"Name":"Alice","Body":"Hello","Time":"2011-01-25T00:00:00Z"}`)

Only data structures that can be represented as valid JSON will be encoded:

  • Pointers will be encoded as the values they point to (or null if the pointer is nil).

  • JSON objects only support strings as keys. Thus, to encode a Go map, it must have a key that encodes as a JSON string, such as map[string]T (where T is any Go type supported by the json package).

  • Channel, complex, and function types cannot be encoded.

  • Cyclic data structures are not supported.

  • Marshal documents the full set of encoding semantics.

The json package only accesses the exported fields of struct types (those that begin with an uppercase letter). Therefore only the exported fields of a struct will be present in the JSON output.

Decoding

To decode JSON data we use the Unmarshal function.

func Unmarshal(in []byte, out any, opts ...Options) (err error)

We must first create a place where the decoded data will be stored

var m Message

and call json.Unmarshal, passing it a []byte of JSON data and a pointer to m

err := json.Unmarshal(b, &m)

If b contains valid JSON that fits in m, after the call err will be nil and the data from b will have been stored in the struct m, as if by an assignment like:

m = Message{
    Name: "Alice",
    Body: "Hello",
    Time: time.Date(2011, 1, 25, 0, 0, 0, 0, time.UTC),
}

How does Unmarshal identify the fields in which to store the decoded data? For a given JSON key "Foo", Unmarshal will look through the destination struct’s fields to find (in order of preference):

  • An exported field with a tag of json:"Foo" (see the Go spec for more on struct tags), or

  • An exported field named Foo.

What happens when the structure of the JSON data doesn’t exactly match the Go type?

b := []byte(`{"Name":"Bob","Food":"Pickle"}`)
var m Message
err := json.Unmarshal(b, &m)

Unmarshal will decode only the fields that it can find in the destination type. In this case, only the Name field of m will be populated, and the Food field will be ignored. This behavior is particularly useful when you wish to pick only a few specific fields out of a large JSON blob. It also means that any unexported fields in the destination struct will be unaffected by Unmarshal.

But what if you don’t know the structure of your JSON data beforehand?

Generic JSON with any

The any type describes any Go type. any is defined as interface{} (empty interface), which describes an interface with zero methods. Every Go type implements at least zero methods and therefore satisfies the empty interface.

any serves as a general container type:

var a any
a = "a string"
a = 2011
a = 2.777

A type assertion accesses the underlying concrete type:

r := a.(float64)
fmt.Println("the circle's area", math.Pi*r*r)

Or, if the underlying type is unknown, a type switch determines the type:

switch v := a.(type) {
case int:
    fmt.Println("twice a is", v*2)
case float64:
    fmt.Println("the reciprocal of a is", 1/v)
case string:
    h := len(v) / 2
    fmt.Println("a swapped by halves is", v[h:]+v[:h])
default:
    // a isn't one of the types above
}

The json package uses map[string]any and []any values to store arbitrary JSON objects and arrays; it will happily unmarshal any valid JSON blob into a plain any value. The default concrete Go types are:

  • bool for JSON booleans,

  • float64 for JSON numbers,

  • string for JSON strings, and

  • nil for JSON null.

Decoding arbitrary data

Consider this JSON data, stored in the variable b:

b := []byte(`{"Name":"Wednesday","Age":6,"Parents":["Gomez","Morticia"]}`)

Without knowing this data’s structure, we can decode it into an any value with Unmarshal:

var f any
err := json.Unmarshal(b, &f)

At this point the Go value in f would be a map whose keys are strings and whose values are themselves stored as any values:

f = map[string]any{
    "Name": "Wednesday",
    "Age":  6,
    "Parents": []any{
        "Gomez",
        "Morticia",
    },
}

To access this data we can use a type assertion to access f’s underlying map[string]any:

m := f.(map[string]any)

We can then iterate through the map with a range statement and use a type switch to access its values as their concrete types:

for k, v := range m {
    switch vv := v.(type) {
    case string:
        fmt.Println(k, "is string", vv)
    case float64:
        fmt.Println(k, "is float64", vv)
    case []any:
        fmt.Println(k, "is an array:")
        for i, u := range vv {
            fmt.Println(i, u)
        }
    default:
        fmt.Println(k, "is of a type I don't know how to handle")
    }
}

In this way you can work with unknown JSON data while still enjoying the benefits of type safety.

Reference Types

Let’s define a Go type to contain the data from the previous example:

type FamilyMember struct {
    Name    string
    Age     int
    Parents []string
}

var m FamilyMember
err := json.Unmarshal(b, &m)

Unmarshaling that data into a FamilyMember value works as expected, but if we look closely we can see a remarkable thing has happened. With the var statement we allocated a FamilyMember struct, and then provided a pointer to that value to Unmarshal, but at that time the Parents field was a nil slice value. To populate the Parents field, Unmarshal allocated a new slice behind the scenes. This is typical of how Unmarshal works with the supported reference types (pointers, slices, and maps).

Consider unmarshaling into this data structure:

type Foo struct {
    Bar *Bar
}

If there were a Bar field in the JSON object, Unmarshal would allocate a new Bar and populate it. If not, Bar would be left as a nil pointer.

From this a useful pattern arises: if you have an application that receives a few distinct message types, you might define “receiver” structure like

type IncomingMessage struct {
    Cmd *Command
    Msg *Message
}

and the sending party can populate the Cmd field and/or the Msg field of the top-level JSON object, depending on the type of message they want to communicate. Unmarshal, when decoding the JSON into an IncomingMessage struct, will only allocate the data structures present in the JSON data. To know which messages to process, the programmer need simply test that either Cmd or Msg is not nil.

Streaming Marshal and Unmarshal

The io.Reader and io.Writer interfaces are ubiquitous in Go, providing streaming access to resources such as HTTP connections, WebSockets, or files. The MarshalWrite and UnmarshalRead functions allow marshaling and unmarshaling directly to and from these streams without requiring an intermediate []byte holding the entire message.

func MarshalWrite(out io.Writer, in any, opts ...Options) (err error)
func UnmarshalRead(in io.Reader, out any, opts ...Options) (err error)

For example, we can write marshal output directly to standard out:

err := json.MarshalWrite(os.Stdout, m)

Custom Marshal and Unmarshal

Sometimes the default marshal behavior does not make sense for your type.

For example, suppose we have a type describing a software version.

type Version struct {
    Major, Minor, Patch int64
}

v := Version{1, 2, 3}
b, err := json.Marshal(s)

This marshals as {"Major":1,"Minor":2,"Patch":3}, as expected based on the struct definition, but a version string like “1.2.3” would likely be a better representation for JSON consumers.

By implementing the MarshalerTo interface, we can provide a custom JSON representation. UnmarshalerFrom provides unmarshaling of the custom JSON representation.

type MarshalerTo interface {
    MarshalJSONTo(*jsontext.Encoder) error
}

type UnmarshalerFrom interface {
    UnmarshalJSONFrom(*jsontext.Decoder) error
}

We can implement these method to convert to and from string representations.

func (v Version) MarshalJSONTo(enc *jsontext.Encoder) error {
    return json.MarshalEncode(enc, fmt.Sprintf("%d.%d.%d", v.Major, v.Minor, v.Patch))
}

With custom marshaling, the Version{1, 2, 3} value now marshals as "1.2.3".

func (v *Version) UnmarshalJSONFrom(dec *jsontext.Decoder) error {
    if k := dec.PeekKind(); k != jsontext.KindString {
        // Value must be a string.
        return &json.SemanticError{JSONKind: k}
    }

    var s string
    if err := json.UnmarshalDecode(dec, &s); err != nil {
        return err
    }

    _, err := fmt.Sscanf(s, "%d.%d.%d", &v.Major, &v.Minor, &v.Patch)
    return err
}

With custom unmarshaling, "1.2.3" now unmarshals as Version{1, 2, 3}. This simple example requires the exact "major.minor.patch" version format, but a more complex UnmarshalJSONFrom could choose to add flexibility, such as making the minor and patch versions optional.

References

For more information see the encoding/json/v2 and encoding/json/jsontext package documentation.

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