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125 lines
3.4 KiB
Go
125 lines
3.4 KiB
Go
// Go offers built-in support for JSON encoding and
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// decoding, including to and from built-in and custom
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// data types.
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package main
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import (
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"encoding/json"
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"fmt"
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"os"
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)
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// We'll use these two structs to demonstrate encoding and
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// decoding of custom types below.
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type response1 struct {
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Page int
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Fruits []string
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}
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// Only exported fields will be encoded/decoded in JSON.
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// Fields must start with capital letters to be exported.
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type response2 struct {
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Page int `json:"page"`
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Fruits []string `json:"fruits"`
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}
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func main() {
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// First we'll look at encoding basic data types to
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// JSON strings. Here are some examples for atomic
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// values.
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bolB, _ := json.Marshal(true)
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fmt.Println(string(bolB))
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intB, _ := json.Marshal(1)
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fmt.Println(string(intB))
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fltB, _ := json.Marshal(2.34)
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fmt.Println(string(fltB))
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strB, _ := json.Marshal("gopher")
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fmt.Println(string(strB))
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// And here are some for slices and maps, which encode
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// to JSON arrays and objects as you'd expect.
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slcD := []string{"apple", "peach", "pear"}
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slcB, _ := json.Marshal(slcD)
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fmt.Println(string(slcB))
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mapD := map[string]int{"apple": 5, "lettuce": 7}
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mapB, _ := json.Marshal(mapD)
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fmt.Println(string(mapB))
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// The JSON package can automatically encode your
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// custom data types. It will only include exported
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// fields in the encoded output and will by default
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// use those names as the JSON keys.
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res1D := &response1{
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Page: 1,
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Fruits: []string{"apple", "peach", "pear"}}
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res1B, _ := json.Marshal(res1D)
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fmt.Println(string(res1B))
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// You can use tags on struct field declarations
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// to customize the encoded JSON key names. Check the
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// definition of `response2` above to see an example
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// of such tags.
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res2D := &response2{
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Page: 1,
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Fruits: []string{"apple", "peach", "pear"}}
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res2B, _ := json.Marshal(res2D)
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fmt.Println(string(res2B))
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// Now let's look at decoding JSON data into Go
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// values. Here's an example for a generic data
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// structure.
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byt := []byte(`{"num":6.13,"strs":["a","b"]}`)
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// We need to provide a variable where the JSON
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// package can put the decoded data. This
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// `map[string]interface{}` will hold a map of strings
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// to arbitrary data types.
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var dat map[string]interface{}
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// Here's the actual decoding, and a check for
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// associated errors.
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if err := json.Unmarshal(byt, &dat); err != nil {
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panic(err)
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}
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fmt.Println(dat)
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// In order to use the values in the decoded map,
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// we'll need to convert them to their appropriate type.
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// For example here we convert the value in `num` to
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// the expected `float64` type.
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num := dat["num"].(float64)
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fmt.Println(num)
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// Accessing nested data requires a series of
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// conversions.
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strs := dat["strs"].([]interface{})
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str1 := strs[0].(string)
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fmt.Println(str1)
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// We can also decode JSON into custom data types.
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// This has the advantages of adding additional
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// type-safety to our programs and eliminating the
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// need for type assertions when accessing the decoded
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// data.
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str := `{"page": 1, "fruits": ["apple", "peach"]}`
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res := response2{}
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json.Unmarshal([]byte(str), &res)
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fmt.Println(res)
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fmt.Println(res.Fruits[0])
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// In the examples above we always used bytes and
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// strings as intermediates between the data and
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// JSON representation on standard out. We can also
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// stream JSON encodings directly to `os.Writer`s like
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// `os.Stdout` or even HTTP response bodies.
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enc := json.NewEncoder(os.Stdout)
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d := map[string]int{"apple": 5, "lettuce": 7}
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enc.Encode(d)
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}
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