From fa33b05e7c560c80fca0fb03c1d16377091ed728 Mon Sep 17 00:00:00 2001
From: "chen.yang" <chen.yang@yuzhen-iot.com>
Date: Tue, 3 Aug 2021 09:25:42 +0800
Subject: [PATCH] =?UTF-8?q?=E5=A2=9E=E5=8A=A0=20Golang=20Map=20=E8=BD=AC?=
 =?UTF-8?q?=20Json=20=E7=9A=84=E9=A1=BA=E5=BA=8F=E9=97=AE=E9=A2=98.?=
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Signed-off-by: chen.yang <chen.yang@yuzhen-iot.com>
---
 .../Go/Basic/Golang_Map_转_Json_的顺序问题.md | 61 +++++++++++++++++++
 1 file changed, 61 insertions(+)
 create mode 100644 Software/Development/Language/Go/Basic/Golang_Map_转_Json_的顺序问题.md

diff --git a/Software/Development/Language/Go/Basic/Golang_Map_转_Json_的顺序问题.md b/Software/Development/Language/Go/Basic/Golang_Map_转_Json_的顺序问题.md
new file mode 100644
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--- /dev/null
+++ b/Software/Development/Language/Go/Basic/Golang_Map_转_Json_的顺序问题.md
@@ -0,0 +1,61 @@
+# [Golang Map 转 Json 的顺序问题](https://blog.csdn.net/xz_studying/article/details/103219478)
+
+我们都知道 map 是无序的，每次取出 key/value 的顺序都可能不一致，但 map 转 json 的顺序是不是也是无序的吗？尽管 json 中的参数顺序大部分情况下对使用没有影响，我们不妨看看源码中怎么处理的。
+
+## 1.结论
+
+* map 转 json 是有序的，按照字符编码升序排列 key。
+
+## 2.溯源
+
+源码位于encoding/json/encode.go中：
+
+```go
+type mapEncoder struct {
+   elemEnc encoderFunc
+}
+
+func (me mapEncoder) encode(e *encodeState, v reflect.Value, opts encOpts) {
+   if v.IsNil() {//为nil时，返回null
+      e.WriteString("null")
+      return
+   }
+   e.WriteByte('{')
+
+   // Extract and sort the keys.
+   keys := v.MapKeys()//获取map中的所有keys
+   sv := make([]reflectWithString, len(keys))
+   for i, v := range keys {
+      sv[i].v = v
+      if err := sv[i].resolve(); err != nil {//处理key，尤其是非string（int/uint）类型的key转string
+         e.error(&MarshalerError{v.Type(), err})
+      }
+   }
+   //排序，升序，直接比较字符串
+   sort.Slice(sv, func(i, j int) bool { return sv[i].s < sv[j].s })
+
+   for i, kv := range sv {
+      if i > 0 {
+         e.WriteByte(',')
+      }
+      e.string(kv.s, opts.escapeHTML)
+      e.WriteByte(':')
+      me.elemEnc(e, v.MapIndex(kv.v), opts)
+   }
+   e.WriteByte('}')
+}
+
+func newMapEncoder(t reflect.Type) encoderFunc {
+   switch t.Key().Kind() {
+   case reflect.String,
+      reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64,
+      reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+   default:
+      if !t.Key().Implements(textMarshalerType) {
+         return unsupportedTypeEncoder
+      }
+   }
+   me := mapEncoder{typeEncoder(t.Elem())}
+   return me.encode
+}
+```