使用 Go 语言时没有关注值传递和误用 for 循环导致的 bug

我们的业务代码中习惯使用 Map 维护一些 LocalCache，前两天发现自己维护的一个 LocalCache 数据有些不对：Cache 的 Key 为某个对象的ID，值为这个ID对应的 PO（即数据库中的对象），调试时发现所有的 Key 对应的值都是一样的，这是因为自己对一些细节没有关注到，还把 Java 那套东西搬来用导致的问题。

为了简化，我就不把业务代码搬上来了，写个简单的示例：

package main

import (
	"encoding/json"
	"fmt"
)

type Student struct {
	ID   int
	Name string
	Age  int
}

func main() {

	var students []Student
	students = append(students, Student{
		ID: 1,
		Name: "张三",
		Age: 18,
	})
	students = append(students, Student{
		ID: 2,
		Name: "李四",
		Age: 19,
	})
	students = append(students, Student{
		ID: 3,
		Name: "王五",
		Age: 20,
	})

	studentMap := make(map[int]*Student, len(students))
	for _, student := range students {
		studentMap[student.ID] = &student
	}

	bs, _ := json.Marshal(studentMap)
	fmt.Println(string(bs))

}

上边代码输出如下：

{"1":{"ID":3,"Name":"王五","Age":20},"2":{"ID":3,"Name":"王五","Age":20},"3":{"ID":3,"Name":"王五","Age":20}}

可以看到所有的 value 是同一个 Student，为什么会出现这样的问题呢？因为 students 存储的是 Student 的值，在给 for 循环中的 student 赋值时，是复制了一个新的值给它，而 for 循环中的 student 变量所指向的地址是不变的。

可以打印 student 的地址看一下：

for _, student := range students {
	fmt.Printf("%p \n", &student)
	studentMap[student.ID] = &student
}

输出为：

0xc0000a6040 
0xc0000a6040 
0xc0000a6040 
{"1":{"ID":3,"Name":"王五","Age":20},"2":{"ID":3,"Name":"王五","Age":20},"3":{"ID":3,"Name":"王五","Age":20}}

这种情况下我们应该用 students 中索引对应数据的指针，上边 for 循环修改如下：

for i, student := range students {
	fmt.Printf("%p \n", &students[i])
	studentMap[student.ID] = &students[i]
}

输出为：

0xc0000b8000 
0xc0000b8020 
0xc0000b8040 
{"1":{"ID":1,"Name":"张三","Age":18},"2":{"ID":2,"Name":"李四","Age":19},"3":{"ID":3,"Name":"王五","Age":20}}

上边的情况给 student 赋值也是有问题的：

for _, student := range students {
	student.Name = "test"
}

bs, _ := json.Marshal(students)
fmt.Println(string(bs))

输出：

[{"ID":1,"Name":"张三","Age":18},{"ID":2,"Name":"李四","Age":19},{"ID":3,"Name":"王五","Age":20}]

---

Java 写习惯了就以为迭代时的 student 指向的是 students 中的地址。