Golang变量赋值的坑

package main

import (
	"database/sql"
	_ "github.com/go-sql-driver/mysql"
	"log"
)

var db *sql.DB

func main() {

	//数据存在校验
	rows, err := db.Query("SELECT uuid FROM account WHERE username=?", "1")
	if err != nil {
		log.Println("QUERY ERROR: ", err)
	}
	defer rows.Close()

	var uuid string

	for rows.Next() {
		err := rows.Scan(&uuid)
		if err != nil {
			log.Fatal("SCAN ERROR: ", err)
		}
		log.Println("UUID: ", uuid)
	}

	err = rows.Err()
	if err != nil {
		log.Fatal("ROWS ERROR", err)
	}
}

func init() {

	//初始化数据库连接池
	db, err := sql.Open("mysql", "root:123456@tcp(127.0.0.1:3306)/loginserver?charset=utf8&loc=Asia%2FShanghai")
	if err != nil {
		log.Fatalln("DB CONNECT: ", err)
	}
	db.SetMaxOpenConns(200)
	db.SetMaxIdleConns(5)
	err = db.Ping()
	if err != nil {
		log.Fatalln("DB PING: ", err)
	}
}

编译通过，运行的时候爆出异常

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xc0000005 code=0x1 addr=0x0 pc=0x465ab0]

goroutine 1 [running]:
panic(0x661d60, 0xc082002080)
        C:/Go/src/runtime/panic.go:481 +0x3f4
database/sql.(*DB).conn(0x0, 0x1, 0xc082002e80, 0x0, 0x0)
        C:/Go/src/database/sql/sql.go:778 +0xad0
database/sql.(*DB).query(0x0, 0x6f37a0, 0x29, 0xc08205de50, 0x1, 0x1, 0x1, 0x0, 0x0, 0x0)
        C:/Go/src/database/sql/sql.go:1073 +0x4d
database/sql.(*DB).Query(0x0, 0x6f37a0, 0x29, 0xc08205de50, 0x1, 0x1, 0x10000c08205de10, 0x0, 0x0)
        C:/Go/src/database/sql/sql.go:1061 +0xaa
main.main()
        D:/Work/Project/golang/src/MySQLStudy/main.go:14 +0x1e1

调试的时候发现，程序到main方法中的时候db为nil，空的

在网上查问题的时候看到http://tonybai.com/2015/01/13/a-hole-about-variable-scope-in-golang/《一个有关Golang变量作用域的坑》 ，关于“:=”的问题， ":=" 使得init方法中的db的值没有作用到全局变量db中。

修改后正常

package main

import (
	"database/sql"
	_ "github.com/go-sql-driver/mysql"
	"log"
)

var db *sql.DB

func main() {

	//数据存在校验
	rows, err := db.Query("SELECT uuid FROM account WHERE username=?", "1")
	if err != nil {
		log.Println("QUERY ERROR: ", err)
	}
	defer rows.Close()

	var uuid string

	for rows.Next() {
		err := rows.Scan(&uuid)
		if err != nil {
			log.Fatal("SCAN ERROR: ", err)
		}
		log.Println("UUID: ", uuid)
	}

	err = rows.Err()
	if err != nil {
		log.Fatal("ROWS ERROR", err)
	}
}

func init() {

	//初始化数据库连接池
	db, _ = sql.Open("mysql", "root:123456@tcp(127.0.0.1:3306)/loginserver?charset=utf8&loc=Asia%2FShanghai")
	db.SetMaxOpenConns(200)
	db.SetMaxIdleConns(5)
	err := db.Ping()
	if err != nil {
		log.Fatalln("DB PING: ", err)
	}
}

 

转载于:https://my.oschina.net/GeY/blog/690781