我有这样的代码:

package main

import (
    "database/sql"
    "flag"
    "fmt"
    "log"
    "net/http"
    "os"

    _ "github.com/go-sql-driver/mysql"
)

// Default constant for configuration
const DefaultHTTPAddr = ":8080"
const DefaultDSN = "root:root@tcp(127.0.0.1:3306)/librarian"

// Parameters
var (
    httpAddr string
    dsn      string
    db       *sql.DB
)

// init initializes this package.
func init() {
    flag.StringVar(&httpAddr, "addr", DefaultHTTPAddr, "Set the HTTP bind address")
    flag.StringVar(&dsn, "dsn", DefaultDSN, "Set the Data Source Name")
    flag.Usage = func() {
        fmt.Fprintf(os.Stderr, "Usage: %s [options]\n", os.Args[0])
        flag.PrintDefaults()
    }

}

type Book struct {
    id     int
    title  string
    author string
}

func main() {
    flag.Parse()

    var err error
    db, err := sql.Open("mysql", DefaultDSN)
    if err != nil {
        log.Fatal(err)
    }
    if err = db.Ping(); err != nil {
        log.Fatal(err)
    }

    // handler
    http.HandleFunc("/", homepage)
    http.HandleFunc("/books", booksIndex)

    log.Println("httpd started successfully")
    http.ListenAndServe(httpAddr, nil)
}

func booksIndex(w http.ResponseWriter, r *http.Request) {
    if r.Method != "GET" {
        http.Error(w, http.StatusText(405), 405)
        return
    }

    rows, err := db.Query("SELECT * FROM books")
    if err != nil {
        log.Fatal(err)
    }
    defer rows.Close()

    books := make([]*Book, 0)
    for rows.Next() {
        bk := new(Book)
        err = rows.Scan(&bk.id, &bk.title, &bk.author)
        if err != nil {
            log.Fatal(err)
        }
        books = append(books, bk)
    }
    if err = rows.Err(); err != nil {
        log.Fatal(err)
    }

    for _, bk := range books {
        fmt.Fprintf(w, "%v, %v, %v\n", bk.id, bk.title, bk.author)
    }
}

func homepage(w http.ResponseWriter, r *http.Request) {
    fmt.Fprintf(w, "Welcome!")
}

每次我尝试访问 /books 时，它总是会崩溃。

$ curl localhost:8080/books
curl: (52) Empty reply from server

像这个:

2016/05/31 11:56:38 http: panic serving 127.0.0.1:51711: runtime error: invalid memory address or nil pointer dereference
goroutine 6 [running]:
net/http.(*conn).serve.func1(0xc820074100)
        /usr/local/opt/go/libexec/src/net/http/server.go:1389 +0xc1
panic(0x357b40, 0xc82000a150)
        /usr/local/opt/go/libexec/src/runtime/panic.go:443 +0x4e9
database/sql.(*DB).conn(0x0, 0xc820010b01, 0xc8200de000, 0x0, 0x0)
        /usr/local/opt/go/libexec/src/database/sql/sql.go:778 +0xac9
database/sql.(*DB).query(0x0, 0x4022c0, 0x13, 0x0, 0x0, 0x0, 0x3c4601, 0x6, 0x0, 0x0)
        /usr/local/opt/go/libexec/src/database/sql/sql.go:1073 +0x46
database/sql.(*DB).Query(0x0, 0x4022c0, 0x13, 0x0, 0x0, 0x0, 0xc820016280, 0x0, 0x0)
        /usr/local/opt/go/libexec/src/database/sql/sql.go:1061 +0xa3
main.booksIndex(0x6979c0, 0xc82006da00, 0xc8200e2000)
        /Users/rahmatawaludin/gocode/src/github.com/rahmatawaludin/librarian/main.go:68 +0xd9
net/http.HandlerFunc.ServeHTTP(0x4666f8, 0x6979c0, 0xc82006da00, 0xc8200e2000)
        /usr/local/opt/go/libexec/src/net/http/server.go:1618 +0x3a
net/http.(*ServeMux).ServeHTTP(0xc820010ba0, 0x6979c0, 0xc82006da00, 0xc8200e2000)
        /usr/local/opt/go/libexec/src/net/http/server.go:1910 +0x17d
net/http.serverHandler.ServeHTTP(0xc820074080, 0x6979c0, 0xc82006da00, 0xc8200e2000)
        /usr/local/opt/go/libexec/src/net/http/server.go:2081 +0x19e
net/http.(*conn).serve(0xc820074100)
        /usr/local/opt/go/libexec/src/net/http/server.go:1472 +0xf2e
created by net/http.(*Server).Serve
        /usr/local/opt/go/libexec/src/net/http/server.go:2137 +0x44e

我认为我可以从 booksIndex 访问 db，因为我将其设置为全局变量。当我将数据库初始化移动到 booksIndex 时，错误没有显示出来。

我的代码中哪些部分有问题？

此外，我是 Golang 的新手。如果您对如何组织我的代码有任何建议，请告诉我。谢谢..:)

最佳答案

函数中的 db 变量正在隐藏全局变量。 当您这样做时:

db,err:=

它将它分配给一个新的局部变量db。 这是因为它不是来自同一 block 。根据标准:

Unlike regular variable declarations, a short variable declaration may redeclare variables provided they were originally declared earlier in the same block (or the parameter lists if the block is the function body) with the same type, and at least one of the non-blank variables is new. As a consequence, redeclaration can only appear in a multi-variable short declaration. Redeclaration does not introduce a new variable; it just assigns a new value to the original.

所以全局变量仍然是 nil 指针。当它被访问时，你会得到 nil 指针解引用

将其更改为 d，然后将其分配给 db。或者哪个更正确(如 rahmat 在评论中给出的那样):=

d , err = 

我建议您为处理程序、模型等创建单独的文件。并查看 https://github.com/mattermost/platform如何组织代码。

关于database - 为什么无法从处理程序访问我的数据库连接？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/37537156/