Go language pack import: Why is my global variable still nil after initialization?

Detailed explanation of Go language pack import and variable initialization: Analysis of global variable initialization failure case

This article analyzes common problems in Go language pack import and variable initialization. In the code, utils.go package defines a global variable esclient and is initialized in init() function. However, after main.go imports utils package, it gets nil value when accessing esclient .

Question description:

The following code shows utils.go and main.go files:

utils.go :

 package utils

import (
    "fmt"
    "log"

    "github.com/elastic/go-elasticsearch/v6"
)

var esclient *elasticsearch.Client

func init() {
    host := cfg.section("es").key("host").string() // Assume that cfg has defined if host == "" {
        log.Fatal("es host not set")
    }

    cfg := elasticsearch.Config{
        Addresses: []string{host},
    }

    client, err := elasticsearch.NewClient(cfg) // Note that NewClient is used here and modified to lowercase c
    if err != nil {
        log.Fatal("Connection failed", err)
    }

    _, err = client.Info()
    if err != nil {
        log.Fatalf("Connection error: %s, %v", host, err)
    }

    esclient = client // Key modification: explicitly assign value to the global variable fmt.Println(esclient) // &{0xc000166000 0xc000162000} (Sample output)
}

main.go :

 package main

import (
    "data_push/utils"
    "fmt"
)

func main() {
    fmt.Println(utils.esclient) // The result may still be nil (depending on the cfg configuration)
}

The root of the problem:

In init() function of utils.go , client, err := elasticsearch.newclient(cfg) uses the := assignment operator. This results in a new local variable client declared inside init() function instead of modifying the global variable esclient . After init() function is finished, the local variable client is destroyed, and the global variable esclient still retains its initial value nil .

Solution:

Modify client, err := elasticsearch.newclient(cfg) in utils.go to esclient, err = elasticsearch.NewClient(cfg) , and explicitly assign the return value of elasticsearch.NewClient(cfg) to the global variable esclient . Note that elasticsearch.NewClient has a capitalized letter, which is consistent with the sample code.

Improvement suggestions:

1. Error handling: In the main function, utils.esclient should also be checked for nil to handle possible initialization failures.
2. Configuration Management: The code assumes that the cfg variable is defined, which requires further explanation of how to configure es host. It is recommended to use a more standard configuration method, such as reading from environment variables or configuration files.
3. Use clearer variable names: esclient can be changed to a more descriptive name, such as elasticSearchClient .

Through the above modification, we can ensure that the global variable esclient is correctly initialized to avoid the problem of nil value. Remember that in Go, := declares a new variable, and = is used to assign a value to the declared variable. Understanding this distinction is essential to avoid such problems.

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