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在管理多模块管理时，可能有的模块还在开发中，还没发布到github上，在Go 1.18之前是通过 go mod 的 replace 来做的。2022 年2 月份正式发布的 go1.18 由于新增了工作区特性，给多模块管理提供另一种更方便的解决途径。

过去的replace模式

hanwei@hanweideMacBook-Air golang]$ go version
go version go1.18 darwin/arm64
hanwei@hanweideMacBook-Air golang]$ mkdir go1.18-workspace/
hanwei@hanweideMacBook-Air golang]$ cd go1.18-workspace/
hanwei@hanweideMacBook-Air go1.18-workspace]$ mkdir mypkg example
hanwei@hanweideMacBook-Air go1.18-workspace]$ tree
.
├── example
└── mypkg

2 directories, 0 files
hanwei@hanweideMacBook-Air go1.18-workspace]$ go mod init github.com/go1.18-workspace/mypkg
go: creating new go.mod: module github.com/go1.18-workspace/mypkg
go: to add module requirements and sums:
        go mod tidy
hanwei@hanweideMacBook-Air go1.18-workspace]$ cd mypkg/
hanwei@hanweideMacBook-Air mypkg]$ touch bar.go
hanwei@hanweideMacBook-Air mypkg]$ vi bar.go 
hanwei@hanweideMacBook-Air mypkg]$ cat bar.go 
package mypkg

func Bar() {
        println("This is package mypkg")
}
hanwei@hanweideMacBook-Air mypkg]$ cd ../example/
hanwei@hanweideMacBook-Air example]$ go mod init github.com/go1.18-workspace/example
go: creating new go.mod: module github.com/go1.18-workspace/example
go: to add module requirements and sums:
        go mod tidy
hanwei@hanweideMacBook-Air example]$ touch main.go
hanwei@hanweideMacBook-Air example]$ vi main.go 
hanwei@hanweideMacBook-Air example]$ cat main.go 
package main

import (
    "github.com/go1.18-workspace/mypkg"
)

func main() {
    mypkg.Bar()
}

这时候，如果我们运行 go mod tidy，肯定会报错，因为我们的 mypkg 包根本没有提交到 github 上，肯定找不到。

hanwei@hanweideMacBook-Air example]$ go mod tidy
go: finding module for package github.com/go1.18-workspace/mypkg
github.com/go1.18-workspace/example imports
        github.com/go1.18-workspace/mypkg: cannot find module providing package github.com/go1.18-workspace/mypkg: module github.com/go1.18-workspace/mypkg: git ls-remote -q origin in /Users/hanwei/GoProjects/pkg/mod/cache/vcs/2c423cac5ebc1b2d018ef93a87560d369abd7dec6c155b46cddb11299415bc09: exit status 128:
        remote: Repository not found.
        fatal: repository 'https://github.com/go1.18-workspace/mypkg/' not found
hanwei@hanweideMacBook-Air example]$ go run main.go
main.go:4:5: no required module provides package github.com/go1.18-workspace/mypkg; to add it:
        go get github.com/go1.18-workspace/mypkg

go run main.go 也就不成功。

我们当然可以提交 mypkg 到 github，但我们每修改一次 mypkg，就需要提交，否则 example 中就没法使用上最新的。

针对这种情况，目前是建议通过 replace 来解决，即在 example 中的 go.mod 增加如下 replace：

hanwei@hanweideMacBook-Air example]$ go mod edit -replace=github.com/go1.18-workspace/mypkg=../mypkg
hanwei@hanweideMacBook-Air example]$ cat go.mod 
module github.com/go1.18-workspace/example

go 1.18

replace github.com/go1.18-workspace/mypkg => ../mypkg

（v1.0.0 根据具体情况修改，还未提交，可以使用 v1.0.0）

hanwei@hanweideMacBook-Air example]$ cat go.mod 
module github.com/go1.18-workspace/example

go 1.18

require github.com/go1.18-workspace/mypkg v1.0.0
replace github.com/go1.18-workspace/mypkg => ../mypkg
hanwei@hanweideMacBook-Air example]$ tree ..
..
├── example
│   ├── go.mod
│   └── main.go
└── mypkg
    ├── bar.go
    └── go.mod

2 directories, 4 files

再次运行 go run main.go，输出如下：

hanwei@hanweideMacBook-Air example]$ go run main.go 
This is package mypkg

工作区模式

将上面的replace注释，再执行go run main.go，报错

hanwei@hanweideMacBook-Air example]$ cat go.mod 
module github.com/go1.18-workspace/example

go 1.18

//require github.com/go1.18-workspace/mypkg v1.0.0
//replace github.com/go1.18-workspace/mypkg => ../mypkg
hanwei@hanweideMacBook-Air example]$ go run main.go 
main.go:4:5: missing go.sum entry for module providing package github.com/go1.18-workspace/mypkg; to add:
        go mod download github.com/go1.18-workspace/mypkg

初始化 workspace

hanwei@hanweideMacBook-Air go1.18-workspace]$ go version
go version go1.18 darwin/arm64
hanwei@hanweideMacBook-Air go1.18-workspace]$ go work init example mypkg
hanwei@hanweideMacBook-Air go1.18-workspace]$ tree
.
├── example
│   ├── go.mod
│   └── main.go
├── go.work
└── mypkg
    ├── bar.go
    └── go.mod

2 directories, 5 files
hanwei@hanweideMacBook-Air go1.18-workspace]$ cat go.work 
go 1.18

use (
        ./example
        ./mypkg
)
hanwei@hanweideMacBook-Air example]$ go run main.go 
This is package mypkg

可见用工作区比replace方便太多了。

go.work 文件的语法和 go.mod 类似，因此也支持 replace。

注意，go.work 不需要提交到 Git 中，因为它只是本地开发使用的。

cat .gitignore
# ---> Go
# If you prefer the allow list template instead of the deny list, see community template:
# https://github.com/github/gitignore/blob/main/community/Golang/Go.AllowList.gitignore
#
# Binaries for programs and plugins
*.exe
*.exe~
*.dll
*.so
*.dylib
# Test binary, built with `go test -c`
*.test
# Output of the go coverage tool, specifically when used with LiteIDE
*.out
# Dependency directories (remove the comment below to include it)
# vendor/
# Go workspace file
go.work

在 GOPATH 年代，多 GOPATH 是一个头疼的问题。当时没有很好的解决，Module 就出现了，多 GOPATH 问题因此消失。但多 Module 问题随之出现。Workspace 方案较好的解决了这个问题。

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