The compiler already does this. All code ends up in package files (.a
), but in the executable binary the Go tool does not include everything from imported packages, only what is needed (or more precisely: it excludes things that it can prove unreachable).
See possible duplicate Splitting client/server code.
One thing to note here: if an imported package (from which you only want to include things you refer to) imports other packages, this has to be done recursively of course.
For example if you import this package:
package subplay
func A() {}
And call nothing from it:
package main
import _ "play/subplay"
func main() {
}
The result binary (Go 1.8, linux, amd64) will be 960,134 bytes (roughly 1 MB).
If you change subplay
to import net/http
:
package subplay
import _ "net/http"
func A() {}
But still don't call anything from net/http
, the result will be: 5,370,935 bytes (roughly 5 MB)! (Note that net/http
also imports 39 other packages!)
Now if you go ahead and start using things from net/http
:
package subplay
import "net/http"
func A() {
http.ListenAndServe("", nil)
}
But in the main
package you still don't call subplay.A()
, the size of the executable binary does not change: remains 5,370,935 bytes!
And when you change the main
package to call subplay.A()
:
package main
import "play/subplay"
func main() {
subplay.A()
}
The result binary grows: 5,877,919 bytes!
If you change http.ListenAndServe()
to http.ListenAndServeTLS()
:
func A() {
http.ListenAndServeTLS("", "", "", nil)
}
Result binary is: 6,041,535 bytes.
As you can see, what gets compiled into the executable binary does depend on what you call / use from the imported packages.
Also don't forget that Go is a statically linked language, the result executable binary has to include everything it needs. See related question: Reason for huge size of compiled executable of Go