It seems the problem is that the java files in golang.org/x/mobile/bind/java are not being copied to the vendor directory.

Which in turn seems to be a very upvoted issue but closed as wontfix: #26366

I guess vendoring golang.org/x/mobile is not supported then? 🤔

This still happens with Go 1.17.6. Is it possible to embed the files with https://pkg.go.dev/embed ? Or are you expecting the behavior of go mod vendor to change in the future?

I don't think the root cause is #26366 (related but not the direct cause).
The go mod vendor vendors only packages the go command identifies during the build process, and golang.org/x/mobile/bind/java is a dependency added in the generated code, which the go command doesn't understand.

Can you please show us how github.com/DataDog/datadog-agent specifies dependency on golang.org/x/mobile currently?

Change https://golang.org/cl/377754 mentions this issue: cmd,bind: add implicit dependencies

I sent a cl that spells out the implicit dependencies in the tools' source code (cl/377754), but the current best practice (especially for gobind) is to check in the generated source code to the repository - does that work for your workflows?

This should be fixed at golang.org/x/mobile@v0.0.0-20220112015953-858099ff7816

% cat main.go
package doc

import (
        _ "golang.org/x/mobile/cmd/gobind"
)
% go mod vendor
% tree vendor/golang.org/x/mobile/bind
vendor/golang.org/x/mobile/bind
├── bind.go
├── gen.go
├── genclasses.go
├── gengo.go
├── genjava.go
├── genobjc.go
├── genobjcw.go
├── implicit.go
├── java
│   ├── ClassesTest.java
│   ├── CustomPkgTest.java
│   ├── Seq.java
│   ├── SeqBench.java
....