gomock

gomock is a mocking framework for theGo programming language. Itintegrates well with Go's built-intesting package, but can be used in othercontexts too.

This project originates from Google'sgolang/mock repo. Unfortunately, Googleno longer maintains this project, and given the heavy usage of gomock projectwithin Uber, we've decided to fork and maintain this going forward at Uber.

Contributions are welcome in the form of GitHub issue or PR!

Supported Go Versions

go.uber.org/mock supports all Go versions supported by the officialGo Release Policy. That is,the two most recent releases of Go.

Installation

Install themockgen tool.

go install go.uber.org/mock/mockgen@latest

To ensure it was installed correctly, use:

mockgen -version

If that fails, make sure your GOPATH/bin is in your PATH. You can add it with:

export PATH=$PATH:$(go env GOPATH)/bin

Running mockgen

mockgen has three modes of operation: archive, source and package.

Archive mode

Archive mode generates mock interfaces from a package archivefile (.a). It is enabled by using the -archive flag. An importpath and a comma-separated list of symbols should be providedas a non-flag argument to the command.

Example:

# Build the package to a archive.go build -o pkg.a database/sql/drivermockgen -archive=pkg.a database/sql/driver Conn,Driver

Source mode

Source mode generates mock interfaces from a source file.It is enabled by using the -source flag. Other flags thatmay be useful in this mode are -imports and -aux_files.

Example:

mockgen -source=foo.go [other options]

Package mode

Package mode works by specifying the package and interface names.It is enabled by passing two non-flag arguments: an import path, and acomma-separated list of symbols.

You can use "." to refer to the current path's package.

Example:

mockgen database/sql/driver Conn,Driver# Convenient for `go:generate`.mockgen . Conn,Driver

Flags

Themockgen command is used to generate source code for a mockclass given a Go source file containing interfaces to be mocked.It supports the following flags:

• -archive: A package archive file containing interfaces to be mocked.

• -source: A file containing interfaces to be mocked.

• -destination: A file to which to write the resulting source code. If youdon't set this, the code is printed to standard output.

• -package: The package to use for the resulting mock classsource code. If you don't set this, the package name ismock_ concatenatedwith the package of the input file.

• -imports: A list of explicit imports that should be used in the resultingsource code, specified as a comma-separated list of elements of the formfoo=bar/baz, wherebar/baz is the package being imported andfoo isthe identifier to use for the package in the generated source code.

• -aux_files: A list of additional files that should be consulted toresolve e.g. embedded interfaces defined in a different file. This isspecified as a comma-separated list of elements of the formfoo=bar/baz.go, wherebar/baz.go is the source file andfoo is thepackage name of that file used by the -source file.

• -build_flags: (package mode only) Flags passed verbatim togo list.

• -mock_names: A list of custom names for generated mocks. This is specifiedas a comma-separated list of elements of the formRepository=MockSensorRepository,Endpoint=MockSensorEndpoint, whereRepository is the interface name andMockSensorRepository is the desiredmock name (mock factory method and mock recorder will be named after the mock).If one of the interfaces has no custom name specified, then default namingconvention will be used.

• -self_package: The full package import path for the generated code. Thepurpose of this flag is to prevent import cycles in the generated code bytrying to include its own package. This can happen if the mock's package isset to one of its inputs (usually the main one) and the output is stdio somockgen cannot detect the final output package. Setting this flag will thentell mockgen which import to exclude.

• -copyright_file: Copyright file used to add copyright header to the resulting source code.

• -debug_parser: Print out parser results only.

• -write_package_comment: Writes package documentation comment (godoc) if true. (default true)

• -write_generate_directive: Add //go:generate directive to regenerate the mock. (default false)

• -write_source_comment: Writes original file (source mode) or interface names (package mode) comment if true. (default true)

• -typed: Generate Type-safe 'Return', 'Do', 'DoAndReturn' function. (default false)

• -exclude_interfaces: Comma-separated names of interfaces to be excluded

For an example of the use ofmockgen, see thesample/ directory. In simplecases, you will need only the-source flag.

Building Mocks

type Foo interface {  Bar(x int) int}func SUT(f Foo) { // ...}

func TestFoo(t *testing.T) {  ctrl := gomock.NewController(t)  m := NewMockFoo(ctrl)  // Asserts that the first and only call to Bar() is passed 99.  // Anything else will fail.  m.    EXPECT().    Bar(gomock.Eq(99)).    Return(101)  SUT(m)}

Building Stubs

type Foo interface {  Bar(x int) int}func SUT(f Foo) { // ...}

func TestFoo(t *testing.T) {  ctrl := gomock.NewController(t)  m := NewMockFoo(ctrl)  // Does not make any assertions. Executes the anonymous functions and returns  // its result when Bar is invoked with 99.  m.    EXPECT().    Bar(gomock.Eq(99)).    DoAndReturn(func(_ int) int {      time.Sleep(1*time.Second)      return 101    }).    AnyTimes()  // Does not make any assertions. Returns 103 when Bar is invoked with 101.  m.    EXPECT().    Bar(gomock.Eq(101)).    Return(103).    AnyTimes()  SUT(m)}

Modifying Failure Messages

When a matcher reports a failure, it prints the received (Got) vs theexpected (Want) value.

Got: [3]Want: is equal to 2Expected call at user_test.go:33 doesn't match the argument at index 1.Got: [0 1 1 2 3]Want: is equal to 1

ModifyingWant

TheWant value comes from the matcher'sString() method. If the matcher'sdefault output doesn't meet your needs, then it can be modified as follows:

gomock.WantFormatter(  gomock.StringerFunc(func() string { return "is equal to fifteen" }),  gomock.Eq(15),)

This modifies thegomock.Eq(15) matcher's output forWant: fromis equal to 15 tois equal to fifteen.

ModifyingGot

TheGot value comes from the object'sString() method if it is available.In some cases the output of an object is difficult to read (e.g.,[]byte) andit would be helpful for the test to print it differently. The followingmodifies how theGot value is formatted:

gomock.GotFormatterAdapter(  gomock.GotFormatterFunc(func(i any) string {    // Leading 0s    return fmt.Sprintf("%02d", i)  }),  gomock.Eq(15),)

If the received value is3, then it will be printed as03.