gomock

GoMock is a mocking framework for the Go programming language. It integrates well with Go's built-in testing package, but can be used in other contexts too.

Installation

Once you have installed Go, run these commands to install the gomock package and the mockgen tool:

go get github.com/golang/mock/gomock
go install github.com/golang/mock/mockgen

Documentation

After installing, you can use go doc to get documentation:

go doc github.com/golang/mock/gomock

Alternatively, there is an online reference for the package hosted on GoPkgDoc here.

Running mockgen

mockgen has two modes of operation: source and reflect. Source mode generates mock interfaces from a source file. It is enabled by using the -source flag. Other flags that may be useful in this mode are -imports and -aux_files.

Example:

mockgen -source=foo.go [other options]

Reflect mode generates mock interfaces by building a program that uses reflection to understand interfaces. It is enabled by passing two non-flag arguments: an import path, and a comma-separated list of symbols.

Example:

mockgen database/sql/driver Conn,Driver

The mockgen command is used to generate source code for a mock class given a Go source file containing interfaces to be mocked. It supports the following flags:

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

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

• -package: The package to use for the resulting mock class source code. If you don't set this, the package name is mock_ concatenated with the package of the input file.

• -imports: A list of explicit imports that should be used in the resulting source code, specified as a comma-separated list of elements of the form foo=bar/baz, where bar/baz is the package being imported and foo is the identifier to use for the package in the generated source code.

• -aux_files: A list of additional files that should be consulted to resolve e.g. embedded interfaces defined in a different file. This is specified as a comma-separated list of elements of the form foo=bar/baz.go, where bar/baz.go is the source file and foo is the package name of that file used by the -source file.

• -build_flags: (reflect mode only) Flags passed verbatim to go build.

• -mock_names: A list of custom names for generated mocks. This is specified as a comma-separated list of elements of the form Repository=MockSensorRepository,Endpoint=MockSensorEndpoint, where Repository is the interface name and MockSensorRepository is the desired mock 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 naming convention will be used.

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

For an example of the use of mockgen, see the sample/ directory. In simple cases, 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)

  // Assert that Bar() is invoked.
  defer ctrl.Finish()

  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)
  defer ctrl.Finish()

  m := NewMockFoo(ctrl)

  // Does not make any assertions. Returns 101 when Bar is invoked with 99.
  m.
    EXPECT().
    Bar(gomock.Eq(99)).
    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)
}