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Description

GoMock is a mocking framework for the Go programming language.

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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, install the

mockgen
tool.

To get the latest released version use:

GO111MODULE=on go get github.com/golang/mock/[email protected]

If you use

mockgen
in your CI pipeline, it may be more appropriate to fixate on a specific mockgen version.

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

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

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.

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

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.
  • -self_package
    : The full package import path for the generated code. The purpose of this flag is to prevent import cycles in the generated code by trying to include its own package. This can happen if the mock's package is set to one of its inputs (usually the main one) and the output is stdio so mockgen cannot detect the final output package. Setting this flag will then tell mockgen which import to exclude.
  • -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) }

If you are using a Go version of 1.14+, a mockgen version of 1.5.0+, and are passing a *testing.T into

gomock.NewController(t)
you no longer need to call
ctrl.Finish()
explicitly. It will be called for you automatically from a self registered Cleanup function.

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. 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 the expected (
Want
) value.
Got: [3]
Want: is equal to 2
Expected 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
Modifying
Want

The

Want
value comes from the matcher's
String()
method. If the matcher's default 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 the

gomock.Eq(15)
matcher's output for
Want:
from
is equal
to 15
to
is equal to fifteen
.
Modifying
Got

The

Got
value comes from the object's
String()
method if it is available. In some cases the output of an object is difficult to read (e.g.,
[]byte
) and it would be helpful for the test to print it differently. The following modifies how the
Got
value is formatted:
gomock.GotFormatterAdapter(
  gomock.GotFormatterFunc(func(i interface{}) string {
    // Leading 0s
    return fmt.Sprintf("%02d", i)
  }),
  gomock.Eq(15),
)

If the received value is

3
, then it will be printed as
03
.

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