Mocking is useful to control the behavior of functions and objects to control the communication with other modules and external endpoints. A mock can be created by defining return values or replacing the entire object or function with a user-defined equivalent. This feature will help you to test the Ballerina code independently from other modules and external endpoints.

Mock objects

The Test module provides capabilities to mock an object for unit testing. This allows you to control the behaviour of the object member functions and values of member fields via stubbing or replacing the entire object with a user-defined equivalent. This feature will help you to test the Ballerina code independently of other modules and external endpoints.

Mocking objects can be done in two ways :

1. Creating a test double (providing an equivalent object in place of the real object)
2. Stubbing the member function or member variable (specifying the behaviour of the functions and values of the variables)

Create a test double

You can write a custom mock object and substitute it in place of the real object. The custom object should be made structurally equivalent to the real object via the mocking features in the test module.

Example:

Consider the following example in which an http:Client interacts with an external endpoint to get a random joke.

main.bal

import ballerina/http;
import ballerina/regex;

http:Client clientEndpoint = check new ("https://api.chucknorris.io/jokes/");

type Joke readonly & record {|
    string value;
|};

// This function performs a `get` request to the Chuck Norris API and returns a random joke 
// with the name replaced by the provided name or an error if the API invocation fails.
function getRandomJoke(string name) returns string|error {
    Joke joke = check clientEndpoint->get("/random");
    string replacedText = regex:replaceAll(joke.value, "Chuck Norris", name);
    return replacedText;
}

Let's write tests for the above main.bal file to define a test double for the clientEndpoint object.

Note: Only the get function is implemented since it is the only function used in the sample. Attempting to call any other member function of the clientEndpoint will result in a runtime error.

main_test.bal

import ballerina/test;
import ballerina/http;

// An instance of this object can be used as the test double for the `clientEndpoint`.
public client class MockHttpClient {

    remote function get(string path, map<string|string[]>? headers = (), http:TargetType targetType = http:Response) returns http:Response|anydata|http:ClientError {
        Joke joke = {"value": "Mock When Chuck Norris wants an egg, he cracks open a chicken."};
        return joke;
    }

}

@test:Config {}
public function testGetRandomJoke() {

    // create and assign a test double to the `clientEndpoint` object
    clientEndpoint = test:mock(http:Client, new MockHttpClient());

    // invoke the function to test
    string|error result = getRandomJoke("Sheldon");

    // verify that the function returns the mock value after replacing the name
    test:assertEquals(result, "Mock When Sheldon wants an egg, he cracks open a chicken.");
}

Stub member functions and variables of an object

Instead of creating a test double, you may also choose to create a default mock object and stub the functions to return a specific value or to do nothing.

Note: It is important to ensure that all member functions of the object being tested are properly stubbed. If any function is called within the implementation that hasn't been stubbed, the test framework will generate an error message in the following format: no cases registered for member function '<member_function_name>' of object type '<object_type>'.

Example:

Let’s make changes to the above example to get a random joke from a specific category (e.g., food or movies).

main.bal

import ballerina/http;
import ballerina/lang.array;
import ballerina/io;

http:Client clientEndpoint = check new ("https://api.chucknorris.io/jokes/");

type Joke readonly & record {|
    string value;
|};

// This function performs a `get` request to the Chuck Norris API and returns a random joke 
// or an error if the API invocations fail.
function getRandomJoke(string name, string category = "food") returns string|error {
    string[] categories = check clientEndpoint->get("/categories");

    if !isCategoryAvailable(categories, category) {
        string errorMsg = "'" + category + "' is not a valid category. ";
        io:println(errorMsg);
        return error(errorMsg);
    }

    // Get a random joke from the provided category
    Joke joke = check clientEndpoint->get("/random?category=" + category);
    return re `Chuck Norris`.replaceAll(joke.value, name);
}

function isCategoryAvailable(string[] categories, string category) returns boolean => array:some(categories, categoryVal => categoryVal == category);

test_utils.bal

The util functions below are used to construct mock responses required for testing.

// Returns a mock Joke to be used for the random joke API invocation.
function getMockResponse() returns Joke {
    Joke joke = {"value": "When Chuck Norris wants an egg, he cracks open a chicken."};
    return joke;
}

// Returns a mock response to be used for the category API invocation.
function getCategoriesResponse() returns string[] {
    return ["animal", "food", "history", "money", "movie"];
}

Stub to return a specific value

main_test.bal

This test stubs the behaviour of the get function to return a specific value in 2 ways:

1. Stubbing to return a specific value in general
2. Stubbing to return a specific value based on the input

import ballerina/test;
import ballerina/http;

@test:Config {}
public function testGetRandomJoke() {
    // Create a default mock HTTP Client and assign it to the `clientEndpoint` object
    clientEndpoint = test:mock(http:Client);

    // Stub to return the specified mock response when the `get` function is called.
    test:prepare(clientEndpoint).when("get").thenReturn(getMockResponse());

    // Stub to return the specified mock response when the specified argument is passed.
    test:prepare(clientEndpoint).when("get").withArguments("/categories")
        .thenReturn(getCategoriesResponse());

    // Invoke the function to test.
    string|error result = getRandomJoke("Sheldon");

    // Verify the return value against the expected string.
    test:assertEquals(result, "When Sheldon wants an egg, he cracks open a chicken.");
}

Stub with multiple values to return sequentially for each function call

main_test.bal

This test stubs the behaviour of the get function to return a specified sequence of values for each get function invocation (i.e., the first call to the get function will return the first argument and the second call will return the second argument).

import ballerina/test;
import ballerina/http;
    
@test:Config {}
public function testGetRandomJoke() {
    // Create a default mock HTTP Client and assign it to the `clientEndpoint` object.
    clientEndpoint = test:mock(http:Client);

    // Stub to return the corresponding value for each invocation 
    test:prepare(clientEndpoint).when("get")
        .thenReturnSequence(getCategoriesResponse(), getMockResponse());

    // Invoke the function to test
    string|error result = getRandomJoke("Sheldon");

    // Verify the return value against the expected string
    test:assertEquals(result, "When Sheldon wants an egg, he cracks open a chicken.");
}

Stub a member variable

If a client object has a public member variable, it can be stubbed to return a mock value for testing.

Example:

main.bal

# A record that represents a Product.
#
# + code - Code used to identify the product
# + name - Product Name
# + quantity - Quantity included in the product
public type Product record {|
    readonly int code;
    string name;
    string quantity;
|};

# A table with a list of Products uniquely identified using the code.
public type ProductInventory table<Product> key(code);

// This is a sample data set in the defined inventory.
ProductInventory inventory = table [
    {code: 1, name: "Milk", quantity: "1l"},
    {code: 2, name: "Bread", quantity: "500g"},
    {code: 3, name: "Apple", quantity: "750g"}
];

# This client represents a product.
#
# + productCode - An int code used to identify the product.
public client class ProductClient {
    public int productCode;

    public function init(int productCode) {
        self.productCode = productCode;
    }
}

// The Client represents the product with the code `1` (i.e. "Milk").
ProductClient productClient = new (1);

# Get the name of the product represented by the ProductClient.
#
# + return - The name of the product
public function getProductName() returns string? {
    if !inventory.hasKey(productClient.productCode) {
        return;
    }
    Product? product = inventory.get(productClient.productCode);
    return product is Product ? product.name : ();
}

main_test.bal

This test stubs the member variable productCode of the ProductClient to set a mock product code.

import ballerina/test;

@test:Config {}
function testMemberVariable() {
    int mockProductCode = 2;
    // Create a mockClient which represents product with the code `mockProductCode`
    ProductClient mockClient = test:mock(ProductClient);
    // Stub the member variable `productCode`
    test:prepare(mockClient).getMember("productCode").thenReturn(mockProductCode);
    // Replace `productClient` with the `mockClient`
    productClient = mockClient;
    // Assert for the mocked product name.
    test:assertEquals(getProductName(), "Bread");
}

Stub to do nothing

If a function has an optional or no return type specified, this function can be mocked to do nothing when writing test cases.

Example:

main.bal

import ballerina/email;

email:SmtpClient smtpClient = check new ("localhost", "admin","admin");

// This function sends out emails to specified email addresses and returns an error if sending failed.
function sendNotification(string[] emailIds) returns error? {
    email:Message msg = {
        'from: "builder@abc.com",
        subject: "Error Alert ...",
        to: emailIds,
        body: ""
    };
    return check smtpClient->sendMessage(msg);
}

main_test.bal

This test stubs the behaviour of the send function to do nothing for testing the sendNotification function.

import ballerina/test;
import ballerina/email;

@test:Config {}
function testSendNotification() {
    string[] emailIds = ["user1@test.com", "user2@test.com"];

    // Create a default mock SMTP client and assign it to the `smtpClient` object.
    smtpClient = test:mock(email:SmtpClient);

    // Stub to do nothing when the`send` function is invoked.
    test:prepare(smtpClient).when("sendMessage").doNothing();

    // Invoke the function to test and verify that no error occurred.
    test:assertEquals(sendNotification(emailIds), ());
}

Mock functions

The Ballerina test framework provides the capability to mock a function. You can easily mock a function in a module that you are testing or a function of an imported module by using the mocking feature. This feature will help you to test your Ballerina code independently from other modules and functions.

The object specified with the @test:Mock{} annotation will be considered as a mock function, which gets triggered in place of the real function.

• moduleName : "<moduleName>" - (optional) Name of the module in which the function to be mocked resides in. If the function is within the same module, this can be left blank or "." (no module) can be passed. If the function is in a different module but within the same package, just passing the module name will suffice. For functions in completely separate modules, the fully-qualified module name must be passed, which includes the packageOrg (i.e., packageOrg/moduleName). For native functions, the Ballerina module needs to be specified.

• functionName : "<functionName>" - Name of the function to be mocked.

Example:

main.bal

// This function returns the result provided by the `intAdd` function.
public function addValues(int a, int b) returns int {
    return intAdd(a, b);
}

// This function adds two integers and returns the result.
public function intAdd(int a, int b) returns int {
    return (a + b);
}

main_test.bal

This is the initialization of the mock function, which should be called in place of the intAdd function.

import ballerina/test;

@test:Mock {functionName: "intAdd"}
test:MockFunction intAddMockFn = new ();

After the initialization, the following options can be used to stub the behaviour of a function written in the module being tested.

Stub to return a specific value

This test stubs the behaviour of the get function to return a specific value in 2 ways:

1. Stubbing to return a specific value in general
2. Stubbing to return a specific value based on the input

import ballerina/test;

@test:Mock {functionName: "intAdd"}
test:MockFunction intAddMockFn = new ();
   
@test:Config {}
function testReturn() {
    // Stub to return the specified value when the `intAdd` is invoked.
    test:when(intAddMockFn).thenReturn(20);
   
    // Stub to return the specified value when the `intAdd` is invoked with the specified arguments.
    test:when(intAddMockFn).withArguments(0, 0).thenReturn(-1);
        
    test:assertEquals(addValues(10, 6), 20, msg = "function mocking failed");
    test:assertEquals(addValues(0, 0), -1, msg = "function mocking with arguments failed");
}

Stub to invoke another function in place of the real

This test stubs the behaviour of the intAdd function to substitute it with a user-defined mock function.

import ballerina/test;

@test:Mock {functionName: "intAdd"}
test:MockFunction intAddMockFn = new ();

@test:Config {}
function testCall() {
    // Stub to call another function when `intAdd` is called.
    test:when(intAddMockFn).call("mockIntAdd");
   
    test:assertEquals(addValues(11, 6), 5, msg = "function mocking failed");
}
    
// The mock function to be used in place of the `intAdd` function
public function mockIntAdd(int a, int b) returns int {
    return (a - b);
}

This test stubs the behaviour of an imported function to substitute it with a user-defined mock function.

import ballerina/test;
import ballerina/io;

@test:Mock {
    moduleName: "ballerina/io",
    functionName: "println"
}
test:MockFunction printlnMockFn = new ();

int tally = 0;

// This is a function that can be called in place of the `io:println` function.
public function mockPrint(any|error... val) {
    tally = tally + 1;
}

@test:Config {}
function testCall() {
    test:when(printlnMockFn).call("mockPrint");

    io:println("Testing 1");
    io:println("Testing 2");
    io:println("Testing 3");

    test:assertEquals(tally, 3);
}

This test calls the original intAdd function after it has been stubbed with a user-defined mock function.

import ballerina/test;
       
@test:Mock {functionName: "intAdd"}
test:MockFunction intAddMockFn = new ();

@test:Config {}
function testCallOriginal() {
    // Stub to call another function when `intAdd` is called.
    test:when(intAddMockFn).call("mockIntAdd");
   
    test:assertEquals(addValues(11, 6), 5, msg = "function mocking failed");
    
    // Stub to call the original `intAdd` function.
    test:when(intAddMockFn).callOriginal();
    test:assertEquals(addValues(11, 6), 17, msg = "function mocking failed");
}
    
// The mock function to be used in place of the `intAdd` function
public function mockIntAdd(int a, int b) returns int {
    return (a - b);
}