java - How to mock classes instantiated as local variables

Java - How to mock classes instantiated as local variables

Mocking classes instantiated as local variables can be challenging because the object creation is tightly coupled to the method. However, with a little bit of refactoring and the use of dependency injection or factory patterns, you can make it possible to mock those objects.

Here are several approaches to achieve this:

1. Dependency Injection

Inject dependencies into your class so that you can provide mocked instances during testing.

Example

Suppose you have the following class:

public class MyService { public void doSomething() { MyDependency myDependency = new MyDependency(); myDependency.doWork(); } }

You can refactor it to use dependency injection:

public class MyService { private final MyDependency myDependency; public MyService(MyDependency myDependency) { this.myDependency = myDependency; } public void doSomething() { myDependency.doWork(); } }

Now you can easily mock MyDependency in your tests:

import static org.mockito.Mockito.*; public class MyServiceTest { @Test public void testDoSomething() { MyDependency mockDependency = mock(MyDependency.class); MyService myService = new MyService(mockDependency); myService.doSomething(); verify(mockDependency).doWork(); } }

2. Factory Pattern

Use a factory to create instances of your dependencies. This way, you can mock the factory to return mocked dependencies.

Example

Original class:

public class MyService { public void doSomething() { MyDependency myDependency = new MyDependency(); myDependency.doWork(); } }

Refactor to use a factory:

public class MyService { private final MyDependencyFactory myDependencyFactory; public MyService(MyDependencyFactory myDependencyFactory) { this.myDependencyFactory = myDependencyFactory; } public void doSomething() { MyDependency myDependency = myDependencyFactory.create(); myDependency.doWork(); } } public class MyDependencyFactory { public MyDependency create() { return new MyDependency(); } }

Now you can mock the factory:

import static org.mockito.Mockito.*; public class MyServiceTest { @Test public void testDoSomething() { MyDependency mockDependency = mock(MyDependency.class); MyDependencyFactory mockFactory = mock(MyDependencyFactory.class); when(mockFactory.create()).thenReturn(mockDependency); MyService myService = new MyService(mockFactory); myService.doSomething(); verify(mockDependency).doWork(); } }

3. Using PowerMock (Not Recommended for New Projects)

If you can't change the code, you can use a tool like PowerMock to mock the creation of new objects. However, this approach is generally not recommended because it requires adding a lot of complexity and may lead to fragile tests.

Example

import static org.mockito.Mockito.*; import org.junit.Test; import org.junit.runner.RunWith; import org.powermock.api.mockito.PowerMockito; import org.powermock.core.classloader.annotations.PrepareForTest; import org.powermock.modules.junit4.PowerMockRunner; @RunWith(PowerMockRunner.class) @PrepareForTest(MyService.class) public class MyServiceTest { @Test public void testDoSomething() throws Exception { MyDependency mockDependency = mock(MyDependency.class); PowerMockito.whenNew(MyDependency.class).withNoArguments().thenReturn(mockDependency); MyService myService = new MyService(); myService.doSomething(); verify(mockDependency).doWork(); } }

Summary

  • Dependency Injection: Refactor your code to use dependency injection, allowing you to pass mock objects during testing.
  • Factory Pattern: Use a factory to create instances, which can be mocked to return test doubles.
  • PowerMock: As a last resort, use PowerMock to mock object creation, though this is not recommended for new projects due to added complexity and fragility.

By applying these patterns, you can decouple your code from the concrete implementations, making it easier to test and maintain.

Examples

  1. How to mock a class instantiated as a local variable using Mockito in Java?

    Code:

    import org.junit.jupiter.api.Test; import org.mockito.Mockito; public class MyClassTest { @Test public void testMethodWithLocalVariable() { // Create a mock instance of LocalClass LocalClass mockLocal = Mockito.mock(LocalClass.class); // Use mockLocal as the local variable in your test method MyClass myClass = new MyClass(); myClass.methodWithLocalVariable(mockLocal); // Verify interactions with mockLocal if necessary Mockito.verify(mockLocal).someMethod(); } }

    Description: This example demonstrates using Mockito to mock LocalClass, which is instantiated as a local variable within MyClass. The mock instance allows testing methodWithLocalVariable without invoking the actual LocalClass.

  2. How to mock a private method in a class with local variable instantiation using PowerMock in Java?

    Code:

    import org.junit.jupiter.api.Test; import org.powermock.api.mockito.PowerMockito; public class MyClassTest { @Test public void testPrivateMethodWithLocalVariable() throws Exception { // Mock the private method of MyClass MyClass mockMyClass = PowerMockito.spy(new MyClass()); PowerMockito.doReturn("mocked result").when(mockMyClass, "privateMethod", Mockito.any()); // Call the method that internally instantiates LocalClass String result = mockMyClass.methodCallingPrivateMethod(); // Assertions or verifications Assert.assertEquals("mocked result", result); PowerMockito.verifyPrivate(mockMyClass).invoke("privateMethod", Mockito.any()); } }

    Description: This code snippet shows how to use PowerMockito to mock a private method (privateMethod) in MyClass, which is called within a method (methodCallingPrivateMethod) that instantiates LocalClass locally.

  3. How to mock a local variable using EasyMock in Java?

    Code:

    import org.easymock.EasyMock; import org.junit.jupiter.api.Test; public class MyClassTest { @Test public void testMethodWithLocalVariable() { // Create a mock instance of LocalClass LocalClass mockLocal = EasyMock.createMock(LocalClass.class); // Use mockLocal as the local variable in your test method MyClass myClass = new MyClass(); myClass.methodWithLocalVariable(mockLocal); // Replay and verify EasyMock.replay(mockLocal); myClass.verifyMethod(mockLocal); // Verify interactions with mockLocal if necessary EasyMock.verify(mockLocal); } }

    Description: This example demonstrates using EasyMock to mock LocalClass instantiated as a local variable within MyClass. It enables testing methodWithLocalVariable and verifying interactions with mockLocal.

  4. How to mock a local variable using JMockit in Java?

    Code:

    import org.junit.jupiter.api.Test; import mockit.Injectable; import mockit.Tested; public class MyClassTest { @Tested MyClass myClass; @Injectable LocalClass mockLocal; @Test public void testMethodWithLocalVariable() { // Use mockLocal as the local variable in your test method myClass.methodWithLocalVariable(mockLocal); // Verification or assertions new Verifications() {{ mockLocal.someMethod(); times = 1; }}; } }

    Description: This snippet demonstrates using JMockit (@Injectable and @Tested) to mock LocalClass, which is instantiated locally within methodWithLocalVariable of MyClass.

  5. How to mock a local variable instantiated in a lambda expression using Mockito in Java?

    Code:

    import org.junit.jupiter.api.Test; import org.mockito.Mockito; public class MyClassTest { @Test public void testLambdaWithLocalVariable() { // Mock instance of LocalClass used in lambda expression LocalClass mockLocal = Mockito.mock(LocalClass.class); // Mocking lambda expression MyClass myClass = new MyClass(); myClass.methodWithLambda(() -> { // Use mockLocal in lambda body mockLocal.someMethod(); }); // Verify interactions with mockLocal if necessary Mockito.verify(mockLocal).someMethod(); } }

    Description: This code shows how to mock LocalClass used in a lambda expression within methodWithLambda of MyClass using Mockito for testing purposes.

  6. How to mock a local variable instantiated in a static method using PowerMock in Java?

    Code:

    import org.junit.jupiter.api.Test; import org.powermock.api.mockito.PowerMockito; public class MyClassTest { @Test public void testStaticMethodWithLocalVariable() throws Exception { // Mock the static method call PowerMockito.mockStatic(LocalClass.class); LocalClass mockLocal = new LocalClass(); // Mock or real instance // Call the method that instantiates LocalClass locally MyClass.methodWithStatic(LocalClass::new); // Assertions or verifications PowerMockito.verifyStatic(LocalClass.class); LocalClass.staticMethod(); } }

    Description: This snippet demonstrates using PowerMockito to mock LocalClass instantiated locally within a static method (methodWithStatic) of MyClass.

  7. How to mock a local variable instantiated in a constructor using Mockito in Java?

    Code:

    import org.junit.jupiter.api.Test; import org.mockito.Mockito; public class MyClassTest { @Test public void testConstructorWithLocalVariable() { // Mock instance of LocalClass used in constructor LocalClass mockLocal = Mockito.mock(LocalClass.class); // Mocking constructor call MyClass myClass = new MyClass(mockLocal); // Verify interactions with mockLocal if necessary Mockito.verify(mockLocal).someMethod(); } }

    Description: This example illustrates using Mockito to mock LocalClass instantiated as a local variable within the constructor of MyClass.

  8. How to mock a local variable instantiated inside a method using Mockito in Java?

    Code:

    import org.junit.jupiter.api.Test; import org.mockito.Mockito; public class MyClassTest { @Test public void testMethodWithLocalVariable() { // Mock instance of LocalClass used inside a method LocalClass mockLocal = Mockito.mock(LocalClass.class); // Mocking method call MyClass myClass = new MyClass(); myClass.methodWithLocalVariable(mockLocal); // Verify interactions with mockLocal if necessary Mockito.verify(mockLocal).someMethod(); } }

    Description: This code snippet demonstrates how to mock LocalClass instantiated as a local variable inside a method (methodWithLocalVariable) of MyClass using Mockito for testing.

  9. How to mock a local variable used in a lambda function using JMockit in Java?

    Code:

    import org.junit.jupiter.api.Test; import mockit.Injectable; import mockit.Tested; public class MyClassTest { @Tested MyClass myClass; @Injectable LocalClass mockLocal; @Test public void testLambdaWithLocalVariable() { // Use mockLocal in lambda function myClass.methodWithLambda(() -> { mockLocal.someMethod(); }); // Verification or assertions new Verifications() {{ mockLocal.someMethod(); times = 1; }}; } }

    Description: This example shows how to mock LocalClass used in a lambda function (methodWithLambda) of MyClass using JMockit (@Injectable and @Tested annotations).

  10. How to mock a local variable inside a try-with-resources block using Mockito in Java?

    Code:

    import org.junit.jupiter.api.Test; import org.mockito.Mockito; public class MyClassTest { @Test public void testTryWithResourcesWithLocalVariable() { // Mock instance of LocalClass used inside try-with-resources LocalClass mockLocal = Mockito.mock(LocalClass.class); // Mocking try-with-resources block try (LocalClass ignored = mockLocal) { // Use mockLocal inside try block mockLocal.someMethod(); } // Verify interactions with mockLocal if necessary Mockito.verify(mockLocal).someMethod(); } }

    Description: This snippet demonstrates how to mock LocalClass instantiated as a local variable inside a try-with-resources block in Java using Mockito for testing purposes.

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