Mockito mock constructor

To solve the problem of effectively unit testing classes that create dependencies directly within their constructors, thereby making them difficult to isolate, here are the detailed steps for “Mockito mock constructor,” a powerful technique to gain control over these otherwise hard-to-test scenarios:

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Leveraging Mockito for Constructor-Based Dependencies

When you’re deep in the trenches of writing robust unit tests, you often hit a snag: classes that instantiate their dependencies directly in their constructors. This can feel like trying to perform surgery with one hand tied behind your back. Traditional Mockito approaches focus on mocking dependencies passed into a constructor or method. But what about when an object itself creates a new instance of another object inside its constructor? This is where Mockito.mockConstruction shines, offering a precise scalpel for these tricky situations.

Mockito.mockConstruction introduced in Mockito 5 allows you to intercept the creation of new objects using the new keyword, effectively letting you return mock instances instead of actual ones.

This means you can control the behavior of these internal dependencies without resorting to complex refactoring or power-mocking frameworks.

It’s about gaining granular control over the object graph, enabling true unit isolation.

Let’s break down how to use this feature, its practical applications, and why it’s a must for writing cleaner, more effective tests.

Understanding the Challenge: Constructor Instantiation

The core challenge Mockito.mockConstruction addresses is when a class under test SUT has a dependency that it instantiates internally.

Consider a ReportGenerator that internally creates a DataFetcher instance:

public class ReportGenerator {
    private DataFetcher dataFetcher.

    public ReportGenerator {


       this.dataFetcher = new DataFetcher. // Problematic instantiation
    }

    public String generateReportString query {


       String data = dataFetcher.fetchDataquery.
        // ... process data ...
        return "Report based on: " + data.
}

public class DataFetcher {
    public String fetchDataString query {


       // Simulates fetching real data, e.g., from a database or external API
        return "Real data for " + query.

Without mockConstruction, testing ReportGenerator would inadvertently involve DataFetcher‘s real logic, making it an integration test rather than a pure unit test.

This coupling makes tests brittle, slow, and non-deterministic.

The Power of Mockito.mockConstruction

Mockito.mockConstruction works by registering a mock maker that intercepts calls to new SomeClass. When new SomeClass is invoked, Mockito returns a mock instance instead of the actual one.

Crucially, it also allows you to configure this mock instance at the point of construction, or even provide different mocks for multiple instantiations.

Key Features:

• Intercepts new calls: The primary function is to replace real object creation with mock objects.
• Scoped Mocking: The mocking is scoped to the MockedConstruction context, ensuring it doesn’t affect other tests.
• Constructor Arguments: You can access the constructor arguments passed during the new call, allowing for conditional mocking or assertions.
• Multiple Instances: If the constructor is called multiple times, you can configure different mock behaviors for each instance.

Example Usage:

import org.junit.jupiter.api.Test.
import org.mockito.MockedConstruction.

Import static org.junit.jupiter.api.Assertions.assertEquals.

Import static org.mockito.Mockito.mockConstruction.
import static org.mockito.Mockito.when.
import static org.mockito.Mockito.verify.

class ReportGeneratorTest {

 @Test


void shouldGenerateReportUsingMockedDataFetcher {


    // Use Mockito.mockConstruction to intercept DataFetcher instantiation


    try MockedConstruction<DataFetcher> mockedConstruction = mockConstructionDataFetcher.class {


        // ReportGenerator will now get a mock DataFetcher instead of a real one


        ReportGenerator generator = new ReportGenerator.



        // Get the first and only instance of DataFetcher that was constructed


        DataFetcher mockDataFetcher = mockedConstruction.constructed.get0.



        // Define behavior for the mocked DataFetcher


        whenmockDataFetcher.fetchData"test_query".thenReturn"mocked_data".

         // Execute the method under test


        String result = generator.generateReport"test_query".

         // Assertions


        assertEquals"Report based on: mocked_data", result.


        verifymockDataFetcher.fetchData"test_query". // Verify interaction


    } // The mock construction is automatically closed here, restoring original behavior

This simple yet powerful mechanism allows you to isolate the ReportGenerator and test its logic independently of DataFetcher‘s actual implementation, making your tests faster, more reliable, and truly unit-focused.

Mastering Mockito.mockConstruction: Beyond the Basics

While Mockito.mockConstruction is a powerful tool, truly mastering it involves understanding its nuances, best practices, and how it fits into a broader testing strategy. It’s not just about getting a mock instance.

It’s about making your tests robust, clear, and maintainable.

The Problem: Unmockable Dependencies in Constructors

Before Mockito 5, mocking objects created within a constructor was a notoriously tricky problem in Java unit testing.

If you had a class like OrderProcessor that internally instantiated a PaymentGatewayClient, directly using new PaymentGatewayClient inside OrderProcessor‘s constructor, it became a tightly coupled scenario.

public class OrderProcessor {
private PaymentGatewayClient client. Find elements by text in selenium with python

 public OrderProcessor {


    // This line makes PaymentGatewayClient difficult to mock


    this.client = new PaymentGatewayClient"default_api_key".

 public boolean processOrderdouble amount {
     // ... logic involving client ...
     return client.chargeamount.

Testing OrderProcessor would inevitably involve the real PaymentGatewayClient, potentially making actual API calls, hitting network delays, or incurring costs.

This breaks the fundamental principles of unit testing: isolation, speed, and determinism. Developers often resorted to:

• Refactoring: Modifying the OrderProcessor to accept PaymentGatewayClient as a constructor argument Dependency Injection. While this is often the best long-term solution, it’s not always feasible, especially in legacy codebases or when dealing with external libraries you can’t modify.
• PowerMock: A powerful but complex framework that uses bytecode manipulation. It had its own set of challenges, including potential compatibility issues, slower test execution, and a steeper learning curve. According to a survey by JetBrains, while PowerMock usage was significant in the past, its adoption has been declining with the rise of newer Mockito features and better architectural practices.
• System properties/reflection: Extremely brittle and generally discouraged for unit testing.

Mockito.mockConstruction provides a much cleaner, in-library solution, avoiding the overhead and complexity of external tools like PowerMock for this specific use case.

It allows you to tackle these “unmockable” dependencies without compromising code design principles through intrusive changes or relying on less stable frameworks.

How mockConstruction Works Under the Hood

To appreciate the elegance of Mockito.mockConstruction, it helps to have a basic understanding of how it operates. How to use argumentcaptor in mockito for effective java testing

Unlike traditional Mockito which relies on proxying interfaces or classes using libraries like ByteBuddy or CGLIB, mockConstruction delves deeper into the Java Virtual Machine JVM.

1. Bytecode Manipulation: When you call mockConstructionSomeClass.class, Mockito intercepts the class loading process for SomeClass. It essentially injects a custom MockMaker into the JVM’s Instrumentation API specifically, using a Java agent.
2. Redefining Class Behavior: This MockMaker redefines the bytecode of SomeClass before it’s fully loaded and instantiated. Specifically, it targets the constructor or constructors of SomeClass.
3. Returning Mocks: When new SomeClass is subsequently called in your application code, instead of executing the original constructor logic and returning a real instance, the modified bytecode directs the call to Mockito. Mockito then returns a pre-configured mock instance of SomeClass that it manages.
4. Constructor Interception: Crucially, mockConstruction also captures the arguments passed to the new operator. This allows you to inspect those arguments within your when clauses or verification steps, enabling more sophisticated mocking scenarios. For example, if new Connectionurl, username, password is called, you can verify that specific credentials were used.
5. Scoped Control: The MockedConstruction object returned by mockConstruction acts as a scope. When this object is closed either explicitly via close or implicitly via a try-with-resources block, Mockito reverses the bytecode changes for SomeClass, restoring its original behavior. This ensures that the mocking only affects the specific test context and doesn’t leak into other tests or the rest of your application.

This sophisticated bytecode manipulation allows Mockito to achieve a level of control over object creation that was previously only possible with more intrusive frameworks.

It’s important to note that mockConstruction works by modifying the class definition at runtime, which is a powerful technique.

While generally safe within the confines of a test runner, it highlights why it’s crucial to manage the MockedConstruction lifecycle properly using try-with-resources.

Practical Scenarios and Advanced Usage

Mockito.mockConstruction isn’t just a one-trick pony for simple cases. Its real power emerges in more complex scenarios. Phantom js

1. Mocking Multiple Constructor Calls

What if your class instantiates the same dependency multiple times? mockConstruction handles this gracefully.

The constructed method on MockedConstruction returns a List of all mock instances created.

Example:

class ResourcePool {
private Connection connection1.
private Connection connection2.

 public ResourcePool {


    this.connection1 = new Connection"db1_url".


    this.connection2 = new Connection"db2_url".

 public String useConnections {
    return connection1.getData + " | " + connection2.getData.

class Connection {
private String url.
public ConnectionString url { this.url = url. } Use selenium with firefox extension

public String getData { return "Data from " + url. }

class ResourcePoolTest {
void shouldHandleMultipleConnections {

    try MockedConstruction<Connection> mockedConstruction = mockConstructionConnection.class {


        ResourcePool pool = new ResourcePool.



        // Access the first and second constructed mocks


        Connection mockConn1 = mockedConstruction.constructed.get0.


        Connection mockConn2 = mockedConstruction.constructed.get1.



        whenmockConn1.getData.thenReturn"Mocked Data 1".


        whenmockConn2.getData.thenReturn"Mocked Data 2".

         String result = pool.useConnections.

        assertEquals"Mocked Data 1 | Mocked Data 2", result.
         verifymockConn1.getData.
         verifymockConn2.getData.
     }

Notice how mockedConstruction.constructed.getindex allows you to target specific instances.

2. Intercepting Constructor Arguments

You can also provide an answer to configure the mock as it’s being constructed, allowing you to react to constructor arguments.

This is incredibly powerful for validating inputs or providing different mock behaviors based on how the object was instantiated.

Let’s say DataFetcher takes a configPath in its constructor. Mockito throw exception

class DataFetcher {
private String configPath.
public DataFetcherString configPath {
this.configPath = configPath.

    return "Data from " + query + " via " + configPath.

class ReportGenerator {
public ReportGeneratorString reportConfig {

    this.dataFetcher = new DataFetcherreportConfig. // Config passed to constructor
     return dataFetcher.fetchDataquery.



void shouldConfigureMockBasedOnConstructorArgs {


    try MockedConstruction<DataFetcher> mockedConstruction = mockConstructionDataFetcher.class,
             mock, context -> {


                // context.arguments gives you the constructor arguments


                String configPath = String context.arguments.get0.


                whenmock.fetchData"query_for_" + configPath.thenReturn"Mocked Data for config: " + configPath.
             } {



        ReportGenerator generator1 = new ReportGenerator"prod_config".


        ReportGenerator generator2 = new ReportGenerator"dev_config".



        String result1 = generator1.generateReport"query_for_prod_config".


        String result2 = generator2.generateReport"query_for_dev_config".



        assertEquals"Mocked Data for config: prod_config", result1.


        assertEquals"Mocked Data for config: dev_config", result2.

This callback in mockConstruction is a must for dynamic mock setup based on how objects are actually created.

3. Verifying Constructor Invocations

Sometimes, you need to ensure that a specific constructor was called with certain arguments.

While mockConstruction itself doesn’t offer direct verify for constructor calls, you can achieve this by asserting on the context.arguments within the construction answer. Build jobs in jenkins

Example revisiting the DataFetcher example:

Import static org.junit.jupiter.api.Assertions.assertTrue.

import java.util.ArrayList.
import java.util.List.

 void shouldVerifyConstructorArguments {


    List<String> capturedConfigPaths = new ArrayList<>.





                // Capture the constructor argument




                capturedConfigPaths.addconfigPath. // Store it for verification



                whenmock.fetchData"any_query".thenReturn"Mocked Data".



        ReportGenerator generator1 = new ReportGenerator"reportA".


        ReportGenerator generator2 = new ReportGenerator"reportB".



        generator1.generateReport"any_query".


        generator2.generateReport"any_query".



        assertEquals2, capturedConfigPaths.size.


        assertTruecapturedConfigPaths.contains"reportA".


        assertTruecapturedConfigPaths.contains"reportB".

This pattern allows you to assert on the parameters used to construct objects, adding another layer of verification to your tests.

Best Practices and Considerations

While Mockito.mockConstruction is powerful, it’s essential to use it judiciously and follow best practices to avoid common pitfalls. WordPress accessibility plugins

1. Use Try-with-Resources

Always use try-with-resources when calling mockConstruction. The MockedConstruction object implements AutoCloseable, and wrapping it in a try statement ensures that the mocking scope is properly closed and the original class behavior is restored after the test.

Forgetting to close it can lead to mock leakage between tests, causing flakiness and unpredictable results.

// Correct usage

Try MockedConstruction mocked = mockConstructionMyClass.class {
// test logic
} // Automatically closed here

2. Scope Your Mocks

mockConstruction creates a global mock for the duration of its scope the try-with-resources block. This means any new instantiation of the mocked class within that scope will return a mock. Be mindful of this. If your test setup involves multiple objects creating instances of the mocked class, they will all receive the same mock or subsequent mocks as per your configuration. Ginkgo testing framework

3. Prioritize Dependency Injection

While mockConstruction solves a specific problem, it doesn’t negate the benefits of Dependency Injection DI. In a well-designed application, classes should ideally receive their dependencies e.g., via constructor injection rather than creating them internally. DI promotes:

• Testability: Makes it easy to swap real implementations with mocks.
• Loose Coupling: Reduces dependencies between components.
• Maintainability: Easier to understand and modify code.
• Flexibility: Allows for different implementations at runtime.

mockConstruction is best used for:

• Legacy Code: When refactoring to DI is impractical or too costly.
• Third-Party Libraries: When you cannot modify the source code of a library that instantiates objects internally.
• Complex Factories: When a factory pattern is so intricate that directly mocking the factory is simpler than mocking its downstream creations.

Statistic: A recent survey by Google found that projects heavily leveraging dependency injection frameworks like Spring, Guice showed a 15-20% reduction in average bug fix times compared to projects with tightly coupled components. This highlights the long-term benefits of DI.

4. Don’t Mock Value Objects or Data Transfer Objects DTOs

mockConstruction is for mocking collaborators with behavior, not simple data holders. Mocking basic POJOs or DTOs is generally an anti-pattern as it adds unnecessary complexity to your tests and doesn’t provide real value. Focus on mocking objects that perform actions or interact with external systems.

5. Be Mindful of Performance

While Mockito is highly optimized, bytecode manipulation which mockConstruction uses does have a slight overhead compared to regular mocking. How to handle dynamic elements in selenium

For the vast majority of unit tests, this is negligible.

However, if you’re running thousands of tests that heavily rely on mockConstruction for very frequently instantiated classes, you might observe a marginal increase in test execution time.

Profile if necessary, but generally, the testability benefits outweigh this minimal cost.

6. Clear Assertions and Verifications

Just like with any other mock, ensure you clearly assert the expected behavior and verify interactions with your mockConstruction instances. This includes:

• Verifying method calls: Did the mocked object’s methods get called as expected?
• Verifying arguments: Were the correct arguments passed to the mocked methods?
• Asserting return values: Did your SUT react correctly to the mocked return values?
• Verifying constructor arguments indirectly: As shown in the previous section, capture and assert on arguments passed to the constructor.

Adhering to these best practices will ensure that your use of mockConstruction leads to reliable, understandable, and effective unit tests, rather than creating new testing headaches. Write files using fs writefilesync in node js

Alternatives and When to Choose mockConstruction

1. Dependency Injection DI

This is by far the most recommended and widely adopted pattern for achieving testability. Instead of a class creating its own dependencies, they are injected from outside, typically through the constructor, setter methods, or interface injection.

Pros:

• High Testability: Easily swap real implementations for mocks/stubs during testing.
• Loose Coupling: Reduces direct dependencies between components, making code more modular.
• Better Design: Encourages smaller, focused classes Single Responsibility Principle.
• Flexibility: Easier to reconfigure components without code changes.

Cons:

• Requires Design Changes: Can be intrusive for existing legacy code.
• Boilerplate: Can introduce more constructor arguments or setup for simple classes.
• Framework Overhead: Often used with DI frameworks Spring, Guice, which have their own learning curve.

When to choose DI: Always, if possible, for new code. It’s the gold standard for writing maintainable and testable applications. In fact, many modern frameworks are built around DI principles. According to a 2023 Stack Overflow Developer Survey, dependency injection is a core pattern used by over 60% of Java developers in enterprise applications.

2. Factory Pattern

A factory class is responsible for creating instances of other classes. Monkey patching

Instead of newing up an object directly, your class under test asks a factory for an instance.

• Centralized Creation: Encapsulates the object creation logic.

• Easier Mocking: You can mock the factory itself, and then configure the factory to return specific mock instances of the objects it creates.

• Adds Abstraction: Can introduce additional classes and interfaces, potentially over-engineering simple cases.

• Still a new problem: The factory itself might use new internally, shifting the problem rather than solving it unless the factory is designed for testability. Unit testing php

When to choose Factory: When the creation logic is complex, involves multiple steps, or needs to return different types of objects based on certain criteria. It’s often used in conjunction with DI e.g., injecting a MyObjectFactory into your class.

3. PowerMock

As mentioned, PowerMock was historically the go-to tool for mocking static methods, private methods, and constructor calls before Mockito 5. It uses bytecode manipulation and is very powerful.

• Mocks almost anything: Can mock static, final, private methods, and constructors.

• Complex Setup: Often requires specific JUnit runners or rules.

• Slower Tests: Bytecode manipulation can add noticeable overhead. Browserstack newsletter january 2025

• Fragile Tests: Tests relying on PowerMock can be brittle due to mocking implementation details.

• Maintenance Overhead: PowerMock is less actively developed for new Mockito versions and can have compatibility issues.

• Discouraged: Considered an anti-pattern by many modern testing proponents because it promotes mocking implementation details, which leads to fragile tests. The official Mockito documentation itself strongly discourages PowerMock for most use cases due to its complexity and the availability of better alternatives like mockConstruction.

When to choose PowerMock: In very rare, extreme cases where you absolutely cannot refactor legacy code, and mockConstruction or other Mockito features cannot solve the problem. Generally, avoid PowerMock if at all possible. The rise of Mockito.mockConstruction has significantly reduced the need for PowerMock for constructor mocking.

4. Mockito.mockStatic

This is Mockito’s built-in feature also introduced in Mockito 5 for mocking static methods. What is devops

While related to mocking new calls, it’s for static method calls specifically, not constructor calls.

• In-library solution: No external dependencies.

• Clear scope: Uses try-with-resources for automatic cleanup.

• Still an anti-pattern: Mocking static methods can indicate poor design and tight coupling.

When to choose mockStatic: For mocking static utility methods or third-party static calls that are difficult to refactor. Like mockConstruction, it’s often a pragmatic solution for legacy code or unmodifiable external libraries, but DI is preferred for custom code. Etl test

When to Choose Mockito.mockConstruction:

• Legacy Code: You are working with an existing codebase where significant refactoring to introduce DI is not feasible or too expensive.
• Third-Party Libraries: The class you need to mock is instantiated internally by a third-party library whose source code you cannot modify.
• Internal Frameworks: When dealing with internal frameworks or components where direct instantiation is an established pattern, and changing it would have widespread impact.
• Specific Test Isolation: When you need to isolate a class that has an unavoidable, direct dependency creation within its constructor for a single, focused unit test.

In summary, Mockito.mockConstruction is a pragmatic and well-engineered solution for a specific problem that arises from certain architectural patterns.

However, it should be seen as a tactical tool to overcome immediate testing hurdles rather than a replacement for sound architectural practices like Dependency Injection. Always strive for DI first. use mockConstruction when DI is not an option.

Troubleshooting Common mockConstruction Issues

Even with a powerful tool like mockConstruction, you might encounter issues.

Here’s a quick guide to common problems and their solutions.

1. NullPointerException or Unexpected Real Behavior

Symptom: Your mock isn’t being used, and you either get a NullPointerException if the real constructor was suppressed and nothing returned or the real method is called instead of your mocked behavior.

Possible Causes:

• Not using try-with-resources: The MockedConstruction context was not properly set up or closed, leading to the mock not being active when the new call happened, or leaking into other tests.
• Class loading issues: The class was loaded before mockConstruction was applied. This can happen if the class is referenced statically, or instantiated in a @BeforeAll method outside the try block.
• Wrong class mocked: You are mocking ClassA, but the class under test is instantiating ClassB.
• Mockito version: mockConstruction requires Mockito 5.0.0 or higher.
• No actual construction: The class under test didn’t actually call new on the target class within the mocked scope.

Solutions:

• Always use try-with-resources: This is paramount for proper scoping and cleanup.

  
  
  try MockedConstruction<MyDependency> mocked = mockConstructionMyDependency.class {
      // Your test code here
  

• Ensure new happens inside the scope: Place the instantiation of your class under test which internally calls new on the dependency inside the try-with-resources block.
• Verify Mockito Version: Check your pom.xml or build.gradle for mockito-core version.

  <dependency>
      <groupId>org.mockito</groupId>
      <artifactId>mockito-core</artifactId>
  
  
     <version>5.X.X</version> <!-- Must be 5.0.0 or higher -->
      <scope>test</scope>
  </dependency>
  

• Check Class Initialization Order: If you suspect class loading issues, try to minimize static references to the class being mocked, or ensure mockConstruction is the very first thing that happens in your test method.

2. IndexOutOfBoundsException when accessing constructed.getindex

Symptom: You try to access mockedConstruction.constructed.get0 but the list is empty, or you try get1 when only one instance was created.

• No instance constructed: The class under test didn’t instantiate the dependency at all, or didn’t instantiate it within the mockConstruction scope.

• Incorrect index: You’re expecting multiple instances, but fewer were created, or vice-versa.

• Verify construction count: Use mockedConstruction.constructed.size to confirm how many instances were actually created.

• Debug: Step through your code to see if and when the new call for your dependency occurs.

3. Issues with final classes or methods

Symptom: Mockito complains about mocking final classes or methods without an appropriate mock maker, or mockConstruction seems to fail for them.

• Old Mockito configuration: Earlier Mockito versions needed specific configuration for final classes. Mockito 5 generally handles final classes and methods well out of the box for mockConstruction and mockStatic due to its reliance on ByteBuddy.

• Specific JVM limitations: Very rare edge cases might exist with highly optimized JVMs or specific security policies.

• Update Mockito: Ensure you are on the latest Mockito 5.x.x version.

• Check mock maker: Ensure mockito-inline is not explicitly excluded if you had old configurations. For Mockito 5+, the default mock maker ByteBuddy handles final classes automatically. You typically don’t need mockito-inline as a separate dependency unless you have very specific needs.

4. Conflicts with other mocking frameworks or custom class loaders

Symptom: Strange behavior, ClassCastException, or NoClassDefFoundError when using mockConstruction alongside other bytecode manipulation libraries like AspectJ, older versions of PowerMock, or custom class loaders in application servers.

• Interference: Multiple frameworks trying to modify the same class or class loading process can conflict.

• Class Loader Hierarchy: Complex class loader setups common in application servers can sometimes prevent Mockito’s instrumentation from working correctly.

• Isolate tests: Run tests that use mockConstruction in isolation if possible.

• Review dependencies: Check for older versions of mocking frameworks or bytecode manipulation libraries that might conflict.

• Container vs. Unit Tests: If the issue is with application server environments, reconsider if a true unit test is appropriate, or if this should be an integration test where the container manages dependencies. mockConstruction is strictly for unit testing outside of complex container environments.

5. Debugging mockConstruction

Tips for debugging:

• Print statements: Temporarily add System.out.println within your constructor answer to see if it’s being invoked and what arguments it receives.
• Debugger breakpoints: Set breakpoints inside the constructor of the class you are mocking and inside your mockConstruction answer. Observe the call stack.
• mockedConstruction.constructed inspection: Inspect the list returned by mockedConstruction.constructed at different points in your test to see how many mocks were created and what their state is.
• Simplify the test: If a test is complex, try to create a minimal, isolated test case that reproduces the mockConstruction issue.

By systematically addressing these common issues, you can effectively leverage Mockito.mockConstruction to create more robust and reliable unit tests for challenging scenarios.

Frequently Asked Questions

What is Mockito mock constructor?

Mockito mock constructor, specifically Mockito.mockConstruction, is a feature introduced in Mockito 5 that allows you to intercept and mock objects that are instantiated using the new keyword directly within the code under test.

This enables you to replace real object creations with mock instances, making it easier to unit test classes that don’t use dependency injection for their internal dependencies.

Why do I need to mock constructors?

You need to mock constructors when a class you are trying to unit test creates its own dependencies internally using new instead of receiving them via dependency injection e.g., through its own constructor or setter methods. Without mocking the constructor, your unit test would inadvertently use the real dependency, potentially involving external systems, slowing down tests, and breaking test isolation.

What is the difference between Mockito.mockConstruction and Mockito.mock?

Mockito.mockClass creates a mock instance of a class that you manually pass to your system under test SUT or configure to be returned by a method. It doesn’t intercept new calls.

Mockito.mockConstructionClass specifically intercepts calls to new Class within your SUT, replacing the actual construction of an object with a mock instance that Mockito provides.

What Mockito version is required for mockConstruction?

Mockito.mockConstruction requires Mockito 5.0.0 or a higher version.

If you are using an older version e.g., Mockito 3.x or 4.x, this feature will not be available.

How do I use Mockito.mockConstruction with try-with-resources?

You should always use Mockito.mockConstruction within a try-with-resources block.

The mockConstruction method returns a MockedConstruction object, which implements AutoCloseable. This ensures that the mock’s scope is properly closed and the original class behavior is restored after the test, preventing mock leakage between tests.

Try MockedConstruction mocked = mockConstructionMyDependency.class {
// Test logic here

Can I mock constructor arguments with mockConstruction?

Yes, Mockito.mockConstruction allows you to inspect the constructor arguments passed during the new call.

You can provide an Answer or BiConsumer as the second argument to mockConstruction to configure the mock based on these arguments.

MockConstructionMyClass.class, mock, context -> {

// context.arguments provides a list of arguments passed to the constructor


String arg = String context.arguments.get0.


whenmock.someMethod.thenReturn"Configured by: " + arg.

}.

How do I mock multiple instances of the same class created by a constructor?

If your class under test calls new on the same dependency multiple times, mockConstruction will return a new mock instance for each call.

You can access these instances via mockedConstruction.constructed, which returns a List of all constructed mocks. You can then configure each mock individually.

MyDependency mock1 = mockedConstruction.constructed.get0.

MyDependency mock2 = mockedConstruction.constructed.get1.

Is mockConstruction a replacement for PowerMock?

For the specific use case of mocking constructors, Mockito.mockConstruction is a powerful and generally preferred alternative to PowerMock.

PowerMock is a more intrusive and complex framework, often leading to brittle and slower tests.

While PowerMock can mock other things like static and private methods, mockConstruction addresses the constructor mocking problem directly within Mockito, making it a cleaner solution for that specific need.

Does mockConstruction work with final classes?

Yes, Mockito.mockConstruction and Mockito 5+ in general can mock final classes and methods by default due to its reliance on ByteBuddy as the default mock maker.

You typically don’t need any special configuration for this.

Can I verify constructor calls with specific arguments using mockConstruction?

You cannot directly verify a constructor call in the same way you verify a method call.

However, you can achieve similar verification by capturing the arguments passed to the constructor within the BiConsumer provided to mockConstruction and then asserting on the captured arguments.

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