JUnit Test in Java: Writing Effective Test Cases and Best Practices

JUnit test is a powerful framework for writing automated tests in Java applications. Developers use this framework to verify that individual code units function correctly. Unit testing helps catch bugs early before they cause problems in production.

The JUnit testing framework makes it simple to create repeatable tests for methods. These automated tests run quickly and provide immediate feedback about code quality. Well-written JUnit test cases serve as documentation for how code should work.

Good testing practices lead to more reliable and maintainable Java applications. The JUnit testing process becomes easier with practice and proper technique. Many professional Java developers consider unit testing an essential development skill. 

Learning how to write JUnit  test cases will improve your coding abilities. Quality unit tests significantly reduce the time spent debugging complex issues later. Several online Software Engineering courses now emphasize JUnit test as fundamental knowledge.

What is JUnit Testing

JUnit testing is a framework for writing and running unit tests in Java. Unit testing focuses on testing individual components or "units" of code. A unit is typically a single method or function. JUnit test cases verify that these units work correctly in isolation.

The junit test framework was created by Kent Beck and Erich Gamma. It follows the xUnit architecture for unit testing. JUnit is open-source and free to use in any Java project. It has become the standard testing tool in the Java ecosystem.

JUnit testing helps developers find bugs early. It validates code behavior against expected outcomes. Well-written junit test cases serve as living documentation. They show how each component should function.

The framework provides assertions to check expected results. It offers test runners to execute test suites. JUnit also includes annotations to configure test behavior. The latest version, JUnit 5, brings modern Java features to testing.

A good junit testing strategy covers all critical code paths. It tests both normal operations and edge cases. Thorough junit test coverage builds confidence in code quality.

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Setting Up JUnit in Java Projects

Setting up JUnit in your Java project is straightforward. You need to add the JUnit library to your project dependencies. The setup process varies slightly based on your build tool.

For Maven projects, add this to your pom.xml file:

<dependencies>
    <!-- JUnit 5 -->
    <dependency>
        <groupId>org.junit.jupiter</groupId>
        <artifactId>junit-jupiter-api</artifactId>
        <version>5.9.2</version>
        <scope>test</scope>
    </dependency>
    <dependency>
        <groupId>org.junit.jupiter</groupId>
        <artifactId>junit-jupiter-engine</artifactId>
        <version>5.9.2</version>
        <scope>test</scope>
    </dependency>
</dependencies>

For Gradle projects, add this to your build.gradle file:
dependencies {
    testImplementation 'org.junit.jupiter:junit-jupiter-api:5.9.2'
    testRuntimeOnly 'org.junit.jupiter:junit-jupiter-engine:5.9.2'
}

test {
    useJUnitPlatform()
}

After adding the dependencies, create a test directory. In Maven projects, use src/test/java. This keeps junit test cases separate from application code. Each test class should match a class in your main code.

Most IDEs like IntelliJ IDEA and Eclipse have built-in junit testing support. They can generate test classes for you. These IDEs also provide tools to run and debug junit test cases easily.

To verify your setup, create a simple test class. Run it to ensure JUnit works correctly. Now you're ready to write junit tests for your application.

Must read: Stack and Heap Memory in Java: Key Differences Explained

How to Write JUnit Test Cases

Writing effective junit test cases requires understanding the test structure. Each junit test focuses on a specific functionality or behavior. Good tests are readable, maintainable, and reliable.

To write junit test cases, follow these general steps:

• Create a test class for each production class
• Add test methods for each behavior to verify
• Setup any required test data or objects
• Call the method being tested
• Assert that the results match expectations

Test classes use naming conventions like ClassNameTest or ClassNameTests. Each test method should have a descriptive name. Names should explain what the test verifies.

Also read: Comprehensive Guide to Exception Handling in Java

JUnit Test Case Structure

A typical junit test case follows a clear structure. This makes tests easier to understand and maintain. The structure includes several key components.

The basic junit test structure looks like this:

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

public class CalculatorTest {

    @BeforeEach
    void setUp() {
        // Initialize objects or data before each test
    }

    @Test
    void testAddition() {
        // 1. Arrange - set up test data
        Calculator calculator = new Calculator();
        int a = 5;
        int b = 3;
        
        // 2. Act - call the method being tested
        int result = calculator.add(a, b);
        
        // 3. Assert - verify the result
        assertEquals(8, result, "5 + 3 should equal 8");
    }
    
    @AfterEach
    void tearDown() {
        // Clean up after each test
    }
}

JUnit tests often follow the Arrange-Act-Assert pattern:

• Arrange: Prepare objects and data for testing
• Act: Call the method being tested
• Assert: Verify the results match expectations

JUnit 5 provides lifecycle annotations to structure tests:

• @BeforeAll: Runs once before all tests in the class
• @BeforeEach: Runs before each test method
• @AfterEach: Runs after each test method
• @AfterAll: Runs once after all tests in the class

These annotations help set up and clean up test environments. They ensure each test runs in a clean state. This makes junit testing more reliable and predictable.

Must explore: Conditional Operators in Java

Basic Assertion Methods

JUnit provides many assertion methods to verify test results. These methods check if actual results match expected values. If they don't, the test fails with a message.

Common assertion methods include:

Here's how to use these assertions in junit test cases:

@Test
void testBasicAssertions() {
    // assertEquals
    assertEquals(4, 2+2, "2+2 should equal 4");
    
    // assertTrue/assertFalse
    int value = 5;
    assertTrue(value > 0, "Value should be positive");
    assertFalse(value == 0, "Value should not be zero");
    
    // assertNull/assertNotNull
    String text = getText();
    assertNotNull(text, "Text should not be null");
    
    // assertThrows
    assertThrows(ArithmeticException.class, () -> {
        int result = 1 / 0;
    }, "Dividing by zero should throw ArithmeticException");
}

Output (if tests pass):

Test 'testBasicAssertions' passed

Test run finished: 1 test passed

Output (if the getText() method returns null):

org.opentest4j.AssertionFailedError: Text should not be null

Expected: not <null>

     but was: <null>

Test 'testBasicAssertions' failed

Use assertion messages to clarify test failures. Good messages explain what went wrong. They help other developers understand test intent. JUnit 5 also supports grouped assertions with assertAll(). This shows all failures instead of stopping at the first one.

Check out: Arithmetic Operators in Java

JUnit Test Case Examples

Let's examine a practical junit test case example. We'll test a StringUtils class with methods for string manipulation. This example shows how to write effective junit test cases.

First, here's the class we want to test:

public class StringUtils {
    
    /**
     * Reverses the given string.
     * Returns empty string if input is null.
     */
    public String reverse(String input) {
        if (input == null) {
            return "";
        }
        StringBuilder reversed = new StringBuilder();
        for (int i = input.length() - 1; i >= 0; i--) {
            reversed.append(input.charAt(i));
        }
        return reversed.toString();
    }
    
    /**
     * Counts occurrences of a character in a string.
     * Returns 0 if input is null.
     */
    public int countOccurrences(String input, char character) {
        if (input == null) {
            return 0;
        }
        int count = 0;
        for (int i = 0; i < input.length(); i++) {
            if (input.charAt(i) == character) {
                count++;
            }
        }
        return count;
    }
}

Now, let's write junit test cases for this class:

import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

public class StringUtilsTest {
    
    private StringUtils stringUtils;
    
    @BeforeEach
    void setUp() {
        // Initialize object before each test
        stringUtils = new StringUtils();
    }
    
    @Test
    void testReverse_NormalString() {
        // Test with a normal string
        String input = "hello";
        String result = stringUtils.reverse(input);
        assertEquals("olleh", result, "Should reverse the string correctly");
    }
    
    @Test
    void testReverse_EmptyString() {
        // Test with an empty string
        String result = stringUtils.reverse("");
        assertEquals("", result, "Should handle empty string");
    }
    
    @Test
    void testReverse_NullInput() {
        // Test with null input
        String result = stringUtils.reverse(null);
        assertEquals("", result, "Should return empty string for null input");
    }
    
    @Test
    void testCountOccurrences_NormalCase() {
        // Test normal case
        String input = "hello world";
        int result = stringUtils.countOccurrences(input, 'l');
        assertEquals(3, result, "Should count 3 occurrences of 'l'");
    }
    
    @Test
    void testCountOccurrences_CharNotPresent() {
        // Test when character is not present
        String input = "hello";
        int result = stringUtils.countOccurrences(input, 'z');
        assertEquals(0, result, "Should return 0 when char not present");
    }
    
    @Test
    void testCountOccurrences_NullInput() {
        // Test with null input
        int result = stringUtils.countOccurrences(null, 'a');
        assertEquals(0, result, "Should return 0 for null input");
    }
}

Output (when all tests pass):

Test run finished: 6 tests passed

StringUtilsTest

  ✓ testReverse_NormalString

  ✓ testReverse_EmptyString

  ✓ testReverse_NullInput

  ✓ testCountOccurrences_NormalCase

  ✓ testCountOccurrences_CharNotPresent

  ✓ testCountOccurrences_NullInput

This example shows several junit testing best practices:

• Each test method focuses on one scenario
• Test method names clearly describe what they test
• Tests cover normal cases and edge cases (empty strings, null values)
• Each test has a clear arrange-act-assert structure
• Assertion messages explain what each test verifies

Writing similar junit test cases for your code ensures thorough coverage. Test both expected behavior and error handling. This approach catches bugs before they reach production.

How to Test Private Methods in JUnit

Testing private methods in JUnit presents a unique challenge. Private methods aren't directly accessible from test classes. However, there are several approaches to test them effectively.

The first approach is indirect testing. Test the public methods that call private methods. If private methods work correctly, public methods should behave as expected. This approach tests private methods through their public interfaces.

However, sometimes you need to test private methods directly. Here are three techniques for how to test private methods in junit:

1. Using Reflection API

Java's Reflection API can access private methods. Here's how to do it:

import java.lang.reflect.Method;
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

public class ClassWithPrivateMethodTest {

    @Test
    void testPrivateMethodUsingReflection() throws Exception {
        // Create instance of the class
        ClassWithPrivateMethod instance = new ClassWithPrivateMethod();
        
        // Get the private method
        Method privateMethod = ClassWithPrivateMethod.class
            .getDeclaredMethod("calculateSecret", int.class);
        
        // Make it accessible
        privateMethod.setAccessible(true);
        
        // Invoke the private method
        int result = (int) privateMethod.invoke(instance, 5);
        
        // Assert the result
        assertEquals(25, result, "Private method should square the input");
    }
}

This approach directly calls the private method. However, it makes tests more fragile. If the private method signature changes, tests will break.

2. Extract Method to Package-Private

Another approach is making the method package-private instead of private. This allows test classes in the same package to access it:

// In production class
class ClassWithPackagePrivateMethod {
    // Changed from private to package-private
    int calculateSecret(int input) {
        return input * input;
    }
    
    public int processValue(int value) {
        return calculateSecret(value) + 10;
    }
}

// In test class (same package)
public class ClassWithPackagePrivateMethodTest {
    @Test
    void testPackagePrivateMethod() {
        ClassWithPackagePrivateMethod instance = new ClassWithPackagePrivateMethod();
        int result = instance.calculateSecret(5);
        assertEquals(25, result);
    }
}

This approach is cleaner than reflection. However, it exposes methods beyond their intended scope.

3. Use Nested Test Classes

JUnit 5 supports nested test classes. A nested class can access private members of its outer class:

public class ClassWithPrivateMethodTest {
    
    // Production code inside test class for demonstration
    class Calculator {
        private int multiply(int a, int b) {
            return a * b;
        }
        
        public int calculate(int x, int y) {
            return multiply(x, y);
        }
    }
    
    // Nested test class
    @Nested
    class CalculatorTests {
        @Test
        void testMultiply() {
            Calculator calc = new Calculator();
            // Access private method from nested class
            int result = calc.multiply(4, 5);
            assertEquals(20, result);
        }
    }
}

This approach only works for demonstration. In real projects, production code is separate.

Though these methods exist, consider if testing private methods is necessary. Private methods should be implementation details. Testing public behavior is usually sufficient. If you need to test a private method directly, it might be better as a separate class.

JUnit Testing in Spring Boot

Spring Boot provides excellent integration with JUnit in Java. This makes junit testing in Spring Boot applications straightforward. The framework offers specialized annotations and tools for testing.

To get started with junit testing in spring boot, add the spring-boot-starter-test dependency:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-test</artifactId>
    <scope>test</scope>
</dependency>

This dependency includes JUnit, Spring Test, AssertJ, Hamcrest, and other testing libraries. It provides everything needed for comprehensive testing.

Spring Boot supports several testing approaches:

1. Unit Testing: Testing individual components in isolation
2. Integration Testing: Testing components working together
3. Mock MVC Testing: Testing web controllers without starting a server
4. WebTestClient: Testing reactive web applications
5. TestRestTemplate: Testing REST controllers with a real server

Let's look at how to do junit testing in spring boot with a controller example:

import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.web.servlet.WebMvcTest;
import org.springframework.boot.test.mock.mockito.MockBean;
import org.springframework.test.web.servlet.MockMvc;

import static org.mockito.Mockito.when;
import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.content;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;

@WebMvcTest(UserController.class)
public class UserControllerTest {

    @Autowired
    private MockMvc mockMvc;
    
    @MockBean
    private UserService userService;
    
    @Test
    void getUserById_ShouldReturnUser() throws Exception {
        // Arrange
        User user = new User(1L, "John Doe", "[email protected]");
        when(userService.findById(1L)).thenReturn(user);
        
        // Act & Assert
        mockMvc.perform(get("/api/users/1"))
               .andExpect(status().isOk())
               .andExpect(content().json("{'id':1,'name':'John Doe','email':'[email protected]'}"));
    }
}

Output (when test passes):

HTTP Method = GET

Request URI = /api/users/1

Parameters = {}

Headers = []

Body = null

MockHttpServletResponse:

Status = 200

Content-Type = application/json

Headers = [Content-Type:"application/json"]

Content = {"id":1,"name":"John Doe","email":"[email protected]"}

Test 'getUserById_ShouldReturnUser' passed

This test uses MockMvc to simulate HTTP requests. It mocks the UserService to isolate the controller logic. The test verifies that the controller returns the expected response.

Spring Boot Test Annotations

Spring Boot provides specialized annotations for junit testing. These annotations configure the test environment. They make it easier to test different application layers.

Here are key annotations for junit testing in spring boot:

Here's an example of a repository test using @DataJpaTest:

import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.orm.jpa.DataJpaTest;

import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.junit.jupiter.api.Assertions.assertNotNull;

@DataJpaTest
public class UserRepositoryTest {

    @Autowired
    private UserRepository userRepository;
    
    @Test
    void findByEmail_ShouldReturnUser() {
        // Arrange
        User user = new User(null, "Jane Doe", "[email protected]");
        userRepository.save(user);
        
        // Act
        User found = userRepository.findByEmail("[email protected]");
        
        // Assert
        assertNotNull(found);
        assertEquals("Jane Doe", found.getName());
    }
}

This test focuses on the repository layer. It uses an in-memory database for testing. The @DataJpaTest annotation configures the test environment automatically.

For testing services with mocked dependencies:

import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.mockito.InjectMocks;
import org.mockito.Mock;
import org.mockito.junit.jupiter.MockitoExtension;

import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.mockito.Mockito.when;

@ExtendWith(MockitoExtension.class)
public class UserServiceTest {

    @Mock
    private UserRepository userRepository;
    
    @InjectMocks
    private UserServiceImpl userService;
    
    @Test
    void getUserByEmail_ShouldReturnUser() {
        // Arrange
        User user = new User(1L, "Alice", "[email protected]");
        when(userRepository.findByEmail("[email protected]")).thenReturn(user);
        
        // Act
        User result = userService.findByEmail("[email protected]");
        
        // Assert
        assertEquals("Alice", result.getName());
    }
}

This test uses Mockito to mock the repository. It focuses on testing service logic in isolation. The combination of JUnit and Mockito is powerful for unit testing.

Best Practices for Writing Effective Test Cases

Following best practices ensures high-quality junit test cases. These practices make tests more reliable and maintainable. They help catch bugs effectively.

Here are key best practices for junit testing:

1. Follow the AAA pattern: Arrange-Act-Assert makes tests clear and consistent. Separate test setup, execution, and verification.
2. Test one thing per test: Each test should verify a single behavior. This makes tests focused and easier to debug.
3. Use descriptive test names: Names should explain what the test verifies. Good format: methodName_stateUnderTest_expectedBehavior.
4. Cover edge cases: Test normal inputs and edge cases like null values, empty collections, and boundary values.
5. Keep tests independent: Tests shouldn't depend on each other. Each test should run in isolation.
6. Don't test private methods directly: Focus on testing public behavior. Private methods are implementation details.
7. Use appropriate assertions: Choose the most specific assertion for each check. Use custom messages to clarify failures.
8. Avoid logic in tests: Tests should be simple and straightforward. Complex logic in tests can introduce its own bugs.
9. Use test fixtures: For common setup code, use @BeforeEach and @BeforeAll. This reduces code duplication.
10. Test exceptions properly: Use assertThrows to verify exceptions. Check exception messages when needed.
11. Keep tests fast: Unit tests should run quickly. Slow tests discourage frequent testing.
12. Use test doubles wisely: Use mocks, stubs, and fakes to isolate the code under test. But don't over-mock.

Applying these junit testing best practices improves test quality. Good tests catch bugs early and serve as documentation. They make development faster and more confident.

JUnit 4 vs JUnit 5 Comparison

JUnit has evolved significantly over time. JUnit 5 brings modern features compared to JUnit 4. Understanding the differences helps choose the right version.

Here's a comparison of JUnit 4 and JUnit 5:

JUnit 5 offers many advantages over JUnit 4:

1. Improved Java 8 support: Uses lambdas and streams
2. Extension model: More flexible than JUnit 4 runners and rules
3. Nested tests: Better organization of related tests
4. Parameter sources: More options for parameterized tests
5. Dynamic tests: Generate tests at runtime
6. Better assertion messages: More informative by default

If starting a new project, JUnit 5 is recommended. It offers modern features and better extensibility. Existing JUnit 4 tests can run on JUnit 5 using the Vintage engine.

How to Run JUnit Tests

Knowing how to run junit tests is essential for efficient testing. Tests can be run from IDEs or build tools. Each approach has advantages for different scenarios.

Running from IDE

Modern IDEs make running junit tests simple. They provide visual test runners and debugging tools.

To run junit tests in IntelliJ IDEA:

• Right-click on a test class or method
• Select "Run" or use keyboard shortcut (Ctrl+Shift+F10)
• View results in the test runner window

To run junit tests in Eclipse:

• Right-click on a test class or package
• Select "Run As" > "JUnit Test"
• View results in the JUnit panel

To run junit tests in VS Code:

• Install the Java Test Runner extension
• Click the test icon in the left sidebar
• Run individual tests or test classes

IDE test runners show detailed results. They highlight passed and failed tests. They also provide stack traces for failures. Most IDEs allow debugging tests directly.

Running with Build Tools

Build tools run junit tests during the build process. This enables continuous integration and automated testing.

To run junit tests with Maven:

# Run all tests
mvn test

# Run a specific test class
mvn test -Dtest=CalculatorTest

# Run a specific method
mvn test -Dtest=CalculatorTest#testAddition

To run junit tests with Gradle:
# Run all tests
./gradlew test

# Run a specific test class
./gradlew test --tests "com.example.CalculatorTest"

# Run a specific method
./gradlew test --tests "com.example.CalculatorTest.testAddition"

Build tools generate test reports. Maven creates reports in the target/surefire-reports directory. Gradle places them in build/reports/tests. These reports show test results and statistics.

For continuous integration, configure your CI server to run tests automatically. Popular CI tools like Jenkins, GitHub Actions, and GitLab CI support JUnit test reports.

Conclusion

JUnit testing in Java forms a critical part of Java development. It ensures code quality and reliability. Learning how to write junit test cases is essential for any Java developer. The framework makes unit testing accessible and efficient. It catches bugs early and provides confidence during refactoring. JUnit testing in Spring Boot extends these benefits to enterprise applications. Following best practices leads to maintainable and effective tests.

The knowledge of how to test private methods and edge cases improves coverage. With the right approach, junit test becomes a powerful tool for software quality.

FAQs

1. What is JUnit testing?

JUnit testing is a framework for writing and running automated tests for Java code. It helps verify that units of code work as expected. JUnit provides annotations, assertions, and test runners to simplify testing. This framework has become the standard for unit testing in the Java ecosystem and is an essential tool for test-driven development.

2. What are the benefits of JUnit testing?

JUnit testing offers many benefits. It catches bugs early in development. It improves code quality through verification. It provides documentation for how code should work. It enables refactoring with confidence. It also supports continuous integration and delivery. Additionally, JUnit tests serve as a safety net when making changes, allowing developers to modify code without fear of breaking existing functionality.

3. How to write JUnit test cases?

To write junit test cases, create a test class with methods annotated with @Test. Use assertions to verify expected behavior. Follow the Arrange-Act-Assert pattern. Test both normal scenarios and edge cases. Name tests descriptively to explain what they verify. Remember to make each test independent and focus on testing a single behavior in each test method.

4. How to test private methods in JUnit?

You can test private methods in JUnit through indirect testing via public methods. Alternatively, use reflection to access private methods directly. You can also make methods package-private for testing. Consider if testing private methods directly is necessary. If you find yourself frequently testing private methods, it may indicate that those methods should be refactored into a separate class.

5. What are the best practices for JUnit testing?

Best practices include testing one concept per test, writing independent tests, using descriptive names, covering edge cases, avoiding logic in tests, using appropriate assertions, and keeping tests fast. Follow the Arrange-Act-Assert pattern for clarity. Additionally, focus on maintainability by keeping tests simple and refactoring test code alongside production code.

6. How to run JUnit tests?

You can run junit tests through your IDE by right-clicking on test classes or methods. Alternatively, use build tools like Maven or Gradle with commands like mvn test or ./gradlew test. CI servers can also run tests automatically. Using test suites allows you to organize and run related tests together for more efficient testing workflows.

7. What is the difference between JUnit 4 and JUnit 5?

JUnit 5 has a modular architecture, requires Java 8+, and introduces new features like nested tests, improved parameterized tests, and a flexible extension model. It changes some annotations like @Before to @BeforeEach and adds features like @DisplayName and assertAll(). JUnit 5 also provides better integration with modern Java features like lambda expressions and method references.

8. How to do JUnit testing in Spring Boot?

To do junit testing in Spring Boot, use spring-boot-starter-test dependency. Use annotations like @SpringBootTest for integration tests or @WebMvcTest for controller tests. Spring Boot provides tools for testing different application layers and mocking dependencies. The framework also offers specialized slices for testing specific parts of the application, such as repositories, REST controllers, and web layers.

9. What are assertion methods in JUnit?

Assertion methods verify test expectations. Common assertions include assertEquals, assertTrue, assertFalse, assertNull, assertNotNull, and assertThrows. JUnit 5 adds assertAll for grouped assertions. Each assertion checks a specific condition. Good assertion messages help identify the exact failure point and make debugging easier when tests fail.

10. How to handle exceptions in JUnit tests?

Use assertThrows to verify that code throws expected exceptions. This method takes an exception class and executable code. It fails if the exception isn't thrown or if a different exception occurs. You can also check exception messages and properties. The assertThrows method returns the exception object, allowing further verification of exception details.

11. Can JUnit test asynchronous code?

Yes, JUnit 5 provides tools for testing asynchronous code. Use assertTimeout for time-limited tests. For truly asynchronous tests, use assertTimeoutPreemptively. Libraries like Awaitility also help with asynchronous testing in JUnit. Testing asynchronous code requires careful attention to timeouts and thread management to avoid flaky tests.

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