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injection
Constructor injection in Spring Boot makes testing easier primarily because it ensures that dependencies are clearly defined, mandatory, and immutable. This design naturally facilitates the use of mock objects in unit tests [1].
Key reasons why it simplifies testing:
Explicit Dependencies: Dependencies are declared in the constructor signature, making it instantly clear what a class needs to function. This transparency helps developers understand the system's structure and ensures all required components are provided during testing. Immutability: By default, constructor-injected fields can be marked as final [1]. This ensures that dependencies cannot be changed after the object is created, leading to more predictable and robust code. In testing, this guarantees that the mock objects you set up won't be replaced by other values accidentally during the test execution. Decoupling: Constructor injection encourages the use of interfaces rather than concrete classes. Your class depends on an abstraction, not a specific implementation. During testing, you can easily provide a mock implementation of that interface instead of the real, complex production dependency, isolating the class under test. No Proxy Issues: Unlike field (or setter) injection, constructor injection ensures that all dependencies are available immediately when the object is instantiated. This avoids potential NullPointerException errors that can sometimes occur in tests with other injection methods if dependencies aren't correctly initialized. Ease of Instantiation in Tests: With constructor injection, you can easily instantiate the class under test in your unit tests using the new keyword, passing in mock or stub objects as parameters [1]. You don't need to rely on the full Spring application context to wire things up, which makes the tests faster and more "pure" unit tests.
Field Setter Constructor
https://insights.orangeandbronze.com/constructor-injection-vs-field-injection-spring-boot/
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