SpringBoot Application – Autowired vs Constructor injection which one is better?

Both autowired and constructor injection have their advantages and can be used effectively in a Spring Boot application. The choice between them depends on your specific needs and preferences. However, in recent years, there has been a shift towards favoring constructor injection as the recommended approach. Here are some factors to consider:

1. Explicit Dependencies: Constructor injection makes dependencies explicit by declaring them as parameters in the constructor. This provides clear visibility of the dependencies required by a class, making the code more readable and understandable. Autowired, on the other hand, can introduce implicit dependencies, which may not be immediately apparent when looking at the code.
2. Testability: Constructor injection simplifies unit testing by allowing dependencies to be easily mocked or stubbed. Since the dependencies are provided through the constructor, it becomes straightforward to create instances of the class with specific dependencies for testing purposes. Autowired injection can make unit testing more challenging, especially if you need to mock or stub dependencies.
3. Immutability: Constructor injection encourages the use of immutable objects since the dependencies are provided at construction time. Immutable objects have advantages such as thread safety and can help in writing more predictable and reliable code.
4. Dependency Injection Framework Agnostic: Constructor injection does not rely on any specific framework annotations, making your code independent of the Spring framework. This can be beneficial if you plan to switch to a different dependency injection framework in the future or if you want to make your code more modular and decoupled from the Spring framework.
5. Compile-Time Safety: Constructor injection provides compile-time safety by ensuring that all required dependencies are provided during object instantiation. If a required dependency is missing, the code will fail to compile, preventing runtime errors. Autowired injection, on the other hand, may result in runtime errors if a required dependency is not available or if there are multiple candidates.

While constructor injection is generally considered the preferred approach, autowired injection can still be useful in certain situations. Autowired can be handy when dealing with optional dependencies or when you have a large number of dependencies that are unlikely to change. It provides convenience and flexibility in wiring dependencies without the need for explicit configuration.

In summary, constructor injection is often recommended due to its explicitness, testability, immutability, and independence from specific frameworks. However, the choice between autowired and constructor injection ultimately depends on the specific needs of your application and your development team’s preferences.