Understanding Dependency Injection Types in Spring: Constructor, Setter, and Field Injection

Spring officially discourages the use of @Autowired field/property injection, and many companies forbid it in new projects.

The article explains the three types of dependency injection supported by Spring 5.1.3: constructor‑based, setter‑based, and field‑based injection, and discusses when each should be used.

Constructor‑based injection marks the class constructor with @Autowired (or omits it) and allows injected fields to be declared final, ensuring immutability.

@Component
public class ConstructorBasedInjection {
    
    private final InjectedBean injectedBean;
    
    @Autowired    
    public ConstructorBasedInjection(InjectedBean injectedBean) {
        this.injectedBean = injectedBean;
    }
}

Setter‑based injection annotates a setter method with @Autowired; the container calls the setter after creating the bean.

@Component
public class SetterBasedInjection {
    
    private InjectedBean injectedBean;
    
    @Autowired
    public void setInjectedBean(InjectedBean injectedBean) {
        this.injectedBean = injectedBean;
    }
}

Field‑based injection directly annotates a member variable with @Autowired. It results in concise code but has several drawbacks.

Drawbacks of field injection include:

Cannot declare immutable (final) dependencies.

Can hide the growing number of dependencies, violating the Single Responsibility Principle.

Creates tight coupling to the Spring container, making the class hard to use outside the container or in unit tests.

Hides dependencies from the public API.

The article recommends preferring constructor injection for required dependencies and setter injection for optional ones, while avoiding field injection whenever possible.

References: “Field injection is not recommended – Spring IOC” by Marc Nuri and Spring official documentation.