Optional and mandatory dependencyies in a Spring component

components.java

public interface MyComponent {
  
  void myBusinessMethod(Parameter parameter);
} 

/**
 * Package protected implementation to avoid clients to use it directly. It contains
 * a mandatory and optional dependency where the mandatory one is exposed as constructor
 * argument and checked to be not {@literal null}. The optional dependency can be either 
 * {@literal null} or set from the outside. It's also fine to default this dependency to 
 * something but one has to adapt the setter then.
 * 
 * Using this design principle it becomes immediately obvious what dependencies are 
 * mandatory (constructor arguments, final fields) and which are optional (setters and
 * non final fields) and have to be checked against {@literal null} before invocation.
 * 
 * @author Oliver Gierke
 */
@Component
class MyComponentImplementation implements MyComponent {

  private final MandatoryDependency mandatoryDependency;
  private OptionalDependency optionalDependency;

  @Autowired
  public MyComponentImplementation(MandatoryDependency dependency) {
    Assert.notNull(dependency, "Mandatory dependency must be given!");
    this.mandatoryDependency = dependency;
  }

  @Autowired(required = false)
  public void setOptionalDependency(OptionalDependency dependency) {
    this.optionalDependency = dependency;
  }

  @Override
  public void myBusinessMethod(Parameter parameter) {

    if (optionalDependency != null) {
      // … do something if optional dependency is set
    }

    // Mandatory dependency can be used without null check
    mandatoryDependency.callSomeMethod(…);
  }
}