Spring recommends: "Always use constructor based dependency injection in your beans. Always use assertions for mandatory dependencies".

Field injection/NullPointerException example:

class MyComponent {

  @Inject MyCollaborator collaborator;

  public void myBusinessMethod() {
    collaborator.doSomething(); // -> NullPointerException
  }
}

Constructor injection should be used:

class MyComponent {

  private final MyCollaborator collaborator;

  @Inject
  public MyComponent(MyCollaborator collaborator) {
    Assert.notNull(collaborator, "MyCollaborator must not be null!");
    this.collaborator = collaborator;
  }

  public void myBusinessMethod() {
    collaborator.doSomething(); // -> safe
  }
}