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

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Field injection/NullPointerException example:

  class MyComponent {
   @Inject MyCollaborator collaborator;

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

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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
   }
  }