Field injection seems like a tidy way to get your classes what they need to do their jobs, but it's really a ``++NullPointerException++`` waiting to happen unless all your class constructors are ``++private++``. That's because any class instances that are constructed by callers, rather than instantiated by a Dependency Injection framework compliant with the JSR-330 (Spring, Guice, ...), won't have the ability to perform the field injection.


Instead ``++@Inject++`` should be moved to the constructor and the fields required as constructor parameters.


This rule raises an issue when classes with non-``++private++`` constructors (including the default constructor) use field injection.


== Noncompliant Code Example

----
class MyComponent {  // Anyone can call the default constructor 

  @Inject MyCollaborator collaborator;  // Noncompliant

  public void myBusinessMethod() {
    collaborator.doSomething();  // this will fail in classes new-ed by a caller
  }
}
----


== Compliant Solution

----
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();
  }
}
----


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