Constructor injection should be used instead of field injection

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();
  }
}