jersey2-guice-impl/src/main/java/com/squarespace/jersey2/guice/BindingUtils.java
public static boolean isNullable(Injectee injectee) {
    // HK2's optional
    if (injectee.isOptional()) {
      return true;
    }
    
    // Guice's optional
    AnnotatedElement element = injectee.getParent();
    if (isGuiceOptional(element)) {
      return true;
    }
    
    // Any @Nullable?
    int position = injectee.getPosition();
    
    if (element instanceof Field) {
      return Nullability.allowsNull(((Field)element).getAnnotations());
      
    } else if (element instanceof Method) {
      Annotation annotations[][] = ((Method)element).getParameterAnnotations();
      return Nullability.allowsNull(annotations[position]);
      
    } else if (element instanceof Constructor<?>) {
      Annotation annotations[][] = ((Constructor<?>)element).getParameterAnnotations();
      return Nullability.allowsNull(annotations[position]);
    }
    
    return false;
  }
public static boolean isNullable(Injectee injectee) {
    // HK2's optional
    if (injectee.isOptional()) {
      return true;
    }
    
    // Guice's optional
    AnnotatedElement element = injectee.getParent();
    if (isGuiceOptional(element)) {
      return true;
    }
    
    // Any @Nullable?
    int position = injectee.getPosition();
    
    if (element instanceof Field) {
      return Nullability.allowsNull(((Field)element).getAnnotations());
      
    } else if (element instanceof Method) {
      Annotation annotations[][] = ((Method)element).getParameterAnnotations();
      return Nullability.allowsNull(annotations[position]);
      
    } else if (element instanceof Constructor<?>) {
      Annotation annotations[][] = ((Constructor<?>)element).getParameterAnnotations();
      return Nullability.allowsNull(annotations[position]);
    }
    
    return false;
  }
Returns {@code true} if the given {@link Injectee} can be {@code null}.

@see Optional
@see Injectee#isOptional()
@see Nullable
@see com.google.inject.Inject#optional()
@see Nullability#allowsNull(Annotation[])