Class that simulates the EJB Dependency Injection mechanism. May be used to inject mock implementations in tests. Originally based on http://blog.bbv.ch/2011/03/24/ejb-dependency-injection-and-unit-testing/, but enhanced for subclassing.

EjbInjector.java

import java.lang.annotation.Annotation;
import java.lang.reflect.Field;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;

import javax.annotation.Resource;
import javax.ejb.EJB;
import javax.persistence.PersistenceContext;
 
public class EjbInjector {
 
  private static final List<Class<? extends Annotation>> EJB_ANNOTATIONS;
  static {
    EJB_ANNOTATIONS = new ArrayList<Class<? extends Annotation>>();
    EJB_ANNOTATIONS.add(EJB.class);
    EJB_ANNOTATIONS.add(PersistenceContext.class);
    EJB_ANNOTATIONS.add(Resource.class);
  }
 
  final Map<Class<?>, Object> mappings = new HashMap<Class<?>, Object>();
 
  public void inject(final Object bean) throws Exception {
    for (final Field field : getEJBAnnotatedFields(bean)) {
       injectField(field, bean);
    }
  }
 
  public void assign(final Class<?> type, final Object instance) {
    mappings.put(type, instance);
  }
 
  private void injectField(final Field field, final Object bean)  throws Exception {
    final Object instanceToInject = mappings.get(field.getType());
    if (!field.isAccessible()) {
      field.setAccessible(true);
    }
    field.set(bean, instanceToInject);
  }
 
  private List<Field> getEJBAnnotatedFields(final Object bean) {
    final Class<? extends Object> beanClass = bean.getClass();
    final List<Field> annotatedFields = new ArrayList<Field>();
    for (final Field field : getAllFields(beanClass)) {
      if (hasEJBAnnotation(field)) {
        annotatedFields.add(field);
      }
    }
    return annotatedFields;
  }
  
  private List<Field> getAllFields(Class<? extends Object> beanClass)
  {
	  final List<Field> result = new LinkedList<Field>();
	  do
	  {
		  result.addAll(Arrays.asList(beanClass.getDeclaredFields()));
		  beanClass = beanClass.getSuperclass();
	  } while (!beanClass.equals(Object.class));
	  return result;
  }
 
  private static boolean hasEJBAnnotation(final Field field) {
    for (final Class<? extends Annotation> annotation : EJB_ANNOTATIONS) {
      if (field.isAnnotationPresent(annotation)) {
        return true;
      }
    }
    return false;
  }
}