Injecting mock objects on an annotation. e.g. You can inject a mock object on a @PersistenceContext annotated field

ClassInjector.java

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

import javax.persistence.PersistenceContext;

public class ClassInjector {
	private static final List<Class<? extends Annotation>> ANNOTATIONS;
	static {
		ANNOTATIONS = new ArrayList<Class<? extends Annotation>>();
		ANNOTATIONS.add(PersistenceContext.class);
	}

	final Map<Class<?>, Object> mappings = new HashMap<Class<?>, Object>();

	public void inject(final Object bean) throws Exception {
		for (final Field field : getClassAnnotatedFields(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> getClassAnnotatedFields(final Object bean) {
		final Class<? extends Object> beanClass = bean.getClass();
		final List<Field> annotatedFields = new ArrayList<Field>();
		for (final Field field : beanClass.getDeclaredFields()) {
			if (hasAnnotation(field)) {
				annotatedFields.add(field);
			}
		}
		return annotatedFields;
	}

	private static boolean hasAnnotation(final Field field) {
		for (final Class<? extends Annotation> annotation : ANNOTATIONS) {
			if (field.isAnnotationPresent(annotation)) {
				return true;
			}
		}
		return false;
	}
}