targetsに作りたい型を放り込んでおけばMockitoがMockを作ってSpringに登録してくれる

MocksRegistry.java

import org.mockito.Mockito;
import org.springframework.beans.BeansException;
import org.springframework.beans.MutablePropertyValues;
import org.springframework.beans.PropertyValue;
import org.springframework.beans.factory.FactoryBean;
import org.springframework.beans.factory.config.BeanFactoryPostProcessor;
import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
import org.springframework.beans.factory.support.BeanDefinitionRegistry;
import org.springframework.beans.factory.support.RootBeanDefinition;

public class MocksRegistry implements BeanFactoryPostProcessor {

    private Class<?>[] targets;

    public void setTargets(Class<?>[] targets) {
        this.targets = targets;
    }

    @Override
    public void postProcessBeanFactory(
            final ConfigurableListableBeanFactory context)
            throws BeansException {
        BeanDefinitionRegistry registry = ((BeanDefinitionRegistry) context);
        for (Class<?> clazz : targets) {
            register(registry, clazz);
        }
    }

    private void register(final BeanDefinitionRegistry registry,
            final Class<?> type) {
        RootBeanDefinition definition = new RootBeanDefinition(
                MocksFactory.class);
        MutablePropertyValues values = new MutablePropertyValues();
        values.addPropertyValue(new PropertyValue("type", type));
        definition.setPropertyValues(values);
        registry.registerBeanDefinition("factoryOf" + type.getSimpleName(),
                definition);
    }

    public static class MocksFactory<T> implements FactoryBean<T> {

        private Class<T> type;

        public void setType(final Class<T> type) {
            this.type = type;
        }

        @Override
        public T getObject() throws Exception {
            return Mockito.mock(type);
        }

        @Override
        public Class<T> getObjectType() {
            return type;
        }

        @Override
        public boolean isSingleton() {
            return true;
        }
    }
}