@Override
	public boolean isSingleton() {
		return true;
	}
Will create a {@link SynchronizingModuleRegistry} for hosting custom modules if necessary, depending on the
protocol scheme (using a synchronizing registry for everything but {@code file://} protocol).

@since 1.2
@author Eric Bottard
@author David Turanski
spring-xd-dirt/src/main/java/org/springframework/xd/dirt/module/CustomModuleRegistryFactoryBean.java