In the activiti REST, we use a ServletContextListener (org.activiti.rest.common.servlet.ActivitiServletContextListener) to get hold of the default process-engine. This is when you're not in a spring-context…
In spring-context, use:
<bean id="processEngine" class="org.activiti.spring.ProcessEngineFactoryBean" destroy-method="destroy">
<property name="processEngineConfiguration" ref="processEngineConfiguration" />
</bean>
If you have services that depend on the process-engine, the engine will be initialized when it's injected in your service for the first time (thats what a factory-bean is for) and will be resued by all services that depend on it. So once the app-context is booted, the engine is booted as wel. I presume you have a ApplicationContextServletContextListener (not sure what the exact name is in Spring) that initialises the spring-context when the webapp boots, so that's what you need…