ProcessDefinition processDefinition = repositoryService.createProcessDefinitionQuery().singleResult();
ProcessInstanceBuilder processInstanceBuilder = runtimeService.createProcessInstanceBuilder();
// by key
ProcessInstance processInstance = processInstanceBuilder.processDefinitionKey("oneTaskProcess").businessKey("123").start();
assertNotNull(processInstance);
assertEquals("123", processInstance.getBusinessKey());
assertEquals(1, runtimeService.createProcessInstanceQuery().processDefinitionKey("oneTaskProcess").count());
processInstanceBuilder = runtimeService.createProcessInstanceBuilder();