如何获取Spring事务管理器实例？

<bean id="transactionManager" class="org.springframework.jdbc.datasource.DataSourceTransactionManager">
    <property name="dataSource" ref="dataSource" />
</bean>

@Autowired
private PlatformTransactionManager transactionManager;

ApplicationContext context = new ClassPathXmlApplicationContext("spring-config.xml");
PlatformTransactionManager transactionManager = context.getBean("transactionManager", PlatformTransactionManager.class);

@Transactional
public void doSomething() {
    // 事务操作
}

TransactionDefinition definition = new DefaultTransactionDefinition();
TransactionStatus status = transactionManager.getTransaction(definition);
try {
    // 事务操作
    transactionManager.commit(status);
} catch (Exception e) {
    transactionManager.rollback(status);
    throw e;
}