Spring事务监听机制---使用@TransactionalEventListener处理数据库事务提交成功后再执行操作（附：Spring4.2新特性讲解）【享学Spring】

@Slf4j
@Service
public class HelloServiceImpl implements HelloService {

    @Autowired
    private JdbcTemplate jdbcTemplate;
    @Autowired
    private ApplicationEventPublisher applicationEventPublisher;

    @Transactional
    @Override
    public Object hello(Integer id) {
        // 向数据库插入一条记录
        String sql = "insert into user (id,name,age) values (" + id + ",'fsx',21)";
        jdbcTemplate.update(sql);

        // 发布一个自定义的事件~~~
        applicationEventPublisher.publishEvent(new MyAfterTransactionEvent("我是和事务相关的事件，请事务提交后执行我~~~", id));
        return "service hello";
    }

    @Slf4j
    @Component
    private static class MyTransactionListener {
        @Autowired
        private JdbcTemplate jdbcTemplate;

        @TransactionalEventListener(phase = TransactionPhase.AFTER_COMMIT)
        private void onHelloEvent(HelloServiceImpl.MyAfterTransactionEvent event) {
            Object source = event.getSource();
            Integer id = event.getId();

            String query = "select count(1) from user where id = " + id;
            Integer count = jdbcTemplate.queryForObject(query, Integer.class);
            
            // 可以看到 这里的count是1  它肯定是在上面事务提交之后才会执行的
            log.info(source + ":" + count.toString()); //我是和事务相关的事件，请事务提交后执行我~~~:1
        }
    }


    // 定一个事件，继承自ApplicationEvent 
    private static class MyAfterTransactionEvent extends ApplicationEvent {

        private Integer id;

        public MyAfterTransactionEvent(Object source, Integer id) {
            super(source);
            this.id = id;
        }

        public Integer getId() {
            return id;
        }
    }
}

// @since 4.2  显然，注解的方式提供得还是挺晚的，而API的方式第一个版本就已经提供了
// 另外最重要的是，它头上有一个注解：`@EventListener`  so  
@Target({ElementType.METHOD, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@EventListener //有类似于注解继承的效果
public @interface TransactionalEventListener {
	// 这个注解取值有：BEFORE_COMMIT、AFTER_COMMIT、AFTER_ROLLBACK、AFTER_COMPLETION
	// 各个值都代表什么意思表达什么功能，非常清晰~
	// 需要注意的是：AFTER_COMMIT + AFTER_COMPLETION是可以同时生效的
	// AFTER_ROLLBACK + AFTER_COMPLETION是可以同时生效的
	TransactionPhase phase() default TransactionPhase.AFTER_COMMIT;

	// 若没有事务的时候，对应的event是否已经执行  默认值为false表示  没事务就不执行了
	boolean fallbackExecution() default false;

	// 这里巧妙的用到了@AliasFor的能力，放到了@EventListener身上
	// 注意：一般我建议都需要指定此值，否则默认可以处理所有类型的事件  范围太广了
	@AliasFor(annotation = EventListener.class, attribute = "classes")
	Class<?>[] value() default {};
	@AliasFor(annotation = EventListener.class, attribute = "classes")
	Class<?>[] classes() default {};
	
	String condition() default "";
}

public class TransactionalEventListenerFactory implements EventListenerFactory, Ordered {
	private int order = 50; // 执行时机还是比较早的~~~（默认的工厂是最低优先级）
	
	// 显然这个工厂只会生成标注有此注解的handler~~~
	@Override
	public boolean supportsMethod(Method method) {
		return AnnotatedElementUtils.hasAnnotation(method, TransactionalEventListener.class);
	}

	// 这里使用的是ApplicationListenerMethodTransactionalAdapter，而非ApplicationListenerMethodAdapter
	// 虽然ApplicationListenerMethodTransactionalAdapter是它的子类
	@Override
	public ApplicationListener<?> createApplicationListener(String beanName, Class<?> type, Method method) {
		return new ApplicationListenerMethodTransactionalAdapter(beanName, type, method);
	}
}

// @since 4.2
class ApplicationListenerMethodTransactionalAdapter extends ApplicationListenerMethodAdapter {

	private final TransactionalEventListener annotation;

	// 构造函数
	public ApplicationListenerMethodTransactionalAdapter(String beanName, Class<?> targetClass, Method method) {
		// 这一步的初始化交给父类，做了很多事情   强烈建议看看上面推荐的事件/监听的博文
		super(beanName, targetClass, method);

		// 自己个性化的：和事务相关
		TransactionalEventListener ann = AnnotatedElementUtils.findMergedAnnotation(method, TransactionalEventListener.class);
		if (ann == null) {
			throw new IllegalStateException("No TransactionalEventListener annotation found on method: " + method);
		}
		this.annotation = ann;
	}

	@Override
	public void onApplicationEvent(ApplicationEvent event) {
		// 若**存在事务**：毫无疑问 就注册一个同步器进去~~
		if (TransactionSynchronizationManager.isSynchronizationActive()) {
			TransactionSynchronization transactionSynchronization = createTransactionSynchronization(event);
			TransactionSynchronizationManager.registerSynchronization(transactionSynchronization);
		}
		// 若fallbackExecution=true，那就是表示即使没有事务  也会执行handler
		else if (this.annotation.fallbackExecution()) {
			if (this.annotation.phase() == TransactionPhase.AFTER_ROLLBACK && logger.isWarnEnabled()) {
				logger.warn("Processing " + event + " as a fallback execution on AFTER_ROLLBACK phase");
			}
			processEvent(event);
		}
		else {
			// No transactional event execution at all
			// 若没有事务，输出一个debug信息，表示这个监听器没有执行~~~~
			if (logger.isDebugEnabled()) {
				logger.debug("No transaction is active - skipping " + event);
			}
		}
	}

	// TransactionSynchronizationEventAdapter是一个内部类，它是一个TransactionSynchronization同步器
	// 此类实现也比较简单，它的order由listener.getOrder();来决定
	private TransactionSynchronization createTransactionSynchronization(ApplicationEvent event) {
		return new TransactionSynchronizationEventAdapter(this, event, this.annotation.phase());
	}


	private static class TransactionSynchronizationEventAdapter extends TransactionSynchronizationAdapter {

		private final ApplicationListenerMethodAdapter listener;
		private final ApplicationEvent event;
		private final TransactionPhase phase;

		public TransactionSynchronizationEventAdapter(ApplicationListenerMethodAdapter listener,
				ApplicationEvent event, TransactionPhase phase) {
			this.listener = listener;
			this.event = event;
			this.phase = phase;
		}

		// 它的order又监听器本身来决定  
		@Override
		public int getOrder() {
			return this.listener.getOrder();
		}

		// 最终都是委托给了listenner来真正的执行处理  来执行最终处理逻辑（也就是解析classes、condtion、执行方法体等等）
		@Override
		public void beforeCommit(boolean readOnly) {
			if (this.phase == TransactionPhase.BEFORE_COMMIT) {
				processEvent();
			}
		}

		// 此处结合status和phase   判断是否应该执行~~~~
		// 此处小技巧：我们发现TransactionPhase.AFTER_COMMIT也是放在了此处执行的，只是它结合了status进行判断而已~~~
		@Override
		public void afterCompletion(int status) {
			if (this.phase == TransactionPhase.AFTER_COMMIT && status == STATUS_COMMITTED) {
				processEvent();
			} else if (this.phase == TransactionPhase.AFTER_ROLLBACK && status == STATUS_ROLLED_BACK) {
				processEvent();
			} else if (this.phase == TransactionPhase.AFTER_COMPLETION) {
				processEvent();
			}
		}

		protected void processEvent() {
			this.listener.processEvent(this.event);
		}
	}
}

@Import(ScopedBean.class) // 使用@Import注入  即使它头上没有任何注解也是ok的
@Configuration
public class Main {

    @Lazy
    @Resource
    ScopedBean bean;

    public static void main(String[] args) {
        AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(Main.class);

        Main bean = context.getBean(Main.class);

        //如果bean上没有@Lazy注解, 则2个获取的bean是一个实例, 
        //加了@Lazy注解后, 则2次获取的是2个实例
        System.out.println(bean.bean);
        System.out.println(bean.bean);

        context.close();
    }

}

// 多例    
@Scope(value = ConfigurableBeanFactory.SCOPE_PROTOTYPE)
class ScopedBean {
}

    @Resource
    ScopedBean bean;
	...
        System.out.println(bean.bean); //com.fsx.maintest.ScopedBean@5fdcaa40
        System.out.println(bean.bean); //com.fsx.maintest.ScopedBean@5e0826e7

@MainBean(beanName = "mainbean")
public class Main {

    public static void main(String[] args) {
        AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(Main.class);
        String[] beannames = context.getBeanNamesForType(Main.class);

        //当加上@AliasFor时, 输出"mainbean"
        //当去掉@AliasFor注解后, 输出"main"（`main`是默认的beanName生成策略生成的）
        System.out.println(beannames[0]);
        context.close();
    }
}

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration
@interface MainBean {
    @AliasFor(annotation = Component.class, attribute = "value")
    String beanName() default "";
}