问题

业务场景

业务需求上经常会有一些边缘操作，比如主流程操作A：用户报名课程操作入库，边缘操作B：发送邮件或短信通知。

业务要求

• 操作A操作数据库失败后，事务回滚，那么操作B不能执行。

• 操作A执行成功后，操作B也必须执行成功

如何实现

• 普通的执行A，之后执行B，是可以满足要求1，对于要求2通常需要设计补偿的操作

• 一般边缘的操作，通常会设置成为异步的，以提升性能，比如发送MQ，业务系统负责事务成功后消息发送成功，然后接收系统负责保证通知成功完成

本文内容

如何在spring事务提交之后进行异步操作，这些异步操作必须得在该事务成功提交后才执行，回滚则不执行。

要点

• 如何在spring事务提交之后操作

• 如何把操作异步化

实现方案

使用TransactionSynchronizationManager在事务提交之后操作

public void insert(TechBook techBook){        bookMapper.insert(techBook);       // send after tx commit but is async        TransactionSynchronizationManager.registerSynchronization(new TransactionSynchronizationAdapter() {            @Override            public void afterCommit() {                System.out.println("send email after transaction commit...");            }        }       );        ThreadLocalRandom random = ThreadLocalRandom.current();        if(random.nextInt() % 2 ==0){            throw new RuntimeException("test email transaction");        }        System.out.println("service end");    }

该方法就可以实现在事务提交之后进行操作

操作异步化

使用mq或线程池来进行异步，比如使用线程池：

private final ExecutorService executorService = Executors.newFixedThreadPool(5);    public void insert(TechBook techBook){        bookMapper.insert(techBook); //        send after tx commit but is async        TransactionSynchronizationManager.registerSynchronization(new TransactionSynchronizationAdapter() {            @Override            public void afterCommit() {                executorService.submit(new Runnable() {                    @Override                    public void run() {                        System.out.println("send email after transaction commit...");                        try {                            Thread.sleep(10*1000);                        } catch (InterruptedException e) {                            e.printStackTrace();                        }                        System.out.println("complete send email after transaction commit...");                    }                });            }        }        ); //        async work but tx not work, execute even when tx is rollback//        asyncService.executeAfterTxComplete();         ThreadLocalRandom random = ThreadLocalRandom.current();        if(random.nextInt() % 2 ==0){            throw new RuntimeException("test email transaction");        }        System.out.println("service end");    }

封装以上两步

对于第二步来说，如果这类方法比较多的话，则写起来重复性太多，因而，抽象出来如下：

public interface AfterCommitExecutor extends Executor {} import org.slf4j.Logger;import org.slf4j.LoggerFactory;import org.springframework.stereotype.Component;import org.springframework.transaction.support.TransactionSynchronizationAdapter;import org.springframework.transaction.support.TransactionSynchronizationManager; import java.util.ArrayList;import java.util.List;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors; @Componentpublic class AfterCommitExecutorImpl extends TransactionSynchronizationAdapter implements AfterCommitExecutor {    private static final Logger LOGGER = LoggerFactory.getLogger(AfterCommitExecutorImpl.class);    private static final ThreadLocal
    
     
      > RUNNABLES = new ThreadLocal
      
       
        >();    private ExecutorService threadPool = Executors.newFixedThreadPool(5);     @Override    public void execute(Runnable runnable) {        LOGGER.info("Submitting new runnable {} to run after commit", runnable);        if (!TransactionSynchronizationManager.isSynchronizationActive()) {            LOGGER.info("Transaction synchronization is NOT ACTIVE. Executing right now runnable {}", runnable);            runnable.run();            return;        }        List
        
          threadRunnables = RUNNABLES.get();        if (threadRunnables == null) {            threadRunnables = new ArrayList
         
          (); RUNNABLES.set(threadRunnables); TransactionSynchronizationManager.registerSynchronization(this); } threadRunnables.add(runnable); } @Override public void afterCommit() { List
          
            threadRunnables = RUNNABLES.get(); LOGGER.info("Transaction successfully committed, executing {} runnables", threadRunnables.size()); for (int i = 0; i < threadRunnables.size(); i++) { Runnable runnable = threadRunnables.get(i); LOGGER.info("Executing runnable {}", runnable); try { threadPool.execute(runnable); } catch (RuntimeException e) { LOGGER.error("Failed to execute runnable " + runnable, e); } } } @Override public void afterCompletion(int status) { LOGGER.info("Transaction completed with status {}", status == STATUS_COMMITTED ? "COMMITTED" : "ROLLED_BACK"); RUNNABLES.remove(); } }public void insert(TechBook techBook){ bookMapper.insert(techBook); // send after tx commit but is async// TransactionSynchronizationManager.registerSynchronization(new TransactionSynchronizationAdapter() {// @Override// public void afterCommit() {// executorService.submit(new Runnable() {// @Override// public void run() {// System.out.println("send email after transaction commit...");// try {// Thread.sleep(10*1000);// } catch (InterruptedException e) {// e.printStackTrace();// }// System.out.println("complete send email after transaction commit...");// }// });// }// }// ); //send after tx commit and is async afterCommitExecutor.execute(new Runnable() { @Override public void run() { try { Thread.sleep(5*1000); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println("send email after transactioin commit"); } }); // async work but tx not work, execute even when tx is rollback// asyncService.executeAfterTxComplete(); ThreadLocalRandom random = ThreadLocalRandom.current(); if(random.nextInt() % 2 ==0){ throw new RuntimeException("test email transaction"); } System.out.println("service end"); }
          
         
        
       
      
     
    

关于Spring的Async

spring为了方便应用使用线程池进行异步化，默认提供了@Async注解，可以整个app使用该线程池，而且只要一个@Async注解在方法上面即可，省去重复的submit操作。关于async要注意的几点：

1、async的配置

这个必须配置在root context里头，而且web context不能扫描controller层外的注解，否则会覆盖掉。

2、async的调用问题

async方法的调用，不能由同类方法内部调用，否则拦截不生效，这是spring默认的拦截问题，必须在其他类里头调用另一个类中带有async的注解方法，才能起到异步效果。

3、事务问题

async方法如果也开始事务的话，要注意事务传播以及事务开销的问题。而且在async方法里头使用如上的TransactionSynchronizationManager.registerSynchronization不起作用，值得注意。