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目录* GlobalTransactionScanner

• 继承AbstractAutoProxyCreator
• 实现InitializingBean接口
• 写在最后

以AT为例，我们使用Seata时只需要在全局事务的方法上加上@GlobalTransactional，就开启了全局事务的支持。那么Seata的底层到底是怎么实现的呢？
首先我们知道，Seata也是一个SpringBoot项目，如果对Seata源码无从下手，那么不妨从Spring切入：

GlobalTransactionScanner

在Seata中有一个重要的bean组件：GlobalTranscationScanner：

全局事务扫描器，它继承了AbstractAutoProxyCreator, InitializingBean, ApplicationContextAware, DisposableBean接口。这四个都是spring的类，所以想要知道这个GlobalTransactionScanner做了什么工作，我们首先得介绍一下spring知识：

• AbstractAutoProxyCreator: 是spring的AOP的一个核心类。
• InitializingBean：此接口为Bean提供了初始化方法的方式，只包含afterPropertiesSet方法
• DisposableBean：这个接口和InitializingBean是一组的.它只包含destroy方法，作用是为Bean生命周期结束前做一些收尾工作。
• ApplicationContextAware：实现了这个接口的类可以方便地获得ApplicationContext中的Bean。 可以简单理解为它就是一个spring容器

可以看出来，GlobalTransactionScanner继承了AOP，可以对Bean进行增强，同时还相当于一个spring容器。接下我们来看它做了实现这些接口都做了什么事。

继承AbstractAutoProxyCreator

实现AOP后，我们重点来看下被重写的wrapIfNecessary, 它是AOP中核心的方法

1.wrapIfNecessary()

 @Override
    protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
        // do checkers
        try {
            synchronized (PROXYED_SET) {
                if (PROXYED_SET.contains(beanName)) {
                    return bean;
                }
                interceptor = null;
                //check TCC proxy：检查是否为TCC(这里我们研究AT，就不看TCC了)
                if (TCCBeanParserUtils.isTccAutoProxy(bean, beanName, applicationContext)) {
                  //...
                } else { 
                // 不是TCC模式：
                    Class<?> serviceInterface = SpringProxyUtils.findTargetClass(bean);
                    Class<?>[] interfacesIfJdk = SpringProxyUtils.findInterfaces(bean);
        //判断是否有相关事务的注解，如GlobalTransactional，如果没有就不会代理，直接返回bean
                    if (!existsAnnotation(new Class[]{serviceInterface})
                        && !existsAnnotation(interfacesIfJdk)) {
                        return bean;
                    }
                //发现存在事务相关注解的bean，则添加拦截器———GlobalTransactionalInterceptor
                    if (globalTransactionalInterceptor == null) {
                        globalTransactionalInterceptor = new GlobalTransactionalInterceptor(failureHandlerHook);
                        ConfigurationCache.addConfigListener(
                                ConfigurationKeys.DISABLE_GLOBAL_TRANSACTION,
                                (ConfigurationChangeListener)globalTransactionalInterceptor);
                    }
                    interceptor = globalTransactionalInterceptor;
                }
                // ...
                PROXYED_SET.add(beanName);
                return bean;
            }
        }
    }

说明： 此方法对被全局事务注解的方法Bean进行了增强。具体实现是将拦截器织入代理对象。

2.invoke()

globalTransactionalInterceptor实现了MethodInterceptor这个接口,此接口只有一个方法#invoke():

   @Override
    public Object invoke(final MethodInvocation methodInvocation) throws Throwable {
    // 获取注解标注的执行方法
        Class<?> targetClass =
            methodInvocation.getThis() != null ? AopUtils.getTargetClass(methodInvocation.getThis()) : null;
        Method specificMethod = ClassUtils.getMostSpecificMethod(methodInvocation.getMethod(), targetClass);
        if (specificMethod != null && !specificMethod.getDeclaringClass().equals(Object.class)) {
            final Method method = BridgeMethodResolver.findBridgedMethod(specificMethod);
            // 获取全局事务GlobalTransactional的元数据
            final GlobalTransactional globalTransactionalAnnotation =
                getAnnotation(method, targetClass, GlobalTransactional.class);
            // 获取全局锁GlobalLock的元数据.全局锁会将本地事务的执行纳入到seata的管理，一起竞争全局锁，
            //保证全局事务在执行时，不会收到本地其他事务的影响。（隔离性）
            final GlobalLock globalLockAnnotation = getAnnotation(method, targetClass, GlobalLock.class);
            boolean localDisable = disable || (degradeCheck && degradeNum >= degradeCheckAllowTimes);
             if (globalTransactionalAnnotation != null) {
                    //执行全局事务
                    return handleGlobalTransaction(methodInvocation, transactional);
                } else if (globalLockAnnotation != null) {
                //执行全局锁
                    return handleGlobalLock(methodInvocation, globalLockAnnotation);
                }
            }
        }
        return methodInvocation.proceed();
    }

说明：

3.handleGlobalTransaction()
我们重点关注执行全局事务的方法handleGlobalTransaction().它的作用是获取事务信息并且执行全局事务:

 Object handleGlobalTransaction(final MethodInvocation methodInvocation,
        final AspectTransactional aspectTransactional) throws Throwable {
        boolean succeed = true;
        try {
            // 调用execute方法,执行全局事务
            return transactionalTemplate.execute(new TransactionalExecutor() {
                @Override
                public Object execute() throws Throwable {
                    return methodInvocation.proceed();
                }
                // 获取事务名称
                public String name() {
                    String name = aspectTransactional.getName();
                    if (!StringUtils.isNullOrEmpty(name)) {
                        return name;
                    }
                    return formatMethod(methodInvocation.getMethod());
                }
                //获取事务信息,并封装成TransactionInfo对象
                @Override
                public TransactionInfo getTransactionInfo() {
                    // reset the value of timeout
                    int timeout = aspectTransactional.getTimeoutMills();
                    if (timeout <= 0 || timeout == DEFAULT_GLOBAL_TRANSACTION_TIMEOUT) {
                        timeout = defaultGlobalTransactionTimeout;
                    }
                    TransactionInfo transactionInfo = new TransactionInfo();
                    transactionInfo.setTimeOut(timeout);
                    transactionInfo.setName(name());
                    transactionInfo.setPropagation(aspectTransactional.getPropagation());
                    transactionInfo.setLockRetryInterval(aspectTransactional.getLockRetryInterval());
                    transactionInfo.setLockRetryTimes(aspectTransactional.getLockRetryTimes());
                    Set<RollbackRule> rollbackRules = new LinkedHashSet<>();
                    for (Class<?> rbRule : aspectTransactional.getRollbackFor()) {
                        rollbackRules.add(new RollbackRule(rbRule));
                    }
                    for (String rbRule : aspectTransactional.getRollbackForClassName()) {
                        rollbackRules.add(new RollbackRule(rbRule));
                    }
                    for (Class<?> rbRule : aspectTransactional.getNoRollbackFor()) {
                        rollbackRules.add(new NoRollbackRule(rbRule));
                    }
                    for (String rbRule : aspectTransactional.getNoRollbackForClassName()) {
                        rollbackRules.add(new NoRollbackRule(rbRule));
                    }
                    transactionInfo.setRollbackRules(rollbackRules);
                    return transactionInfo;
                }
            });
        } catch (TransactionalExecutor.ExecutionException e) {
             //...
            }
        } finally {
          //...
        }
    }

说明:

1. 这个方法的主要工作是获取事务的名称和信息,并开启全局事务.
2. 全局事务的开启调用了transactionalTemplate中的execute()方法.下面继续进入execute方法:

4.execute()

 public Object execute(TransactionalExecutor business) throws Throwable {
        // 1. Get transactionInfo 获取事务信息
        TransactionInfo txInfo = business.getTransactionInfo();
        // 1.1 Get current transaction, if not null, the tx role is 'GlobalTransactionRole.Participant'.
        // 获取当前事务,主要获取XId
        GlobalTransaction tx = GlobalTransactionContext.getCurrent();

        // 1.2 Handle the transaction propagation. 配置不同事务的传播行为,执行不同逻辑.
        Propagation propagation = txInfo.getPropagation();
        SuspendedResourcesHolder suspendedResourcesHolder = null;
        try {
        //Spring事务的7种传播行为
            switch (propagation) {
                case NOT_SUPPORTED:
                    // If transaction is existing, suspend it.
                    if (existingTransaction(tx)) {
                        suspendedResourcesHolder = tx.suspend();
                    }
                    // Execute without transaction and return.
                    return business.execute();
                case REQUIRES_NEW:
                    // If transaction is existing, suspend it, and then begin new transaction.
                    if (existingTransaction(tx)) {
                        suspendedResourcesHolder = tx.suspend();
                        tx = GlobalTransactionContext.createNew();
                    }
                    // Continue and execute with new transaction
                    break;
                case SUPPORTS:
                    // If transaction is not existing, execute without transaction.
                    if (notExistingTransaction(tx)) {
                        return business.execute();
                    }
                    // Continue and execute with new transaction
                    break;
                case REQUIRED:
                    // If current transaction is existing, execute with current transaction,
                    // else continue and execute with new transaction.
                    break;
                case NEVER:
                    // If transaction is existing, throw exception.
                    if (existingTransaction(tx)) {
                        throw new TransactionException(
                            String.format("Existing transaction found for transaction marked with propagation 'never', xid = %s"
                                    , tx.getXid()));
                    } else {
                        // Execute without transaction and return.
                        return business.execute();
                    }
                case MANDATORY:
                    // If transaction is not existing, throw exception.
                    if (notExistingTransaction(tx)) {
                        throw new TransactionException("No existing transaction found for transaction marked with propagation 'mandatory'");
                    }
                    // Continue and execute with current transaction.
                    break;
                default:
                    throw new TransactionException("Not Supported Propagation:" + propagation);
            }

            // 1.3 如果当前事务未空,则新创建一个.
            if (tx == null) {
                tx = GlobalTransactionContext.createNew();
            }
            try {
                // 2. If the tx role is 'GlobalTransactionRole.Launcher', send the request of beginTransaction to TC,
                // else do nothing. Of course, the hooks will still be triggered.
                // 开启全局事务.
                beginTransaction(txInfo, tx);

                Object rs;
                try {
                    // Do Your Business
                    rs = business.execute();
                } catch (Throwable ex) {
                    // 3. The needed business exception to rollback.
                    // 发生异常,全局回滚.各个数据根据undo\_log进行补偿.
                    completeTransactionAfterThrowing(txInfo, tx, ex);
                    throw ex;
                }
                // 如果没有异常发生,则提交全局事务
                // 4. everything is fine, commit.
                commitTransaction(tx);

                return rs;
            } finally {
                //5. clear
                resumeGlobalLockConfig(previousConfig);
                triggerAfterCompletion();
                cleanUp();
            }
        }
}

说明:

1. 看到这里可能有些似曾相似,这个流程下来不就是AT模式的2个阶段嘛.我们探究到了AT模式的具体实现!
2. 在此方法中,我们终于看到了开启全局事务的关键方法: beginTransaction(). 不过,我们知道TM要开启全局事务,首先得向TC发起请求. 说明我们还得进入beginTransaction()方法中一探究竟,这里面还调用了不少方法,大家看的时候可以只看注释的一行往下推进:

4.1 beginTransaction()

   private void beginTransaction(TransactionInfo txInfo, GlobalTransaction tx) throws TransactionalExecutor.ExecutionException {
        try {
            triggerBeforeBegin();
            //对TC发起请求
            tx.begin(txInfo.getTimeOut(), txInfo.getName());
            triggerAfterBegin();
        } //...
    }

   @Override
    public void begin(int timeout, String name) throws TransactionException {
        // 判断事务的发起者是不是TM,如果不是抛异常 
        if (role != GlobalTransactionRole.Launcher) {
            assertXIDNotNull();
            if (LOGGER.isDebugEnabled()) {
                LOGGER.debug("Ignore Begin(): just involved in global transaction [{}]", xid);
            }
            return;
        }
        assertXIDNull();
        String currentXid = RootContext.getXID();  //获取Xid
        if (currentXid != null) {
            throw new IllegalStateException("Global transaction already exists," +
                " can't begin a new global transaction, currentXid = " + currentXid);
        }
        //调用transactionManager.begin()
        xid = transactionManager.begin(null, null, name, timeout);
        status = GlobalStatus.Begin;
        RootContext.bind(xid);
        if (LOGGER.isInfoEnabled()) {
            LOGGER.info("Begin new global transaction [{}]", xid);
        }
    }

    @Override
    public String begin(String applicationId, String transactionServiceGroup, String name, int timeout)
        throws TransactionException {
        GlobalBeginRequest request = new GlobalBeginRequest();
        request.setTransactionName(name);
        request.setTimeout(timeout);
        // 关键:syncCall 同步请求
        GlobalBeginResponse response = (GlobalBeginResponse) syncCall(request);
        if (response.getResultCode() == ResultCode.Failed) {
            throw new TmTransactionException(TransactionExceptionCode.BeginFailed, response.getMsg());
        }
        return response.getXid();
    }

     private AbstractTransactionResponse syncCall(AbstractTransactionRequest request) throws TransactionException {
        try {
        // 通过Netty发起请求
            return (AbstractTransactionResponse) TmNettyRemotingClient.getInstance().sendSyncRequest(request);
        } catch (TimeoutException toe) {
            throw new TmTransactionException(TransactionExceptionCode.IO, "RPC timeout", toe);
        }
    }

实现InitializingBean接口

InitializingBean接口只有一个方法afterPropertiesSet(),GlobalTransactionScanner对它进行了重写:

说明: 调用了initCLient方法：初始化了TM和RM

   private void initClient() {
         // ....
        //init TM
        TMClient.init(applicationId, txServiceGroup, accessKey, secretKey);
        // 日志
        //init RM
        RMClient.init(applicationId, txServiceGroup);
        // 日志
        registerSpringShutdownHook();

    }

初始化TM:

    public static void init(String applicationId, String transactionServiceGroup, String accessKey, String secretKey) {
    // 获取TMRpc客户端实例
        TmNettyRemotingClient tmNettyRemotingClient = TmNettyRemotingClient.getInstance(applicationId, transactionServiceGroup, accessKey, secretKey);
        //初始化 TM Client
        tmNettyRemotingClient.init();
    }

调用 TmRpcClient.getInstance() 方法会获取一个 TM 客户端实例.
在获取过程中，会创建 Netty 客户端配置文件对象，以及创建 messageExecutor 线程池，该线程池用于在处理各种与服务端的消息交互，在创建 TmRpcClient 实例时，创建 ClientBootstrap，用于管理 Netty 服务的启停，以及 ClientChannelManager，它是专门用于管理 Netty 客户端对象池.

初始化TM客户端:
To Be Continue...

写在最后

博主也是刚开始学习Seata,编程功力不太深,很多代码的精妙之处也看不出来.

参考链接:
Seata AT 模式启动源码分析
视频,本文很多都是从这里整理的

转载请注明：xuhss » Seata源码分析(一). AT模式底层实现