分布式事务（五）源码详解

系列目录

引子

本节我们将会从上一节的”简单样例“入手：Spring Boot+Atomikos(TM)+Mybatis(ORM)+Mysql(DB)，深入源码，看看这个分布式事务是怎么定义、执行的。

先来回忆一下第二节讲的JTA规范，如下图。Atomikos是什么角色？起到什么作用？

角色：

Atomikos根本上是一个事务管理器(TM)也就是JTA模型的核心，上图扇形的中心位置。

作用：

TM调用【Resource Manager资源管理器】的XAResource接口来实现事务操作。

TM依赖【Application Server应用服务器】的TransactionManager接口当然如果服务器不支持事务管理，自然也就只能使用第三方包，例如Atomikos。

TM依赖【Application应用程序】设置事务边界、属性，application调用UserTransaction接口控制事务开始、提交、回滚。

一、bean定义

1.1 JtaTransactionManager

org.springframework.transaction.jta.JtaTransactionManager类是spring提供的分布式事务管理器。

JtaTransactionManager类图如下：

实现了接口如下：

PlatformTransactionManager ：获取事务，提交事务，回滚事务
TransactionFactory：创建事务
InitializingBean：初始化bean

JtaTransactionManager实现了InitializingBean接口的afterPropertiesSet()方法，处于bean生命周期的容器初始化->实例化期->初始化中期，如下图：

下面我们看一下JtaTransactionManager在bean初始化中期InitializingBean接口的afterPropertiesSet()做了什么：

 1 /**
 2  * Initialize the UserTransaction as well as the TransactionManager handle.
 3  * @see #initUserTransactionAndTransactionManager()
 4  */
 5 @Override
 6 public void afterPropertiesSet() throws TransactionSystemException {
 7     initUserTransactionAndTransactionManager();
 8     checkUserTransactionAndTransactionManager();
 9     initTransactionSynchronizationRegistry();
10 }

1.initUserTransactionAndTransactionManager：初始化UserTransaction和TransactionManager接口。主要是如果没有定义的话，可以支持JNDI。
2.checkUserTransactionAndTransactionManager：校验2个接口是否存在。UserTransaction必须定义，TransactionManager可以不定义。

　　　　　　源码如下：

　　　　　　对应控制台打印：

o.s.t.jta.JtaTransactionManager          : Using JTA UserTransaction: com.atomikos.icatch.jta.UserTransactionImp@614aeccc
o.s.t.jta.JtaTransactionManager          : Using JTA TransactionManager: com.atomikos.icatch.jta.UserTransactionManager@5116ac09

3.initTransactionSynchronizationRegistry：初始化事务同步注册，这个不使用JNDI的话没啥用。

上一节分布式事务（三）简单样例中我们配置了JtaTransactionManagerConfig类，如下：

 1 package study.config.datasource;
 2 
 3 import com.atomikos.icatch.jta.UserTransactionImp;
 4 import com.atomikos.icatch.jta.UserTransactionManager;
 5 import org.springframework.context.annotation.Bean;
 6 import org.springframework.context.annotation.Configuration;
 7 import org.springframework.transaction.jta.JtaTransactionManager;
 8 
 9 import javax.transaction.UserTransaction;
10 
11 /**
12  * 事务管理器配置类
13  *
14  * @author denny
15  */
16 @Configuration
17 public class JtaTransactionManagerConfig {
18 
19     @Bean(name = "atomikosTransactionManager")
20     public JtaTransactionManager regTransactionManager() {
21         UserTransactionManager userTransactionManager = new UserTransactionManager();
22         UserTransaction userTransaction = new UserTransactionImp();
23         return new JtaTransactionManager(userTransaction, userTransactionManager);
24     }
25 }

如上图，我们定义了一个name = "atomikosTransactionManager"的bean,具体类型为JtaTransactionManager。其中构造了2个实现类UserTransactionImp（javax.transaction.UserTransaction接口）、UserTransactionManager（javax.transaction.TransactionManager接口）。并用这2个实现类构造了一个JtaTransactionManager。

1.UserTransaction接口

提供给用户操控事务的：开启，提交，回滚等等。源码如下：

2 TransactionManager接口

源码如下：

相比UserTransaction，TransactionManager接口多了接口的挂起、恢复、获取事务3个接口。这3个方法明显是留给系统自己调用的。

1.2 AtomikosDataSourceBean

Spring 为Atomikos定制了一个org.springframework.boot.jta.atomikos.AtomikosDataSourceBean，提供了bean生命周期的一些接口：

BeanNameAware：设置bean名称
InitializingBean：初始化bean
DisposableBean:销毁bean

我们只需要定义这个bean即可轻松使得spring来维护。

com.atomikos.jdbc.AtomikosDataSourceBean类图如下:

其中核心接口：

DataSource接口：getConnection获取数据库连接

ConnectionPoolProperties接口：用于载入连接池的属性

二、源码剖析

2.1 自动配置类

老套路哈，spring boot就这么点花花肠子，既然使用@Transactional这种注解的方式，那么我们就从springboot 容器启动时的自动配置载入（spring boot容器启动详解）开始看。在/META-INF/spring.factories中配置文件中查找，如下图：

载入2个关于事务的自动配置类：

org.springframework.boot.autoconfigure.transaction.TransactionAutoConfiguration,
org.springframework.boot.autoconfigure.transaction.jta.JtaAutoConfiguration,

由于本文是分布式事务，故2个配置文件都生效了，我们先看JtaAutoConfiguration

2.2 JtaAutoConfiguration

 1 /**
 2  * {@link EnableAutoConfiguration Auto-configuration} for JTA.
 3  *
 4  * @author Josh Long
 5  * @author Phillip Webb
 6  * @since 1.2.0
 7  */
 8 @ConditionalOnClass(javax.transaction.Transaction.class)
 9 @ConditionalOnProperty(prefix = "spring.jta", value = "enabled", matchIfMissing = true)
10 @AutoConfigureBefore({ XADataSourceAutoConfiguration.class,
11         ActiveMQAutoConfiguration.class, ArtemisAutoConfiguration.class,
12         HibernateJpaAutoConfiguration.class })
13 @Import({ JndiJtaConfiguration.class, BitronixJtaConfiguration.class,
14         AtomikosJtaConfiguration.class, NarayanaJtaConfiguration.class })
15 @EnableConfigurationProperties(JtaProperties.class)
16 public class JtaAutoConfiguration {
17 
18 }

如上，JtaAutoConfiguration这个类竟然是个空壳，只有一堆注解，挑几个重要的讲一讲：

1.@ConditionalOnClass(javax.transaction.Transaction.class)：代表类路径下存在javax.transaction.Transaction.class这个类，那么JtaAutoConfiguration生效。

2.@ConditionalOnProperty(prefix = "spring.jta", value = "enabled", matchIfMissing = true)，自动开启spring.jta.enabled=true.

3.@Import({ JndiJtaConfiguration.class, BitronixJtaConfiguration.class, AtomikosJtaConfiguration.class, NarayanaJtaConfiguration.class }),又是spring套路哈，用来导入类。这里导入了4个配置类，可见支持4种第三方事务管理器。AtomikosJtaConfiguration.class自然就是Atomikos了。

AtomikosJtaConfiguration.class这个配置类

 1 @Configuration
 2 @EnableConfigurationProperties({ AtomikosProperties.class, JtaProperties.class })
 3 @ConditionalOnClass({ JtaTransactionManager.class, UserTransactionManager.class })
 4 @ConditionalOnMissingBean(PlatformTransactionManager.class)
 5 class AtomikosJtaConfiguration {
 6 
 7     private final JtaProperties jtaProperties;
 8 
 9     private final TransactionManagerCustomizers transactionManagerCustomizers;
10 
11     AtomikosJtaConfiguration(JtaProperties jtaProperties,
12             ObjectProvider<TransactionManagerCustomizers> transactionManagerCustomizers) {
13         this.jtaProperties = jtaProperties;
14         this.transactionManagerCustomizers = transactionManagerCustomizers
15                 .getIfAvailable();
16     }
17 
18     @Bean(initMethod = "init", destroyMethod = "shutdownForce")
19     @ConditionalOnMissingBean(UserTransactionService.class)
20     public UserTransactionServiceImp userTransactionService(
21             AtomikosProperties atomikosProperties) {
22         Properties properties = new Properties();
23         if (StringUtils.hasText(this.jtaProperties.getTransactionManagerId())) {
24             properties.setProperty("com.atomikos.icatch.tm_unique_name",
25                     this.jtaProperties.getTransactionManagerId());
26         }
27         properties.setProperty("com.atomikos.icatch.log_base_dir", getLogBaseDir());
28         properties.putAll(atomikosProperties.asProperties());
29         return new UserTransactionServiceImp(properties);
30     }
31 
32     private String getLogBaseDir() {
33         if (StringUtils.hasLength(this.jtaProperties.getLogDir())) {
34             return this.jtaProperties.getLogDir();
35         }
36         File home = new ApplicationHome().getDir();
37         return new File(home, "transaction-logs").getAbsolutePath();
38     }
39 
40     @Bean(initMethod = "init", destroyMethod = "close")
41     @ConditionalOnMissingBean
42     public UserTransactionManager atomikosTransactionManager(
43             UserTransactionService userTransactionService) throws Exception {
44         UserTransactionManager manager = new UserTransactionManager();
45         manager.setStartupTransactionService(false);
46         manager.setForceShutdown(true);
47         return manager;
48     }
49 
50     @Bean
51     @ConditionalOnMissingBean(XADataSourceWrapper.class)
52     public AtomikosXADataSourceWrapper xaDataSourceWrapper() {
53         return new AtomikosXADataSourceWrapper();
54     }
55 
56     @Bean
57     @ConditionalOnMissingBean
58     public static AtomikosDependsOnBeanFactoryPostProcessor atomikosDependsOnBeanFactoryPostProcessor() {
59         return new AtomikosDependsOnBeanFactoryPostProcessor();
60     }
61 
62     @Bean
63     public JtaTransactionManager transactionManager(UserTransaction userTransaction,
64             TransactionManager transactionManager) {
65         JtaTransactionManager jtaTransactionManager = new JtaTransactionManager(
66                 userTransaction, transactionManager);
67         if (this.transactionManagerCustomizers != null) {
68             this.transactionManagerCustomizers.customize(jtaTransactionManager);
69         }
70         return jtaTransactionManager;
71     }
72 
73     @Configuration
74     @ConditionalOnClass(Message.class)
75     static class AtomikosJtaJmsConfiguration {
76 
77         @Bean
78         @ConditionalOnMissingBean(XAConnectionFactoryWrapper.class)
79         public AtomikosXAConnectionFactoryWrapper xaConnectionFactoryWrapper() {
80             return new AtomikosXAConnectionFactoryWrapper();
81         }
82 
83     }
84 
85 }

2.3 TransactionAutoConfiguration

这里和本地事务分析过程一致，就不再重复，飞机票spring事务详解（三）源码详解，一直看到第二节结束.这里只截个图：

最终源码调用具体事务管理器的PlatformTransactionManager接口的3个方法：

1 public interface PlatformTransactionManager {
2     // 获取事务状态
3     TransactionStatus getTransaction(TransactionDefinition definition) throws TransactionException;
4 　　// 事务提交
5     void commit(TransactionStatus status) throws TransactionException;
6 　　// 事务回滚
7     void rollback(TransactionStatus status) throws TransactionException;
8 }

三、核心源码

核心实现类图：

如上提所示，PlatformTransactionManager顶级接口定义了最核心的事务管理方法，下面一层是AbstractPlatformTransactionManager抽象类，实现了PlatformTransactionManager接口的方法并定义了一些抽象方法，供子类拓展。最下面一层是2个经典事务管理器：

1.DataSourceTransactionmanager: 即本地单资源事务管理器。

2.JtaTransactionManager: 即多资源事务管理器（又叫做分布式事务管理器），其实现了JTA规范，使用XA协议进行两阶段提交。

我们这里自然是JTA分布式环境，我们只需要从JtaTransactionManager这个实现类入手即可。

3.1 getTransaction获取事务

AbstractPlatformTransactionManager实现了getTransaction()方法如下：

 1 @Override
 2     public final TransactionStatus getTransaction(TransactionDefinition definition) throws TransactionException {
 3         Object transaction = doGetTransaction();
 4 
 5         // Cache debug flag to avoid repeated checks.
 6         boolean debugEnabled = logger.isDebugEnabled();
 7 
 8         if (definition == null) {
 9             // Use defaults if no transaction definition given.
10             definition = new DefaultTransactionDefinition();
11         }
12 　　　　  // 如果当前已经存在事务
13         if (isExistingTransaction(transaction)) {
14             // 根据不同传播机制不同处理
15             return handleExistingTransaction(definition, transaction, debugEnabled);
16         }
17 
18         // 超时不能小于默认值
19         if (definition.getTimeout() < TransactionDefinition.TIMEOUT_DEFAULT) {
20             throw new InvalidTimeoutException("Invalid transaction timeout", definition.getTimeout());
21         }
22 
23         // 当前不存在事务，传播机制=MANDATORY（支持当前事务，没事务报错），报错
24         if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_MANDATORY) {
25             throw new IllegalTransactionStateException(
26                     "No existing transaction found for transaction marked with propagation 'mandatory'");
27         }// 当前不存在事务，传播机制=REQUIRED/REQUIRED_NEW/NESTED,这三种情况，需要新开启事务，且加上事务同步
28         else if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRED ||
29                 definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW ||
30                 definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {
31             SuspendedResourcesHolder suspendedResources = suspend(null);
32             if (debugEnabled) {
33                 logger.debug("Creating new transaction with name [" + definition.getName() + "]: " + definition);
34             }
35             try {// 是否需要新开启同步// 开启// 开启
36                 boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
37                 DefaultTransactionStatus status = newTransactionStatus(
38                         definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
39                 doBegin(transaction, definition);// 开启新事务
40                 prepareSynchronization(status, definition);//预备同步
41                 return status;
42             }
43             catch (RuntimeException ex) {
44                 resume(null, suspendedResources);
45                 throw ex;
46             }
47             catch (Error err) {
48                 resume(null, suspendedResources);
49                 throw err;
50             }
51         }
52         else {
53             // 当前不存在事务当前不存在事务，且传播机制=PROPAGATION_SUPPORTS/PROPAGATION_NOT_SUPPORTED/PROPAGATION_NEVER，这三种情况，创建“空”事务:没有实际事务，但可能是同步。警告：定义了隔离级别，但并没有真实的事务初始化，隔离级别被忽略有隔离级别但是并没有定义实际的事务初始化，有隔离级别但是并没有定义实际的事务初始化，
54             if (definition.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT && logger.isWarnEnabled()) {
55                 logger.warn("Custom isolation level specified but no actual transaction initiated; " +
56                         "isolation level will effectively be ignored: " + definition);
57             }
58             boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
59             return prepareTransactionStatus(definition, null, true, newSynchronization, debugEnabled, null);
60         }
61     }

上图核心步骤就是：

1.doGetTransaction()：获取事务
2.doBegin：准备工作

3.1.1 JtaTransactionManager的doGetTransaction()

其实也就是把UserTransaction封装成一个JtaTransactionObject返回。

 1     @Override
 2     protected Object doGetTransaction() {
 3         UserTransaction ut = getUserTransaction();
 4         if (ut == null) {
 5             throw new CannotCreateTransactionException("No JTA UserTransaction available - " +
 6                     "programmatic PlatformTransactionManager.getTransaction usage not supported");
 7         }
 8         if (!this.cacheUserTransaction) {
 9             ut = lookupUserTransaction(
10                     this.userTransactionName != null ? this.userTransactionName : DEFAULT_USER_TRANSACTION_NAME);
11         }
12         return doGetJtaTransaction(ut);
13     }
14 
15     /**
16      * Get a JTA transaction object for the given current UserTransaction.
17      * <p>Subclasses can override this to provide a JtaTransactionObject
18      * subclass, for example holding some additional JTA handle needed.
19      * @param ut the UserTransaction handle to use for the current transaction
20      * @return the JtaTransactionObject holding the UserTransaction
21      */
22     protected JtaTransactionObject doGetJtaTransaction(UserTransaction ut) {
23         return new JtaTransactionObject(ut);
24     }

3.1.2 JtaTransactionManager.doBegin

 1 @Override
 2     protected void doBegin(Object transaction, TransactionDefinition definition) {
 3         JtaTransactionObject txObject = (JtaTransactionObject) transaction;
 4         try {
 5             doJtaBegin(txObject, definition);
 6         }
 7         catch (NotSupportedException ex) {
 8             // assume nested transaction not supported
 9             throw new NestedTransactionNotSupportedException(
10                     "JTA implementation does not support nested transactions", ex);
11         }
12         catch (UnsupportedOperationException ex) {
13             // assume nested transaction not supported
14             throw new NestedTransactionNotSupportedException(
15                     "JTA implementation does not support nested transactions", ex);
16         }
17         catch (SystemException ex) {
18             throw new CannotCreateTransactionException("JTA failure on begin", ex);
19         }
20     }

调用JtaTransactionManager.doJtaBegin：

1 protected void doJtaBegin(JtaTransactionObject txObject, TransactionDefinition definition)
2             throws NotSupportedException, SystemException {
3         
4         applyIsolationLevel(txObject, definition.getIsolationLevel());
5         int timeout = determineTimeout(definition);
6         applyTimeout(txObject, timeout);
7         txObject.getUserTransaction().begin();
8     }

UserTransactionImp.begin->TransactionManagerImp.begin

 1 public void begin ( int timeout ) throws NotSupportedException,
 2             SystemException
 3     {
 4         CompositeTransaction ct = null;
 5         ResumePreviousTransactionSubTxAwareParticipant resumeParticipant = null;
 6         
 7         ct = compositeTransactionManager.getCompositeTransaction();
 8         if ( ct != null && ct.getProperty (  JTA_PROPERTY_NAME ) == null ) {
 9             LOGGER.logWarning ( "JTA: temporarily suspending incompatible transaction: " + ct.getTid() +
10                     " (will be resumed after JTA transaction ends)" );
11             ct = compositeTransactionManager.suspend();
12             resumeParticipant = new ResumePreviousTransactionSubTxAwareParticipant ( ct );
13         }
14         
15         try {
16             ct = compositeTransactionManager.createCompositeTransaction ( ( ( long ) timeout ) * 1000 );
17             if ( resumeParticipant != null ) ct.addSubTxAwareParticipant ( resumeParticipant );
18             if ( ct.isRoot () && getDefaultSerial () )
19                 ct.getTransactionControl ().setSerial ();
20             ct.setProperty ( JTA_PROPERTY_NAME , "true" );
21         } catch ( SysException se ) {
22             String msg = "Error in begin()";
23             LOGGER.logWarning( msg , se );
24             throw new ExtendedSystemException ( msg , se
25                     .getErrors () );
26         }
27         recreateCompositeTransactionAsJtaTransaction(ct);
28     }

createCompositeTransaction创建混合事务

 1 public CompositeTransaction createCompositeTransaction ( long timeout ) throws SysException
 2     {
 3         Stack errors = new Stack();
 4         CompositeTransaction ct = null , ret = null;
 5         // 获取当前线程绑定的事务
 6         ct = getCurrentTx ();
　　　　　　 // 当前线程不存在事务
 7         if ( ct == null ) {
               // 创建组合事务
 8             ret = service_.createCompositeTransaction ( timeout );
 9             if(LOGGER.isInfoEnabled()){
10                 LOGGER.logInfo("createCompositeTransaction ( " + timeout + " ): "
11                     + "created new ROOT transaction with id " + ret.getTid ());
12             }
　　　　　　　　// 当前线程存在事务
13         } else {
14              if(LOGGER.isInfoEnabled()) LOGGER.logInfo("createCompositeTransaction ( " + timeout + " )");
　　　　　　　　　　// 创建子事务
15             ret = ct.getTransactionControl ().createSubTransaction ();
16 
17         }
18         Thread thread = Thread.currentThread ();
　　　　　　 // 绑定当前线程和事务的2个映射map
19         setThreadMappings ( ret, thread );
20 
21         return ret;
22     }

如果当前线程不存在事务，创建组合事务。如果当前线程存在事务，创建子事务。

调用TransactionServiceImp的createCompositeTransaction创建混合事务

 1 public CompositeTransaction createCompositeTransaction ( long timeout ) throws SysException
 2     {
 3         if ( !initialized_ ) throw new IllegalStateException ( "Not initialized" );
 4 
 5         if ( maxNumberOfActiveTransactions_ >= 0 && 
 6              tidToTransactionMap_.size () >= maxNumberOfActiveTransactions_ ) {
 7             throw new IllegalStateException ( "Max number of active transactions reached:" + maxNumberOfActiveTransactions_ );
 8         }
 9         
10         String tid = tidmgr_.get ();
11         Stack lineage = new Stack ();
12         //创建协调者
15         CoordinatorImp cc = createCC ( null, tid, true, false, timeout );
　　　　　　 // 创建组合事务
16         CompositeTransaction ct = createCT ( tid, cc, lineage, false );
17         return ct;
18     }

3.2 commit 提交事务

事务提交流程图如下：

AbstractPlatformTransactionManager的commit源码如下：

 1 @Override
 2     public final void commit(TransactionStatus status) throws TransactionException {
 3         if (status.isCompleted()) {// 如果事务已完结，报错无法再次提交
 4             throw new IllegalTransactionStateException(
 5                     "Transaction is already completed - do not call commit or rollback more than once per transaction");
 6         }
 7 
 8         DefaultTransactionStatus defStatus = (DefaultTransactionStatus) status;
 9         if (defStatus.isLocalRollbackOnly()) {// 如果事务明确标记为回滚，
10             if (defStatus.isDebug()) {
11                 logger.debug("Transactional code has requested rollback");
12             }
13             processRollback(defStatus);//执行回滚
14             return;
15         }//如果不需要全局回滚时提交 且 全局回滚
16         if (!shouldCommitOnGlobalRollbackOnly() && defStatus.isGlobalRollbackOnly()) {
17             if (defStatus.isDebug()) {
18                 logger.debug("Global transaction is marked as rollback-only but transactional code requested commit");
19             }//执行回滚
20             processRollback(defStatus);
21             // 仅在最外层事务边界（新事务）或显式地请求时抛出“未期望的回滚异常”
23             if (status.isNewTransaction() || isFailEarlyOnGlobalRollbackOnly()) {
24                 throw new UnexpectedRollbackException(
25                         "Transaction rolled back because it has been marked as rollback-only");
26             }
27             return;
28         }
29 　　　　 // 执行提交事务
30         processCommit(defStatus);
31     }

如上图，各种判断：

1.如果事务明确标记为本地回滚，-》执行回滚
2.如果不需要全局回滚时提交且全局回滚-》执行回滚
3.提交事务，核心方法processCommit()

processCommit如下：

 1 private void processCommit(DefaultTransactionStatus status) throws TransactionException {
 2         try {
 3             boolean beforeCompletionInvoked = false;
 4             try {//3个前置操作
 5                 prepareForCommit(status);
 6                 triggerBeforeCommit(status);
 7                 triggerBeforeCompletion(status);
 8                 beforeCompletionInvoked = true;//3个前置操作已调用
 9                 boolean globalRollbackOnly = false;//新事务 或 全局回滚失败
10                 if (status.isNewTransaction() || isFailEarlyOnGlobalRollbackOnly()) {
11                     globalRollbackOnly = status.isGlobalRollbackOnly();
12                 }//1.有保存点，即嵌套事务
13                 if (status.hasSavepoint()) {
14                     if (status.isDebug()) {
15                         logger.debug("Releasing transaction savepoint");
16                     }//释放保存点
17                     status.releaseHeldSavepoint();
18                 }//2.新事务
19                 else if (status.isNewTransaction()) {
20                     if (status.isDebug()) {
21                         logger.debug("Initiating transaction commit");
22                     }//调用事务处理器提交事务
23                     doCommit(status);
24                 }
25                 // 3.非新事务，且全局回滚失败，但是提交时没有得到异常，抛出异常
27                 if (globalRollbackOnly) {
28                     throw new UnexpectedRollbackException(
29                             "Transaction silently rolled back because it has been marked as rollback-only");
30                 }
31             }
32             catch (UnexpectedRollbackException ex) {
33                 // 触发完成后事务同步，状态为回滚
34                 triggerAfterCompletion(status, TransactionSynchronization.STATUS_ROLLED_BACK);
35                 throw ex;
36             }// 事务异常
37             catch (TransactionException ex) {
38                 // 提交失败回滚
39                 if (isRollbackOnCommitFailure()) {
40                     doRollbackOnCommitException(status, ex);
41                 }// 触发完成后回调，事务同步状态为未知
42                 else {
43                     triggerAfterCompletion(status, TransactionSynchronization.STATUS_UNKNOWN);
44                 }
45                 throw ex;
46             }// 运行时异常
47             catch (RuntimeException ex) {
　　　　　　　　　　　　// 如果3个前置步骤未完成，调用前置的最后一步操作
48                 if (!beforeCompletionInvoked) {
49                     triggerBeforeCompletion(status);
50                 }// 提交异常回滚
51                 doRollbackOnCommitException(status, ex);
52                 throw ex;
53             }// 其它异常
54             catch (Error err) {　　
　　　　　　　　　　　　// 如果3个前置步骤未完成，调用前置的最后一步操作
55                 if (!beforeCompletionInvoked) {
56                     triggerBeforeCompletion(status);
57                 }// 提交异常回滚
58                 doRollbackOnCommitException(status, err);
59                 throw err;
60             }
61 
62             // Trigger afterCommit callbacks, with an exception thrown there
63             // propagated to callers but the transaction still considered as committed.
64             try {
65                 triggerAfterCommit(status);
66             }
67             finally {
68                 triggerAfterCompletion(status, TransactionSynchronization.STATUS_COMMITTED);
69             }
70 
71         }
72         finally {
73             cleanupAfterCompletion(status);
74         }
75     }

如上图，commit事务时，有6个核心操作，分别是3个前置操作，3个后置操作，如下：

1.prepareForCommit(status);源码是空的，没有拓展目前。

2.triggerBeforeCommit(status); 提交前触发操作

1 protected final void triggerBeforeCommit(DefaultTransactionStatus status) {
2         if (status.isNewSynchronization()) {
3             if (status.isDebug()) {
4                 logger.trace("Triggering beforeCommit synchronization");
5             }
6             TransactionSynchronizationUtils.triggerBeforeCommit(status.isReadOnly());
7         }
8     }

triggerBeforeCommit源码如下：

1 public static void triggerBeforeCommit(boolean readOnly) {
2         for (TransactionSynchronization synchronization : TransactionSynchronizationManager.getSynchronizations()) {
3             synchronization.beforeCommit(readOnly);
4         }
5     }

如上图，TransactionSynchronizationManager类定义了多个ThreadLocal（线程本地变量），其中一个用以保存当前线程的事务同步：

private static final ThreadLocal<Set<TransactionSynchronization>> synchronizations = new NamedThreadLocal<Set<TransactionSynchronization>>("Transaction synchronizations");

遍历事务同步器，把每个事务同步器都执行“提交前”操作，比如咱们用的jdbc事务，那么最终就是SqlSessionUtils.beforeCommit()->this.holder.getSqlSession().commit();提交会话。

3.triggerBeforeCompletion(status);完成前触发操作，如果是jdbc事务，那么最终就是

SqlSessionUtils.beforeCompletion->

TransactionSynchronizationManager.unbindResource(sessionFactory); 解绑当前线程的会话工厂

this.holder.getSqlSession().close();关闭会话。

4.triggerAfterCommit(status);提交事务后触发操作。TransactionSynchronizationUtils.triggerAfterCommit();->TransactionSynchronizationUtils.invokeAfterCommit，如下：

1 public static void invokeAfterCommit(List<TransactionSynchronization> synchronizations) {
2         if (synchronizations != null) {
3             for (TransactionSynchronization synchronization : synchronizations) {
4                 synchronization.afterCommit();
5             }
6         }
7     }

好吧，一顿找，最后在TransactionSynchronizationAdapter中复写过，并且是空的....SqlSessionSynchronization继承了TransactionSynchronizationAdapter但是没有复写这个方法。

5. triggerAfterCompletion(status, TransactionSynchronization.STATUS_COMMITTED);

TransactionSynchronizationUtils.TransactionSynchronizationUtils.invokeAfterCompletion,如下：

 1 public static void invokeAfterCompletion(List<TransactionSynchronization> synchronizations, int completionStatus) {
 2         if (synchronizations != null) {
 3             for (TransactionSynchronization synchronization : synchronizations) {
 4                 try {
 5                     synchronization.afterCompletion(completionStatus);
 6                 }
 7                 catch (Throwable tsex) {
 8                     logger.error("TransactionSynchronization.afterCompletion threw exception", tsex);
 9                 }
10             }
11         }
12     }

afterCompletion：对于JDBC事务来说，最终：

1）如果会话任然活着，关闭会话，

2）重置各种属性：SQL会话同步器（SqlSessionSynchronization）的SQL会话持有者（SqlSessionHolder）的referenceCount引用计数、synchronizedWithTransaction同步事务、rollbackOnly只回滚、deadline超时时间点。

6.cleanupAfterCompletion(status);

1）设置事务状态为已完成。

2) 如果是新的事务同步，解绑当前线程绑定的数据库资源，重置数据库连接

3）如果存在挂起的事务（嵌套事务），唤醒挂起的老事务的各种资源：数据库资源、同步器。

 1     private void cleanupAfterCompletion(DefaultTransactionStatus status) {
 2         status.setCompleted();//设置事务状态完成
　　　　　　 //如果是新的同步，清空当前线程绑定的除了资源外的全部线程本地变量：包括事务同步器、事务名称、只读属性、隔离级别、真实的事务激活状态
 3         if (status.isNewSynchronization()) {
 4             TransactionSynchronizationManager.clear();
 5         }//如果是新的事务同步
 6         if (status.isNewTransaction()) {
 7             doCleanupAfterCompletion(status.getTransaction());
 8         }//如果存在挂起的资源
 9         if (status.getSuspendedResources() != null) {
10             if (status.isDebug()) {
11                 logger.debug("Resuming suspended transaction after completion of inner transaction");
12             }//唤醒挂起的事务和资源（重新绑定之前挂起的数据库资源，唤醒同步器，注册同步器到TransactionSynchronizationManager）
13             resume(status.getTransaction(), (SuspendedResourcesHolder) status.getSuspendedResources());
14         }
15     }

对于DataSourceTransactionManager，doCleanupAfterCompletion源码如下：

 1     protected void doCleanupAfterCompletion(Object transaction) {
 2         DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
 3 
 4         // 如果是最新的连接持有者，解绑当前线程绑定的<数据库资源，ConnectionHolder>
 5         if (txObject.isNewConnectionHolder()) {
 6             TransactionSynchronizationManager.unbindResource(this.dataSource);
 7         }
 8 
 9         // 重置数据库连接（隔离级别、只读）
10         Connection con = txObject.getConnectionHolder().getConnection();
11         try {
12             if (txObject.isMustRestoreAutoCommit()) {
13                 con.setAutoCommit(true);
14             }
15             DataSourceUtils.resetConnectionAfterTransaction(con, txObject.getPreviousIsolationLevel());
16         }
17         catch (Throwable ex) {
18             logger.debug("Could not reset JDBC Connection after transaction", ex);
19         }
20 
21         if (txObject.isNewConnectionHolder()) {
22             if (logger.isDebugEnabled()) {
23                 logger.debug("Releasing JDBC Connection [" + con + "] after transaction");
24             }// 资源引用计数-1，关闭数据库连接
25             DataSourceUtils.releaseConnection(con, this.dataSource);
26         }
27         // 重置连接持有者的全部属性
28         txObject.getConnectionHolder().clear();
29     }

上面这6个方法是AbstractPlatformTransactionManager做的事，本地事务和分布式事务都会执行。

doCommit就是调用事务管理器来实现事务提交。分布式事务环境下调用的是：JtaTransactionManager.doCommit()。

3.2.1 JtaTransactionManager.doCommit

 1 @Override
 2     protected void doCommit(DefaultTransactionStatus status) {
 3         JtaTransactionObject txObject = (JtaTransactionObject) status.getTransaction();
 4         try {
 5             int jtaStatus = txObject.getUserTransaction().getStatus();
 6             if (jtaStatus == Status.STATUS_NO_TRANSACTION) {
 7                 // 事务状态=已完结，非事务状态。一般不会触发
10                 throw new UnexpectedRollbackException("JTA transaction already completed - probably rolled back");
11             }
12             if (jtaStatus == Status.STATUS_ROLLEDBACK) {
13                 // 回滚
16                 try {
17                     txObject.getUserTransaction().rollback();
18                 }
19                 catch (IllegalStateException ex) {
20                     if (logger.isDebugEnabled()) {
21                         logger.debug("Rollback failure with transaction already marked as rolled back: " + ex);
22                     }
23                 }
24                 throw new UnexpectedRollbackException("JTA transaction already rolled back (probably due to a timeout)");
25             }//核心操作：提交事务
26             txObject.getUserTransaction().commit();
27         }
28         catch (RollbackException ex) {
29             throw new UnexpectedRollbackException(
30                     "JTA transaction unexpectedly rolled back (maybe due to a timeout)", ex);
31         }
32         catch (HeuristicMixedException ex) {
33             throw new HeuristicCompletionException(HeuristicCompletionException.STATE_MIXED, ex);
34         }
35         catch (HeuristicRollbackException ex) {
36             throw new HeuristicCompletionException(HeuristicCompletionException.STATE_ROLLED_BACK, ex);
37         }
38         catch (IllegalStateException ex) {
39             throw new TransactionSystemException("Unexpected internal transaction state", ex);
40         }
41         catch (SystemException ex) {
42             throw new TransactionSystemException("JTA failure on commit", ex);
43         }
44     }

txObject.getUserTransaction().commit()-->调用UserTransactionImp的commit()

1 public void commit () throws javax.transaction.RollbackException,
2             javax.transaction.HeuristicMixedException,
3             javax.transaction.HeuristicRollbackException,
4             javax.transaction.SystemException, java.lang.IllegalStateException,
5             java.lang.SecurityException
6     {
7         checkSetup ();
8         txmgr_.commit ();
9     }

TransactionManager的commit

 1 public void commit () throws javax.transaction.RollbackException,
 2             javax.transaction.HeuristicMixedException,
 3             javax.transaction.HeuristicRollbackException,
 4             javax.transaction.SystemException, java.lang.IllegalStateException,
 5             java.lang.SecurityException
 6     {
 7         Transaction tx = getTransaction();
 8         if ( tx == null ) raiseNoTransaction();
 9         tx.commit();
10     }

最终调用的是TransactionImp的commit()

 1 public void commit() throws javax.transaction.RollbackException,
 2             javax.transaction.HeuristicMixedException,
 3             javax.transaction.HeuristicRollbackException,
 4             javax.transaction.SystemException, java.lang.SecurityException
 5     {
 6         try {
 7             ct_.commit();
 8         } catch ( HeurHazardException hh ) {
 9             rethrowAsJtaHeuristicMixedException ( hh.getMessage () , hh );
10         } catch ( HeurRollbackException hr ) {
11             rethrowAsJtaHeuristicRollbackException ( hr.getMessage () , hr );
12         } catch ( HeurMixedException hm ) {
13             rethrowAsJtaHeuristicMixedException ( hm.getMessage () , hm );
14         } catch ( SysException se ) {
15             LOGGER.logWarning ( se.getMessage() , se );
16             throw new ExtendedSystemException ( se.getMessage (), se
17                     .getErrors () );
18         } catch ( com.atomikos.icatch.RollbackException rb ) {
19             //see case 29708: all statements have been closed
20             String msg = rb.getMessage ();
21             Throwable cause = rb.getCause();
22             if (cause == null) cause = rb;
23             rethrowAsJtaRollbackException (msg , cause);        
24         }
25     }

这里就是调用CompositeTransaction接口的实现类CompositeTransactionImp的commit()

1 public void commit () throws HeurRollbackException, HeurMixedException,
2             HeurHazardException, SysException, SecurityException,
3             RollbackException
4     {
5         getTerminator().commit();
6     }

调用“组合命令行实现类”CompositeTerminatorImp的commit()

 1 public void commit () throws HeurRollbackException, HeurMixedException,
 2             HeurHazardException, SysException, java.lang.SecurityException,
 3             RollbackException
 4     {
 5         Stack errors = new Stack ();
 6         // 1.标记事务状态
 7         transaction_.doCommit ();
 8         setSiblingInfoForIncoming1pcRequestFromRemoteClient();
 9         
10         if ( transaction_.isRoot () ) {
11             try {//2.提交事务，核心操作
12                 coordinator_.terminate ( true );
13             }
14 
15             catch ( RollbackException rb ) {
16                 throw rb;
17             } catch ( HeurHazardException hh ) {
18                 throw hh;
19             } catch ( HeurRollbackException hr ) {
20                 throw hr;
21             } catch ( HeurMixedException hm ) {
22                 throw hm;
23             } catch ( SysException se ) {
24                 throw se;
25             } catch ( Exception e ) {
26                 errors.push ( e );
27                 throw new SysException (
28                         "Unexpected error: " + e.getMessage (), errors );
29             }
30         }
31 
32     }

如上，1.调用“组合事务实现类”CompositeTransactionImp的doCommit()这里只做标记为非活动，没有提交事务。

2.调用CoordinatorImp的terminate()终结事务。

 1 protected void terminate ( boolean commit ) throws HeurRollbackException,
 2             HeurMixedException, SysException, java.lang.SecurityException,
 3             HeurCommitException, HeurHazardException, RollbackException,
 4             IllegalStateException
 5 
 6     {    
 7         synchronized ( fsm_ ) {
 8             if ( commit ) {// 如果只有一个参与者，直接一阶段提交
 9                 if ( participants_.size () <= 1 ) {
10                     commit ( true );
11                 } else {//二阶段提交：prepare阶段
12                     int prepareResult = prepare ();
13                     // 二阶段提交：commit阶段。非只读事务，才需要提交事务，
14                     if ( prepareResult != Participant.READ_ONLY )
15                         commit ( false );
16                 }
17             } else {
18                 rollback ();
19             }
20         }
21     }

如上图是源码的优化精华：

1.根据参与者判断，如果只有一个参与者，直接优化成一阶段提交。

2.prepare完后，commit阶段如果是只读事务，不用commit。

咱们是分布式事务，插入2个库，肯定是两阶段提交。

CoordinatorImp的commit

 1 public HeuristicMessage[] commit ( boolean onePhase )
 2             throws HeurRollbackException, HeurMixedException,
 3             HeurHazardException, java.lang.IllegalStateException,
 4             RollbackException, SysException
 5     {
 6         HeuristicMessage[] ret = null;
 7         synchronized ( fsm_ ) {
 8             ret = stateHandler_.commit(onePhase);
 9         }
10         return ret;
11     }

追踪到IndoubtStateHandler的commit，这个操作加了同步锁。具体实现如下：

 1 protected HeuristicMessage[] commit ( boolean onePhase )
 2             throws HeurRollbackException, HeurMixedException,
 3             HeurHazardException, java.lang.IllegalStateException,
 4             RollbackException, SysException
 5     {
 6 
 7         return commitWithAfterCompletionNotification ( new CommitCallback() {
 8             public HeuristicMessage[] doCommit()
 9                     throws HeurRollbackException, HeurMixedException,
10                     HeurHazardException, IllegalStateException,
11                     RollbackException, SysException {
12                 return commitFromWithinCallback ( false, false );
13             }              
14         });
15 
16     }

如上，核心接口就是CommitCallback的doCommit方法，方法体就是commitFromWithinCallback。

  1 protected HeuristicMessage[] commitFromWithinCallback ( boolean heuristic ,
  2             boolean onePhase ) throws HeurRollbackException,
  3             HeurMixedException, HeurHazardException,
  4             java.lang.IllegalStateException, RollbackException, SysException
  5     {
  6         Stack<Exception> errors = new Stack<Exception> ();
  7         CoordinatorStateHandler nextStateHandler = null;
  8 
  9         try {
 10 
 11             Vector<Participant> participants = coordinator_.getParticipants();
 12             int count = (participants.size () - readOnlyTable_.size ());
 13             TerminationResult commitresult = new TerminationResult ( count );
 14 
 15             // cf bug 64546: avoid committed_ being null upon recovery!
 16             committed_ = new Boolean ( true );
 17             // for replaying completion: commit decision was reached
 18             // otherwise, replay requests might only see TERMINATED!
 19 
 20             try {
 21                 coordinator_.setState ( TxState.COMMITTING );
 22             } catch ( RuntimeException error ) {
 23                 //See case 23334
 24                 String msg = "Error in committing: " + error.getMessage() + " - rolling back instead";
 25                 LOGGER.logWarning ( msg , error );
 26                 try {
 27                     rollbackFromWithinCallback(getCoordinator().isRecoverableWhileActive().booleanValue(),false);
 28                     throw new RollbackException ( msg , error );
 29                 } catch ( HeurCommitException e ) {
 30                     LOGGER.logWarning ( "Illegal heuristic commit during rollback:" + e );
 31                     throw new HeurMixedException ( e.getHeuristicMessages() );
 32                 }
 33             }
 34 
 35 
 36             // start messages
 37             Enumeration<Participant> enumm = participants.elements ();
 38             while ( enumm.hasMoreElements () ) {
 39                 Participant p = enumm.nextElement ();
 40                 if ( !readOnlyTable_.containsKey ( p ) ) {
 41                     CommitMessage cm = new CommitMessage ( p, commitresult,
 42                             onePhase );
 43 
 44                     // if onephase: set cascadelist anyway, because if the
 45                     // participant is a REMOTE one, then it might have
 46                     // multiple participants that are not visible here!
 47 
 48                     if ( onePhase && cascadeList_ != null ) { // null for OTS
 49                         Integer sibnum = (Integer) cascadeList_.get ( p );
 50                         if ( sibnum != null ) // null for local participant!
 51                             p.setGlobalSiblingCount ( sibnum.intValue () );
 52                         p.setCascadeList ( cascadeList_ );
 53                     }
 54                     propagator_.submitPropagationMessage ( cm );
 55                 }
 56             } // while
 57 
 58             commitresult.waitForReplies ();
 59             int res = commitresult.getResult ();
 60 
 61             if ( res != TerminationResult.ALL_OK ) {
 62 
 63                 if ( res == TerminationResult.HEUR_MIXED ) {
 64                     Hashtable<Participant,TxState> hazards = commitresult.getPossiblyIndoubts ();
 65                     Hashtable heuristics = commitresult
 66                             .getHeuristicParticipants ();
 67                     addToHeuristicMap ( heuristics );
 68                     enumm = participants.elements ();
 69                     while ( enumm.hasMoreElements () ) {
 70                         Participant p = (Participant) enumm.nextElement ();
 71                         if ( !heuristics.containsKey ( p ) )
 72                             addToHeuristicMap ( p, TxState.TERMINATED );
 73                     }
 74                     nextStateHandler = new HeurMixedStateHandler ( this,
 75                             hazards );
 76 
 77                     coordinator_.setStateHandler ( nextStateHandler );
 78                     throw new HeurMixedException ( getHeuristicMessages () );
 79                 }
 80 
 81                 else if ( res == TerminationResult.ROLLBACK ) {
 82                     // 1PC and rolled back before commit arrived.
 83                     nextStateHandler = new TerminatedStateHandler ( this );
 84                     coordinator_.setStateHandler ( nextStateHandler );
 85                     throw new RollbackException ( "Rolled back already." );
 86                 } else if ( res == TerminationResult.HEUR_ROLLBACK ) {
 87                     nextStateHandler = new HeurAbortedStateHandler ( this );
 88                     coordinator_.setStateHandler ( nextStateHandler );
 89                     // Here, we do NOT need to add extra information, since ALL
 90                     // participants agreed to rollback. 
 91                     // Therefore, we need not worry about who aborted and who committed.
 92                     throw new HeurRollbackException ( getHeuristicMessages () );
 93 
 94                 }
 95 
 96                 else if ( res == TerminationResult.HEUR_HAZARD ) {
 97                     Hashtable hazards = commitresult.getPossiblyIndoubts ();
 98                     Hashtable heuristics = commitresult
 99                             .getHeuristicParticipants ();
100                     addToHeuristicMap ( heuristics );
101                     enumm = participants.elements ();
102                     while ( enumm.hasMoreElements () ) {
103                         Participant p = (Participant) enumm.nextElement ();
104                         if ( !heuristics.containsKey ( p ) )
105                             addToHeuristicMap ( p, TxState.TERMINATED );
106                     }
107                     nextStateHandler = new HeurHazardStateHandler ( this,
108                             hazards );
109                     coordinator_.setStateHandler ( nextStateHandler );
110                     throw new HeurHazardException ( getHeuristicMessages () );
111                 }
112 
113             } else {
114                 // all OK
115                 if ( heuristic ) {
116                     nextStateHandler = new HeurCommittedStateHandler ( this );
117                     // again, here we do NOT need to preserve extra per-participant
118                     // state mappings, since ALL participants were heur. committed.
119                 } else
120                     nextStateHandler = new TerminatedStateHandler ( this );
121 
122                 coordinator_.setStateHandler ( nextStateHandler );
123             }
124         } catch ( RuntimeException runerr ) {
125             errors.push ( runerr );
126             throw new SysException ( "Error in commit: " + runerr.getMessage (), errors );
127         }
128 
129         catch ( InterruptedException intr ) {
130             // cf bug 67457
131             InterruptedExceptionHelper.handleInterruptedException ( intr );
132             errors.push ( intr );
133             throw new SysException ( "Error in commit" + intr.getMessage (), errors );
134         }
135 
136         return getHeuristicMessages ();
137 
138     }

如上，构造了一个CommitMessage，调用传播者Propagator的submitPropagationMessage()提交传播消息。-》CommitMessage的send()方法-》Participant的commit().

-》XAResourceTransaction的commit提交XA资源-》XAResource的commit ( xid_, onePhase );饶了一大圈终于到了最最核心的代码了....我们这里XAResource接口的实现类是MysqlXAConnection。

 1 public void commit(Xid xid, boolean onePhase) throws XAException {
 2         StringBuilder commandBuf = new StringBuilder(MAX_COMMAND_LENGTH);
 3         commandBuf.append("XA COMMIT ");
 4         appendXid(commandBuf, xid);
 5 
 6         if (onePhase) {
 7             commandBuf.append(" ONE PHASE");
 8         }
 9 
10         try {
11             dispatchCommand(commandBuf.toString());
12         } finally {
13             this.underlyingConnection.setInGlobalTx(false);
14         }
15     }

dispatchCommand调度命令如下：

 1 private ResultSet dispatchCommand(String command) throws XAException {
 2         Statement stmt = null;
 3 
 4         try {
 5             if (this.logXaCommands) {
 6                 this.log.logDebug("Executing XA statement: " + command);
 7             }
 8 
 9             // TODO: Cache this for lifetime of XAConnection这里还规划要做缓存 - -！
10             stmt = this.underlyingConnection.createStatement();
11 　　　　　　　// 核心执行事务提交
12             stmt.execute(command);
13 
14             ResultSet rs = stmt.getResultSet();
15 
16             return rs;
17         } catch (SQLException sqlEx) {
18             throw mapXAExceptionFromSQLException(sqlEx);
19         } finally {
20             if (stmt != null) {
21                 try {
22                     stmt.close();
23                 } catch (SQLException sqlEx) {
24                 }
25             }
26         }
27     }

如上就是一个经典的使用jdbc执行sql语句的过程：

1.使用com.mysql.jdbc.Connection创建Statement。

2.Statement执行sql命令.

3.得到结果。

3.3 rollback回滚事务

AbstractPlatformTransactionManager中rollback源码如下：

1     public final void rollback(TransactionStatus status) throws TransactionException {
2         if (status.isCompleted()) {
3             throw new IllegalTransactionStateException(
4                     "Transaction is already completed - do not call commit or rollback more than once per transaction");
5         }
6 
7         DefaultTransactionStatus defStatus = (DefaultTransactionStatus) status;
8         processRollback(defStatus);
9     }

processRollback源码如下：

 1     private void processRollback(DefaultTransactionStatus status) {
 2         try {
 3             try {// 解绑当前线程绑定的会话工厂，并关闭会话
 4                 triggerBeforeCompletion(status);
 5                 if (status.hasSavepoint()) {// 1.如果有保存点，即嵌套式事务
 6                     if (status.isDebug()) {
 7                         logger.debug("Rolling back transaction to savepoint");
 8                     }//回滚到保存点
 9                     status.rollbackToHeldSavepoint();
10                 }//2.如果就是一个简单事务
11                 else if (status.isNewTransaction()) {
12                     if (status.isDebug()) {
13                         logger.debug("Initiating transaction rollback");
14                     }//回滚核心方法
15                     doRollback(status);
16                 }//3.当前存在事务且没有保存点，即加入当前事务的
17                 else if (status.hasTransaction()) {//如果已经标记为回滚 或 当加入事务失败时全局回滚（默认true）
18                     if (status.isLocalRollbackOnly() || isGlobalRollbackOnParticipationFailure()) {
19                         if (status.isDebug()) {//debug时会打印：加入事务失败-标记已存在事务为回滚
20                             logger.debug("Participating transaction failed - marking existing transaction as rollback-only");
21                         }//设置当前connectionHolder：当加入一个已存在事务时回滚
22                         doSetRollbackOnly(status);
23                     }
24                     else {
25                         if (status.isDebug()) {
26                             logger.debug("Participating transaction failed - letting transaction originator decide on rollback");
27                         }
28                     }
29                 }
30                 else {
31                     logger.debug("Should roll back transaction but cannot - no transaction available");
32                 }
33             }
34             catch (RuntimeException ex) {//关闭会话，重置SqlSessionHolder属性
35                 triggerAfterCompletion(status, TransactionSynchronization.STATUS_UNKNOWN);
36                 throw ex;
37             }
38             catch (Error err) {
39                 triggerAfterCompletion(status, TransactionSynchronization.STATUS_UNKNOWN);
40                 throw err;
41             }
42             triggerAfterCompletion(status, TransactionSynchronization.STATUS_ROLLED_BACK);
43         }
44         finally {、、解绑当前线程
45             cleanupAfterCompletion(status);
46         }
47     }

核心操作是doRollback,分布式环境下调用的是JtaTransactionManager的doRollback。

 1 @Override
 2     protected void doRollback(DefaultTransactionStatus status) {
 3         JtaTransactionObject txObject = (JtaTransactionObject) status.getTransaction();
 4         try {
 5             int jtaStatus = txObject.getUserTransaction().getStatus();
 6             if (jtaStatus != Status.STATUS_NO_TRANSACTION) {
 7                 try {
 8                     txObject.getUserTransaction().rollback();
 9                 }
10                 catch (IllegalStateException ex) {
11                     if (jtaStatus == Status.STATUS_ROLLEDBACK) {
12                         // Only really happens on JBoss 4.2 in case of an early timeout...
13                         if (logger.isDebugEnabled()) {
14                             logger.debug("Rollback failure with transaction already marked as rolled back: " + ex);
15                         }
16                     }
17                     else {
18                         throw new TransactionSystemException("Unexpected internal transaction state", ex);
19                     }
20                 }
21             }
22         }
23         catch (SystemException ex) {
24             throw new TransactionSystemException("JTA failure on rollback", ex);
25         }
26     }

调用的UserTransactionImp的rollback

1 public void rollback () throws IllegalStateException, SystemException,
2             SecurityException
3     {
4         checkSetup ();
5         txmgr_.rollback ();
6     }

TransactionManagerImp的rollback

1 public void rollback () throws IllegalStateException, SystemException,
2             SecurityException
3     {
4         Transaction tx = getTransaction();
5         if ( tx == null ) raiseNoTransaction();
6         tx.rollback();
7        
8     }

TransactionImp的rollback

 1 public void rollback() throws IllegalStateException, SystemException
 2     {
 3         try {
 4             ct_.rollback();
 5         } catch ( SysException se ) {
 6             LOGGER.logWarning ( se.getMessage() , se );
 7             throw new ExtendedSystemException ( se.getMessage (), se
 8                     .getErrors () );
 9         }
10 
11     }

CompositeTransactionImp的rollback

1  public void rollback () throws IllegalStateException, SysException
2     {
3         getTerminator().rollback();
4     }

CompositeTerminatorImp的rollback

 1 public void rollback () throws IllegalStateException, SysException
 2     {
 3         Stack errors = new Stack ();
 4 
 5         transaction_.doRollback ();
 6 
 7         if ( transaction_.isRoot () )
 8             try {
 9                 coordinator_.terminate ( false );
10             } catch ( Exception e ) {
11                 errors.push ( e );
12                 throw new SysException ( "Unexpected error in rollback: " + e.getMessage (), errors );
13             }
14     }

一直追踪到

RollbackMessage.send

 1 protected Object send () throws PropagationException
 2     {
 3         Participant part = getParticipant ();
 4         HeuristicMessage[] msgs = null;
 5         try {
 6             msgs = part.rollback ();
 7 
 8         } catch ( HeurCommitException heurc ) {
 9             throw new PropagationException ( heurc, false );
10         } catch ( HeurMixedException heurm ) {
11             throw new PropagationException ( heurm, false );
12         }
13 
14         catch ( Exception e ) {
15             // only retry if might be indoubt. Otherwise ignore.
16             if ( indoubt_ ) {
17                 // here, participant might be indoubt!
18                 // fill in exact heuristic msgs by using buffered effect of proxies
19                 HeurHazardException heurh = new HeurHazardException ( part.getHeuristicMessages () );
20                 throw new PropagationException ( heurh, true );
21             }
22         }
23         return msgs;
24     }

XAResourceTransaction.rollback->MysqlXAConnection.rollback

 1 public void rollback(Xid xid) throws XAException {
 2         StringBuilder commandBuf = new StringBuilder(MAX_COMMAND_LENGTH);
 3         commandBuf.append("XA ROLLBACK ");
 4         appendXid(commandBuf, xid);
 5 
 6         try {
 7             dispatchCommand(commandBuf.toString());
 8         } finally {
 9             this.underlyingConnection.setInGlobalTx(false);
10         }
11     }

dispatchCommand执行sql

 1 private ResultSet dispatchCommand(String command) throws XAException {
 2         Statement stmt = null;
 3 
 4         try {
 5             if (this.logXaCommands) {
 6                 this.log.logDebug("Executing XA statement: " + command);
 7             }
 8 
 9             // TODO: Cache this for lifetime of XAConnection
10             stmt = this.underlyingConnection.createStatement();
11 
12             stmt.execute(command);
13 
14             ResultSet rs = stmt.getResultSet();
15 
16             return rs;
17         } catch (SQLException sqlEx) {
18             throw mapXAExceptionFromSQLException(sqlEx);
19         } finally {
20             if (stmt != null) {
21                 try {
22                     stmt.close();
23                 } catch (SQLException sqlEx) {
24                 }
25             }
26         }
27     }

debug得到command：XA ROLLBACK 0x3139322e3136382e36302e31312e746d30303030313030303437,0x3139322e3136382e36302e31312e746d31,0x41544f4d
至此，回滚完毕。

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原文地址：https://www.cnblogs.com/dennyzhangdd/p/10858414.html