Apache ShardingSphere整合Atomikos源码解析

Shardingsphere整合Atomikos对XA分布式事务的支持

Apache ShardingSphere 是一套开源的分布式数据库中间件解决方案组成的生态圈，它由 JDBC、Proxy 和 Sidecar（规划中）这 3 款相互独立，却又能够混合部署配合使用的产品组成。它们均提供标准化的数据分片、分布式事务和数据库治理功能，可适用于如 Java 同构、异构语言、云原生等各种多样化的应用场景。

ShardingSphere 已于2020年4月16日成为 Apache 软件基金会的顶级项目。

咋们话不多，接上篇，我们直接进入正题。

Atomikos简单介绍

Atomikos（https://www.atomikos.com/），其实是一家公司的名字，提供了基于JTA规范的XA分布式事务TM的实现。其旗下最著名的产品就是事务管理器。产品分两个版本：

TransactionEssentials：开源的免费产品；
ExtremeTransactions：上商业版，需要收费。

这两个产品的关系如下图所示： Apache ShardingSphere整合Atomikos源码解析

ExtremeTransactions在TransactionEssentials的基础上额外提供了以下功能（重要的）：

支持TCC：这是一种柔性事务
支持通过RMI、IIOP、SOAP这些远程过程调用技术，进行事务传播。
事务日志云存储，云端对事务进行恢复，并且提供了完善的管理后台。

org.apache.shardingsphere.transaction.xa.XAShardingTransactionManager详解

我们简单的来回顾下org.apache.shardingsphere.transaction.spi.ShardingTransactionManager

public interface ShardingTransactionManager extends AutoCloseable {

    /**
     * Initialize sharding transaction manager.
     *
     * @param databaseType database type
     * @param resourceDataSources resource data sources
     */
    void init(DatabaseType databaseType, Collection<ResourceDataSource> resourceDataSources);

    /**
     * Get transaction type.
     *
     * @return transaction type
     */
    TransactionType getTransactionType();

    /**
     * Judge is in transaction or not.
     *
     * @return in transaction or not
     */
    boolean isInTransaction();

    /**
     * Get transactional connection.
     *
     * @param dataSourceName data source name
     * @return connection
     * @throws SQLException SQL exception
     */
    Connection getConnection(String dataSourceName) throws SQLException;

    /**
     * Begin transaction.
     */
    void begin();

    /**
     * Commit transaction.
     */
    void commit();

    /**
     * Rollback transaction.
     */
    void rollback();
}

我们重点县关注init方法，从它的命名，你就应该能够看出来，这是整个框架的初始化方法，让我们来看看它是如何进行初始化的。

 private final Map<String, XATransactionDataSource> cachedDataSources = new HashMap<>();

 private final XATransactionManager xaTransactionManager = XATransactionManagerLoader.getInstance().getTransactionManager();

    @Override
    public void init(final DatabaseType databaseType, final Collection<ResourceDataSource> resourceDataSources) {
        for (ResourceDataSource each : resourceDataSources) {
            cachedDataSources.put(each.getOriginalName(), new XATransactionDataSource(databaseType, each.getUniqueResourceName(), each.getDataSource(), xaTransactionManager));
        }
        xaTransactionManager.init();
    }

首先SPI的方式加载XATransactionManager的具体实现类，这里返回的就是org.apache.shardingsphere.transaction.xa.atomikos.manager.AtomikosTransactionManager。
我们在关注下 new XATransactionDataSource() , 进入 org.apache.shardingsphere.transaction.xa.jta.datasource。XATransactionDataSource类的构造方法。

public XATransactionDataSource(final DatabaseType databaseType, final String resourceName, final DataSource dataSource, final XATransactionManager xaTransactionManager) { this.databaseType = databaseType; this.resourceName = resourceName; this.dataSource = dataSource; if (!CONTAINER_DATASOURCE_NAMES.contains(dataSource.getClass().getSimpleName())) { // 重点关注 1 ,返回了xaDatasource xaDataSource = XADataSourceFactory.build(databaseType, dataSource); this.xaTransactionManager = xaTransactionManager; // 重点关注2 注册资源 xaTransactionManager.registerRecoveryResource(resourceName, xaDataSource); } }
我们重点来关注 XADataSourceFactory.build(databaseType, dataSource)，从名字我们就可以看出，这应该是返回JTA规范里面的XADataSource,在ShardingSphere里面很多的功能，可以从代码风格的命名上就能猜出来，这就是优雅代码（吹一波）。不多逼逼，我们进入该方法。

public final class XADataSourceFactory {
```
public static XADataSource build(final DatabaseType databaseType, final DataSource dataSource) {
    return new DataSourceSwapper(XADataSourceDefinitionFactory.getXADataSourceDefinition(databaseType)).swap(dataSource);
}
```
}
首先又是一个SPI定义的 XADataSourceDefinitionFactory，它根据不同的数据库类型，来加载不同的方言。然后我们进入 swap方法。

public XADataSource swap(final DataSource dataSource) { XADataSource result = createXADataSource(); setProperties(result, getDatabaseAccessConfiguration(dataSource)); return result; }
很简明，第一步创建，XADataSource,第二步给它设置属性（包含数据的连接，用户名密码等），然后返回。
返回 XATransactionDataSource 类，关注 xaTransactionManager.registerRecoveryResource(resourceName, xaDataSource); 从名字可以看出，这是注册事务恢复资源。这个我们在事务恢复的时候详解。
返回 XAShardingTransactionManager.init() ，我们重点来关注： xaTransactionManager.init();,最后进入AtomikosTransactionManager.init()。流程图如下：

Apache ShardingSphere整合Atomikos源码解析

代码：

public final class AtomikosTransactionManager implements XATransactionManager {

    private final UserTransactionManager transactionManager = new UserTransactionManager();

    private final UserTransactionService userTransactionService = new UserTransactionServiceImp();

    @Override
    public void init() {
        userTransactionService.init();
    }

}

进入UserTransactionServiceImp.init()

private void initialize() { //添加恢复资源不用关心 for (RecoverableResource resource : resources_) { Configuration.addResource ( resource ); } for (LogAdministrator logAdministrator : logAdministrators_) { Configuration.addLogAdministrator ( logAdministrator ); } //注册插件不用关心 for (TransactionServicePlugin nxt : tsListeners_) { Configuration.registerTransactionServicePlugin ( nxt ); } //获取配置属性重点关心 ConfigProperties configProps = Configuration.getConfigProperties(); configProps.applyUserSpecificProperties(properties_); //进行初始化 Configuration.init(); }

我们重点关注，获取配置属性。最后进入com.atomikos.icatch.provider.imp.AssemblerImp.initializeProperties()方法。

@Override
public ConfigProperties initializeProperties() {
     //读取classpath下的默认配置transactions-defaults.properties
    Properties defaults = new Properties();
    loadPropertiesFromClasspath(defaults, DEFAULT_PROPERTIES_FILE_NAME);
    //读取classpath下，transactions.properties配置，覆盖transactions-defaults.properties中相同key的值
    Properties transactionsProperties = new Properties(defaults);
    loadPropertiesFromClasspath(transactionsProperties, TRANSACTIONS_PROPERTIES_FILE_NAME);
    //读取classpath下，jta.properties，覆盖transactions-defaults.properties、transactions.properties中相同key的值
    Properties jtaProperties = new Properties(transactionsProperties);
    loadPropertiesFromClasspath(jtaProperties, JTA_PROPERTIES_FILE_NAME);

    //读取通过java -Dcom.atomikos.icatch.file方式指定的自定义配置文件路径，覆盖之前的同名配置
    Properties customProperties = new Properties(jtaProperties);
    loadPropertiesFromCustomFilePath(customProperties);
    //最终构造一个ConfigProperties对象，来表示实际要使用的配置
    Properties finalProperties = new Properties(customProperties);
    return new ConfigProperties(finalProperties);
}

接下来重点关注， Configuration.init()，进行初始化。

ublic static synchronized boolean init() { boolean startupInitiated = false; if (service_ == null) { startupInitiated = true; //SPI方式加载插件注册，无需过多关心 addAllTransactionServicePluginServicesFromClasspath(); ConfigProperties configProperties = getConfigProperties(); //调用插件的beforeInit方法进行初始化话，无需过多关心 notifyBeforeInit(configProperties); //进行事务日志恢复的初始化，很重要，接下来详解 assembleSystemComponents(configProperties); //进入系统注解的初始化，一般重要 initializeSystemComponents(configProperties); notifyAfterInit(); if (configProperties.getForceShutdownOnVmExit()) { addShutdownHook(new ForceShutdownHook()); } } return startupInitiated; }
我们先来关注 assembleSystemComponents(configProperties); 进入它，进入com.atomikos.icatch.provider.imp.AssemblerImp.assembleTransactionService()方法：

@Override public TransactionServiceProvider assembleTransactionService( ConfigProperties configProperties) { RecoveryLog recoveryLog =null; //打印日志 logProperties(configProperties.getCompletedProperties()); //生成唯一名字 String tmUniqueName = configProperties.getTmUniqueName(); long maxTimeout = configProperties.getMaxTimeout(); int maxActives = configProperties.getMaxActives(); boolean threaded2pc = configProperties.getThreaded2pc(); //SPI方式加载OltpLog ，这是最重要的扩展地方，如果用户没有SPI的方式去扩展那么就为null OltpLog oltpLog = createOltpLogFromClasspath(); if (oltpLog == null) { LOGGER.logInfo("Using default (local) logging and recovery..."); //创建事务日志存储资源 Repository repository = createRepository(configProperties); oltpLog = createOltpLog(repository); //??? Assemble recoveryLog recoveryLog = createRecoveryLog(repository); } StateRecoveryManagerImp recoveryManager = new StateRecoveryManagerImp(); recoveryManager.setOltpLog(oltpLog); //生成唯一id生成器，以后生成XID会用的到 UniqueIdMgr idMgr = new UniqueIdMgr ( tmUniqueName ); int overflow = idMgr.getMaxIdLengthInBytes() - MAX_TID_LENGTH; if ( overflow > 0 ) { // see case 73086 String msg = "Value too long : " + tmUniqueName; LOGGER.logFatal ( msg ); throw new SysException(msg); } return new TransactionServiceImp(tmUniqueName, recoveryManager, idMgr, maxTimeout, maxActives, !threaded2pc, recoveryLog); }
我们重点来分析createOltpLogFromClasspath(), 采用SPI的加载方式来获取，默认这里会返回 null, 什么意思呢？就是当没有扩展的时候，atomikos，会创建框架自定义的资源，来存储事务日志。

private OltpLog createOltpLogFromClasspath() { OltpLog ret = null; ServiceLoader loader = ServiceLoader.load(OltpLogFactory.class,Configuration.class.getClassLoader()); int i = 0; for (OltpLogFactory l : loader ) { ret = l.createOltpLog(); i++; } if (i > 1) { String msg = "More than one OltpLogFactory found in classpath - error in configuration!"; LOGGER.logFatal(msg); throw new SysException(msg); } return ret; }

我们跟着进入 Repository repository = createRepository(configProperties);

private CachedRepository createCoordinatorLogEntryRepository(
        ConfigProperties configProperties) throws LogException {
    //创建内存资源存储
    InMemoryRepository inMemoryCoordinatorLogEntryRepository = new InMemoryRepository();
   //进行初始化
    inMemoryCoordinatorLogEntryRepository.init();
   //创建使用文件存储资源作为backup
    FileSystemRepository backupCoordinatorLogEntryRepository = new FileSystemRepository();
   //进行初始化
    backupCoordinatorLogEntryRepository.init();
  //内存与file资源进行合并
    CachedRepository repository = new CachedRepository(inMemoryCoordinatorLogEntryRepository, backupCoordinatorLogEntryRepository);
    repository.init();
    return repository;
}

这里就会创建出 CachedRepository，里面包含了 InMemoryRepository 与 FileSystemRepository

回到主线 com.atomikos.icatch.config.Configuration.init(), 最后来分析下notifyAfterInit();

private static void notifyAfterInit() {
     //进行插件的初始化
    for (TransactionServicePlugin p : tsListenersList_) {
        p.afterInit();
    }
    for (LogAdministrator a : logAdministrators_) {
        a.registerLogControl(service_.getLogControl());
    }
     //设置事务恢复服务，进行事务的恢复
    for (RecoverableResource r : resourceList_ ) {
        r.setRecoveryService(recoveryService_);
    }

}

插件的初始化会进入com.atomikos.icatch.jta.JtaTransactionServicePlugin.afterInit()

public void afterInit() {
    TransactionManagerImp.installTransactionManager(Configuration.getCompositeTransactionManager(), autoRegisterResources);
      //如果我们自定义扩展了 OltpLog ，这里就会返回null，如果是null，那么XaResourceRecoveryManager就是null
    RecoveryLog recoveryLog = Configuration.getRecoveryLog();
    long maxTimeout = Configuration.getConfigProperties().getMaxTimeout();
    if (recoveryLog != null) {
        XaResourceRecoveryManager.installXaResourceRecoveryManager(new DefaultXaRecoveryLog(recoveryLog, maxTimeout),Configuration.getConfigProperties().getTmUniqueName());
    }

}

重点注意 RecoveryLog recoveryLog = Configuration.getRecoveryLog(); ,如果用户采用SPI的方式，扩展了com.atomikos.recovery.OltpLog ，这里就会返回 null。如果是null，则不会对 XaResourceRecoveryManager 进行初始化。
回到 notifyAfterInit(), 我们来分析 setRecoveryService。

public void setRecoveryService ( RecoveryService recoveryService ) throws ResourceException { if ( recoveryService != null ) { if ( LOGGER.isTraceEnabled() ) LOGGER.logTrace ( "Installing recovery service on resource " + getName () ); this.branchIdentifier=recoveryService.getName(); recover(); } }
我们进入 recover() 方法：

public void recover() { XaResourceRecoveryManager xaResourceRecoveryManager = XaResourceRecoveryManager.getInstance(); //null for LogCloud recovery if (xaResourceRecoveryManager != null) { try { xaResourceRecoveryManager.recover(getXAResource()); } catch (Exception e) { refreshXAResource(); //cf case 156968 } } }
看到最关键的注释了吗，如果用户采用SPI的方式，扩展了com.atomikos.recovery.OltpLog，那么XaResourceRecoveryManager 为null，则就会进行云端恢复，反之则进行事务恢复。事务恢复很复杂，我们会单独来讲。

到这里atomikos的基本的初始化已经完成。

atomikos事务begin流程

我们知道，本地的事务，都会有一个 trainsaction.begin, 对应XA分布式事务来说也不另外，我们再把思路切换回XAShardingTransactionManager.begin(), 会调用com.atomikos.icatch.jta.TransactionManagerImp.begin()。流程图如下: Apache ShardingSphere整合Atomikos源码解析

代码：

  public void begin ( int timeout ) throws NotSupportedException,
            SystemException
    {
        CompositeTransaction ct = null;
        ResumePreviousTransactionSubTxAwareParticipant resumeParticipant = null;

        ct = compositeTransactionManager.getCompositeTransaction();
        if ( ct != null && ct.getProperty (  JTA_PROPERTY_NAME ) == null ) {
            LOGGER.logWarning ( "JTA: temporarily suspending incompatible transaction: " + ct.getTid() +
                    " (will be resumed after JTA transaction ends)" );
            ct = compositeTransactionManager.suspend();
            resumeParticipant = new ResumePreviousTransactionSubTxAwareParticipant ( ct );
        }

        try {
      //创建事务补偿点
            ct = compositeTransactionManager.createCompositeTransaction ( ( ( long ) timeout ) * 1000 );
            if ( resumeParticipant != null ) ct.addSubTxAwareParticipant ( resumeParticipant );
            if ( ct.isRoot () && getDefaultSerial () )
                ct.setSerial ();
            ct.setProperty ( JTA_PROPERTY_NAME , "true" );
        } catch ( SysException se ) {
            String msg = "Error in begin()";
            LOGGER.logError( msg , se );
            throw new ExtendedSystemException ( msg , se );
        }
        recreateCompositeTransactionAsJtaTransaction(ct);
    }

这里我们主要关注 compositeTransactionManager.createCompositeTransaction(),

public CompositeTransaction createCompositeTransaction ( long timeout ) throws SysException { CompositeTransaction ct = null , ret = null; ct = getCurrentTx (); if ( ct == null ) { ret = getTransactionService().createCompositeTransaction ( timeout ); if(LOGGER.isDebugEnabled()){ LOGGER.logDebug("createCompositeTransaction ( " + timeout + " ): " + "created new ROOT transaction with id " + ret.getTid ()); } } else { if(LOGGER.isDebugEnabled()) LOGGER.logDebug("createCompositeTransaction ( " + timeout + " )"); ret = ct.createSubTransaction (); } Thread thread = Thread.currentThread (); setThreadMappings ( ret, thread ); return ret; }
创建了事务补偿点，然后把他放到了用当前线程作为key的Map当中，这里思考，为啥它不用 threadLocal。

到这里atomikos的事务begin流程已经完成。大家可能有些疑惑，begin好像什么都没有做，XA start 也没调用？别慌，下一节继续来讲。

XATransactionDataSource getConnection() 流程

我们都知道想要执行SQL语句，必须要获取到数据库的connection。让我们再回到 XAShardingTransactionManager.getConnection() 最后会调用到org.apache.shardingsphere.transaction.xa.jta.datasourceXATransactionDataSource.getConnection()。流程图如下： Apache ShardingSphere整合Atomikos源码解析

代码 :

 public Connection getConnection() throws SQLException, SystemException, RollbackException {
      //先检查是否已经有存在的connection，这一步很关心，也是XA的关键，因为XA事务，必须在同一个connection
        if (CONTAINER_DATASOURCE_NAMES.contains(dataSource.getClass().getSimpleName())) {
            return dataSource.getConnection();
        }
      //获取数据库连接
        Connection result = dataSource.getConnection();
      //转成XAConnection，其实是同一个连接
        XAConnection xaConnection = XAConnectionFactory.createXAConnection(databaseType, xaDataSource, result);
      //获取JTA事务定义接口
        Transaction transaction = xaTransactionManager.getTransactionManager().getTransaction();
        if (!enlistedTransactions.get().contains(transaction)) {
      //进行资源注册
            transaction.enlistResource(new SingleXAResource(resourceName, xaConnection.getXAResource()));
            transaction.registerSynchronization(new Synchronization() {
                @Override
                public void beforeCompletion() {
                    enlistedTransactions.get().remove(transaction);
                }

                @Override
                public void afterCompletion(final int status) {
                    enlistedTransactions.get().clear();
                }
            });
            enlistedTransactions.get().add(transaction);
        }
        return result;
    }

首先第一步很关心，尤其是对shardingsphere来说，因为在一个事务里面，会有多个SQL语句，打到相同的数据库，所以对相同的数据库，必须获取同一个XAConnection，这样才能进行XA事务的提交与回滚。
我们接下来关心 transaction.enlistResource(new SingleXAResource(resourceName, xaConnection.getXAResource()));, 会进入com.atomikos.icatch.jta.TransactionImp.enlistResource(), 代码太长，截取一部分。

try { restx = (XAResourceTransaction) res .getResourceTransaction(this.compositeTransaction); // next, we MUST set the xa resource again, // because ONLY the instance we got as argument // is available for use now ! // older instances (set in restx from previous sibling) // have connections that may be in reuse already // ->old xares not valid except for 2pc operations restx.setXAResource(xares); restx.resume(); } catch (ResourceException re) { throw new ExtendedSystemException( "Unexpected error during enlist", re); } catch (RuntimeException e) { throw e; } addXAResourceTransaction(restx, xares);
我们直接看 restx.resume();

public synchronized void resume() throws ResourceException { int flag = 0; String logFlag = ""; if (this.state.equals(TxState.LOCALLY_DONE)) {// reused instance flag = XAResource.TMJOIN; logFlag = "XAResource.TMJOIN"; } else if (!this.knownInResource) {// new instance flag = XAResource.TMNOFLAGS; logFlag = "XAResource.TMNOFLAGS"; } else throw new IllegalStateException("Wrong state for resume: " + this.state); try { if (LOGGER.isDebugEnabled()) { LOGGER.logDebug("XAResource.start ( " + this.xidToHexString + " , " + logFlag + " ) on resource " + this.resourcename + " represented by XAResource instance " + this.xaresource); } this.xaresource.start(this.xid, flag); } catch (XAException xaerr) { String msg = interpretErrorCode(this.resourcename, "resume", this.xid, xaerr.errorCode); LOGGER.logWarning(msg, xaerr); throw new ResourceException(msg, xaerr); } setState(TxState.ACTIVE); this.knownInResource = true; }
哦多尅，看见了吗，各位，看见了 this.xaresource.start(this.xid, flag); 了吗？？？？，我们进去，假设我们使用的Mysql数据库：

public void start(Xid xid, int flags) throws XAException { StringBuilder commandBuf = new StringBuilder(300); commandBuf.append("XA START "); appendXid(commandBuf, xid); switch(flags) { case 0: break; case 2097152: commandBuf.append(" JOIN"); break; case 134217728: commandBuf.append(" RESUME"); break; default: throw new XAException(-5); } this.dispatchCommand(commandBuf.toString()); this.underlyingConnection.setInGlobalTx(true); }
组装XA start Xid SQL语句，进行执行。

到这里，我们总结下，在获取数据库连接的时候，我们执行了XA协议接口中的 XA start xid

atomikos事务commit流程

好了，上面我们已经开启了事务，现在我们来分析下事务commit流程，我们再把视角切换回XAShardingTransactionManager.commit(),最后我们会进入com.atomikos.icatch.imp.CompositeTransactionImp.commit() 方法。流程图如下: Apache ShardingSphere整合Atomikos源码解析

代码：

 public void commit () throws HeurRollbackException, HeurMixedException,
            HeurHazardException, SysException, SecurityException,
            RollbackException
    {
       //首先更新下事务日志的状态
        doCommit ();
        setSiblingInfoForIncoming1pcRequestFromRemoteClient();

        if ( isRoot () ) {
         //真正的commit操作
          coordinator.terminate ( true );
        }
    }

我们关注 coordinator.terminate ( true );

protected void terminate ( boolean commit ) throws HeurRollbackException, HeurMixedException, SysException, java.lang.SecurityException, HeurCommitException, HeurHazardException, RollbackException, IllegalStateException { synchronized ( fsm_ ) { if ( commit ) { //判断有几个参与者，如果只有一个，直接提交 if ( participants_.size () <= 1 ) { commit ( true ); } else { //否则，走XA 2阶段提交流程，先prepare，再提交 int prepareResult = prepare (); // make sure to only do commit if NOT read only if ( prepareResult != Participant.READ_ONLY ) commit ( false ); } } else { rollback (); } } }
首先会判断参与者的个数，这里我们可以理解为MySQL的database数量，如果只有一个，退化成一阶段，直接提交。如果有多个，则走标准的XA二阶段提交流程。
我们来看 prepare (); 流程，最后会走到com.atomikos.icatch.imp.PrepareMessage.send() ---> com.atomikos.datasource.xa.XAResourceTransaction.prepare()

int ret = 0; terminateInResource(); if (TxState.ACTIVE == this.state) { // tolerate non-delisting apps/servers suspend(); } // duplicate prepares can happen for siblings in serial subtxs!!! // in that case, the second prepare just returns READONLY if (this.state == TxState.IN_DOUBT) return Participant.READ_ONLY; else if (!(this.state == TxState.LOCALLY_DONE)) throw new SysException("Wrong state for prepare: " + this.state); try { // refresh xaresource for MQSeries: seems to close XAResource after // suspend??? testOrRefreshXAResourceFor2PC(); if (LOGGER.isTraceEnabled()) { LOGGER.logTrace("About to call prepare on XAResource instance: " + this.xaresource); } ret = this.xaresource.prepare(this.xid); } catch (XAException xaerr) { String msg = interpretErrorCode(this.resourcename, "prepare", this.xid, xaerr.errorCode); if (XAException.XA_RBBASE <= xaerr.errorCode && xaerr.errorCode <= XAException.XA_RBEND) { LOGGER.logWarning(msg, xaerr); // see case 84253 throw new RollbackException(msg); } else { LOGGER.logError(msg, xaerr); throw new SysException(msg, xaerr); } } setState(TxState.IN_DOUBT); if (ret == XAResource.XA_RDONLY) { if (LOGGER.isDebugEnabled()) { LOGGER.logDebug("XAResource.prepare ( " + this.xidToHexString + " ) returning XAResource.XA_RDONLY " + "on resource " + this.resourcename + " represented by XAResource instance " + this.xaresource); } return Participant.READ_ONLY; } else { if (LOGGER.isDebugEnabled()) { LOGGER.logDebug("XAResource.prepare ( " + this.xidToHexString + " ) returning OK " + "on resource " + this.resourcename + " represented by XAResource instance " + this.xaresource); } return Participant.READ_ONLY + 1; }

终于，我们看到了这么一句 ret = this.xaresource.prepare(this.xid); 但是等等，我们之前不是说了，XA start xid 以后要先 XA end xid 吗？答案就在 suspend(); 里面。

public synchronized void suspend() throws ResourceException {

    // BugzID: 20545
    // State may be IN_DOUBT or TERMINATED when a connection is closed AFTER
    // commit!
    // In that case, don't call END again, and also don't generate any
    // error!
    // This is required for some hibernate connection release strategies.
    if (this.state.equals(TxState.ACTIVE)) {
        try {
            if (LOGGER.isDebugEnabled()) {
                LOGGER.logDebug("XAResource.end ( " + this.xidToHexString
                        + " , XAResource.TMSUCCESS ) on resource "
                        + this.resourcename
                        + " represented by XAResource instance "
                        + this.xaresource);
            }
             //执行了 xa end 语句
            this.xaresource.end(this.xid, XAResource.TMSUCCESS);

        } catch (XAException xaerr) {
            String msg = interpretErrorCode(this.resourcename, "end",
                    this.xid, xaerr.errorCode);
            if (LOGGER.isTraceEnabled())
                LOGGER.logTrace(msg, xaerr);
            // don't throw: fix for case 102827
        }
        setState(TxState.LOCALLY_DONE);
    }
}

到了这里，我们已经执行了 XA start xid -> XA end xid --> XA prepare xid，接下来就是最后一步 commit

我们再回到 terminate（false） 方法，来看 commit()流程。其实和 prepare流程一样，最后会走到 com.atomikos.datasource.xa.XAResourceTransaction.commit()。 commit执行完，数据提交

//繁杂代码过多，就显示核心的 this.xaresource.commit(this.xid, onePhase);

思考：这里的参与者提交是在一个循环里面，一个一个提交的，如果之前的提交了，后面的参与者提交的时候，挂了，就会造成数据的不一致性。

Atomikos rollback() 流程

Apache ShardingSphere整合Atomikos源码解析上面我们已经分析了commit流程，其实rollback流程和commit流程一样，我们在把目光切换回 org.apache.shardingsphere.transaction.xa.XAShardingTransactionManager.rollback() ，最后会执行到com.atomikos.icatch.imp.CompositeTransactionImp.rollback()。

    public void rollback () throws IllegalStateException, SysException
    {
        //清空资源，更新事务日志状态等
        doRollback ();
        if ( isRoot () ) {
            try {
                coordinator.terminate ( false );
            } catch ( Exception e ) {
                throw new SysException ( "Unexpected error in rollback: " + e.getMessage (), e );
            }
        }
    }

重点关注 coordinator.terminate ( false ); ，这个和 commit流程是一样的，只不过在 commit流程里面，参数传的是true。

protected void terminate ( boolean commit ) throws HeurRollbackException, HeurMixedException, SysException, java.lang.SecurityException, HeurCommitException, HeurHazardException, RollbackException, IllegalStateException { synchronized ( fsm_ ) { if ( commit ) { if ( participants_.size () <= 1 ) { commit ( true ); } else { int prepareResult = prepare (); // make sure to only do commit if NOT read only if ( prepareResult != Participant.READ_ONLY ) commit ( false ); } } else { //如果是false，走的是rollback rollback (); } } }
我们重点关注 rollback() ,最后会走到com.atomikos.datasource.xa.XAResourceTransaction.rollback()。

public synchronized void rollback() throws HeurCommitException, HeurMixedException, HeurHazardException, SysException { terminateInResource(); if (rollbackShouldDoNothing()) { return; } if (this.state.equals(TxState.TERMINATED)) { return; } if (this.state.equals(TxState.HEUR_MIXED)) throw new HeurMixedException(); if (this.state.equals(TxState.HEUR_COMMITTED)) throw new HeurCommitException(); if (this.xaresource == null) { throw new HeurHazardException("XAResourceTransaction " + getXid() + ": no XAResource to rollback?"); } try { if (this.state.equals(TxState.ACTIVE)) { // first suspend xid suspend(); } // refresh xaresource for MQSeries: seems to close XAResource after // suspend??? testOrRefreshXAResourceFor2PC(); if (LOGGER.isDebugEnabled()) { LOGGER.logDebug("XAResource.rollback ( " + this.xidToHexString + " ) " + "on resource " + this.resourcename + " represented by XAResource instance " + this.xaresource); } this.xaresource.rollback(this.xid);
先在supend()方法里面执行了 XA end xid 语句, 接下来执行 this.xaresource.rollback(this.xid); 进行数据的回滚。