Spring 事務原理

Spring事務攔截器為

org.springframework.transaction.interceptor.TransactionInterceptor

當Spring對函數(shù)進行事務攔截時，會調(diào)用到TransactionInterceptor的invoke函數(shù)。

@Override
public Object invoke(final MethodInvocation invocation) throws Throwable {
    Class<?> targetClass = (invocation.getThis() != null ? AopUtils.getTargetClass(invocation.getThis()) : null);

    // 在事務中調(diào)用函數(shù)，使函數(shù)支持事務
    return invokeWithinTransaction(invocation.getMethod(), targetClass, new InvocationCallback() {
        @Override
        public Object proceedWithInvocation() throws Throwable {
            return invocation.proceed();
        }
    });
}

invokeWithTransaction函數(shù)

protected Object invokeWithinTransaction(Method method, Class<?> targetClass, final InvocationCallback invocation)
        throws Throwable {

    // 數(shù)據(jù)準備
    final TransactionAttribute txAttr = getTransactionAttributeSource().getTransactionAttribute(method, targetClass);
    final PlatformTransactionManager tm = determineTransactionManager(txAttr);
    final String joinpointIdentification = methodIdentification(method, targetClass);

    if (txAttr == null || !(tm instanceof CallbackPreferringPlatformTransactionManager)) {
        // 1.新建一個事務
        TransactionInfo txInfo = createTransactionIfNecessary(tm, txAttr, joinpointIdentification);
        Object retVal = null;
        try {
            //2.執(zhí)行業(yè)務代碼
            retVal = invocation.proceedWithInvocation();
        }
        catch (Throwable ex) {
            // 異常處理
            completeTransactionAfterThrowing(txInfo, ex);
            throw ex;
        }
        finally {//
            cleanupTransactionInfo(txInfo);
        }
        //3.提交事務
        commitTransactionAfterReturning(txInfo);
        return retVal;
    }
//......其他代碼
}

整個事務處理過程中主要涉及到四個類

  1. TransactionAttribute
  2. PlatformTransactionManager
  3. TransactionStatus
  4. TransactionInfo

TransactionAttribute

public interface TransactionAttribute extends TransactionDefinition {
String getQualifier();
boolean rollbackOn(Throwable ex);
}

父類TransactionDefinition的定義

public interface TransactionDefinition {
  int getIsolationLevel();
  String getName();
  int getPropagationBehavior();
  boolean isReadOnly();
  int getTimeout();
}

TransactionAttribute用于保存@Transactional注解上的屬性內(nèi)容。

PlatformTransactionManager

package org.springframework.transaction;
public interface PlatformTransactionManager {
  //啟動一個新事務
  TransactionStatus getTransaction(TransactionDefinition definition)
  //提交事務
  void commit(TransactionStatus status)
  //回滾事務
  void rollback(TransactionStatus status)
  }

PlatformTransactionManager是事務的具體操作類，類似于JDBC的事務操作，使用PlatformTransactionManager進行事務處理的偽代碼如下：

PlatformTransactionManager pm;
TransactionDefinition definition
//啟動事務
TransactionStatus  status=pm.getTransaction(definition);
  try{
  //處理業(yè)務邏輯
  .....
  //提交事務
  pm.commit(status);
  }catch(Throwable e){
    //回滾事務
    pm.rollback(status);
  }
}

**TransactionStatus **

public interface TransactionStatus{
  boolean isNewTransaction();
  boolean hasSavepoint();
  void setRollbackOnly();
  boolean isRollbackOnly();
  void flush();
  boolean isCompleted();
}

TransactionStatus表征一次事務操作。

TransactionInfo

protected final class TransactionInfo {

    private final PlatformTransactionManager transactionManager;

    private final TransactionAttribute transactionAttribute;

    private final String joinpointIdentification;

    private TransactionStatus transactionStatus;

    private TransactionInfo oldTransactionInfo;
}

TransactionInfo將上面的三個類的內(nèi)容融合在一起，即當前事務的相關(guān)信息，
oldTransactionInfo為父事務的相關(guān)信息。

Spring的事務的具體操作是PlatformTransactionManager，下面來具體說明PlatformTransactionManager得三個函數(shù)。
1. getTransaction

    TransactionStatus org.springframework.transaction.support.AbstractPlatformTransactionManager.getTransaction(TransactionDefinition definition)

getTransaction函數(shù)的業(yè)務邏輯可簡化為

    public final TransactionStatus getTransaction(TransactionDefinition definition) throws TransactionException {
        //1.獲取當前的事務對象，如數(shù)據(jù)庫的connection對象
        Object transaction = doGetTransaction();
        //2.如果當前的事務對象可用，根據(jù)當前的傳遞方式，構(gòu)建TransactionStatus
        if (isExistingTransaction(transaction)) {
            return handleExistingTransaction(definition, transaction, debugEnabled);
        }//3.如果不存在，根據(jù)當前的傳遞方式，構(gòu)建TransactionStatus
        else if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRED ||
                definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW ||
                definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {

            DefaultTransactionStatus status = newTransactionStatus(
                    definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
            return status;
            
        }
    }

doGetTransaction函數(shù)
以PlatformTransactionManager的實現(xiàn)之一DataSourceTransactionManager為例，

    protected Object doGetTransaction() {
        DataSourceTransactionObject txObject = new DataSourceTransactionObject();
        txObject.setSavepointAllowed(isNestedTransactionAllowed());
        ConnectionHolder conHolder =
                (ConnectionHolder) TransactionSynchronizationManager.getResource(this.dataSource);
        txObject.setConnectionHolder(conHolder, false);
        return txObject;
    }

TransactionSynchronizationManager.getResource是從線程上下文獲取數(shù)據(jù)庫連接信息，如果存在，直接返回，否則返回null.

isExistingTransaction
判斷當前事務對象是否可用，在DataSourceTransactionManager中，只需要簡單判斷ConnectionHolder是否可用即可。

    protected boolean isExistingTransaction(Object transaction) {
        DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
        return (txObject.getConnectionHolder() != null && txObject.getConnectionHolder().isTransactionActive());
    }

handleExistingTransaction
是處理事務傳遞的關(guān)鍵類，在handleExistingTransaction中會根據(jù)當前不同的事務傳遞方式創(chuàng)建不同的TransactionStatus

1.PROPAGATION_NEVER
當前方法不應在Transaction中運行，如果存在已經(jīng)定義的Transaction則拋出異常。

if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NEVER) {
        throw new IllegalTransactionStateException(
                "Existing transaction found for transaction marked with propagation 'never'");
    }

所以代碼中直接拋出異常

2.PROPAGATION_NOT_SUPPORTED
當前方法不應在Transaction中運行，如果存在已經(jīng)定義的Transaction，則該Transaction暫停(掛起)直至該方法運行完畢。

    if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NOT_SUPPORTED) {
        if (debugEnabled) {
            logger.debug("Suspending current transaction");
        }
        Object suspendedResources = suspend(transaction);
        boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
        return prepareTransactionStatus(
                definition, null, false, newSynchronization, debugEnabled, suspendedResources);
    }

前面介紹過transaction是從上下文中獲取的，suspend函數(shù)掛起transaction的主要作用是將transaction的ConnectionHolder和事務的相關(guān)信息從線程上下文移除，這樣在業(yè)務代碼中獲取數(shù)據(jù)庫連接時，就不能從線程上下文中獲取到父事務的連接。

DefaultTransactionStatus prepareTransactionStatus(
        TransactionDefinition definition, Object transaction, boolean newTransaction,
        boolean newSynchronization, boolean debug, Object suspendedResources)

prepareTransactionStatus的作用有兩個

  1. 組裝DefaultTransactionStatus
  2. 如果newTransaction==true，更新當前線程上下文，保存當前事務的信息，如事務隔離級別，傳遞方式等。newTransaction還有另外一個作用：表示當前事務需不需要進行提交，回滾操作。

當傳遞方式為PROPAGATION_NOT_SUPPORTED時，prepareTransactionStatus函數(shù)newTransaction參數(shù)的值為false，不會更新線程上下文信息。業(yè)務代碼中獲取數(shù)據(jù)庫連接時會從數(shù)據(jù)源中獲取一個全新的connection使用。

3.Propagation.REQUIRES_NEW
當前方法必須在新開的Transaction中運行。如果存在已經(jīng)定義的Transaction，則該已定義的Transaction暫停直至新開的Transaction執(zhí)行完畢。

    if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW) {
        SuspendedResourcesHolder suspendedResources = suspend(transaction);
        try {
            boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
            DefaultTransactionStatus status = newTransactionStatus(
                    definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
            doBegin(transaction, definition);
            prepareSynchronization(status, definition);
            return status;
        }
        catch (RuntimeException beginEx) {
            resumeAfterBeginException(transaction, suspendedResources, beginEx);
            throw beginEx;
        }
        catch (Error beginErr) {
            resumeAfterBeginException(transaction, suspendedResources, beginErr);
            throw beginErr;
        }
    }

操作分為3步：

  1. 掛起當前事務
  2. 從數(shù)據(jù)源中獲取一個新的連接，將connection的AutoCommit設置false.并將該connection綁定到當前線程（doBegin函數(shù)），以便業(yè)務代碼能獲取到該connection。
  3. 更新線程上下文關(guān)于當前事務的相關(guān)信息。

........
其他傳遞方式略

2.commit
commit 就比較簡單，主要做兩件事

  1. 如果當前事務是newTransaction，提交事務
  2. 恢復當前connection的屬性，釋放connection
  3. 恢復被掛起的事務

提交connection

    protected void doCommit(DefaultTransactionStatus status) {
        DataSourceTransactionObject txObject = (DataSourceTransactionObject) status.getTransaction();
        Connection con = txObject.getConnectionHolder().getConnection();
        if (status.isDebug()) {
            logger.debug("Committing JDBC transaction on Connection [" + con + "]");
        }
        try {
            //數(shù)據(jù)庫connection提交
            con.commit();
        }
        catch (SQLException ex) {
            throw new TransactionSystemException("Could not commit JDBC transaction", ex);
        }
    }

恢復connection屬性，釋放connection

    protected void doCleanupAfterCompletion(Object transaction) {
        DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;

        // Remove the connection holder from the thread, if exposed.
        if (txObject.isNewConnectionHolder()) {
            TransactionSynchronizationManager.unbindResource(this.dataSource);
        }

        // Reset connection.
        Connection con = txObject.getConnectionHolder().getConnection();
        try {
            if (txObject.isMustRestoreAutoCommit()) {
                con.setAutoCommit(true);
            }
            DataSourceUtils.resetConnectionAfterTransaction(con, txObject.getPreviousIsolationLevel());
        }
        catch (Throwable ex) {
            logger.debug("Could not reset JDBC Connection after transaction", ex);
        }

        if (txObject.isNewConnectionHolder()) {
            if (logger.isDebugEnabled()) {
                logger.debug("Releasing JDBC Connection [" + con + "] after transaction");
            }
            DataSourceUtils.releaseConnection(con, this.dataSource);
        }

        txObject.getConnectionHolder().clear();
    }

恢復被掛起的事務

    protected final void resume(Object transaction, SuspendedResourcesHolder resourcesHolder)
            throws TransactionException {

        if (resourcesHolder != null) {
            Object suspendedResources = resourcesHolder.suspendedResources;
            if (suspendedResources != null) {
                doResume(transaction, suspendedResources);
            }
            List<TransactionSynchronization> suspendedSynchronizations = resourcesHolder.suspendedSynchronizations;
            if (suspendedSynchronizations != null) {
                TransactionSynchronizationManager.setActualTransactionActive(resourcesHolder.wasActive);
                TransactionSynchronizationManager.setCurrentTransactionIsolationLevel(resourcesHolder.isolationLevel);
                TransactionSynchronizationManager.setCurrentTransactionReadOnly(resourcesHolder.readOnly);
                TransactionSynchronizationManager.setCurrentTransactionName(resourcesHolder.name);
                doResumeSynchronization(suspendedSynchronizations);
            }
        }
    }

3.rollback
與commit雷同，不做解釋。

其他

Spring的Transactional注解是如何在mybatis中生效的？
Spring通過控制mybatis獲取的connection來起作用。

mybatis的數(shù)據(jù)庫連接是從Mybatis的事務中獲取的

    package org.apache.ibatis.executor;
    public abstract class BaseExecutor{
        protected Connection getConnection(Log statementLog) throws SQLException {
            Connection connection = transaction.getConnection();
            if (statementLog.isDebugEnabled()) {
              return ConnectionLogger.newInstance(connection, statementLog, queryStack);
            } else {
              return connection;
            }
        }
    }

mybatis的事務對象為：

org.apache.ibatis.transaction.Transaction

Transaction是有mybatis的事務工廠產(chǎn)生的

org.apache.ibatis.transaction.TransactionFactory

而mybatis默認的事務工廠為

org.mybatis.spring.transaction.SpringManagedTransactionFactory

見下面mybatis代碼

    package org.mybatis.spring;
    public class SqlSessionFactoryBean{
        protected SqlSessionFactory buildSqlSessionFactory(){
            //部分代碼略..........
            if (this.transactionFactory == null) {
                this.transactionFactory = new SpringManagedTransactionFactory();
            //部分代碼略.............
        }
        }
    }

SpringManagedTransactionFactory生產(chǎn)的事務的類為

org.mybatis.spring.transaction.SpringManagedTransaction

SpringManagedTransaction的getConnection會調(diào)用到openConnection函數(shù)。

    private void openConnection() throws SQLException {
        this.connection = DataSourceUtils.getConnection(this.dataSource);
        this.autoCommit = this.connection.getAutoCommit();
        this.isConnectionTransactional = DataSourceUtils.isConnectionTransactional(this.connection, this.dataSource);

        if (logger.isDebugEnabled()) {
          logger.debug(
              "JDBC Connection ["
                  + this.connection
                  + "] will"
                  + (this.isConnectionTransactional ? " " : " not ")
                  + "be managed by Spring");
        }
    }

DataSourceUtils.getConnection最終會調(diào)用到DataSourceUtils.doGetConnection函數(shù)

    public static Connection doGetConnection(DataSource dataSource) throws SQLException {
        Assert.notNull(dataSource, "No DataSource specified");

        ConnectionHolder conHolder = (ConnectionHolder) TransactionSynchronizationManager.getResource(dataSource);
        if (conHolder != null && (conHolder.hasConnection() || conHolder.isSynchronizedWithTransaction())) {
            conHolder.requested();
            if (!conHolder.hasConnection()) {
                logger.debug("Fetching resumed JDBC Connection from DataSource");
                conHolder.setConnection(dataSource.getConnection());
            }
            return conHolder.getConnection();
        }
        // Else we either got no holder or an empty thread-bound holder here.

        logger.debug("Fetching JDBC Connection from DataSource");
        Connection con = dataSource.getConnection();

        if (TransactionSynchronizationManager.isSynchronizationActive()) {
            logger.debug("Registering transaction synchronization for JDBC Connection");
            // Use same Connection for further JDBC actions within the transaction.
            // Thread-bound object will get removed by synchronization at transaction completion.
            ConnectionHolder holderToUse = conHolder;
            if (holderToUse == null) {
                holderToUse = new ConnectionHolder(con);
            }
            else {
                holderToUse.setConnection(con);
            }
            holderToUse.requested();
            TransactionSynchronizationManager.registerSynchronization(
                    new ConnectionSynchronization(holderToUse, dataSource));
            holderToUse.setSynchronizedWithTransaction(true);
            if (holderToUse != conHolder) {
                TransactionSynchronizationManager.bindResource(dataSource, holderToUse);
            }
        }

        return con;
    }

上面的代碼首先會從線程上下文中獲取數(shù)據(jù)庫連接，如果不能獲取到，就直接從數(shù)據(jù)源中獲取。

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