1.引

上一節(jié)已經(jīng)分析了了在單service單方法下的事物創(chuàng)建過程以及其后續(xù)處理工作娶桦，當然還缺少真正創(chuàng)建事物的分析，本篇接續(xù)蓬蝶。先來回顧一下事物創(chuàng)建的代碼片段：

try {
    boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
    // 創(chuàng)建DefaultTransactionStatus對象實例
    DefaultTransactionStatus status = newTransactionStatus(
            definition,
            transaction,
            true,
            newSynchronization,
            debugEnabled,
            suspendedResources);
    // 開啟事物
    doBegin(transaction, definition);
    // 初始化事務同步耳贬。
    prepareSynchronization(status, definition);
    return status;
}

2.創(chuàng)建DefaultTransactionStatus對象

/**
 * 創(chuàng)建DefaultTransactionStatus實例
 * Create a TransactionStatus instance for the given arguments.
 */
protected DefaultTransactionStatus newTransactionStatus(
        TransactionDefinition definition,
        @Nullable Object transaction,
        boolean newTransaction,
        boolean newSynchronization,
        boolean debug,
        @Nullable Object suspendedResources) {

    boolean actualNewSynchronization = newSynchronization && !TransactionSynchronizationManager.isSynchronizationActive();

    return new DefaultTransactionStatus(
            transaction,
            newTransaction,
            actualNewSynchronization,
            definition.isReadOnly(),
            debug,
            suspendedResources);
}

//構造方法
public DefaultTransactionStatus(
            @Nullable Object transaction,
            boolean newTransaction,
            boolean newSynchronization,
            boolean readOnly,
            boolean debug,
            @Nullable Object suspendedResources) {

    this.transaction = transaction;
    this.newTransaction = newTransaction;
    this.newSynchronization = newSynchronization;
    this.readOnly = readOnly;
    this.debug = debug;
    this.suspendedResources = suspendedResources;
}

3.開啟事物

針對不同的事物管理器會有不同的事物開啟過程，本例我們只看DataSourceTransactionManager下的事物開啟過程：

protected void doBegin(Object transaction, TransactionDefinition definition) {
    DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
    Connection con = null;

    try {
        // ConnectionHolder簡介:包裝JDBC連接的資源容器碌燕。DataSourceTransactionManager將該類的實例綁定到特定數(shù)據(jù)源的線程误证。
        // 如果txObject沒有ConnectionHolder或者connectionHolder并沒有加同步鎖,則為其設置ConnectionHolder并加同步鎖
        if (!txObject.hasConnectionHolder() || txObject.getConnectionHolder().isSynchronizedWithTransaction()) {
            // 從數(shù)據(jù)源獲取連接
            Connection newCon = obtainDataSource().getConnection();
            if (logger.isDebugEnabled()) {
                logger.debug("Acquired Connection [" + newCon + "] for JDBC transaction");
            }
            // 設置DataSourceTransactionObject的ConnectionHolder對象
            txObject.setConnectionHolder(new ConnectionHolder(newCon), true);
        }

        // 設置同步鎖標記
        txObject.getConnectionHolder().setSynchronizedWithTransaction(true);

        // 從ConnectionHolder對象中獲取連接
        con = txObject.getConnectionHolder().getConnection();

        // 設置連接的只讀屬性和數(shù)據(jù)庫事物隔離級別
        Integer previousIsolationLevel = DataSourceUtils.prepareConnectionForTransaction(con, definition);
        txObject.setPreviousIsolationLevel(previousIsolationLevel);

        // Switch to manual commit if necessary. This is very expensive in some JDBC drivers,
        // so we don't want to do it unnecessarily
        // (for example if we've explicitly configured the connection pool to set it already).
        // 如果需要，切換到手動提交修壕。
        // 在某些JDBC驅動程序中愈捅，這是非常昂貴的，所以我們不想做不必要的事情
        // (例如慈鸠，如果我們已經(jīng)顯式地配置了連接池來設置它)蓝谨。
        // 所以:如果連接設置了自動提交,這里要將其轉換為手動提交
        if (con.getAutoCommit()) {
            txObject.setMustRestoreAutoCommit(true);
            if (logger.isDebugEnabled()) {
                logger.debug("Switching JDBC Connection [" + con + "] to manual commit");
            }
            con.setAutoCommit(false);
        }

        // 設置當前事物只讀,如果定義了只讀屬性為true
        prepareTransactionalConnection(con, definition);
        // 設置當前事物為已激活
        txObject.getConnectionHolder().setTransactionActive(true);

        // 設置超時時間(如果超時時間不等于默認超時時間)
        int timeout = determineTimeout(definition);
        if (timeout != TransactionDefinition.TIMEOUT_DEFAULT) {
            txObject.getConnectionHolder().setTimeoutInSeconds(timeout);
        }

        // Bind the connection holder to the thread.
        // 綁定ConnectionHolder到當前線程
        if (txObject.isNewConnectionHolder()) {
            // 即TransactionSynchronizationManager類的resources對象:
            // 該對象保存每個事物線程對應的connection或session等類型的資源
            // private static final ThreadLocal<Map<Object, Object>> resources = new NamedThreadLocal<>("Transactional resources");
            TransactionSynchronizationManager.bindResource(obtainDataSource(), txObject.getConnectionHolder());
        }
    }

    // 異常處理
    catch (Throwable ex) {
        if (txObject.isNewConnectionHolder()) {
            DataSourceUtils.releaseConnection(con, obtainDataSource());
            txObject.setConnectionHolder(null, false);
        }
        throw new CannotCreateTransactionException("Could not open JDBC Connection for transaction", ex);
    }
}

該方法也比較簡單，相信大家都能看懂青团，其中關于ConnectionHolder（前文沒有講解）對象譬巫，也已經(jīng)在注釋里做過介紹了。

4.初始化事物同步

/**
 * 根據(jù)需要初始化事務同步督笆。
 * Initialize transaction synchronization as appropriate.
 */
protected void prepareSynchronization(DefaultTransactionStatus status, TransactionDefinition definition) {
    if (status.isNewSynchronization()) {
        // 設置事物激活狀態(tài)
        TransactionSynchronizationManager.setActualTransactionActive(status.hasTransaction());
        // 設置事物隔離級別
        TransactionSynchronizationManager.setCurrentTransactionIsolationLevel(
                definition.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT ? definition.getIsolationLevel() : null);
        // 設置事物只讀屬性
        TransactionSynchronizationManager.setCurrentTransactionReadOnly(definition.isReadOnly());
        // 設置事物名稱
        TransactionSynchronizationManager.setCurrentTransactionName(definition.getName());
        // 激活當前線程的事務同步芦昔。事務管理器在事務開始時調用。
        TransactionSynchronizationManager.initSynchronization();
    }
}

總體來講上面的內容還是比較簡單的娃肿，不做過多的分析了烟零。

5.其他事物特性處理

以上分析的都是基于單service下單個方法調用的事物創(chuàng)建過程，且當前不存在事物咸作，那么對于其它的事物的特性呢？接下來就分析一下其他的特性宵睦，其實前面的章節(jié)已經(jīng)有過簡單的介紹了记罚，這里再詳細分析下。對于PROPAGATION_NESTED特性會直接拋出異常壳嚎。

那么下面介紹一下PROPAGATION_SUPPORTS桐智、PROPAGATION_NOT_SUPPORTED、PROPAGATION_NEVER特性事物處理過程烟馅，先來看一下代碼片段：

// Create "empty" transaction: no actual transaction, but potentially synchronization.
if (definition.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT && logger.isWarnEnabled()) {
    logger.warn("Custom isolation level specified but no actual transaction initiated; "
            + "isolation level will effectively be ignored: " + definition);
}
boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
return prepareTransactionStatus(
        definition,
        null,
        true,
        newSynchronization,
        debugEnabled,
        null);

protected final DefaultTransactionStatus prepareTransactionStatus(
        TransactionDefinition definition,
        @Nullable Object transaction,
        boolean newTransaction,
        boolean newSynchronization,
        boolean debug,
        @Nullable Object suspendedResources) {

    DefaultTransactionStatus status = newTransactionStatus(
            definition,
            transaction,
            newTransaction,
            newSynchronization,
            debug,
            suspendedResources);

    prepareSynchronization(status, definition);
    return status;
}

從代碼里可以看到说庭，該處理與創(chuàng)建真正的事物的過程相似，只是傳遞的參數(shù)不同罷了郑趁。大家可以多設定幾種傳播特性刊驴，跟蹤一下代碼即可。