Spring 資料庫連接（Connection）綁定線程（Thread）的實現

Spring 資料庫連接（Connection）綁定線程（Thread）的實現

最近在看spring事務的時候在想一個問題：spring中的很多bean都是單例的，是非狀態的，而資料庫連接是一種有狀態的對象，所以spring一定在建立出connection之後在threadlocal中儲存了它。今天正好有空，就看了一下源碼：

    /**
    * Bind the given resource for the given key to the current thread.
    * @param key the key to bind the value to (usually the resource factory)
    * @param value the value to bind (usually the active resource object)
    * @throws IllegalStateException if there is already a value bound to the thread
    * @see ResourceTransactionManager#getResourceFactory()
    */
    public static void bindResource(Object key, Object value) throws IllegalStateException {
        Object actualKey = TransactionSynchronizationUtils.unwrapResourceIfNecessary(key);
        Assert.notNull(value, "Value must not be null");
        Map<Object, Object> map = resources.get();
        // set ThreadLocal Map if none found
        if (map == null) {
            map = new HashMap<>();
            resources.set(map);
        }
        Object oldValue = map.put(actualKey, value);
        // Transparently suppress a ResourceHolder that was marked as void...
        if (oldValue instanceof ResourceHolder && ((ResourceHolder) oldValue).isVoid()) {
            oldValue = null;
        }
        if (oldValue != null) {
            throw new IllegalStateException("Already value [" + oldValue + "] for key [" +
                    actualKey + "] bound to thread [" + Thread.currentThread().getName() + "]");
        }
        if (logger.isTraceEnabled()) {
            logger.trace("Bound value [" + value + "] for key [" + actualKey + "] to thread [" +
                    Thread.currentThread().getName() + "]");
        }
    }

代碼很簡單，以dataSource為key，ConnectionHolder為value存進了一個map裡，而這個叫做resources的map是一個Threadlocal變數，存在於當前線程的ThreadlocalMap裡。

bindResource是在DataSourceTransactionManager.doBegin()方法中被調用的，來看看這個方法

/**
    * This implementation sets the isolation level but ignores the timeout.
    */
    @Override
    protected void doBegin(Object transaction, TransactionDefinition definition) {
        DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
        Connection con = null;

        try {
            if (!txObject.hasConnectionHolder() ||
                    txObject.getConnectionHolder().isSynchronizedWithTransaction()) {
                Connection newCon = obtainDataSource().getConnection();
                if (logger.isDebugEnabled()) {
                    logger.debug("Acquired Connection [" + newCon + "] for JDBC transaction");
                }
                txObject.setConnectionHolder(new ConnectionHolder(newCon), true);
            }

            txObject.getConnectionHolder().setSynchronizedWithTransaction(true);
            con = txObject.getConnectionHolder().getConnection();
　　　　　　  //這裡設定隔離等級
            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).
　　　　　　  //這裡設定自動認可由spring控制

            if (con.getAutoCommit()) {
                txObject.setMustRestoreAutoCommit(true);
                if (logger.isDebugEnabled()) {
                    logger.debug("Switching JDBC Connection [" + con + "] to manual commit");
                }
                con.setAutoCommit(false);
            }

            prepareTransactionalConnection(con, definition);
            //設定該串連現在已經有事務了
            txObject.getConnectionHolder().setTransactionActive(true);

            int timeout = determineTimeout(definition);
            if (timeout != TransactionDefinition.TIMEOUT_DEFAULT) {
                txObject.getConnectionHolder().setTimeoutInSeconds(timeout);
            }
　　　　　　
            // Bind the connection holder to the thread.
　　　　　　  //這裡把新串連綁定到當前線程

            if (txObject.isNewConnectionHolder()) {
                TransactionSynchronizationManager.bindResource(obtainDataSource(), txObject.getConnectionHolder());
            }
        }

        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);
        }
    }

該方法的主要作用都寫在注釋裡了，看一下就好。

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