Spring 資料庫連線(Connection)繫結執行緒(Thread)的實現
阿新 • • 發佈:2022-05-04
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,而資料庫連線是一種有狀態的物件,所以spring一定在創建出connection之後在threadlocal中儲存了它。今天正好有空,就看了一下原始碼:
/** * 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() + "]"); } }
程式碼很簡單,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,存在於當前執行緒的ThreadlocalMap裡。
bindResource是在DataSourceTransactionManager.doBegin()方法中被呼叫的,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). //這裡設定自動提交由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); //設定該連線現在已經有事務了 txObject.getConnectionHolder().setTransactionActive(true); int timeout = determineTimeout(definition); if (timeout != TransactionDefinition.TIMEOUT_DEFAULT) { txObject.getConnectionHolder().setTimeoutInSeconds(timeout); } // Bind the connection holder to the thread. //這裡把新連線繫結到當前執行緒 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註釋裡了,看一下就好。