结合ThreadLocal来看spring事务源码，感受下清泉般的洗涤！

public class ThreadLocalTest
{
    ThreadLocal<Long> longLocal = new ThreadLocal<Long>();
    ThreadLocal<String> stringLocal = new ThreadLocal<String>();
    
    public void set()
    {
        longLocal.set(1L);
        stringLocal.set(Thread.currentThread().getName());
    }
    
    public long getLong()
    {
        return longLocal.get();
    }
    
    public String getString()
    {
        return stringLocal.get();
    }
    
    public static void main(String[] args) throws InterruptedException
    {
        final ThreadLocalTest test = new ThreadLocalTest();
        
        test.set();     // 初始化ThreadLocal
        for (int i=0; i<10; i++)
        {
            System.out.println(test.getString() + " : " + test.getLong() + i);
        }
        
        Thread thread1 = new Thread(){
            public void run() {
                test.set();
                for (int i=0; i<10; i++)
                {
                    System.out.println(test.getString() + " : " + test.getLong() + i);
                }
            };
        };
        thread1.start();
        
        Thread thread2 = new Thread(){
            public void run() {
                test.set();
                for (int i=0; i<10; i++)
                {
                    System.out.println(test.getString() + " : " + test.getLong() + i);
                }
            };
        };
        thread2.start();
    }
}

Thread与ThreadLocal对象之间的引用关系图
 

ApplicationContext ac = new ClassPathXmlApplicationContext("applicationContext-jdbc.xml");
DaoImpl daoImpl = (DaoImpl) ac.getBean("daoImpl");
System.out.println(daoImpl.insertUser("yes", 25));

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

    try {
        
        if (txObject.getConnectionHolder() == null ||
                txObject.getConnectionHolder().isSynchronizedWithTransaction()) {
            // 从连接池获取一个connection
            Connection newCon = this.dataSource.getConnection();
            if (logger.isDebugEnabled()) {
                logger.debug("Acquired Connection [" + newCon + "] for JDBC transaction");
            }
            // 包装newCon，并赋值到txObject，并标记是新的ConnectionHolder
            txObject.setConnectionHolder(new ConnectionHolder(newCon), true);
        }

        txObject.getConnectionHolder().setSynchronizedWithTransaction(true);
        con = txObject.getConnectionHolder().getConnection();

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

        int timeout = determineTimeout(definition);
        if (timeout != TransactionDefinition.TIMEOUT_DEFAULT) {
            txObject.getConnectionHolder().setTimeoutInSeconds(timeout);
        }

        // 若是新的ConnectionHolder，则将它绑定到当前线程中
        // Bind the session holder to the thread.
        if (txObject.isNewConnectionHolder()) {
            TransactionSynchronizationManager.bindResource(getDataSource(), txObject.getConnectionHolder());
        }
    }

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

/**
 * 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 {        //key：通常指资源工厂，也就是connection工厂，value：通常指活动的资源，也就是活动的ConnectionHolder
    
    // 必要时unwrap给定的连接池; 否则按原样返回给定的连接池。
    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   如果ThreadLocal Map不存在则新建，并将其设置到resources中
    // private static final ThreadLocal<Map<Object, Object>> resources = new NamedThreadLocal<Map<Object, Object>>("Transactional resources");
　　// 这就到了ThreadLocal流程了
    if (map == null) {
        map = new HashMap<Object, Object>();
        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() + "]");
    }
}