最近在看spring事务的时候在想一个问题：spring中的很多bean都是单例的，是非状态的，而数据库连接是一种有状态的对象，所以spring一定在创建出connection之后在threadlocal中保存了它。今天正好有空，就看了一下源码：

    /**
    * 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() + "]");
        }
    }

代码很简单，以dataSource为key，ConnectionHolder为value存进了一个map里，而这个叫做resources的map是一个Threadlocal变量，存在于当前线程的ThreadlocalMap里。

bindResource是在DataSourceTransactionManager.doBegin()方法中被调用的，来看看这个方法

/**
    * 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;