OpenSessionInViewFilter——>TransactionSynchronizationManager

public abstract class TransactionSynchronizationManager {

TransactionSynchronizationManager是管理需要同步的事务资源，也可以管理线程,可以把资源绑定到当前线程当中。

下面介绍一下TransactionSynchronizationManager类的一些关键方法:

	public static boolean hasResource(Object key) {
		Object actualKey = TransactionSynchronizationUtils.unwrapResourceIfNecessary(key);
		Object value = doGetResource(actualKey);
		return (value != null);
	}

上面的方法时判断在当前线程中能不能找到session, 判断当前线程是否已经绑定了session,key是sessionFactory对象,一个sessionFactory可以绑定一个session

TransactionSynchronizationUtils工具类的unwrapResourceIfNecessary()方法是解除包装的方法。当一个包装好了的类丢进去则返回的是原始的类，当传进去是一个没包装的类，返回的还是它自己，也就是传进sessionFactory进去，返回的还是sessionFactory。

	public static void bindResource(Object key, Object value) throws IllegalStateException {
		Object actualKey = TransactionSynchronizationUtils.unwrapResourceIfNecessary(key);
		Assert.notNull(value, "Value must not be null");
		Map map = resources.get();
		// set ThreadLocal Map if none found
		if (map == null) {
			map = new HashMap();
			resources.set(map);
		}//得到一个绑定线程的干净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;
		}
            ..............................有些省略
}

这里的bindResouce方法是 绑定session到当前线程，这里的resouces是TransactionSynchronizationManager的一个属性如下,是一个ThreadLocal类,里面的泛型T是一个Map传到ThreadLocal里面去。TransactionSynchronizationManager类中还有好几个ThreadLocal,例如ThreadLocal>,ThreadLocal等

	private static final ThreadLocal> resources =
			new NamedThreadLocal>("Transactional resources");

public class ThreadLocal {

public class NamedThreadLocal extends ThreadLocal {

这里ThreadLocal和NamedThreadLocal大概都是一样的,只是 NamedThreadLocal可以通过名字来创建。

我们来到ThreadLocal类里面:

    public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }

    void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }

这里有点难理解,在 TransactionSynchronizationManager里面把T也就是map传进来，也就相当是ThreadLocal

t.threadLocals=new ThreadLocalMap(this,firstValue); 这里的this就是这个类本身ThreadLocal

ThreadLocal	Map
ThreadLocal	List
ThreadLocal	String
ThreadLocal	Boolean

这些都放到了threadLocals里面

    public T get() {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);//把类本身传进去当key,得到value(Map,List