OpenSessionInViewFilter——>TransactionSynchronizationManager

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public abstract class TransactionSynchronizationManager {

TransactionSynchronizationManager是管理需要同步的事务资源，也可以管理线程。

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

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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。

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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<Object, Object>();

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<List<TransactionSynchronization>>,ThreadLocal<String>等

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private static final ThreadLocal<Map<Object, Object>> resources =

new NamedThreadLocal<Map<Object, Object>>("Transactional resources");

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public class ThreadLocal<T> {

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public class NamedThreadLocal<T> extends ThreadLocal<T> {

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:

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public void set(T value) {

Thread t = Thread.currentThread();

ThreadLocalMap map = getMap(t);

if (map != null)

map.set(this, value);

else

createMap(t, value);

}

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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<Map>,createMap方法将当前线程t和T(Map)传进来,

t.threadLocals=new
ThreadLocalMap(this,firstValue); 这里的this就是这个类本身ThreadLocal<Map>,firstValue就是传进来的Map,下表就是一个t.threadLocals,key是类本身,value是T(也就是map),你传进来的T是什么,value就是什么。这里ThreadLocal<Map>也就是ThreadLocal<Map<Object,Object>>

ThreadLocal<Map>	Map
ThreadLocal<List>	List
ThreadLocal<String>	String
ThreadLocal<Boolean>	Boolean

这些都放到了threadLocals里面

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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<Object,Object>,List<Object>)

if (e != null)

return (T)e.value;

}

return setInitialValue();

}

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ThreadLocalMap getMap(Thread t) {

return t.threadLocals;

}

所以这里可以看到在上面的TransactionSynchronizationManager中resouces.get()得到的就是传进去的Map,但是现在的这个Map就是已经绑定到线程里面当中去了的Map,

Object
oldValue = map.put(actualKey, value);这里则是把SessionFactory和new new SessionHolder(Session)装进Map里面去了,这里也就绑定到线程里面去了。以上这几段代码就是绑定到线程中。

下面是解绑:

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public static Object unbindResource(Object key) throws IllegalStateException {

Object actualKey = TransactionSynchronizationUtils.unwrapResourceIfNecessary(key);

Object value = doUnbindResource(actualKey);

if (value == null) {

throw new IllegalStateException(

"No value for key [" + actualKey + "] bound to thread [" + Thread.currentThread().getName() + "]");

}

return value;

}

unbindResource方法时将该sessionFactory对应的session从线程里面移除，也就是把t.threadLocals这个Map中移除ThreadLocal<Map>,而这个ThreadLocal<Map>里面已经存进了SessionFactory和session,移除了ThreadLocal<Map>，也就相当于移除了session。