SessionHolder

DEFAULT_KEY 存储Threadlocal Session(可能由OpenSessionInViewFilter/Interceptor获取，也可能由Spring激活时SessionFactoryUtils直接或间接调用获取)
sessionMap  除了存放DEFAULT_KEY指定线程绑定的Session,还有hibernate配置中指定的 TransactionManager事务激活时的对应的javax.transaction.Transaction的Session,当Spring同步不处于激活状态且有JTA激活事务时创建
previousFlushMode Spring管理的Session要求在事务提交时Flush,可能会将Flush.Never或Flush.Manual覆盖掉，当事务处理(或同步)完成后,会回复过来

/*
 * Copyright 2002-2005 the original author or authors.
 * 
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 * 
 *      http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.orm.hibernate3;

import java.util.Collections;
import java.util.HashMap;
import java.util.Map;

import org.hibernate.FlushMode;
import org.hibernate.Session;
import org.hibernate.Transaction;

import org.springframework.transaction.support.ResourceHolderSupport;
import org.springframework.util.Assert;

/**
 * Session holder, wrapping a Hibernate Session and a Hibernate Transaction.
 * HibernateTransactionManager binds instances of this class
 * to the thread, for a given SessionFactory.
 *
 * <p>Note: This is an SPI class, not intended to be used by applications.
 *
 * @author Juergen Hoeller
 * @since 1.2
 * @see HibernateTransactionManager
 * @see SessionFactoryUtils
 */
public class SessionHolder extends ResourceHolderSupport {

	private static final Object DEFAULT_KEY = new Object();

	/**
	 * This Map needs to be synchronized because there might be multi-threaded
	 * access in the case of JTA with remote transaction propagation.
	 */
	private final Map sessionMap = Collections.synchronizedMap(new HashMap(1));

	private Transaction transaction;

	private FlushMode previousFlushMode;


	public SessionHolder(Session session) {
		addSession(session);
	}

	public SessionHolder(Object key, Session session) {
		addSession(key, session);
	}


	public Session getSession() {
		return getSession(DEFAULT_KEY);
	}

	public Session getSession(Object key) {
		return (Session) this.sessionMap.get(key);
	}

	public Session getValidatedSession() {
		return getValidatedSession(DEFAULT_KEY);
	}

	public Session getValidatedSession(Object key) {
		Session session = (Session) this.sessionMap.get(key);
		// Check for dangling Session that's around but already closed.
		// Effectively an assertion: that should never happen in practice.
		// We'll seamlessly remove the Session here, to not let it cause
		// any side effects.
		if (session != null && !session.isOpen()) {
			this.sessionMap.remove(key);
			session = null;
		}
		return session;
	}

	public Session getAnySession() {
		synchronized (this.sessionMap) {
			if (!this.sessionMap.isEmpty()) {
				return (Session) this.sessionMap.values().iterator().next();
			}
			return null;
		}
	}

	public void addSession(Session session) {
		addSession(DEFAULT_KEY, session);
	}

	public void addSession(Object key, Session session) {
		Assert.notNull(key, "Key must not be null");
		Assert.notNull(session, "Session must not be null");
		this.sessionMap.put(key, session);
	}

	public Session removeSession(Object key) {
		return (Session) this.sessionMap.remove(key);
	}

	public boolean containsSession(Session session) {
		return this.sessionMap.containsValue(session);
	}

	public boolean isEmpty() {
		return this.sessionMap.isEmpty();
	}

	public boolean doesNotHoldNonDefaultSession() {
		synchronized (this.sessionMap) {
			return this.sessionMap.isEmpty() ||
					(this.sessionMap.size() == 1 && this.sessionMap.containsKey(DEFAULT_KEY));
		}
	}


	public void setTransaction(Transaction transaction) {
		this.transaction = transaction;
	}

	public Transaction getTransaction() {
		return transaction;
	}

	public void setPreviousFlushMode(FlushMode previousFlushMode) {
		this.previousFlushMode = previousFlushMode;
	}

	public FlushMode getPreviousFlushMode() {
		return previousFlushMode;
	}


	public void clear() {
		super.clear();
		this.transaction = null;
		this.previousFlushMode = null;
	}

}