ThreadLocal学习2

典型实例

HiberanteHibernateSessionFactory

package com.zoucq.stock.factory;

 

import org.hibernate.HibernateException;

import org.hibernate.Session;

import org.hibernate.cfg.Configuration;

import org.hibernate.cfg.AnnotationConfiguration;

 

/**

 * Configures and provides access to Hibernate sessions, tied to the

 * current thread of execution.  Follows the Thread Local Session

 * pattern, see {@link http://hibernate.org/42.html }.

 */

public class HibernateSessionFactory {

    private static String CONFIG_FILE_LOCATION = "/config/hibernate.cfg.xml";

    //ThreadLocal实例

    private static final ThreadLocal<Session> threadLocal =

new ThreadLocal<Session>();

    private  static Configuration configuration = new AnnotationConfiguration();   

    private static org.hibernate.SessionFactory sessionFactory;

    private static String configFile = CONFIG_FILE_LOCATION;

    //静态初始化sessionFactory

    static {

    try {

            configuration.configure(configFile);

            sessionFactory = configuration.buildSessionFactory();

        } catch (Exception e) {

            System.err.println("%%%% Error Creating SessionFactory %%%%");

            e.printStackTrace();

        }

    }

    private HibernateSessionFactory() {

    }

    /**

     * 获取session

     * @return

     * @throws HibernateException

     */

    public static Session getSession() throws HibernateException {

        Session session = (Session) threadLocal.get();

        //如果TreadLocal中不存在当前线程对应的session,就重新创建并保存

        if (session == null || !session.isOpen()) {

            if (sessionFactory == null) {

                rebuildSessionFactory();

            }

            session =

(sessionFactory != null) ? sessionFactory.openSession(): null;

            threadLocal.set(session);

        }

        return session;

    }

 

    /**

     *  Rebuild hibernate session factory

     *

     */

    public static void rebuildSessionFactory() {

        try {

            configuration.configure(configFile);

            sessionFactory = configuration.buildSessionFactory();

        } catch (Exception e) {

            System.err.println("%%%% Error Creating SessionFactory %%%%");

            e.printStackTrace();

        }

    }

 

    /**

     *  Close the single hibernate session instance.

     *

     *  @throws HibernateException

     */

    public static void closeSession() throws HibernateException {

        Session session = (Session) threadLocal.get();

        threadLocal.set(null);

        if (session != null) {

            session.close();

        }

}

//一些简单的方法就省略

}

总结

ThreadLocal使用场合主要解决多线程中数据数据因并发产生不一致问题。ThreadLocal为每个线程的中并发访问的数据提供一个副本,通过访问副本来运行业务,这样的结果是耗费了内存,单大大减少了线程同步所带来性能消耗,也减少了线程并发控制的复杂度。ThreadLocal不能使用原子类型,只能使用Object类型。ThreadLocal的使用比synchronized要简单得多。 ThreadLocalSynchonized都用于解决多线程并发访问。但是ThreadLocalsynchronized有本质的区别。synchronized是利用锁的机制,使变量或代码块在某一时该只能被一个线程访问。而ThreadLocal为每一个线程都提供了变量的副本,使得每个线程在某一时间访问到的并不是同一个对象,这样就隔离了多个线程对数据的数据共享。而Synchronized却正好相反,它用于在多个线程间通信时能够获得数据共享。 Synchronized用于线程间的数据共享,而ThreadLocal则用于线程间的数据隔离 当然ThreadLocal并不能替代synchronized,它们处理不同的问题域。Synchronized用于实现同步机制,比ThreadLocal更加复杂。

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