初始化hibernate 时可能出现重复读取hibernate配置文件导致报错的解决办法

假设你有一个应用程序，登录系统时会初始化很多信息，比如使用ajax同时初始化两类信息，这时如果你的hibernate还没初始化信息，就会有两个线程同时进行初始化操作。这时hibernate就会发现两个相同的hbm配置文件被读取，所以，问题发生了。

这怪谁？其实是我们自己的问题，如果你出现了上面的问题，就需要看看你的获取session的机制是否正常了

这是使用myeclipse自动生成的session工厂类：

package com.dcsoft.sbgl.hibernate;

import org.hibernate.HibernateException;
import org.hibernate.Session;
import org.hibernate.cfg.Configuration;

/**
* 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 {

/**
* Location of hibernate.cfg.xml file.
* Location should be on the classpath as Hibernate uses
* #resourceAsStream style lookup for its configuration file.
* The default classpath location of the hibernate config file is
* in the default package. Use #setConfigFile() to update
* the location of the configuration file for the current session.
*/
private static String CONFIG_FILE_LOCATION = "/hibernate.cfg.xml";
private static final ThreadLocal threadLocal = new ThreadLocal();
private static Configuration configuration = new Configuration();
private static org.hibernate.SessionFactory sessionFactory;
private static String configFile = CONFIG_FILE_LOCATION;

private HibernateSessionFactory() {
}

/**
* Returns the ThreadLocal Session instance. Lazy initialize
* the <code>SessionFactory</code> if needed.
*
* @return Session
* @throws HibernateException
*/
public static Session getSession() throws HibernateException {
Session session = (Session) threadLocal.get();

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();
}
}

/**
* return session factory
*
*/
public static org.hibernate.SessionFactory getSessionFactory() {
return sessionFactory;
}

/**
* return session factory
*
*session factory will be rebuilded in the next call
*/
public static void setConfigFile(String configFile) {
HibernateSessionFactory.configFile = configFile;
sessionFactory = null;
}

/**
* return hibernate configuration
*
*/
public static Configuration getConfiguration() {
return configuration;
}

}

请看一下标红的部分：

if (sessionFactory == null)

这时就会进行初始化操作，如果两个线程同时到了这里，嘿嘿…………

解决方法：

1 初始化：在启动应用的时候配置一个servlet进行初始化hibernate操作，无论做什么都行，只要使用hibernate进行数据库操作。

如果你觉得这种方法还不保险，那就使用第二招：加锁！

2 将红色部分加上：

public static Session getSession() throws HibernateException {
Session session = (Session) threadLocal.get();

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

synchronized (lockObj) {
if (sessionFactory == null) {
rebuildSessionFactory();
}

}
session = (sessionFactory != null) ? sessionFactory.openSession()
: null;
threadLocal.set(session);
}

return session;
}

但总感觉这种方法会影响效率，感觉能用第一种方法最好