近来参与一个Java的web办公系统，碰到一个bug，开始猜测是线程池管理的问题，最后发现是单例模式的问题。
即，当同时发起两个事务请求时，当一个事务完成后，另一个事务会抛出session is closed异常。具体见下图：
至于，下面这种情况，当时也测试过，但问题情形忘了，手上没有数据库环境，无法进行测试：
最开始，个人认为是session管理的问题，比如，在关闭session的时候，会同时关闭先前打开的session。由于下面采用的是其他公司的框架，所以就反馈给了技术总监。后来，反馈给我，竟然是单例的问题。
简单看了一下本系统，其在框架基础上又封装了一层，涉及这个bug的类关系如下：
发现原来设想复杂了，本框架并没有一个session的线程池管理，仅仅是对每个请求新建一个ThreadLocal对象（在DaoFactoryClass中实现），其中的initValue方法中新建了一个session对象。
问题出现在自己封装的DaoBaseClass类中，此类实现了一个单例模式，需要一个Dao参数，这个参数是通过ActionFrameClass的方法getDao()获得的，于是乎，原来实现的每个线程一个session变量，现在又被单例模式给破坏了。
附注：
ThreadLocal和Synchonized都用于解决多线程并发访问。但是ThreadLocal与synchronized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。
一、ThreadLocal使用一般步骤：
1、在多线程的类（如ThreadDemo类）中，创建一个ThreadLocal对象threadXxx，用来保存线程间需要隔离处理的对象xxx。
2、在ThreadDemo类中，创建一个获取要隔离访问的数据的方法getXxx()，在方法中判断，若ThreadLocal对象为null时候，应该new()一个隔离访问类型的对象，并强制转换为要应用的类型。
3、在ThreadDemo类的run()方法中，通过getXxx()方法获取要操作的数据，这样可以保证每个线程对应一个数据对象，在任何时刻都操作的是这个对象。
二、Hibernate中的使用：
private static final ThreadLocal threadSession = new ThreadLocal();
public static Session getSession() throws InfrastructureException { Session s = (Session) threadSession.get(); try { if (s == null) { s = getSessionFactory().openSession(); threadSession.set(s); } } catch (HibernateException ex) { throw new InfrastructureException(ex); } return s; }
三、ThreadLocal实现原理（JDK1.5中）
public class ThreadLocal<T> { /** * ThreadLocals rely on per-thread hash maps attached to each thread * (Thread.threadLocals and inheritableThreadLocals). The ThreadLocal * objects act as keys, searched via threadLocalHashCode. This is a * custom hash code (useful only within ThreadLocalMaps) that eliminates * collisions in the common case where consecutively constructed * ThreadLocals are used by the same threads, while remaining well-behaved * in less common cases. */ private final int threadLocalHashCode = nextHashCode();
/** * The next hash code to be given out. Accessed only by like-named method. */ private static int nextHashCode = 0;
/** * The difference between successively generated hash codes - turns * implicit sequential thread-local IDs into near-optimally spread * multiplicative hash values for power-of-two-sized tables. */ private static final int HASH_INCREMENT = 0x61c88647;
/** * Compute the next hash code. The static synchronization used here * should not be a performance bottleneck. When ThreadLocals are * generated in different threads at a fast enough rate to regularly * contend on this lock, memory contention is by far a more serious * problem than lock contention. */ private static synchronized int nextHashCode() { int h = nextHashCode; nextHashCode = h + HASH_INCREMENT; return h; }
/** * Creates a thread local variable. */ public ThreadLocal() { }
/** * Returns the value in the current thread's copy of this thread-local * variable. Creates and initializes the copy if this is the first time * the thread has called this method. * * @return the current thread's value of this thread-local */ public T get() { Thread t = Thread.currentThread(); ThreadLocalMap map = getMap(t); if (map != null) return (T)map.get(this);
// Maps are constructed lazily. if the map for this thread // doesn't exist, create it, with this ThreadLocal and its // initial value as its only entry. T value = initialValue(); createMap(t, value); return value; }
/** * Sets the current thread's copy of this thread-local variable * to the specified value. Many applications will have no need for * this functionality, relying solely on the {@link #initialValue} * method to set the values of thread-locals. * * @param value the value to be stored in the current threads' copy of * this thread-local. */ public void set(T value) { Thread t = Thread.currentThread(); ThreadLocalMap map = getMap(t); if (map != null) map.set(this, value); else createMap(t, value); }
/** * Get the map associated with a ThreadLocal. Overridden in * InheritableThreadLocal. * * @param t the current thread * @return the map */ ThreadLocalMap getMap(Thread t) { return t.threadLocals; }
/** * Create the map associated with a ThreadLocal. Overridden in * InheritableThreadLocal. * * @param t the current thread * @param firstValue value for the initial entry of the map * @param map the map to store. */ void createMap(Thread t, T firstValue) { t.threadLocals = new ThreadLocalMap(this, firstValue); }
.......
/** * ThreadLocalMap is a customized hash map suitable only for * maintaining thread local values. No operations are exported * outside of the ThreadLocal class. The class is package private to * allow declaration of fields in class Thread. To help deal with * very large and long-lived usages, the hash table entries use * WeakReferences for keys. However, since reference queues are not * used, stale entries are guaranteed to be removed only when * the table starts running out of space. */ static class ThreadLocalMap {
........
}
}
public class Thread implements Runnable { ......
/* ThreadLocal values pertaining to this thread. This map is maintained * by the ThreadLocal class. */ ThreadLocal.ThreadLocalMap threadLocals = null; ...... }