使用ThreadLocal管理Mybatis中SqlSession对象

public class MybatisUtil {

    private static SqlSessionFactory factory;
    // 解决资源争抢问题.
    private static ThreadLocal<SqlSession> localSessions = new ThreadLocal<SqlSession>();

    static {
        Reader reader = null;
        try {
            // 加载Mybatis的配置文件
            reader = Resources.getResourceAsReader("mybatis.xml");
            // 创建SqlSessionFactory对象.
            factory = new SqlSessionFactoryBuilder().build(reader);
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (reader != null) {
                try {
                    reader.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }

    // 获取Session对象
    public static SqlSession getSession() {
        SqlSession session = localSessions.get();
        if (session == null) {
            session = factory.openSession();
            localSessions.set(session);
        }
        return session;
    }

    // 关闭session的方法
    public static void closeSession() {
        SqlSession session = localSessions.get();
        if (session != null) {
            session.close();
            localSessions.remove();
        }
    }
}

     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;  
    }  

     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 {  
      
        ........  
      
        }  
      
    }  

    private final int threadLocalHashCode = nextHashCode();  
    private static int nextHashCode = 0;  
    private static final int HASH_INCREMENT = 0x61c88647;  

private final int threadLocalHashCode = nextHashCode(); 

     private static synchronized int nextHashCode() {  
        int h = nextHashCode;  
        nextHashCode = h + HASH_INCREMENT;  
        return h;  
    }  

ThreadLocalMap map = Thread.currentThread().threadLocals;  

     public class Thread implements Runnable {  
        ......  
      
        /* ThreadLocal values pertaining to this thread. This map is maintained 
         * by the ThreadLocal class. */  
        ThreadLocal.ThreadLocalMap threadLocals = null;    
        ......  
    }  

     if (map != null)  
        map.set(this, value);