public class HibernateUtil {
    private static Log log = LogFactory.getLog(HibernateUtil.class);
    private static final SessionFactory sessionFactory;     //定义SessionFactory
    static {
        try {
            // 通过默认配置文件hibernate.cfg.xml创建SessionFactory
            sessionFactory = new Configuration().configure().buildSessionFactory();
        } catch (Throwable ex) {
            log.error("初始化SessionFactory失败！", ex);
            throw new ExceptionInInitializerError(ex);
        }
    }
    //创建线程局部变量session，用来保存Hibernate的Session
    public static final ThreadLocal session = new ThreadLocal();
    /**
     * 获取当前线程中的Session
     * @return Session
     * @throws HibernateException
     */
    public static Session currentSession() throws HibernateException {
        Session s = (Session) session.get();
        // 如果Session还没有打开，则新开一个Session
        if (s == null) {
            s = sessionFactory.openSession();
            session.set(s);         //将新开的Session保存到线程局部变量中
        }
        return s;
    }
    public static void closeSession() throws HibernateException {
        //获取线程局部变量，并强制转换为Session类型
        Session s = (Session) session.get();
        session.set(null);
        if (s != null)
            s.close();
    }
}

public T get() {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null)
                return (T)e.value;
        }
        return setInitialValue();
    }

private T setInitialValue() {
        T value = initialValue();
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
        return value;
}
ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
}
void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }

/* ThreadLocal values pertaining to this thread. This map is maintained
     * by the ThreadLocal class. */
    ThreadLocal.ThreadLocalMap threadLocals = null;

/**
     * 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 {

/**
         * The entries in this hash map extend WeakReference, using
         * its main ref field as the key (which is always a
         * ThreadLocal object).  Note that null keys (i.e. entry.get()
         * == null) mean that the key is no longer referenced, so the
         * entry can be expunged from table.  Such entries are referred to
         * as "stale entries" in the code that follows.
         */
        static class Entry extends WeakReference<ThreadLocal> {
            /** The value associated with this ThreadLocal. */
            Object value;
            Entry(ThreadLocal k, Object v) {
                super(k);
                value = v;
            }
        }
/**
         * Construct a new map initially containing (firstKey, firstValue).
         * ThreadLocalMaps are constructed lazily, so we only create
         * one when we have at least one entry to put in it.
         */
        ThreadLocalMap(ThreadLocal firstKey, Object firstValue) {
            //默认16长度
            table = new Entry[INITIAL_CAPACITY];
            //hashcode取模，得到坑位
            int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
            table[i] = new Entry(firstKey, firstValue);
            size = 1;
            setThreshold(INITIAL_CAPACITY);
        }
        /**
         * Set the resize threshold to maintain at worst a 2/3 load factor.
         */
        private void setThreshold(int len) {
            threshold = len * 2 / 3;
        }
        private void set(ThreadLocal key, Object value) {
            // We don't use a fast path as with get() because it is at
            // least as common to use set() to create new entries as
            // it is to replace existing ones, in which case, a fast
            // path would fail more often than not.
            Entry[] tab = table;
            int len = tab.length;
            int i = key.threadLocalHashCode & (len - 1);
            for (Entry e = tab[i]; e != null; e = tab[i = nextIndex(i, len)]) {
                ThreadLocal k = e.get();
                if (k == key) {
                    e.value = value;
                    return;
                }
                //key在gc时，会被回收，变成null
                if (k == null) {
                    replaceStaleEntry(key, value, i);
                    return;
                }
            }
            tab[i] = new Entry(key, value);
            int sz = ++size;
            if (!cleanSomeSlots(i, sz) && sz >= threshold)
                rehash();
        }
private void replaceStaleEntry(ThreadLocal key, Object value, int staleSlot) {
            Entry[] tab = table;
            int len = tab.length;
            Entry e;
            // Back up to check for prior stale entry in current run.
            // We clean out whole runs at a time to avoid continual
            // incremental rehashing due to garbage collector freeing
            // up refs in bunches (i.e., whenever the collector runs).
            int slotToExpunge = staleSlot;
                        //替换掉之前有key=null的
            for (int i = prevIndex(staleSlot, len); (e = tab[i]) != null; i = prevIndex(i, len))
                if (e.get() == null)
                    slotToExpunge = i;
            // Find either the key or trailing null slot of run, whichever
            // occurs first
            for (int i = nextIndex(staleSlot, len); (e = tab[i]) != null; i = nextIndex(i, len)) {
                ThreadLocal k = e.get();
                // If we find key, then we need to swap it
                // with the stale entry to maintain hash table order.
                // The newly stale slot, or any other stale slot
                // encountered above it, can then be sent to expungeStaleEntry
                // to remove or rehash all of the other entries in run.
                if (k == key) {
                    e.value = value;
                    tab[i] = tab[staleSlot];
                    tab[staleSlot] = e;
                    // Start expunge at preceding stale entry if it exists
                    if (slotToExpunge == staleSlot)
                        slotToExpunge = i;
                    cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
                    return;
                }
                // If we didn't find stale entry on backward scan, the
                // first stale entry seen while scanning for key is the
                // first still present in the run.
                if (k == null && slotToExpunge == staleSlot)
                    slotToExpunge = i;
            }
            // If key not found, put new entry in stale slot
            tab[staleSlot].value = null;
            tab[staleSlot] = new Entry(key, value);
            // If there are any other stale entries in run, expunge them
            if (slotToExpunge != staleSlot)
                cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
        }