线程封闭

public class ConnectionManager {
    private static ThreadLocal connectionHolder = new ThreadLocal() {
        public Connection initialValue() {
            Connection conn = null;
            try {
                conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/test", "username", "password");
            } catch (SQLException e) {
                e.printStackTrace();
            }
            return conn;
        }
    };

    public static Connection getConnection() {
        return connectionHolder.get();
    }

    public static void setConnection(Connection conn) {
        connectionHolder.set(conn);
    }
}

 public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
}

ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
}

void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
}

public
class Thread implements Runnable {
           ......
    ThreadLocal.ThreadLocalMap threadLocals = null;
 .         ......
}

 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> {
            /** The value associated with this ThreadLocal. */
            Object value;

            Entry(ThreadLocal> k, Object v) {
                super(k);
                value = v;
            }
        }
}

 ThreadLocalMap(ThreadLocal> firstKey, Object firstValue) {
            table = new Entry[INITIAL_CAPACITY];
            int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
            table[i] = new Entry(firstKey, firstValue);
            size = 1;
            setThreshold(INITIAL_CAPACITY);
}

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;
            //获取要插入的位置
            int i = key.threadLocalHashCode & (len-1);
            //遍历数组查找ThreadLocal实例相同的key,如果找到，则就会保存这个value
            for (Entry e = tab[i];
                 e != null;
                 e = tab[i = nextIndex(i, len)]) {
                ThreadLocal> k = e.get();

                if (k == key) {
                    e.value = value;
                    return;
                }

                if (k == null) {
                   //会清理其中的没有key的enter
                    replaceStaleEntry(key, value, i);
                    return;
                }
            }

            tab[i] = new Entry(key, value);
            int sz = ++size;
            //如果清理了其中没有key的enter，并且数据的长度大于负载因子，会进行rehash
            if (!cleanSomeSlots(i, sz) && sz >= threshold)
                rehash();
}