ReentrantReadWriteLock

读写锁的例子，半个ConcurrentHashMap

class RWDictionary {
   private final Map<String, Data> m = new TreeMap<>();
   private final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();
   private final Lock r = rwl.readLock();
   private final Lock w = rwl.writeLock();

   public Data get(String key) {
     r.lock();
     try { return m.get(key); }
     finally { r.unlock(); }
   }
   public List<String> allKeys() {
     r.lock();
     try { return new ArrayList<>(m.keySet()); }
     finally { r.unlock(); }
   }
   public Data put(String key, Data value) {
     w.lock();
     try { return m.put(key, value); }
     finally { w.unlock(); }
   }
   public void clear() {
     w.lock();
     try { m.clear(); }
     finally { w.unlock(); }
   }
 }