首先，大致要考虑线程安全，锁（重量级锁Synchronized或读写锁），LRU
具体可以用HashMap、ConcurrentHashMap
HashMap线程不安全，见源码注释：
(The <tt>HashMap</tt>
* class is roughly equivalent to <tt>Hashtable</tt>, except that it is
* unsynchronized and permits nulls.) ConcurrentHashMap线程安全，见源码截图：
/**
* A hash table supporting full concurrency of retrievals and
* adjustable expected concurrency for updates. This class obeys the
* same functional specification as {@link java.util.Hashtable}, and
* includes versions of methods corresponding to each method of
* <tt>Hashtable</tt>. However, even though all operations are
* thread-safe, retrieval operations do <em>not</em> entail locking,
* and there is <em>not</em> any support for locking the entire table
* in a way that prevents all access. This class is fully
* interoperable with <tt>Hashtable</tt> in programs that rely on its
* thread safety but not on its synchronization details.ReentrantReadWriteLock，读写锁
* ReentrantReadWriteLocks can be used to improve concurrency in some
* uses of some kinds of Collections. This is typically worthwhile
* only when the collections are expected to be large, accessed by
* more reader threads than writer threads, and entail operations with
* overhead that outweighs synchronization overhead. For example, here
* is a class using a TreeMap that is expected to be large and
* concurrently accessed.
*
* <pre>{@code
* class RWDictionary {
* private final Map<String, Data> m = new TreeMap<String, Data>();
* 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 String[] allKeys() {
* r.lock();
* try { return m.keySet().toArray(); }
* 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(); }
* }
* }}</pre>
