Guava Cache之MapMaker

MapMaker: 超级强大的 Map 构造工具
所处的包 package com.google.common.collect
MapMaker 是用来构造 ConcurrentMap 的工具类。为什么可以把 MapMaker 叫做超级强大？看了下面的例子你就知道了。首先，它可以用来构造ConcurrentHashMap:得到线程安全的hashMap

ConcurrentMap<String, Object> map1 = new MapMaker() .concurrencyLevel(8) .makeMap();

对应的源码为

 @Override
  public <K, V> ConcurrentMap<K, V> makeMap() {
    if (!useCustomMap) {
      return new ConcurrentHashMap<K, V>(getInitialCapacity(), 0.75f, getConcurrencyLevel());
    }
    return (nullRemovalCause == null)
        ? new MapMakerInternalMap<K, V>(this)
        : new NullConcurrentMap<K, V>(this);
  }

构造用各种不同 reference 作为 key 和 value 的 Map:

ConcurrentMap<String, Object> map2 = new MapMaker() 
        .weakValues() 
        .makeMap();

构造有自动移除时间过期项的 Map:存放30秒后移除，可以用于存放请求的场景

ConcurrentMap<String, Object> map3 = new MapMaker() 
        .expireAfterWrite(30, TimeUnit.SECONDS) 
        .makeMap();

构造有最大限制数目的 Map：限制存放数据

//Map size grows close to the 100, the map will evict 
//entries that are less likely to be used again 
 ConcurrentMap<String, Object> map4 = new MapMaker() 
            .maximumSize(100) 
            .makeMap();

提供当 Map 里面 get 不到数据，它能够自动加入到 Map 的功能。这个功能当 Map 作为缓存的时候很有用 :

ConcurrentMap<String, Object> map5 = new MapMaker() 
        .makeComputingMap( 
          new Function<String, Object>() { 
            public Object apply(String key) { 
              return createObject(key); 
        }
private Object createObject(String key) {
// TODO Auto-generated method stub
      return "";
}});

提供拥有以上所有特性的 Map:

  ConcurrentMap<String, Object> mapAll = new MapMaker() 
        .concurrencyLevel(8) 
        .weakValues() 
        .expireAfterWrite(30, TimeUnit.SECONDS) 
        .maximumSize(100) 
        .makeComputingMap( 
          new Function<String, Object>() { 
            public Object apply(String key) { 
              return createObject(key); 
         }

            private Object createObject(String key) {
                // TODO Auto-generated method stub
                return null;
            }});