linkedhashmap中关于LRU算法的实现
//LinkedHashMap的一个构造函数,当参数accessOrder为true时,即会按照访问顺序排序,最近访问的放在最前,最早访问的放在后面 public LinkedHashMap(int initialCapacity, float loadFactor, boolean accessOrder) { super(initialCapacity, loadFactor); this.accessOrder = accessOrder; }
LRU是Least Recently Used 的缩写,翻译过来就是“最近最少使用”,LRU缓存就是使用这种原理实现,简单的说就是缓存一定量的数据,当超过设定的阈值时就把一些过期的数据删除掉,比如我们缓存10000条数据,当数据小于10000时可以随意添加,当超过10000时就需要把新的数据添加进来,同时要把过期数据删除,以确保我们最大缓存10000条,那怎么确定删除哪条过期数据呢,采用LRU算法实现的话就是将最老的数据删掉,Java版的LRU缓存实现就数linkedHashmap了
LinkedHashMap<Integer, String> linkedHashMap = new LinkedHashMap<>(new CountingMapData(9)); print(linkedHashMap); print("====="); linkedHashMap = new LinkedHashMap<>(16, 0.75f, true); linkedHashMap.putAll(new CountingMapData(9)); print(linkedHashMap); System.out.println("====="); for (int i = 0; i < 6; i++) { linkedHashMap.get(i); } print("--"+linkedHashMap); linkedHashMap.get(0); print("--"+linkedHashMap);
public class CountingMapData extends AbstractMap<Integer,String> { private int size; private static String[] chars = "A B C D E F G H I J K L M N O P Q R S T U V W X Y Z" .split(" "); public CountingMapData(int size) { if(size < 0) this.size = 0; this.size = size; } private static class Entry implements Map.Entry<Integer,String> { int index; Entry(int index) { this.index = index; } public boolean equals(Object o) { return Integer.valueOf(index).equals(o); } public Integer getKey() { return index; } public String getValue() { return chars[index % chars.length] + Integer.toString(index / chars.length); } public String setValue(String value) { throw new UnsupportedOperationException(); } public int hashCode() { return Integer.valueOf(index).hashCode(); } } public Set<Map.Entry<Integer,String>> entrySet() { // LinkedHashSet retains initialization order: Set<Map.Entry<Integer,String>> entries = new LinkedHashSet<Map.Entry<Integer,String>>(); for(int i = 0; i < size; i++) entries.add(new Entry(i)); return entries; } public static void main(String[] args) { System.out.println(new CountingMapData(60)); } }
输出:
{0=A0, 1=B0, 2=C0, 3=D0, 4=E0, 5=F0, 6=G0, 7=H0, 8=I0} ===== {0=A0, 1=B0, 2=C0, 3=D0, 4=E0, 5=F0, 6=G0, 7=H0, 8=I0} ===== --{6=G0, 7=H0, 8=I0, 0=A0, 1=B0, 2=C0, 3=D0, 4=E0, 5=F0} --{6=G0, 7=H0, 8=I0, 1=B0, 2=C0, 3=D0, 4=E0, 5=F0, 0=A0}