Android缓存机制——LruCache的详解

public LinkedHashMap(int initialCapacity,
 float loadFactor,
 boolean accessOrder) {
 super(initialCapacity, loadFactor);
 this.accessOrder = accessOrder;
}

LinkedHashMap  map = new LinkedHashMap <> (0, 0.75f, true);
map.put(0, 0);
map.put(1, 1);
map.put(2, 2);
map.put(3, 3);
map.get(1);
map.get(2);

for (Map.Entry  entry: map.entrySet()) {
 System.out.println(entry.getKey() + ":" + entry.getValue());

}

/**
  * @param maxSize for caches that do not override {@link #sizeOf}, this is
  *  the maximum number of entries in the cache. For all other caches,
  *  this is the maximum sum of the sizes of the entries in this cache.
  */
 public LruCache(int maxSize) {
  if (maxSize <= 0) {
   throw new IllegalArgumentException("maxSize <= 0");
  }
  this.maxSize = maxSize;
  this.map = new LinkedHashMap(0, 0.75f, true);//accessOrder被设置为true
 }

/**
  * Caches {@code value} for {@code key}. The value is moved to the head of
  * the queue.
  *
  * @return the previous value mapped by {@code key}.
  */
 public final V put(K key, V value) {
  if (key == null || value == null) {//判空，不可为空
   throw new NullPointerException("key == null || value == null");
  }

  V previous;
  synchronized (this) {
   putCount++;//插入缓存对象加1
   size += safeSizeOf(key, value);//增加已有缓存的大小
   previous = map.put(key, value);//向map中加入缓存对象
   if (previous != null) {//如果已有缓存对象，则缓存大小恢复到之前
    size -= safeSizeOf(key, previous);
   }
  }

  if (previous != null) {//entryRemoved()是个空方法，可以自行实现
   entryRemoved(false, key, previous, value);
  }

  trimToSize(maxSize);//调整缓存大小(关键方法)
  return previous;
 }

/**
  * Remove the eldest entries until the total of remaining entries is at or
  * below the requested size.
  *
  * @param maxSize the maximum size of the cache before returning. May be -1
  *   to evict even 0-sized elements.
  */
 public void trimToSize(int maxSize) {
  while (true) {//死循环
   K key;
   V value;
   synchronized (this) {
　　　　　　　　 //如果map为空并且缓存size不等于0或者缓存size小于0，抛出异常
    if (size <0 || (map.isEmpty() && size != 0)) {
     throw new IllegalStateException(getClass().getName()
       + ".sizeOf() is reporting inconsistent results!");
    }
　　　　　　　　　　//如果缓存大小size小于最大缓存，或者map为空，不需要再删除缓存对象，跳出循环
    if (size <= maxSize) {
     break;
    }
　　　　　　　　　　// 取出 map 中最老的映射
    Map.Entry toEvict = map.eldest();
    if (toEvict == null) {
     break;
    }

    key = toEvict.getKey();
    value = toEvict.getValue();
    map.remove(key);
    size -= safeSizeOf(key, value);
    evictionCount++;
   }

   entryRemoved(true, key, value, null);
  }
 }

/**
  * Returns the value for {@code key} if it exists in the cache or can be
  * created by {@code #create}. If a value was returned, it is moved to the
  * head of the queue. This returns null if a value is not cached and cannot
  * be created.
  */
 public final V get(K key) {
  if (key == null) {//key不能为空
   throw new NullPointerException("key == null");
  }

  V mapValue;
  synchronized (this) {
　　　　　　　　/获取对应的缓存对象 
   mapValue = map.get(key);
   if (mapValue != null) {
    hitCount++;
    return mapValue;
   }
   missCount++;
  }

public V get(Object key) {
  LinkedHashMapEntry e = (LinkedHashMapEntry)getEntry(key);
  if (e == null)
   return null;
  e.recordAccess(this);//实现排序的关键
  return e.value;
 }

/**
   * This method is invoked by the superclass whenever the value
   * of a pre-existing entry is read by Map.get or modified by Map.set.
   * If the enclosing Map is access-ordered, it moves the entry
   * to the end of the list; otherwise, it does nothing.
   */
  void recordAccess(HashMap m) {
   LinkedHashMap lm = (LinkedHashMap)m;
   if (lm.accessOrder) {//判断是否是访问顺序
    lm.modCount++;
    remove();//删除此元素
    addBefore(lm.header);//将此元素移到队尾
   }
  }