源码: /frameworks/base/core/java/android/util/LruCache.java
文件开篇注释如下:
A cache that holds strong references to a limited number of values. Each time a value is accessed, it is moved to the head of a queue. When a value is added to a full cache, the value at the end of that queue is evicted and may become eligible for garbage collection.
含义是:
一个只对有限个value保持强引用的Cache.
每次访问一个value, 它都会被移到队头.
当一个Cache满了之后, 如果再向它添加元素, 队尾的元素将会被回收.
<p>If your cached values hold resources that need to be explicitly released, override {@link #entryRemoved}.
如果有一个元素需要被显式的释放, 重写entryRemoved方法
protected void entryRemoved(boolean evicted, K key, V oldValue, V newValue) {}
<p>If a cache miss should be computed on demand for the corresponding keys,override {@link #create}.
This simplifies the calling code, allowing it to assume a value will always be returned, even when there's a cache miss.
对这段理解的一直不是很好, 读完源码以后再说
protected V create(K key) {
return null;
}
<p>This class does not allow null to be used as a key or value.
A return value of null from {@link #get}, {@link #put} or {@link #remove} is unambiguous: the key was not in the cache.
这个类不允许Key 和 Value 是 null. 如果 get, put, remove中出现了null, 那么将会返回 该Cache中不存在这个key
关键代码:
public class LruCache<K, V> {
private final LinkedHashMap<K, V> map;
/** Size of this cache in units. Not necessarily the number of elements. */
private int size;
private int maxSize;
private int putCount;
private int createCount;
private int evictionCount;
private int hitCount;
private int missCount;
/**
* @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<K, V>(0, 0.75f, true);
}
}
使用 LinkedHashMap来管理整个Cache
使用 LruCache 必须重写 sizeOf 方法, sizeOf方法用来计算 value的大小
protected int sizeOf(K key, V value) {
return 1;
}
使用方法
// 继承LruCache时，必须要复写sizeof方法，用于计算每个条目的大小
// cacheMemory 表示可用缓存的大小, value 也不能超出 cacheMemory
mLruCache = new LruCache<String, Bitmap>(cacheMemory) {
@Override
protected int sizeOf(String key, Bitmap value) {
return value.getByteCount();
}
};