package com.example.helloworld;
import java.util.LinkedHashMap;
import java.util.Map;
/**
* Cache保存一个强引用来限制内容数量，每当Item被访问的时候，此Item就会移动到队列的头部。
* 当cache已满的时候加入新的item时，在队列尾部的item会被回收。
*
* 如果你cache的某个值需要明确释放，重写entryRemoved()
*
* 如果key相对应的item丢掉啦，重写create().这简化了调用代码，即使丢失了也总会返回。
*
* 默认cache大小是测量的item的数量，重写sizeof计算不同item的大小。
*
* int cacheSize = 4 * 1024 * 1024; // 4MiB
* LruCache<String, Bitmap> bitmapCache = new LruCache<String, Bitmap>(cacheSize) {
* protected int sizeOf(String key, Bitmap value) {
* return value.getByteCount();
* }
* }
*
* 这个类是安全的，并发操作时有同步块控制
* This class is thread-safe. Perform multiple cache operations atomically by
* synchronizing on the cache:
*
* synchronized (cache) {
* if (cache.get(key) == null) {
* cache.put(key, value);
* }
* }
*
* ·不允许key或者value为null
* 当get（），put（），remove（）返回值为null时，key相应的项不在cache中
*
* 最少最近使用算法
*/
public class LruCache<K, V> {
private final LinkedHashMap<K, V> map; //LruCache 本质上是一个map
/** Size of this cache in units. Not necessarily the number of elements. */
private int size; //已经存储的大小
private int maxSize; //规定的最大存储空间
private int putCount; //put的次数
private int createCount; //create的次数
private int evictionCount; //回收的次数
private int hitCount; //命中的次数
private int missCount; //丢失的次数
/**
* maxSize规定了缓存的最大值，缓存实体的最大值不能超过maxSize
*/
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);
}
/**
* 通过key返回相应的item，或者创建返回相应的item。
* 相应的item会移动到队列的头部，
* 如果item的value没有被cache或者不能被创建，则返回null。
*/
public final V get(K key) {
if (key == null) {
throw new NullPointerException("key == null");
}
V mapValue;
synchronized (this) {
mapValue = map.get(key);
if (mapValue != null) {
hitCount++; //命中
return mapValue;
}
missCount++; //丢失
}
/*
* 根据key没找到item -->就试图创建一个item
* 实际中 create()一般由我们来实现。 LruCache的create()方法默认返回null
*/
V createdValue = create(key);
if (createdValue == null) {
return null;
}
synchronized (this) {
createCount++; //创建++
mapValue = map.put(key, createdValue);
if (mapValue != null) {
// There was a conflict so undo that last put
//如果前面存在oldValue，那么撤销put（）
map.put(key, mapValue);
} else {
size += safeSizeOf(key, createdValue);
}
}
if (mapValue != null) {
entryRemoved(false, key, createdValue, mapValue); //
return mapValue;
} else {
trimToSize(maxSize); //重新缓存区计算大小
return createdValue;
}
}
/**
* 存入元素--> 刚存入的元素在最前面
* 返回值：先前存的值 之前没有值返回null
*/
public final V put(K key, V value) {
if (key == null || value == null) {
throw new NullPointerException("key == null || value == null");
}
V previous;
synchronized (this) {
putCount++;
size += safeSizeOf(key, value);
previous = map.put(key, value); //map-->linkedHashMap
if (previous != null) { //返回的先前的value值
size -= safeSizeOf(key, previous);
}
}
if (previous != null) {
entryRemoved(false, key, previous, value); //当新值替换原来的值时调用的方法 默认实现为空
}
trimToSize(maxSize); //重新缓存区计算大小
return previous;
}
/**
* @param maxSize the maximum size of the cache before returning. May be -1
* to evict even 0-sized elements.
* 清空cache空间
*/
private void trimToSize(int maxSize) {
while (true) {
K key;
V value;
synchronized (this) {
if (size < 0 || (map.isEmpty() && size != 0)) {
throw new IllegalStateException(getClass().getName()
+ ".sizeOf() is reporting inconsistent results!");
}
if (size <= maxSize) {
break;
}
Map.Entry<K, V> toEvict = map.eldest(); //拿到最老的 即最不常用到的
if (toEvict == null) {
break;
}
key = toEvict.getKey();
value = toEvict.getValue();
map.remove(key);
size -= safeSizeOf(key, value);
evictionCount++;
}
entryRemoved(true, key, value, null);
}
}
/**
* Removes the entry for {@code key} if it exists.
* 删除key相应的cache项，返回相应的value
* @return the previous value mapped by {@code key}.
*/
public final V remove(K key) {
if (key == null) {
throw new NullPointerException("key == null");
}
V previous;
synchronized (this) {
previous = map.remove(key);
if (previous != null) {
size -= safeSizeOf(key, previous);
}
}
if (previous != null) {
entryRemoved(false, key, previous, null);
}
return previous;
}
/**
* 当item被回收或者删掉时调用。
* 改方法当value被回收释放存储空间时被remove调用，
* 或者替换item值时put调用，默认实现什么都没做。
*
* 这个方法没有同步 当这个方法执行时 其他线程可以访问缓存
*
* true---为释放空间被删除；
*
* false---put或remove导致
* @param newValue the new value for key, if it exists. If non-null,
* this removal was caused by a put. Otherwise it was caused by
* an eviction or a remove.
*
*/
protected void entryRemoved(boolean evicted, K key, V oldValue, V newValue) {}
/**
* 用户自己实现
* 当某Item丢失时会调用到，返回计算的相应的value或者null
* 这个方法没有同步 当这个方法执行时 其他线程可以访问缓存
*/
protected V create(K key) {
return null;
}
//检查size 是否合理
private int safeSizeOf(K key, V value) {
int result = sizeOf(key, value);
if (result < 0) {
throw new IllegalStateException("Negative size: " + key + "=" + value);
}
return result;
}
/**
* 返回用户定义的item的大小，默认返回1代表item的数量，最大size就是最大item值
* 一般用户自己实现
*/
protected int sizeOf(K key, V value) {
return 1;
}
/**
* Clear the cache, calling entryRemoved on each removed entry.
* 清空cacke
*/
public final void evictAll() {
trimToSize(-1); // -1 will evict 0-sized elements
}
/**
* For caches that do not override {@link #sizeOf}, this returns the number
* of entries in the cache. For all other caches, this returns the sum of
* the sizes of the entries in this cache.
*
* 返回 缓存中集合的数量 final类型 不允许重写
*/
public synchronized final int size() {
return size;
}
/**
* For caches that do not override {@link #sizeOf}, this returns the maximum
* number of entries in the cache. For all other caches, this returns the
* maximum sum of the sizes of the entries in this cache.
*/
public synchronized final int maxSize() {
return maxSize;
}
/**
* Returns the number of times {@link #get} returned a value that was
* already present in the cache.
*/
public synchronized final int hitCount() {
return hitCount;
}
/**
* Returns the number of times {@link #get} returned null or required a new
* value to be created.
*/
public synchronized final int missCount() {
return missCount;
}
/**
* Returns the number of times {@link #create(Object)} returned a value.
*/
public synchronized final int createCount() {
return createCount;
}
/**
* Returns the number of times {@link #put} was called.
*/
public synchronized final int putCount() {
return putCount;
}
/**
* Returns the number of values that have been evicted.
* 返回被回收的数量
*/
public synchronized final int evictionCount() {
return evictionCount;
}
/**
* Returns a copy of the current contents of the cache, ordered from least
* recently accessed to most recently accessed. 返回当前cache的副本，从最近最少访问到最多访问
*/
public synchronized final Map<K, V> snapshot() {
return new LinkedHashMap<K, V>(map);
}
@Override
public synchronized final String toString() {
int accesses = hitCount + missCount;
int hitPercent = accesses != 0 ? (100 * hitCount / accesses) : 0;
return String.format("LruCache[maxSize=%d,hits=%d,misses=%d,hitRate=%d%%]",
maxSize, hitCount, missCount, hitPercent);
}
}
