public class LruCache<K, V> {
private final LinkedHashMap<K, V> map;
//缓存的当前容量
private int size;
//缓存的最大容量
private int maxSize;
//插入操作次数
private int putCount;
//未命中-创建数
private int createCount;
//容量满了之后，移除操作的次数
private int evictionCount;
//命中数
private int hitCount;
//没有命中数
private int missCount;
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);
}
//调整map的容量
public void resize(int maxSize) {
if (maxSize <= 0) {
throw new IllegalArgumentException("maxSize <= 0");
}
synchronized (this) {
this.maxSize = maxSize;
}
trimToSize(maxSize);
}
public final V get(K key) {
//key为空抛出异常
if (key == null) {
throw new NullPointerException("key == null");
}
V mapValue;
synchronized (this) {
//获取对应的缓存对象
//get()方法会实现将访问的元素更新到队列头部的功能
mapValue = map.get(key);
if (mapValue != null) {
//命中数加一
hitCount++;
return mapValue;
}
//没有命中数加一
missCount++;
}
//如果没有命中，是否要自定义创建
V createdValue = create(key);
if (createdValue == null) {
return null;
}
synchronized (this) {
//未命中-创建数
createCount++;
mapValue = map.put(key, createdValue);
if (mapValue != null) {
//创建的时候，如果另一个元素刚加入进去，则不使用创建的，使用之前添加的
//后面不再计算大小，因为正常添加的流程已经计算过大小了，
map.put(key, mapValue);
} else {
//使用创建的之后计算容量大小
size += safeSizeOf(key, createdValue);
}
}
//如果之前刚添加，则回调，自定义生成的对象，正常添加的对象
if (mapValue != null) {
entryRemoved(false, key, createdValue, mapValue);
return mapValue;
} else {
//使用自定义生成的对象，需要校验map的容量
trimToSize(maxSize);
return createdValue;
}
}
//
public final V put(K key, V value) {
//不可为空，否则抛出异常
if (key == null || value == null) {
throw new NullPointerException("key == null || value == null");
}
V previous;
//同步添加缓存
synchronized (this) {
//插入的缓存对象值加1
putCount++;
//增加已有缓存的大小
size += safeSizeOf(key, value);
//向map中加入缓存对象
previous = map.put(key, value);
//如果已有缓存对象，则缓存大小恢复到之前
if (previous != null) {
size -= safeSizeOf(key, previous);
}
}
//entryRemoved()是个空方法，可以自行实现
if (previous != null) {
entryRemoved(false, key, previous, value);
}
trimToSize(maxSize);
return previous;
}
public void trimToSize(int maxSize) {
////死循环
while (true) {
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，不需要再删除缓存对象，跳出循环
if (size <= maxSize) {
break;
}
//迭代器获取第一个对象，即队尾的元素，近期最少访问的元素
Map.Entry<K, V> toEvict = map.eldest();
if (toEvict == null) {
break;
}
key = toEvict.getKey();
value = toEvict.getValue();
//删除该对象，并更新缓存大小
map.remove(key);
size -= safeSizeOf(key, value);
//移除数加一
evictionCount++;
}
entryRemoved(true, key, value, null);
//移除之后循环操作，直到实际容量小于最大容量
}
}
public final V remove(K key) {
if (key == null) {
throw new NullPointerException("key == null");
}
V previous;
synchronized (this) {
//移除key对应的元素
previous = map.remove(key);
//如果该元素有值，则重新计算大小
if (previous != null) {
size -= safeSizeOf(key, previous);
}
}
if (previous != null) {
entryRemoved(false, key, previous, null);
}
return previous;
}
//每次remove都会调用
protected void entryRemoved(boolean evicted, K key, V oldValue, V newValue) {}
//数据未命中，自定义创建V
protected V create(K key) {
return null;
}
private int safeSizeOf(K key, V value) {
int result = sizeOf(key, value);
if (result < 0) {
throw new IllegalStateException("Negative size: " + key + "=" + value);
}
return result;
}
/**
* Returns the size of the entry for {@code key} and {@code value} in
* user-defined units. The default implementation returns 1 so that size
* is the number of entries and max size is the maximum number of entries.
*
* <p>An entry's size must not change while it is in the cache.
*/
protected int sizeOf(K key, V value) {
return 1;
}
/**
* Clear the cache, calling {@link #entryRemoved} on each removed entry.
*/
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.
*/
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.
*/
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.
*/
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);
}
}
