LruCache的用法及分析

素颜马尾好姑娘i 2022-06-04 08:16 173阅读 0赞

package com.jack.test; 
 import java.util.LinkedHashMap; 
 import java.util.Map; 
 import java.util.Map.Entry;
 /** * 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. * 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。 * If your cached values hold resources that need to be explicitly released, * override { @link #entryRemoved}. * 如果你cache的某个值需要明确释放，重写entryRemoved() * 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. * 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。 * By default, the cache size is measured in the number of entries. Override * { @link #sizeOf} to size the cache in different units. For example, this cache * is limited to 4MiB of bitmaps: 默认cache大小是测量的item的数量，重写sizeof计算不同item的 * 大小。 * <pre> { @code * 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(); * } * }}</pre> * * This class is thread-safe. Perform multiple cache operations atomically by * synchronizing on the cache: <pre> { @code * synchronized (cache) { * if (cache.get(key) == null) { * cache.put(key, value); * } * }}</pre> * * 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. * 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 */ 
 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; //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
 
 /** * @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); 
 } 
 
 /** * 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. 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
 } 
 
 /* * Attempt to create a value. This may take a long time, and the map * may be different when create() returns. If a conflicting value was * added to the map while create() was working, we leave that value in * the map and release the created value. * 如果丢失了就试图创建一个item */ 
 
 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); 
 return createdValue; 
 } 
 } 
 
 /** * 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) { 
 throw new NullPointerException("key == null || value == null"); 
 } 
 
 V previous; 
 synchronized (this) { 
 putCount++; 
 size += safeSizeOf(key, value); 
 previous = map.put(key, value); 
 if (previous != null) { //返回的先前的value值
 size -= safeSizeOf(key, previous); 
 } 
 } 
 
 if (previous != null) { 
 entryRemoved(false, key, previous, value); 
 } 
 
 trimToSize(maxSize); 
 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 = (Entry<K, V>) map.entrySet(); 
 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; 
 } 
 
 /** * Called for entries that have been evicted or removed. This method is * invoked when a value is evicted to make space, removed by a call to * { @link #remove}, or replaced by a call to { @link #put}. The default * implementation does nothing. * 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。 * The method is called without synchronization: other threads may * access the cache while this method is executing. * * @param evicted true if the entry is being removed to make space, false * if the removal was caused by a { @link #put} or { @link #remove}. * true---为释放空间被删除；false---put或remove导致 * @param newValue the new value for { @code key}, if it exists. If non-null, * this removal was caused by a { @link #put}. Otherwise it was caused by * an eviction or a { @link #remove}. */ 
 protected void entryRemoved(boolean evicted, K key, V oldValue, V newValue) {} 
 
 /** * Called after a cache miss to compute a value for the corresponding key. * Returns the computed value or null if no value can be computed. The * default implementation returns null. * 当某Item丢失时会调用到，返回计算的相应的value或者null * The method is called without synchronization: other threads may * access the cache while this method is executing. * * If a value for { @code key} exists in the cache when this method * returns, the created value will be released with { @link #entryRemoved} * and discarded. This can occur when multiple threads request the same key * at the same time (causing multiple values to be created), or when one * thread calls { @link #put} while another is creating a value for the same * key. */ 
 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. * 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 * 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. * 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. */ 
 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); 
 } 
 }