Android提供的LruCache类简介

分类： Android开发 2013-02-06 15:26 26733人阅读评论(10) 收藏举报

package android.util;
import import /**
* 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。
* 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
* <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.
*/ publicclass privatefinal
privateint size; //已经存储的大小
privateint maxSize; //规定的最大存储空间
privateint putCount; //put的次数
privateint createCount; //create的次数
privateint evictionCount; //回收的次数
privateint hitCount; //命中的次数
privateint 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.
*/ publicint if) {
thrownew this thisnew, .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，
*/ publicfinal ifnull thrownew synchronizedthis ifnull return * 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
*/ ifnull returnnull synchronizedthis ifnull
//如果前面存在oldValue，那么撤销put（）
else ifnull false return else return * Caches {@code value} for {@code key}. The value is moved to the head of
* the queue.
*
* @return the previous value mapped by {@code key}.
*/ publicfinal ifnullnull thrownew synchronizedthis ifnull) { //返回的先前的value值
ifnull false return * @param maxSize the maximum size of the cache before returning. May be -1
* to evict even 0-sized elements.
*/ privatevoidint whiletrue synchronizedthis if || (map.isEmpty() && size != )) {
thrownew if break ifnull break truenull * 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}.
*/ publicfinal ifnull thrownew synchronizedthis ifnull ifnull falsenull return * 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，
* 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}.
* @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}.
*/ protectedvoidboolean * 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 returnnull privateint int if) {
thrownew + value);
return * 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.
*/ protectedint return;
* Clear the cache, calling {@link #entryRemoved} on each removed entry.
*/ publicfinalvoid );
* 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.
*/ publicsynchronizedfinalint return * 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.
*/ publicsynchronizedfinalint return * Returns the number of times {@link #get} returned a value that was
* already present in the cache.
*/ publicsynchronizedfinalint return * Returns the number of times {@link #get} returned null or required a new
* value to be created.
*/ publicsynchronizedfinalint return * Returns the number of times {@link #create(Object)} returned a value.
*/ publicsynchronizedfinalint return * Returns the number of times {@link #put} was called.
*/ publicsynchronizedfinalint return * Returns the number of values that have been evicted.
*/ publicsynchronizedfinalint return * 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
*/ publicsynchronizedfinal returnnew publicsynchronizedfinal int int ? ( * hitCount / accesses) : ;
return,
}

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Android提供的LruCache类简介

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