Simple read of the LruCache source code in the android. support. v4 package, androidlrucache

Simple read of the LruCache source code in the android. support. v4 package, androidlrucache

Package android. util; import java. util. linkedHashMap; import java. util. map;/*** 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 stores a strong reference to limit the number of content. When an Item is accessed, the Item will be moved The header of the queue. * When a new item is added when the cache is full, items at the end of the queue will be recycled. * <P> If your cached values hold resources that need to be explicitly released, * override {@ link # entryRemoved }. * If a value of your cache needs to be explicitly released, rewrite entryRemoved () * <p> If a cache miss shocould 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. * If the item corresponding to the key is lost Write create (). This simplifies the call code, and will always return even if it is lost. * <P> 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: the default cache size is the number of measured items. Override sizeof to calculate the * size of different items. * <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> ** <p> 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> ** <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. * key or value is not allowed to be null * When get (), put (), remove () returns null, the corresponding key item is not in the cache */public class LruCache <K, V> {private final LinkedHashMap <K, V> map;/** Si Ze of this cache in units. not necessarily the number of elements. */private int size; // the size of the private int maxSize that has been stored; // the specified maximum storage space private int putCount; // The number of put Times private int createCount; // create Times private int evictionCount; // times of recovery private int hitCount; // times of hit private int missCount; // number of lost times/*** @ param maxSize for caches that do not override {@ link # sizeOf}, this is * the maximum number of ent Ries 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. return the corresponding item through the key, or create and return the corresponding item. The corresponding item is moved to the queue header. * If the item value is not cached or cannot be created, null is returned. */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 ++; // return mapValue hit;} missCount ++; // lost}/** 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. * if it is lost, try to create an item */V createdValue = create (key); if (createdValue = null) {retu Rn null;} synchronized (this) {createCount ++; // create ++ mapValue = map. put (key, createdValue); if (mapValue! = Null) {// There was a conflict so undo that last put // if There is oldValue in front, cancel 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 | v Alue = null ");} V previous; synchronized (this) {putCount ++; size + = safeSizeOf (key, value); previous = map. put (key, value); if (previous! = Null) {// The returned previous value 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. * clear cache space */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 (); 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. * Delete the cache entry corresponding to the key and return the corresponding value * @ return the previous value mapped by {@ code k Ey }. */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. * called when items are recycled or deleted. This method is used when the value is recycled and the bucket is removed and called * or put when the item value is replaced. By default, nothing is done. * <P> 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 --- indicates that the space is deleted; false --- put or remove causes * @ param newValue the new value for {@ code key}, if it exists. if non-null, * this removal was cause D 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. * when an Item is lost, it is called, and The corresponding value or null calculated is returned * <p> The me Thod is called without synchronization: other threads may * access the cache while this method is executing. ** <p> 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 CILS {@ 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 u Nits. the default implementation returns 1 so that size * is the number of entries and max size is the maximum number of entries. * returns the size of a user-defined item. By default, 1 indicates the number of items, the maximum size is the maximum item value * <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. * clear cacke */public final voi D 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 {@ lin K # 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. * return the number of recycled items */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. returns the copy of the current cache, from the least recent access to the maximum access */public synchronized final Map <K, V> snapshot () {return new LinkedHashMap <K, v> (map) ;}@ Override pu Blic 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 );}}