Java資本緩存 之 LruCache。本站提示廣大學習愛好者：（Java資本緩存 之 LruCache）文章只能為提供參考，不一定能成為您想要的結果。以下是Java資本緩存 之 LruCache正文
例如對 收集加載圖片停止緩存 ：
// 獲得 運用法式 被分派的最年夜的內存
int maxMemory=(int) Runtime.getRuntime().maxMemory();
// 取處內存的 1/5 用來當 緩存 年夜小
int cachSize=maxMemory/5;
// 實例化 LruCache
lruCache=new lruCache<String, Bitmap>(cachSize){
//外部辦法sizeOf設置每張圖片的緩存年夜小
protected int sizeOf(String key, Bitmap value) {
//在每次存入緩存時挪用，告知體系這張緩存圖片有多年夜
// 相當於 為每次 要緩存的 資本 分派 年夜小空間
return value.getByteCount();
}
};
下面的 代碼 普通 放在初始化的 辦法 外面
其實 可以將 LurCache 類 懂得 為 Map 類 map 有 put和 get 辦法
接下去就挪用put 和 get 辦法 停止須要緩存資本的存取
LurCache 的 add ：
public void putBitmapToCache(String url,Bitmap bitmap){
if (getBitmapfromCache(url)==null) {//斷定以後緩存能否存在
lruCache.put(url, bitmap);
}
}
LurCache 的 get：
public Bitmap getBitmapfromCache(String url){
return lruCache.get(url);//可將lruCache算作map
}
挪用下面的 add 和 get 辦法 便可以對資本停止緩存的 ，照樣挺簡略的，
但要留意一點 LruCache lruCache=new LruCache<String, Bitmap>(cachSize) 只能被new 一次 ，否則分歧對象就分歧的緩存了
附上Android的Lrucache類
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保留一個強援用來限制內容數目，每當Item被拜訪的時刻，此Item就會挪動到隊列的頭部。
* 當cache已滿的時刻參加新的item時，在隊列尾部的item會被收受接管。
* <p>If your cached values hold resources that need to be explicitly released,
* override {@link #entryRemoved}.
* 假如你cache的某個值須要明白釋放，重寫entryRemoved()
* <p>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.
* 假如key絕對應的item丟失落啦，重寫create().這簡化了挪用代碼，即便喪失了也總會前往。
* <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: 默許cache年夜小是丈量的item的數目，重寫sizeof盤算分歧item的
* 年夜小。
* <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>
*
* <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.
* 不許可key或許value為null
* 當get（），put（），remove（）前往值為null時，key響應的項不在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; //曾經存儲的年夜小
private int maxSize; //劃定的最年夜存儲空間
private int putCount; //put的次數
private int createCount; //create的次數
private int evictionCount; //收受接管的次數
private int hitCount; //射中的次數
private int missCount; //喪失的次數
/**
* @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. 經由過程key前往響應的item，或許創立前往響應的item。響應的item會挪動到隊列的頭部，
* 假如item的value沒有被cache或許不克不及被創立，則前往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++; //射中
return mapValue;
}
missCount++; //喪失
}
/*
* 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.
* 假如喪失了就試圖創立一個item
*/
V createdValue = create(key);
if (createdValue == null) {
return null;
}
synchronized (this) {
createCount++;//創立++
mapValue = map.put(key, createdValue);
if (mapValue != null) {
// There was a conflict so undo that last put
//假如後面存在oldValue，那末撤消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) { //前往的先前的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空間
*/
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.
* 刪除key響應的cache項，前往響應的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.
* 當item被收受接管或許刪失落時挪用。改辦法當value被收受接管釋放存儲空間時被remove挪用，
* 或許調換item值時put挪用，默許完成甚麼都沒做。
* <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---為釋放空間被刪除；false---put或remove招致
* @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.
* 當某Item喪失時會挪用到，前往盤算的響應的value或許null
* <p>The method 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 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.
* 前往用戶界說的item的年夜小，默許前往1代表item的數目，最年夜size就是最年夜item值
* <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.
* 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.
* 前往被收受接管的數目
*/
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. 前往以後cache的正本，從比來起碼拜訪到最多拜訪
*/
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);
}
}
