LruCache我想對於做Android的人來說肯定是不會陌生的，在各種用到大量加載圖片的地方就能看的這個類的身影,各大圖片框架裏面都有用到LruCache。最近項目一階段的活已經做完了，發現自己好久沒學習了，所以隨便寫點東西使自己學習學習。
LruCache是Android中專門用來做圖片緩存處理的，它有一個特點，當圖片達到了預先設定的值的時候，那麼最近使用次數最少的圖片就會被回收釋放掉。LruCache位於android.support.v4.util這個包裏面，下面我們來簡單分析分析它的源碼：
/**
* Static library version of {@link android.util.LruCache}. Used to write apps
* that run on API levels prior to 12. When running on API level 12 or above,
* this implementation is still used; it does not try to switch to the
* framework's implementation. See the framework SDK documentation for a class
* overview.
*/
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; //已存儲大小
private int maxSize; //規定的最大存儲空間
private int putCount; //put的次數
private int createCount; //create的次數
private int evictionCount; //回收次數
private int hitCount; //命中次數
private int missCount; //丟失次數
/**
* @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. //設置緩存中最大存儲量
*/
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);
}
/**
* Sets the size of the cache. //設置緩存的大小
*
* @param maxSize The new maximum size.
*/
public void resize(int maxSize) {
if (maxSize <= 0) {
throw new IllegalArgumentException("maxSize <= 0");
}
synchronized (this) {
this.maxSize = maxSize;
}
trimToSize(maxSize);
}
/**
* 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.
*/
//通過key返回相應的值，或者創建相應的值。如果該值有被返回則將其移動到隊列的頭部
//如果該值返回爲null,則將不會被創建和緩存起來。
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++;//未命中
}
/*
* 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.
*/
V createdValue = create(key);
if (createdValue == null) {
return null;
}
synchronized (this) {
createCount++;
mapValue = map.put(key, createdValue);
if (mapValue != null) {
// There was a conflict so undo that last 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}.
*/
// 通過key緩存的value,這個value將會被移動到隊列的頭部
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) {//排除之前已經插入了相同的key
size -= safeSizeOf(key, previous);//那麼減去該entry的容量，因爲發生覆蓋
}
}
if (previous != null) {
entryRemoved(false, key, previous, value);//這個方法默認空實現
}
trimToSize(maxSize);//若容量超過maxsize，將會刪除最近很少訪問的entry
return previous;
}
/**
* Remove the eldest entries until the total of remaining entries is at or
* below the requested size.
*
* @param maxSize the maximum size of the cache before returning. May be -1
* to evict even 0-sized elements.
*/
//移除最早或最老的條目（最少使用的），直到當前緩存的大小數量不大於請求的大小（maxSize）
public void trimToSize(int maxSize) {
//不斷循環刪除鏈表首部元素，也就是最近最少訪問的元素，直到容量不超過預先定義的最大值爲止
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 || map.isEmpty()) {
break;
}
Map.Entry<K, V> toEvict = map.entrySet().iterator().next();
key = toEvict.getKey();
value = toEvict.getValue();
map.remove(key);//刪除最少訪問的entry
size -= safeSizeOf(key, value);
evictionCount++;
}
entryRemoved(true, key, value, null);
}
}
/**
* Removes the entry for {@code key} if it exists.
*
* @return the previous value mapped by {@code key}.
*/
//移除緩存中存在key相對應的值
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.
*
* <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}.
* @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}.
*/
// 使用的條目被回收或刪除時調用，該方法當value被回收釋放存儲空間時被remove調用，
// 或者替換item值時被put調用。默認什麼都沒做，evicted:true爲釋放空間；false爲put或remove導致
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.
*
* <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.
*/
//某個key對應的item丟失時，或者同一個key值時(一個LruCache只能存儲一個相同的key)調用
//避免多線程操作同一個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.
*
* <p>An entry's size must not change while it is in the cache.
*/
//返回用戶定義的item的大小，默認返回1代表item的數量，最大size就是最大item值
protected int sizeOf(K key, V value) {
return 1;
}
/**
* Clear the cache, calling {@link #entryRemoved} on each removed entry.
*/
//清空隊列中的緩存
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.
*/
//這裏返回被回收的數量
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.
*/
//返回一個當前的所有緩存的副本條目(從最近最少訪問到最多訪問)
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);
}
}
通過上面簡單的分析，我們基本上可以知道：
1.LruCache封裝了LinkedHashMap（雙向鏈表），提供了LRU（最近最少使用的緩存）功能。
2.LruCache中不允許空鍵值。
3.LruCache中不存儲重複的鍵值。
4.LruCache中各處都加了synchronized，它的線程是安全的。
5.LruCache通過trimToSize方法自動刪除最近最少訪問的鍵值對。
6.繼承LruCache時，必須要複寫sizeof()方法，用於計算每個條目的大小。（注意）
源碼解析就暫時到這，後面再來講訴如何使用LruCache進行圖片緩存…
pass:上面解析有錯誤的地方歡迎指出，麼麼噠！
