LruMemoryCache和LruCache

#LruMemoryCache和LruCache

在UniversalImageLoader中緩存在內存中的數據就使用了LruCache類，叫LruMemoryCache，存在內存中的圖片就是放在該類中了，想一想早年咱們用軟引用的方式保存，也是弱爆了...

Lru

Lru（Least recently used，最近最少使用）是一個緩存淘汰算法的簡寫，若是數據長時間沒有被訪問，那麼緩存數據容量滿的的時候，將會淘汰最少使用的數據。 咱們直接看LruMemoryCache的代碼。

/**
 * A cache that holds strong references to a limited number of Bitmaps. Each time a Bitmap is accessed, it is moved to
 * the head of a queue. When a Bitmap is added to a full cache, the Bitmap at the end of that queue is evicted and may
 * become eligible for garbage collection.<br />
 * <br />
 * <b>NOTE:</b> This cache uses only strong references for stored Bitmaps.
 *
 * @author Sergey Tarasevich (nostra13[at]gmail[dot]com)
 * @since 1.8.1
 */
public class LruMemoryCache implements MemoryCache {

   //LinkedHashMap accessOrder設爲true之後，最近訪問的數據將會被放到前面，
	private final LinkedHashMap<String, Bitmap> map;

	private final int maxSize;
	/** Size of this cache in bytes */
	private int size;

	/** @param maxSize Maximum sum of the sizes of the Bitmaps in this cache */
	public LruMemoryCache(int maxSize) {
		if (maxSize <= 0) {
			throw new IllegalArgumentException("maxSize <= 0");
		}
		
		//maxSize universal分配的策略是分配給應用的內存的1/8
		this.maxSize = maxSize;
		
		//設爲true了
		this.map = new LinkedHashMap<String, Bitmap>(0, 0.75f, true);
	}

	/**
	 * Returns the Bitmap for {@code key} if it exists in the cache. If a Bitmap was returned, it is moved to the head
	 * of the queue. This returns null if a Bitmap is not cached.
	 */
	@Override
	public final Bitmap get(String key) {
		if (key == null) {
			throw new NullPointerException("key == null");
		}

     //LinkHashMap是非線程安全的，因此要本身同步，這裏get之後，LinkedHashMap就會把數據放在最前面
		synchronized (this) {
			return map.get(key);
		}
	}

	/** Caches {@code Bitmap} for {@code key}. The Bitmap is moved to the head of the queue. */
	@Override
	public final boolean put(String key, Bitmap value) {
		if (key == null || value == null) {
			throw new NullPointerException("key == null || value == null");
		}

		synchronized (this) {
		   //加上當前的size
			size += sizeOf(key, value);
			Bitmap previous = map.put(key, value);
			
			//若是key存在，要減去之前的size
			if (previous != null) {
				size -= sizeOf(key, previous);
			}
		}

      //清理緩存，若是超出最大值，則須要刪除老的數據
		trimToSize(maxSize);
		return true;
	}

	/**
	 * 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.
	 */
	private void trimToSize(int maxSize) {
		while (true) {
			String key;
			Bitmap 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;
				}
				
           //刪除老的數據直至 size<maxSize
				Map.Entry<String, Bitmap> toEvict = map.entrySet().iterator().next();
				if (toEvict == null) {
					break;
				}
				key = toEvict.getKey();
				value = toEvict.getValue();
				map.remove(key);
				size -= sizeOf(key, value);
			}
		}
	}

	/** Removes the entry for {@code key} if it exists. */
	@Override
	public final Bitmap remove(String key) {
		if (key == null) {
			throw new NullPointerException("key == null");
		}

		synchronized (this) {
			Bitmap previous = map.remove(key);
			if (previous != null) {
				size -= sizeOf(key, previous);
			}
			return previous;
		}
	}

	@Override
	public Collection<String> keys() {
		synchronized (this) {
			return new HashSet<String>(map.keySet());
		}
	}

	@Override
	public void clear() {
		trimToSize(-1); // -1 will evict 0-sized elements
	}

	/**
	 * Returns the size {@code Bitmap} in bytes.
	 * <p/>
	 * An entry's size must not change while it is in the cache.
	 */
	private int sizeOf(String key, Bitmap value) {
		return value.getRowBytes() * value.getHeight();
	}

	@Override
	public synchronized final String toString() {
		return String.format("LruCache[maxSize=%d]", maxSize);
	}
}

LruCache

Android 4.0之後官方也引入了一個緩存類叫LruCache，能夠直接在源碼中找到該類，具體實現和上相似，只是這是一個泛型的實現，具體若是之後你們用的到，能夠去源碼中查詢。