ImageLoader內存緩存解析

接下來我要說的就是鼎鼎大名的Universal Image Loader，上一篇講了ImageLoader硬盤緩存解析，本篇將介紹ImageLoader的內存緩存。

先來看看ImageLoader內存緩存部分的代碼結構。

首先是MemoryCache接口

public interface MemoryCache {
	/**
	 * Puts value into cache by key
	 *
	 * @return <b>true</b> - if value was put into cache successfully, <b>false</b> - if value was <b>not</b> put into
	 * cache
	 */
	boolean put(String key, Bitmap value);

	/** Returns value by key. If there is no value for key then null will be returned. */
	Bitmap get(String key);

	/** Removes item by key */
	Bitmap remove(String key);

	/** Returns all keys of cache */
	Collection<String> keys();

	/** Remove all items from cache */
	void clear();
}

接口很簡單，就是5個方法，對內存緩存進行處理。

接下來是BaseMemoryCache，在實現MemoryCache接口的同時做了下拓展。

/*******************************************************************************
 * Copyright 2011-2014 Sergey Tarasevich
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *******************************************************************************/
package com.nostra13.universalimageloader.cache.memory;

import android.graphics.Bitmap;

import java.lang.ref.Reference;
import java.util.*;

/**
 * Base memory cache. Implements common functionality for memory cache. Provides object references (
 * {@linkplain Reference not strong}) storing.
 *
 * @author Sergey Tarasevich (nostra13[at]gmail[dot]com)
 * @since 1.0.0
 */
public abstract class BaseMemoryCache implements MemoryCache {

	/** Stores not strong references to objects */
	private final Map<String, Reference<Bitmap>> softMap = Collections.synchronizedMap(new HashMap<String, Reference<Bitmap>>());

	@Override
	public Bitmap get(String key) {
		Bitmap result = null;
		Reference<Bitmap> reference = softMap.get(key);
		if (reference != null) {
			result = reference.get();
		}
		return result;
	}

	@Override
	public boolean put(String key, Bitmap value) {
		softMap.put(key, createReference(value));
		return true;
	}

	@Override
	public Bitmap remove(String key) {
		Reference<Bitmap> bmpRef = softMap.remove(key);
		return bmpRef == null ? null : bmpRef.get();
	}

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

	@Override
	public void clear() {
		softMap.clear();
	}

	/** Creates {@linkplain Reference not strong} reference of value */
	protected abstract Reference<Bitmap> createReference(Bitmap value);
}

增加了一個softMap的成員變量，實現了MemoryCache對softMap的操作，並且增加了一個Reference<Bitmap> createReference(Bitmap value)的抽象方法。

/*******************************************************************************
 * Copyright 2011-2014 Sergey Tarasevich
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *******************************************************************************/
package com.nostra13.universalimageloader.cache.memory;

import android.graphics.Bitmap;

import com.nostra13.universalimageloader.utils.L;

import java.util.Collections;
import java.util.LinkedList;
import java.util.List;
import java.util.concurrent.atomic.AtomicInteger;

/**
 * Limited cache. Provides object storing. Size of all stored bitmaps will not to exceed size limit (
 * {@link #getSizeLimit()}).<br />
 * <br />
 * <b>NOTE:</b> This cache uses strong and weak references for stored Bitmaps. Strong references - for limited count of
 * Bitmaps (depends on cache size), weak references - for all other cached Bitmaps.
 *
 * @author Sergey Tarasevich (nostra13[at]gmail[dot]com)
 * @see BaseMemoryCache
 * @since 1.0.0
 */
public abstract class LimitedMemoryCache extends BaseMemoryCache {

	private static final int MAX_NORMAL_CACHE_SIZE_IN_MB = 16;
	private static final int MAX_NORMAL_CACHE_SIZE = MAX_NORMAL_CACHE_SIZE_IN_MB * 1024 * 1024;

	private final int sizeLimit;

	private final AtomicInteger cacheSize;

	/**
	 * Contains strong references to stored objects. Each next object is added last. If hard cache size will exceed
	 * limit then first object is deleted (but it continue exist at {@link #softMap} and can be collected by GC at any
	 * time)
	 */
	private final List<Bitmap> hardCache = Collections.synchronizedList(new LinkedList<Bitmap>());

	/** @param sizeLimit Maximum size for cache (in bytes) */
	public LimitedMemoryCache(int sizeLimit) {
		this.sizeLimit = sizeLimit;
		cacheSize = new AtomicInteger();
		if (sizeLimit > MAX_NORMAL_CACHE_SIZE) {
			L.w("You set too large memory cache size (more than %1$d Mb)", MAX_NORMAL_CACHE_SIZE_IN_MB);
		}
	}

	@Override
	public boolean put(String key, Bitmap value) {
		boolean putSuccessfully = false;
		// Try to add value to hard cache
		int valueSize = getSize(value);
		int sizeLimit = getSizeLimit();
		int curCacheSize = cacheSize.get();
		if (valueSize < sizeLimit) {
			while (curCacheSize + valueSize > sizeLimit) {
				Bitmap removedValue = removeNext();
				if (hardCache.remove(removedValue)) {
					curCacheSize = cacheSize.addAndGet(-getSize(removedValue));
				}
			}
			hardCache.add(value);
			cacheSize.addAndGet(valueSize);

			putSuccessfully = true;
		}
		// Add value to soft cache
		super.put(key, value);
		return putSuccessfully;
	}

	@Override
	public Bitmap remove(String key) {
		Bitmap value = super.get(key);
		if (value != null) {
			if (hardCache.remove(value)) {
				cacheSize.addAndGet(-getSize(value));
			}
		}
		return super.remove(key);
	}

	@Override
	public void clear() {
		hardCache.clear();
		cacheSize.set(0);
		super.clear();
	}

	protected int getSizeLimit() {
		return sizeLimit;
	}

	protected abstract int getSize(Bitmap value);

	protected abstract Bitmap removeNext();
}

接下來再看看LimitedMemoryCache，LimitedMemoryCache繼承於BaseMemoryCache，並且有自己的抽象方法。

後面基於LimitedMemoryCache派生出來的類，我們可以注意到LimitedMemoryCache的put(String key, Bitmap value)方法裏面調用到removeNext()方法，我們就可以通過改變removeNext()抽象方法來實現各個不同的LimitedMemoryCache。

這是FIFOLimitedMemoryCache的removeNext()方法。

@Override
	protected Bitmap removeNext() {
		return queue.remove(0);
	}

直接將LinkedList的queue調用remove(0)方法從而實現FIFOLimitedMemoryCache。

這是LargestLimitedMemoryCache的removeNext()方法。

@Override
	protected Bitmap removeNext() {
		Integer maxSize = null;
		Bitmap largestValue = null;
		Set<Entry<Bitmap, Integer>> entries = valueSizes.entrySet();
		synchronized (valueSizes) {
			for (Entry<Bitmap, Integer> entry : entries) {
				if (largestValue == null) {
					largestValue = entry.getKey();
					maxSize = entry.getValue();
				} else {
					Integer size = entry.getValue();
					if (size > maxSize) {
						maxSize = size;
						largestValue = entry.getKey();
					}
				}
			}
		}
		valueSizes.remove(largestValue);
		return largestValue;
	}

可以看到這裏對valueSizes裏面的元素做了對比找出最大的那個，然後返回出來。

LRULimitedMemoryCache的removeNext()方法。

@Override
	protected Bitmap removeNext() {
		Bitmap mostLongUsedValue = null;
		synchronized (lruCache) {
			Iterator<Entry<String, Bitmap>> it = lruCache.entrySet().iterator();
			if (it.hasNext()) {
				Entry<String, Bitmap> entry = it.next();
				mostLongUsedValue = entry.getValue();
				it.remove();
			}
		}
		return mostLongUsedValue;
	}

這裏用到了LinkedHashMap實現removeNext方法，從而實現LRU算法。

UsingFreqLimitedMemoryCache的removeNext()方法。

@Override
	protected Bitmap removeNext() {
		Integer minUsageCount = null;
		Bitmap leastUsedValue = null;
		Set<Entry<Bitmap, Integer>> entries = usingCounts.entrySet();
		synchronized (usingCounts) {
			for (Entry<Bitmap, Integer> entry : entries) {
				if (leastUsedValue == null) {
					leastUsedValue = entry.getKey();
					minUsageCount = entry.getValue();
				} else {
					Integer lastValueUsage = entry.getValue();
					if (lastValueUsage < minUsageCount) {
						minUsageCount = lastValueUsage;
						leastUsedValue = entry.getKey();
					}
				}
			}
		}
		usingCounts.remove(leastUsedValue);
		return leastUsedValue;
	}

這裏使用的是以Bitmap和Integer爲鍵值對的Map，Integer記錄了Bitmap的使用頻率，removeNext裏面查找最少使用頻率，從而刪除它。

總之MemoryCache部分的接口設計得挺好的，值得我們學習，有關內存緩存部分就說到這裏。