内存缓存
缓存与内存回收机制有关，java中有四种与垃圾回收（gc）有关的引用：强引用（StrongReference）、软引用（SoftReference）、弱引用（WeakReference）和虚引用（PhantomReference）。
强引用（StrongReference）
强引用是最普遍的一种引用，在java中使用new关键字生成的对象就是强引用，对于强引用，java垃圾回收机制不会将其回收，当内存不足时，java虚拟机宁肯抛出OutOfMemoryError错误也不会随意回收强引用的对象。
软引用（SoftReference）
对于软引用，当内存足够时，不会回收软引用的对象，但是当内存空间不足时，就会回收这些对象。
弱引用（WeakReference）
弱引用，相对于软引用来说，更容易被回收机制回收，当垃圾回收机制扫描内存空间时，如果发现弱引用对象，不论当前的内存是否充足，都会立即将其回收，释放内存空间。
虚引用（PhantomReference）
虚引用不同于其他引用，其并不会决定对象的生命周期。如果一个对象仅持有虚引用，那么它就和没有任何引用一样，在任何时候都可能被垃圾回收器回收。
Universal-Image-Loader中的缓存机制
1、LRU（Least Recently Used）最近最久未使用页面置换算法
LRU是操作系统中页面置换算法的一种，其核心是，对于每一个页面，设置一个时间字段，用来记录这个页面自上次被访问以来所经历的时间t，当出现缺页中断（也就是内存不足）时，将t值最大的页面删除。
2、Universal-Image-Loader中缓存机制
（1）只使用的是强引用缓存
LruMemoryCache（使用LRU算法，缓存对象的是bitmap的强引用）
（2）使用强引用和弱引用相结合的缓存有
UsingFreqLimitedMemoryCache（如果缓存的图片总量超过限定值，先删除使用频率最小的bitmap）
LRULimitedMemoryCache（这个也是使用的lru算法，和LruMemoryCache不同的是，他缓存的是bitmap的弱引用）
FIFOLimitedMemoryCache（先进先出的缓存策略，当超过设定值，先删除最先加入缓存的bitmap）
LargestLimitedMemoryCache(当超过缓存限定值，先删除最大的bitmap对象)
LimitedAgeMemoryCache（当 bitmap加入缓存中的时间超过我们设定的值，将其删除）
（3）只使用弱引用缓存
WeakMemoryCache（这个类缓存bitmap的总大小没有限制，唯一不足的地方就是不稳定，缓存的图片容易被回收掉）
LruMemoryCache源码分析
package com.nostra13.universalimageloader.cache.memory.impl;
import android.graphics.Bitmap;
import com.nostra13.universalimageloader.cache.memory.MemoryCache;
import java.util.Collection;
import java.util.HashSet;
import java.util.LinkedHashMap;
import java.util.Map;
/**
* 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.
*
* NOTE: 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 {
private final LinkedHashMap 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");
}
this.maxSize = maxSize;
this.map = new LinkedHashMap(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");
}
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) {
size += sizeOf(key, value);
Bitmap previous = map.put(key, value);
if (previous != null) {
size -= sizeOf(key, previous);
}
}
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;
}
Map.Entry 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 keys() {
synchronized (this) {
return new HashSet(map.keySet());
}
}
@Override
public void clear() {
trimToSize(-1); // -1 will evict 0-sized elements
}
/**
* Returns the size {@code Bitmap} in bytes.
*
* 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);
}
}
在源码中LinkedHashMap作为LRU算法的缓存，LinkedHashMap是HashMap的一个子类，它保留插入的顺序，LinkedHashMap实现与HashMap的不同之处在于，后者维护着一个运行于所有条目的双重链接列表。此链接列表定义了迭代顺序，该迭代顺序可以是插入顺序或者是访问顺序。
默认是按插入顺序排序，如果指定按访问顺序排序，那么调用get方法后，会将这次访问的元素移至链表尾部，不断访问可以形成按访问顺序排序的链表。可以重写removeEldestEntry方法返回true值指定插入元素时移除最老的元素。
this.map = new LinkedHashMap(0, 0.75f, true);
构造方法中第一个参数代表LinkedHashMap初始容量，第二个参数0.75f是加载因子，第三个参数是访问顺序，默认为false即插入顺序，true为访问顺序。
LruMemoryCache中维护了一个所有bitmap的size和，如果size超过设定的maxSize，就会将最近最久未使用的图片缓存删除。
在源码的put方法中调用了HashMap中的put方法：
public V put(K key, V value) {
if (key == null)
return putForNullKey(value);
int hash = hash(key.hashCode());
int i = indexFor(hash, table.length);
for (Entry e = table[i]; e != null; e = e.next) {
Object k;
if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
V oldValue = e.value;
e.value = value;
e.recordAccess(this);
return oldValue;
}
}
modCount++;
addEntry(hash, key, value, i);
return null;
}
void recordAccess(HashMap m) {
LinkedHashMap lm = (LinkedHashMap)m;
//当LinkedHashMap按访问排序时
if (lm.accessOrder) {
lm.modCount++;
//移除当前节点
remove();
//将当前节点插入到头结点前面
addBefore(lm.header);
}
}
/**
* 移除节点，并修改前后引用
*/
private void remove() {
before.after = after;
after.before = before;
}
/**
* 将当前节点插入到existingEntry的前面
*/
private void addBefore(Entry existingEntry) {
after = existingEntry;
before = existingEntry.before;
before.after = this;
after.before = this;
}
HashMap中put方法源码，当HashMap中存在这个value时，会将这个value返回，否则返回null，所以当map.put(key, value)返回值不为空时，需要减去刚加入的图片size。当返回为null时，将bitmap加入到LinkedHashMap中，返回为bitmap时，直接使用该bitmap。recordAccess(HashMap在HashMap的put和get方法中，会调用该方法，在HashMap中该方法为空，在LinkedHashMap中，当按访问顺序排序时，该方法会将当前节点插入到链表尾部(头结点的前一个节点)，否则不做任何事（最近使用的bitmap会移动到双向链表的尾部）。
LRU算法实现的核心是private void trimToSize(int maxSize)函数中不断判断当前size是否大于maxSize，当超过maxSize时，会将LinkedHashMap中的第一个元素删除，其就是最近最久未使用的bitmap。
使用LinkedHashMap可以很容易构造一个基于LRU算法的缓存。
UsingFreqLimitedMemoryCache源码分析
/*******************************************************************************
* 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> softMap = Collections.synchronizedMap(new HashMap>());
@Override
public Bitmap get(String key) {
Bitmap result = null;
Reference 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 bmpRef = softMap.remove(key);
return bmpRef == null ? null : bmpRef.get();
}
@Override
public Collection keys() {
synchronized (softMap) {
return new HashSet(softMap.keySet());
}
}
@Override
public void clear() {
softMap.clear();
}
/** Creates {@linkplain Reference not strong} reference of value */
protected abstract Reference createReference(Bitmap value);
}
/*******************************************************************************
* 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()}).
*
* NOTE: 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 hardCache = Collections.synchronizedList(new LinkedList());
/** @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();
}
/*******************************************************************************
* 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.impl;
import android.graphics.Bitmap;
import com.nostra13.universalimageloader.cache.memory.LimitedMemoryCache;
import java.lang.ref.Reference;
import java.lang.ref.WeakReference;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;
/**
* Limited {@link Bitmap bitmap} cache. Provides {@link Bitmap bitmaps} storing. Size of all stored bitmaps will not to
* exceed size limit. When cache reaches limit size then the bitmap which used the least frequently is deleted from
* cache.
*
* NOTE: 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)
* @since 1.0.0
*/
public class UsingFreqLimitedMemoryCache extends LimitedMemoryCache {
/**
* Contains strong references to stored objects (keys) and last object usage date (in milliseconds). If hard cache
* size will exceed limit then object with the least frequently usage is deleted (but it continue exist at
* {@link #softMap} and can be collected by GC at any time)
*/
private final Map usingCounts = Collections.synchronizedMap(new HashMap());
public UsingFreqLimitedMemoryCache(int sizeLimit) {
super(sizeLimit);
}
@Override
public boolean put(String key, Bitmap value) {
if (super.put(key, value)) {
usingCounts.put(value, 0);
return true;
} else {
return false;
}
}
@Override
public Bitmap get(String key) {
Bitmap value = super.get(key);
// Increment usage count for value if value is contained in hardCahe
if (value != null) {
Integer usageCount = usingCounts.get(value);
if (usageCount != null) {
usingCounts.put(value, usageCount + 1);
}
}
return value;
}
@Override
public Bitmap remove(String key) {
Bitmap value = super.get(key);
if (value != null) {
usingCounts.remove(value);
}
return super.remove(key);
}
@Override
public void clear() {
usingCounts.clear();
super.clear();
}
@Override
protected int getSize(Bitmap value) {
return value.getRowBytes() * value.getHeight();
}
@Override
protected Bitmap removeNext() {
Integer minUsageCount = null;
Bitmap leastUsedValue = null;
Set> entries = usingCounts.entrySet();
synchronized (usingCounts) {
for (Entry 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;
}
@Override
protected Reference createReference(Bitmap value) {
return new WeakReference(value);
}
}
UsingFreqLimitedMemoryCache继承自LimitedMemoryCach，而LimitedMemoryCach继承自BaseMemoryCache。BaseMemoryCache中维护了Map包含软引用类型bitmap的softMap，定义了softMap的get、put、remove和clear方法。
LimitedMemoryCache继承BaseMemoryCache，其中定义了MAX_NORMAL_CACHE_SIZE_IN_MB、MAX_NORMAL_CACHE_SIZE、内存大小限制sizeLimit和List
removeNext()，在UsingFreqLimitedMemoryCache中实现，其使用Map
FIFOLimitedMemoryCache源码分析
/*******************************************************************************
* 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.impl;
import android.graphics.Bitmap;
import com.nostra13.universalimageloader.cache.memory.LimitedMemoryCache;
import java.lang.ref.Reference;
import java.lang.ref.WeakReference;
import java.util.Collections;
import java.util.LinkedList;
import java.util.List;
/**
* Limited {@link Bitmap bitmap} cache. Provides {@link Bitmap bitmaps} storing. Size of all stored bitmaps will not to
* exceed size limit. When cache reaches limit size then cache clearing is processed by FIFO principle.
*
* NOTE: 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)
* @since 1.0.0
*/
public class FIFOLimitedMemoryCache extends LimitedMemoryCache {
private final List queue = Collections.synchronizedList(new LinkedList());
public FIFOLimitedMemoryCache(int sizeLimit) {
super(sizeLimit);
}
@Override
public boolean put(String key, Bitmap value) {
if (super.put(key, value)) {
queue.add(value);
return true;
} else {
return false;
}
}
@Override
public Bitmap remove(String key) {
Bitmap value = super.get(key);
if (value != null) {
queue.remove(value);
}
return super.remove(key);
}
@Override
public void clear() {
queue.clear();
super.clear();
}
@Override
protected int getSize(Bitmap value) {
return value.getRowBytes() * value.getHeight();
}
@Override
protected Bitmap removeNext() {
return queue.remove(0);
}
@Override
protected Reference createReference(Bitmap value) {
return new WeakReference(value);
}
}
FIFOLimitedMemoryCache也继承了LimitedMemoryCache，并维护了一个LinkedList
LinkedList的add方法：
public boolean add(E e) {
addBefore(e, header);
return true;
}
private Entry addBefore(E e, Entry entry) {
Entry newEntry = new Entry(e, entry, entry.previous);
newEntry.previous.next = newEntry;
newEntry.next.previous = newEntry;
size++;
modCount++;
return newEntry;
}
向LinkedList中插入元素时，是将元素插入到了header的后面addBefore(e, header);，即将元素插入到链表的尾部。
public E remove(int index) {
return remove(entry(index));
}
private Entry entry(int index) {
if (index < 0 || index >= size)
throw new IndexOutOfBoundsException("Index: "+index+
", Size: "+size);
Entry e = header;
// 根据这个判断决定从哪个方向遍历这个链表
if (index < (size >> 1)) {
for (int i = 0; i <= index; i++)
e = e.next;
} else {
// 可以通过header节点向前遍历，说明这个一个循环双向链表，header的previous指向链表的最后一个节点，这也验证了构造方法中对于header节点的前后节点均指向自己的解释
for (int i = size; i > index; i--)
e = e.previous;
}
return e;
}
FIFOLimitedMemoryCache的removeNext()方法中queue.remove(0);是将双向循环列表的第一个元素删除。由此可知由于插入元素时是向双向循环列表中尾部插入新元素，删除时，删除列表的第一个元素，即先进先出算法。
除此之外，LargestLimitedMemoryCache和LimitedAgeMemoryCach都继承了LimitedMemoryCache，是强引用和弱引用相结合的缓存，因为BaseMemoryCache中维护了一个软引用的HashMap
WeakMemoryCache源码分析
/*******************************************************************************
* 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.impl;
import android.graphics.Bitmap;
import com.nostra13.universalimageloader.cache.memory.BaseMemoryCache;
import java.lang.ref.Reference;
import java.lang.ref.WeakReference;
/**
* Memory cache with {@linkplain WeakReference weak references} to {@linkplain android.graphics.Bitmap bitmaps}
*
* NOTE: This cache uses only weak references for stored Bitmaps.
*
* @author Sergey Tarasevich (nostra13[at]gmail[dot]com)
* @since 1.5.3
*/
public class WeakMemoryCache extends BaseMemoryCache {
@Override
protected Reference createReference(Bitmap value) {
return new WeakReference(value);
}
}
WeakMemoryCache继承BaseMemoryCache，返回一个 WeakReference
参考文章：
http://blog.csdn.net/xiaanming/article/details/27525741
http://www.cnblogs.com/children/archive/2012/10/02/2710624.html
http://blog.csdn.net/jzhf2012/article/details/8540543