LruCache源码解读
浏览数：24 /
时间：2015年06月20日
Lrucache是Android4.0以后提供的一个用于缓存的类，这个类抛弃了原有的softReference（软引用）形式，因为android4.0以后虚拟机更倾向与回收软引用，也就是一旦虚拟机发现软引用，就会进行回收，这使得软引用变得不再可靠。
Lrucache的源码并不长，内部是使用LinkedHashMap<K, V> map来存储缓存的键值对，下面我带大家一起看一下lrucache的源码，希望大家以后再使用lrucache时思路更加清晰。
要说明Lrucache,我们首先要看一下LinkedHashMap的规则。
文档说明：
Map 接口的哈希表和链接列表实现，具有可预知的迭代顺序。此实现与 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，该迭代顺序通常就是将键插入到映射中的顺序（插入顺序）。注意，如果在映射中重新插入 键，则插入顺序不受影响。（如果在调用m.put(k, v) 前m.containsKey(k) 返回了true，则调用时会将键k 重新插入到映射m 中。）
此实现可以让客户避免未指定的、由
HashMap（及
Hashtable）所提供的通常为杂乱无章的排序工作，同时无需增加与
TreeMap 相关的成本。使用它可以生成一个与原来顺序相同的映射副本，而与原映射的实现无关：
void foo(Map m) {
Map copy = new LinkedHashMap(m);
...
}
如果模块通过输入得到一个映射，复制这个映射，然后返回由此副本确定其顺序的结果，这种情况下这项技术特别有用。（客户通常期望返回的内容与其出现的顺序相同。）
提供特殊的构造方法来创建链接哈希映射，该哈希映射的迭代顺序就是最后访问其条目的顺序，从近期访问最少到近期访问最多的顺序（访问顺序）。这种映射很适合构建
LRU 缓存。
OK，注意上面的黑体字，linkedhashmap维护了这样的一个迭代顺序，一看上就应该意识到，这样的特性非常适合LRU缓存，我们来看它的构造函数
LinkedHashMap(int initialCapacity, float loadFactor, boolean accessOrder)
构造一个带指定初始容量、加载因子和排序模式的空 LinkedHashMap 实例。
最后一个参数accessOrder设置为true,就可以设置其排序模式
有了是上面的基本知识，我们来理解Lrucache就不难了。
首先看构造方法：
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;//当前缓存大小
private int maxSize;//最大缓存大小
private int putCount;//成功put的数目
private int createCount;//成功create的数目
private int evictionCount;//成功移除的数目
private int hitCount;//成功get数
private int missCount;//失败get数
/**
* @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);
}类里面的属性非常直观，我加了注释。
构造方法里面，我们可以看到要设置一个maxSize最大容量参数，然后就是初始化一个LinkedHashMap
使用new LinkedHashMap<K, V>(0, 0.75f, true);这个构造函数的原因我们已经知道了。好，构造函数非常简单。
下面我们来看最重要的两个方法
/**
* 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.
*/
public final V get(K key) {
if (key == null) {
throw new NullPointerException("key == null");
}
V mapValue;
synchronized (this) {//锁，保证线程安全
mapValue = map.get(key);
if (mapValue != null) {//获取的值不为空说明获取成功，修改属性
hitCount++;
return mapValue;
}
missCount++;
}
/*
* 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);//默认返回null,子类
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;
}
上面可以看出，我们简单的通过key来获取value,成功返回，不成功就先调用了create(key)方法，那么我们来看这个方法做了什么
另外值得一提的就是我们使用map.get方法时，是在同步块内的，这样会降低程序的效率（原则上来说，get方法是不应该加锁的，但是LinkedHashMap要维护一个Entry顺序链表，使得我们不得不对整个map加上锁，这可以说是Lrucache的一个缺点！）
/**
* 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.
*/
protected V create(K key) {
return null;
}结果发现是说了一大段话，却只返回了null！
OK，我们再在get()方法接着看
V createdValue = create(key);
if (createdValue == null) {
return null;
}如果create(key)返回的值是null,这是get方法才返回null，那么什么时候create不是返回null呢
注意要create()是一个protected方法，说明我可以在子类中复写这个方法，我们可以根据自己的需要创建新的value(注意create方法不是线程安全的)，当然可以默认返回null
假设我们在复写了create()方法，并且返回了value，才能接下去执行
synchronized (this) {
createCount++;//创建数加一
mapValue = map.put(key, createdValue);//试图存入map中
if (mapValue != null) {//是否存在旧值
// There was a conflict so undo that last put
map.put(key, mapValue);//取消更新
} else {
size += safeSizeOf(key, createdValue);
}
}map.put返回的mapValue是map中原本存储的值(如果原本map中没有这个key-value对，就返回空，如果原本就有,返回原本的值)如果原本就有值，取消新值的写入，否则存储成功，更新size
接着我们看怎么更新size的
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.
*/
protected int sizeOf(K key, V value) {
return 1;
}
发现是默认返回1，所谓safeSizeOf不是指线程安全的意思，而是防止使用者错误将item大小设置为非正数（通过复写sizeOf方法）
get方法里面接下去看
if (mapValue != null) {
entryRemoved(false, key, createdValue, mapValue);
return mapValue;
} else {
trimToSize(maxSize);
return createdValue;
}这里还是出来处理旧值的情况，如果存在旧值，调用entryRemoved(false, key, createdValue, mapValue);并且返回旧值，我们来看这个方法
/**
* 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}.
*/
protected void entryRemoved(boolean evicted, K key, V oldValue, V newValue) {}这个方法线程不安全，evicted参数表示是否被回收(true---为释放空间被删除；false---put或remove导致)，接下来是key,旧值，新值。我们可以覆写这个方法
然后如果新值存储成功调用trimToSize()
/**
* @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) {
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()) {//如果没有达到最大缓存，结束
break;
}
Map.Entry<K, V> toEvict = map.entrySet().iterator().next();
key = toEvict.getKey();
value = toEvict.getValue();
map.remove(key);
size -= safeSizeOf(key, value);//清除缓存直到小于maxsize
evictionCount++;
}
entryRemoved(true, key, value, null);//注意我们每次清除都调用了这个方法，不过这个evicted为true
}
}这个方法用于清空cache空间，也就是当超过maxsize时，用LRU算法腾出缓存空间注意这里再while循环里面，目的是为了清除缓存直到小于maxsize
看完get方法，看put方法就容易了
/**
* 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) {//判断是否存在旧值，有则计算新的size
size -= safeSizeOf(key, previous);
}
}
if (previous != null) {
entryRemoved(false, key, previous, value);
}
trimToSize(maxSize);//清理缓存
return previous;
}现在看put方法感觉已经没有什么好说的了，里面的方法几乎都说明过了
OK，除了上述方法以外，Lrucahce里面还有一些get方法用来获取Lrucache的属性，就不仔细说了，另外还有一个值得注意的remove方法
/**
* Removes the entry for {@code key} if it exists.
*
* @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;
}通过上面的说明，我也可以一眼看出remove的执行过程，so easy!
Lrucahce源码就给大家介绍到这里，其实lrucache原理就是维护一个LinkedHashMap，然后put,get方法，每次都有调用trimToSize()方法清理缓存
由于LinkedHashMap最少使用先出的原则，我们也不必担心选择清理哪些旧缓存
最后贴上Lrucache完整源码
/*
* Copyright (C) 2011 The Android Open Source Project
*
* 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.duowan.mobile.netroid.cache;
import java.util.LinkedHashMap;
import java.util.Map;
/**
* This class copy from android support v4.
* 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;
private int maxSize;
private int putCount;
private int createCount;
private int evictionCount;
private int hitCount;
private int missCount;
/**
* @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.
*/
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++;
return mapValue;
}
missCount++;
}
/*
* 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}.
*/
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) {
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.
*/
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 || map.isEmpty()) {
break;
}
Map.Entry<K, V> toEvict = map.entrySet().iterator().next();
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.
*
* @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.
*
* <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}.
*/
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.
*/
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.
*/
protected int sizeOf(K key, V value) {
return 1;
}
/**
* Clear the cache, calling {@link #entryRemoved} on each removed entry.
*/
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.
*/
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.
*/
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.
*/
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);
}
}
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