Android中使用的LruCache

   <p>LRU是Least Recently Used的缩写，即“最近最少使用”，说明LRU缓存算法的淘汰策略是把最近最少使用的数据移除，让出内存给最新读取的数据。下面看一下Android中的LruCache。</p>    <h2>android.util.LruCache</h2>    <p>这个LruCache在android.util包下，是API level 12引入的，对于API level 12之前的系统可以使用support library中的LruCache。先来看看android.util.LruCache的源码。</p>    <p>首先是成员变量：</p>    <pre>  <code class="language-java">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;</code></pre>    <p>LruCache内部使用一个LinkedHashMap作为存储容器，并对各种操作进行计次。</p>    <p>构造器：</p>    <pre>  <code class="language-java">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);      }</code></pre>    <p>构造器的参数maxSize用于指定缓存的最大容量，并初始化一个LinkedHashMap，顺便看看这个LinkedHashMap的构造函数：</p>    <pre>  <code class="language-java">public LinkedHashMap(int initialCapacity, float loadFactor, boolean accessOrder) {          super(initialCapacity, loadFactor);          init();          this.accessOrder = accessOrder;      }</code></pre>    <p>initialCapacity即初始容量设为0，装填因子loadFactor设为0.75，accessOrder设为true，即链表中的元素按照最近最少访问到最多访问排序。这里设置的装填因子为0.75，设置其它值行不行呢？在LinkedHashMap这个构造器中只是将loadFactor作为参数传给了父类构造器，该父类构造器如下：</p>    <pre>  <code class="language-java">public HashMap(int capacity, float loadFactor) {          this(capacity);            if (loadFactor <= 0 || Float.isNaN(loadFactor)) {              throw new IllegalArgumentException("Load factor: " + loadFactor);          }            /*           * Note that this implementation ignores loadFactor; it always uses           * a load factor of 3/4. This simplifies the code and generally           * improves performance.           */      }</code></pre>    <p>调用了HashMap的构造器，可以看到只是对loadFactor进行了合法检查，除此之外没有其他调用或赋值操作，Note中解释了，这个loadFactor没用，装填因子永远使用3/4，也就是0.75。所以在构造LinkedHashMap时，设了装填因子也没什么用。</p>    <p>继续看LruCache，resize方法更新链表容量，调用trimToSize方法。</p>    <pre>  <code class="language-java">public void resize(int maxSize) {          if (maxSize <= 0) {              throw new IllegalArgumentException("maxSize <= 0");          }            synchronized (this) {              this.maxSize = maxSize;          }          trimToSize(maxSize);      }</code></pre>    <p>先看get方法，key为空会抛异常，取出对应的value，若不为空则命中次数hitCount加1并return这个value，否则missCount加1。该value为空时继续向下执行，根据key尝试创建value，如果创建返回的createdValue是null，那就确实没有该值，若创建操作返回的createdValue不为null，则尝试把createdValue放回map，若存在旧值则返回旧值，否则返回这个createdValue。</p>    <pre>  <code class="language-java">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++;          }            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;          }      }</code></pre>    <p>put方法将键值对放入map，重新计算大小之后调用trimToSize方法，删除访问次数最少的元素。</p>    <pre>  <code class="language-java">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;      }</code></pre>    <p>trimToSize方法中会一直尝试删除队首元素即访问次数最少的元素，直到size不超过最大容量或将要删除的对象为空。</p>    <pre>  <code class="language-java">public 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) {                      break;                  }                    Map.Entry<K, V> toEvict = map.eldest();                  if (toEvict == null) {                      break;                  }                    key = toEvict.getKey();                  value = toEvict.getValue();                  map.remove(key);                  size -= safeSizeOf(key, value);                  evictionCount++;              }                entryRemoved(true, key, value, null);          }      }</code></pre>    <h2>android.support.v4.util.LruCache</h2>    <p>support v4包中的LruCache可以用于API level 12之前的系统，和android.util包的LruCache的区别是在trimToSize中获取将要删除元素的方法不一样：</p>    <ul>     <li>android.util.LruCache</li>    </ul>    <pre>  <code class="language-java">Map.Entry<K, V> toEvict = map.eldest();</code></pre>    <ul>     <li>android.support.v4.util.LruCache</li>    </ul>    <pre>  <code class="language-java">Map.Entry<K, V> toEvict = map.entrySet().iterator().next();</code></pre>    <p>LinkedHashMap的eldest()方法已经被标注为@hide，所以使用android.support.v4.util.LruCache更加保险一点。</p>    <p> </p>    <p>来自：http://www.jianshu.com/p/f32e5d435d6c</p>    <p> </p>