LruCache详解之 Android 内存优化

2019-11-09 18:11:02

字体：大中小

来源：转载

供稿：网友

概念：

LruCache 什么是LruCache？ LruCache实现原理是什么？

这两个问题其实可以作为一个问题来回答，知道了什么是 LruCache，就只然而然的知道 LruCache 的实现原理；Lru的全称是Least Recently Used ，近期最少使用的！所以我们可以推断出 LruCache 的实现原理：把近期最少使用的数据从缓存中移除，保留使用最频繁的数据，那具体代码要怎么实现呢，我们进入到源码中看看。

LruCache源码分析

public class LruCache<K, V> { //缓存 map 集合，为什么要用LinkedHashMap //因为没错取了缓存值之后，都要进行排序，以确保 //下次移除的是最少使用的值 PRivate final LinkedHashMap<K, V> map; //当前缓存的值 private int size; //最大值 private int maxSize; //添加到缓存中的个数 private int putCount; //创建的个数 private int createCount; //被移除的个数 private int evictionCount; //命中个数 private int hitCount; //丢失个数 private int missCount; //实例化 Lru，需要传入缓存的最大值 //这个最大值可以是个数，比如对象的个数，也可以是内存的大小 //比如，最大内存只能缓存5兆 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); } //重置最大缓存的值 public void resize(int maxSize) { if (maxSize <= 0) { throw new IllegalArgumentException("maxSize <= 0"); } synchronized (this) { this.maxSize = maxSize; } trimToSize(maxSize); } //通过 key 获取缓存值 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++; } //如果没有，用户可以去创建 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 { //缓存的大小改变 size += safeSizeOf(key, createdValue); } } //这里没有移除，只是改变了位置 if (mapValue != null) { entryRemoved(false, key, createdValue, mapValue); return mapValue; } else { //判断缓存是否越界 trimToSize(maxSize); return createdValue; } } //添加缓存，跟上面这个方法的 create 之后的代码一样的 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; } //检测缓存是否越界 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!"); } //如果没有，则返回 if (size <= maxSize) { break; } //以下代码表示已经超出了最大范围 Map.Entry<K, V> toEvict = null; for (Map.Entry<K, V> entry : map.entrySet()) { toEvict = entry; } if (toEvict == null) { break; } //移除最后一个，也就是最少使用的缓存 key = toEvict.getKey(); value = toEvict.getValue(); map.remove(key); size -= safeSizeOf(key, value); evictionCount++; } entryRemoved(true, key, value, null); } } //手动移除，用户调用 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; } //这里用户可以重写它，实现数据和内存回收操作 protected void entryRemoved(boolean evicted, K key, V oldValue, V newValue) {} 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; } //这个方法要特别注意，跟我们实例化 LruCache 的 maxSize 要呼应，怎么做到呼应呢，比如 maxSize 的大小为缓存的个数，这里就是 return 1就 ok，如果是内存的大小，如果5M，这个就不能是个数了，这是应该是每个缓存 value 的 size 大小，如果是 Bitmap，这应该是 bitmap.getByteCount(); protected int sizeOf(K key, V value) { return 1; } //清空缓存 public final void evictAll() { trimToSize(-1); // -1 will evict 0-sized elements } public synchronized final int size() { return size; } public synchronized final int maxSize() { return maxSize; } public synchronized final int hitCount() { return hitCount; } public synchronized final int missCount() { return missCount; } public synchronized final int createCount() { return createCount; } public synchronized final int putCount() { return putCount; } public synchronized final int evictionCount() { return evictionCount; } public synchronized final Map<K, V> snapshot() { return new LinkedHashMap<K, V>(map); }}

LruCache 使用

先来看两张内存使用的图

这里写图片描述

/** * Created by gyzhong on 15/4/5. */public class LruPageAdapter extends PagerAdapter { private List<String> mData ; private LruCache<String,Bitmap> mLruCache ; private int mTotalSize = (int) Runtime.getRuntime().totalMemory(); private ViewPager mViewPager ; public LruPageAdapter(ViewPager viewPager ,List<String> data){ mData = data ; mViewPager = viewPager ; /*实例化LruCache*/ mLruCache = new LruCache<String,Bitmap>(mTotalSize/5){ /*当缓存大于我们设定的最大值时，会调用这个方法，我们可以用来做内存释放操作*/ @Override protected void entryRemoved(boolean evicted, String key, Bitmap oldValue, Bitmap newValue) { super.entryRemoved(evicted, key, oldValue, newValue); if (evicted && oldValue != null){ oldValue.recycle(); } } /*创建 bitmap*/ @Override protected Bitmap create(String key) { final int resId = mViewPager.getResources().getIdentifier(key,"drawable", mViewPager.getContext().getPackageName()) ; return BitmapFactory.decodeResource(mViewPager.getResources(),resId) ; } /*获取每个 value 的大小*/ @Override protected int sizeOf(String key, Bitmap value) { return value.getByteCount(); } } ; } @Override public Object instantiateItem(ViewGroup container, int position) { View view = LayoutInflater.from(container.getContext()).inflate(R.layout.view_pager_item, null) ; ImageView imageView = (ImageView) view.findViewById(R.id.id_view_pager_item); Bitmap bitmap = mLruCache.get(mData.get(position)); imageView.setImageBitmap(bitmap); container.addView(view); return view; } @Override public void destroyItem(ViewGroup container, int position, Object object) { container.removeView((View) object); } @Override public int getCount() { return mData.size(); } @Override public boolean isViewFromObject(View view, Object object) { return view == object; }}