在你应用程序的UI界面加载一张图片是一件很简单的事情，但是当你需要在界面上加载一大堆图片的时候，情况就变得复杂起来。Android为我们提供了LruCache，今天我们就来学习这个缓存的知识以及原理。
目录导航
LruCache缓存的实例代码
一、 我们建立一个简单的项目去体会LruCache的使用过程
通过http请求网络上的图片文件，然后保存在缓存中。显示图片时，先从缓存中取，如果没有，就发送请求向服务器取。项目结构如下：
二、 在AndroidManifest.xml文件中，加入网络权限的声明：
<uses-permission android:name="android.permission.INTERNET"/>
三、 创建一个图片加载的类，用于对缓存的一些操作，重写LruCache的sizeOf方法：
package com.example.linux.lrucachetest; import android.graphics.Bitmap; import android.util.LruCache; /** * Created by huhx on 2016/4/12. */ public class ImageDownloader { private static final String TAG = "TextDownload"; private LruCache<String, Bitmap> lruCache; public ImageDownloader() { long maxMemory = Runtime.getRuntime().maxMemory(); int cacheSize = (int) (maxMemory / 8); lruCache = new LruCache<String, Bitmap>(cacheSize) { @Override protected int sizeOf(String key, Bitmap value) { return value.getByteCount(); } }; } // 把Bitmap对象加入到缓存中 public void addBitmapToMemory(String key, Bitmap bitmap) { if (getBitmapFromMemCache(key) == null) { lruCache.put(key, bitmap); } } // 从缓存中得到Bitmap对象 public Bitmap getBitmapFromMemCache(String key) {
Log.i(TAG, "lrucache size: " + lruCache.size()); return lruCache.get(key); } // 从缓存中删除指定的Bitmap public void removeBitmapFromMemory(String key) { lruCache.remove(key); } }
四、 在MainActivity中使用并测试LruCache：showBitmap方法是先从缓存中取，如果没有就发送http请求取得。
public void showBitmap(View view) { Bitmap bitmap = imageDownloader.getBitmapFromMemCache("bitmap"); if (bitmap == null) { new BitmapThread(bitmapUrl).start(); } else { imageView.setImageBitmap(bitmap); } }
五、 BitmapThread的线程：从服务器拿到Bitmap对象，并加入到缓存中。
class BitmapThread extends Thread { private String bitmapUrl; BitmapThread(String bitmapUrl) { this.bitmapUrl = bitmapUrl; } @Override public void run() { Log.i(TAG, "run: " + Thread.currentThread().getName()); Bitmap bitmap = null; HttpURLConnection connection = null; InputStream inputStream = null; try { URL url = new URL(bitmapUrl); connection = (HttpURLConnection) url.openConnection(); connection.setConnectTimeout(5000); connection.setRequestMethod("GET"); if (connection.getResponseCode() == HttpURLConnection.HTTP_OK) { inputStream = connection.getInputStream(); bitmap = BitmapFactory.decodeStream(inputStream); } imageDownloader.addBitmapToMemory("bitmap", bitmap); handler.obtainMessage(DOWNLOAD_IMAGE, bitmap).sendToTarget(); } catch (Exception e) { e.printStackTrace(); } finally { if (connection != null) { connection.disconnect(); } if (inputStream != null) { try { inputStream.close(); } catch (IOException e) { e.printStackTrace(); } } } } }
六、 handler处理消息，并显示图片：
private Handler handler = new Handler() { @Override public void handleMessage(Message msg) { Log.i(TAG, "hanlder handleMessage: " + Thread.currentThread().getName()); switch (msg.what) { case DOWNLOAD_IMAGE: imageView.setImageBitmap((Bitmap) msg.obj); break; } } };
七、 从缓存中删除图片：
public void remove(View view) { imageDownloader.removeBitmapFromMemory("bitmap"); }
八、 输出日志结果如下：
第一次点击showBitmap：
04-12 19:51:21.768 15941-15941/com.example.linux.lrucachetest I/TextDownload: lrucache size: 0 04-12 19:51:21.771 15941-19434/com.example.linux.lrucachetest I/MainActivity: run: Thread-2264 04-12 19:51:21.816 15941-19434/com.example.linux.lrucachetest I/TextDownload: lrucache size: 0 04-12 19:51:21.817 15941-15941/com.example.linux.lrucachetest I/MainActivity: hanlder handleMessage: main
第二次点击showBitmap：
04-12 19:52:11.128 15941-15941/com.example.linux.lrucachetest I/TextDownload: lrucache size: 256000
点击remove之后，再点击showBitmap：
04-12 19:52:47.834 15941-15941/com.example.linux.lrucachetest I/TextDownload: lrucache size: 0 04-12 19:52:47.839 15941-20689/com.example.linux.lrucachetest I/MainActivity: run: Thread-2266 04-12 19:52:47.895 15941-20689/com.example.linux.lrucachetest I/TextDownload: lrucache size: 0 04-12 19:52:47.895 15941-15941/com.example.linux.lrucachetest I/MainActivity: hanlder handleMessage: main
LruCache缓存的原理分析
通过上述的案例，我们已经知道了LruCache的使用方法。接下来，我们一步步的分析它的过程以及原理。
一、 LruCache的文档描述如下：
A cache that holds strong references to a limited number of values. Each time a value is accessed, it is moved to the head of a queue. When a value is added to a full cache, the value at the end of that queue is evicted and may become eligible for garbage collection.
二、 它的属性一方法说明如下：
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; }
文档上一些对LruCache方法的描述：
If your cached values hold resources that need to be explicitly released, override entryRemoved(boolean, K, V, V)
If a cache miss should be computed on demand for the corresponding keys, override create(K). This simplifies the calling code, allowing it to assume a value will always be returned, even when there's a cache miss.
By default, the cache size is measured in the number of entries. Override sizeOf(K, V) to size the cache in different units. For example, this cache is limited to 4MiB of bitmaps:
三、 LruCache只有一个构造方法，LruCache(int maxSize)代码如下：初始化一个LinkedHashMap
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); }
四、 LruCache的put方法是把内容放入到缓存中去，代码如下：
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; }
其中safeSizeOf方法，是计算LruCache的已经缓存的大小，以下的sizeOf（默认返回1）方法是我们要重写的。
private int safeSizeOf(K key, V value) { int result = sizeOf(key, value); if (result < 0) { throw new IllegalStateException("Negative size: " + key + "=" + value); } return result; }
我们要重写sizeOf方法：
protected int sizeOf(K key, V value) { return 1; }
五、 LruCache的get方法是从缓存中去取得内容，代码如下：
public final V get(K key) { if (key == null) { throw new NullPointerException("key == null"); } V mapValue; synchronized (this) {
// 如果根据相应的key得到value，就增加一次命中hitCount，并且返回结果 mapValue = map.get(key); if (mapValue != null) { hitCount++; return mapValue; }
// 否则增加一次missCount 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. */ // 试图根据这个key，创建一个value。这里的create(key)默认是返回null，当然这个方法是可以重写的 V createdValue = create(key); if (createdValue == null) { return null; } synchronized (this) { createCount++;
// 如果我们重写了create(key)方法而且返回值不为空，那么将上述的key与这个返回值写入到map当中 mapValue = map.put(key, createdValue); if (mapValue != null) { // There was a conflict so undo that last put
// 方法放入最后put的key，value值
map.put(key, mapValue); } else { size += safeSizeOf(key, createdValue); } } if (mapValue != null) {
// 这个方法也可以重写 entryRemoved(false, key, createdValue, mapValue); return mapValue; } else { trimToSize(maxSize); return createdValue; } }
六、 LruCache的remove方法是从缓存中去删除内容，并更新已经缓存的大小，代码如下：
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; }