public class LruCache<K, V> {
  private final LinkedHashMap<K, V> map;

  ...
  
  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);
  }
  ...
}

//获取当前进程的可用内存（单位KB）
int maxMemory = (int) (Runtime.getRuntime().maxMemory() /1024);
int memoryCacheSize = maxMemory / 8;
mMemoryCache = new LruCache<String, Bitmap>(memoryCacheSize) {
  @Override
  protected int sizeOf(String key, Bitmap bitmap) {
    return bitmap.getByteCount() / 1024;
  }
};  

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;
    }
  }

  private String getKeyFromUrl(String url) {
    String key;
    try {
      MessageDigest messageDigest = MessageDigest.getInstance("MD5");
      messageDigest.update(url.getBytes());
      byte[] m = messageDigest.digest();
      return getString(m);
    } catch (NoSuchAlgorithmException e) {
      key = String.valueOf(url.hashCode());
    }
    return key;
  }
  private static String getString(byte[] b){
    StringBuffer sb = new StringBuffer();
    for(int i = 0; i < b.length; i ++){
      sb.append(b[i]);
    }
    return sb.toString();
  }

 

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;
}

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;
}

private DiskLruCache(File directory, int appVersion, int valueCount, long maxSize) {
    this.directory = directory;
    this.appVersion = appVersion;
    this.journalFile = new File(directory, JOURNAL_FILE);
    this.journalFileTmp = new File(directory, JOURNAL_FILE_TMP);
    this.valueCount = valueCount;
    this.maxSize = maxSize;
}

public static DiskLruCache open(File directory, int appVersion, int valueCount, long maxSize)
      throws IOException {
    if (maxSize <= 0) {
      throw new IllegalArgumentException("maxSize <= 0");
    }
    if (valueCount <= 0) {
      throw new IllegalArgumentException("valueCount <= 0");
    }
 
    // prefer to pick up where we left off
    DiskLruCache cache = new DiskLruCache(directory, appVersion, valueCount, maxSize);
    if (cache.journalFile.exists()) {
      try {
        cache.readJournal();
        cache.processJournal();
        cache.journalWriter = new BufferedWriter(new FileWriter(cache.journalFile, true),
            IO_BUFFER_SIZE);
        return cache;
      } catch (IOException journalIsCorrupt) {
//        System.logW("DiskLruCache " + directory + " is corrupt: "
//            + journalIsCorrupt.getMessage() + ", removing");
        cache.delete();
      }
    }
 
    // create a new empty cache
    directory.mkdirs();
    cache = new DiskLruCache(directory, appVersion, valueCount, maxSize);
    cache.rebuildJournal();
    return cache;
}

File diskCacheDir= getAppCacheDir(mContext, "images");
if (!diskCacheDir.exists()) {
  diskCacheDir.mkdirs();
}
mDiskLruCache = DiskLruCache.open(diskCacheDir, 1, 1, DISK_CACHE_SIZE); 

public static File getAppCacheDir(Context context, String dirName) {
  String cacheDirString;
  if (Environment.MEDIA_MOUNTED.equals(Environment.getExternalStorageState())) {
    cacheDirString = context.getExternalCacheDir().getPath();
  } else {
    cacheDirString = context.getCacheDir().getPath();
  }
  return new File(cacheDirString + File.separator + dirName);
}

if (editor != null) {
  OutputStream outputStream = editor.newOutputStream(0); //参数为索引，由于我们创建时指定一个节点只有一个缓存对象，所以传入0即可
}

//getStream为我们自定义的方法，它通过URL获取输入流并写入outputStream，具体实现后文会给出
if (getStreamFromUrl(url, outputStream)) {
  editor.commit();
} else {
  //返回false表示写入outputStream未成功，因此调用abort方法回退整个操作
  editor.abort();
}
mDiskLruCache.flush(); //将内存中的操作记录同步到日志文件中

public boolean getStreamFromUrl(String urlString, OutputStream outputStream) {
  HttpURLConnection urlCOnnection = null;
  BufferedInputStream bis = null;
  BufferedOutputStream bos = null;
  
  try {
    final URL url = new URL(urlString);
    urlConnection = (HttpURLConnection) url.openConnection();
    bis = new BufferedInputStream(urlConnection.getInputStream(), BUF_SIZE);
    
    int byteRead;
    while ((byteRead = bis.read()) != -1) {
      bos.write(byteRead);
    }
    return true;
  }catch (IOException e) {
    e.printStackTrace();
  } finally {
    if (urlConnection != null) {
      urlConnection.disconnect();
    }
    //HttpUtils为一个自定义工具类
    HttpUtils.close(bis);
    HttpUtils.close(bos);
  }
  return false;
}

public synchronized Snapshot get(String key) throws IOException {
    checkNotClosed();
    validateKey(key);
    Entry entry = lruEntries.get(key);
    if (entry == null) {
      return null;
    }
 
    if (!entry.readable) {
      return null;
    }
 
    /*
     * Open all streams eagerly to guarantee that we see a single published
     * snapshot. If we opened streams lazily then the streams could come
     * from different edits.
     */
    InputStream[] ins = new InputStream[valueCount];19     ... 
    return new Snapshot(key, entry.sequenceNumber, ins);
 }

Bitmap bitmap = null;
String key = getKeyFromUrl(url);
DiskLruCache.Snapshot snapShot = mDiskLruCache.get(key);
if (snapShot != null) {
  FileInputStream fileInputStream = (FileInputStream) snapShot.getInputStream(0); //参数表示索引，同之前的newOutputStream一样
  FileDescriptor fileDescriptor = fileInputStream.getFD();
  bitmap = decodeSampledBitmapFromFD(fileDescriptor, dstWidth, dstHeight);
  if (bitmap != null) {
    addBitmapToMemoryCache(key, bitmap);
  }
}

public Bitmap decodeSampledBitmapFromFD(FileDescriptor fd, int dstWidth, int dstHeight) {
  final BitmapFactory.Options options = new BitmapFactory.Options();
  options.inJustDecodeBounds = true;
  BitmapFactory.decodeFileDescriptor(fd, null, options);
  //calInSampleSize方法的实现请见“Android开发之高效加载Bitmap”这篇博文
  options.inSampleSize = calInSampleSize(options, dstWidth, dstHeight);
  options.inJustDecodeBounds = false;
  return BitmapFactory.decodeFileDescriptor(fd, null, options);
}

public Bitmap loadBitmap(String url, int dstWidth, int dstHeight) {
  Bitmap bitmap = loadFromMemory(url);
  if (bitmap != null) {
    return bitmap;
  }
  //内存缓存中不存在相应图片
  try {
    bitmap = loadFromDisk(url, dstWidth, dstHeight);
    if (bitmap != null) {
      return bitmap;
    }
    //磁盘缓存中也不存在相应图片
    bitmap = loadFromNet(url, dstWidth, dstHeight);
  } catch (IOException e) {
    e.printStackTrace();
  }

  return bitmap;
}

public void displayImage(String url, ImageView imageView, int dstWidth, int widthHeight) {
  imageView.setTag(IMG_URL, url);
  Bitmap bitmap = loadFromMemory(url);
  if (bitmap != null) {
    imageView.setImageBitmap(bitmap);
    return;
  }
  
  Runnable loadBitmapTask = new Runnable() {
    @Override
    public void run() {
      Bitmap bitmap = loadBitmap(url, dstWidth, dstHeigth);
      if (bitmap != null) {
        //Result是我们自定义的类，封装了返回的Bitmap以及它的URL和作为它的容器的ImageView
        Result result = new Result(bitmap, url, imageView);
        //mMainHandler为主线程中创建的Handler
        Message msg = mMainHandler.obtainMessage(MESSAGE_SEND_RESULT, result);
        msg.sendToTarget();
       }
    }
  };
  threadPoolExecutor.execute(loadBitmapTask);
}

private static final int CORE_POOL_SIZE = CPU_COUNT + 1; //corePoolSize为CPU数加1
private static final int MAX_POOL_SIZE = 2 * CPU_COUNT + 1; //maxPoolSize为2倍的CPU数加1
private static final long KEEP_ALIVE = 5L; //存活时间为5s

public static final Executor threadPoolExecutor = new ThreadPoolExecutor(CORE_POOL_SIZE, MAX_POOL_SIZE, KEEP_ALIVE, TimeUnit.SECONDS, new LinkedBlockingQueue<Runnable>());

private Handler mMainHandler = new Handler(Looper.getMainLooper()) {
  @Override
  public void handleMessage(Message msg) {
    Result result = (Result) msg.what;
    ImageView imageView = result.imageView;
    String url = (String) imageView.getTag(IMG_URL);
    if (url.equals(result.url)) {
      imageView.setImageBitmap(result.bitmap);
    } else {
      Log.w(TAG, "The url associated with imageView has changed");
    }
  };
};

private LruCache<String, Bitmap> mMemoryCache;
private DiskLruCache mDiskLruCache;

private ImageLoader(Context context) {
  mContext = context.getApplicationContext();
  int maxMemory = (int) (Runtime.getRuntime().maxMemory() /1024);
  int cacheSize = maxMemory / 8;
  mMemorySize = new LruCache<String, Bitmap>(cacheSize) {
    @Override
    protected int sizeof(String key, Bitmap bitmap) {
      return bitmap.getByteCount() / 1024;
    }
  };
  File diskCacheDir = getAppCacheDir(mContext, "images");
  if (!diskCacheDir.exists()) {
    diskCacheDir.mkdirs();
  }
  if (diskCacheDir.getUsableSpace() > DISK_CACHE_SIZE) { 
    //剩余空间大于我们指定的磁盘缓存大小
    try {
      mDiskLruCache = DiskLruCache.open(diskCacheDir, 1, 1, DISK_CACHE_SIZE);
    } catch （IOException e) {
      e.printStackTrace();
    }
  }
}

private void addToMemoryCache(String key, Bitmap bitmap) {
  if (getFromMemoryCache(key) == null) {
    //不存在时才添加
    mMemoryCache.put(key, bitmap);
  }
}

private Bitmap getFromMemoryCache(String key) {
  return mMemoryCache.get(key);
}
 

private loadFromDiskCache(String url, int dstWidth, int dstHeight) throws IOException {
  if (Looper.myLooper() == Looper.getMainLooper()) {
    //当前运行在主线程，报错
    Log.w(TAG, "should not Bitmap in main thread");
  }
  
  if (mDiskLruCache == null) {
    return null;
  }

  Bitmap bitmap = null;
  String key = getKeyFromUrl(url);
  DiskLruCache.Snapshot snapshot = mDiskLruCache.get(key);
  if (snapshot != null) {
    FileInputStream fileInputStream = (FileInputStream) snapshot.getInputStream(0);
    FileDescriptor fileDescriptor = fileInputStream.getFD();
    bitmap = decodeSampledBitmapFromFD(fileDescriptor, dstWidth, dstHeight);
    if (bitmap != null) {
      addToMemoryCache(key, bitmap);
    }
  }

  return bitmap;
}
  

private Bitmap loadFromNet(String url, int dstWidth, int dstHeight) throws IOException {
  if (Looper.myLooper() == Looper.getMainLooper()) {
    throw new RuntimeException("Do not load Bitmap in main thread.");
  }
  
  if (mDiskLruCache == null) {
    return null;
  }
  
  String key = getKeyFromUrl(url);
  DiskLruCache.Editor editor = mDiskLruCache.edit(key);
  if (editor != null) {
    OutputStream outputStream = editor.newOutputStream(0);
    if (getStreamFromUrl(url, outputStream)) {
      editor.commit();
    } else {
      editor.abort();
    }
    mDiskLruCache.flush();
  }
  return loadFromDiskCache(url, dstWidth, dstHeight);
} 

//加载图片前的时间点
long beforeTime = System.currentTimeMillis();
//加载图片完成的时间点
long afterTime = System.currentTimeMillis();
//total为图片的总数，averTime为加载每张图片所需的平均时间
int averTime = (int) ((afterTime - beforeTime) / total)