Android实现图片缓存与异步加载

ImageManager2这个类具有异步从网络下载图片，从sd读取本地图片，内存缓存，硬盘缓存，图片使用动画渐现等功能，已经将其应用在包含大量图片的应用中一年多，没有出现oom。

Android程序常常会内存溢出，网上也有很多解决方案，如软引用，手动调用recycle等等。但经过我们实践发现这些方案，都没能起到很好的效果，我们的应用依然会出现很多oom，尤其我们的应用包含大量的图片。android3.0之后软引用基本已经失效，因为虚拟机只要碰到软引用就回收，所以带不来任何性能的提升。

我这里的解决方案是HandlerThread(异步加载)+LruCache(内存缓存)+DiskLruCache(硬盘缓存)。

作为程序员，我也不多说，直接和大家共享我的代码，用代码交流更方便些。

package com.example.util;
import java.io.File;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.Queue;
import java.util.Stack;
import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.util.EntityUtils;
import android.app.ActivityManager;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.drawable.BitmapDrawable;
import android.graphics.drawable.ColorDrawable;
import android.graphics.drawable.Drawable;
import android.graphics.drawable.TransitionDrawable;
import android.media.ThumbnailUtils;
import android.os.Handler;
import android.os.HandlerThread;
import android.os.Looper;
import android.os.Message;
import android.support.v4.util.LruCache;
import android.widget.ImageView;
import com.example.MyApplication;

public class ImageManager2 {
  private static ImageManager2 imageManager;
  public LruCache<String, Bitmap> mMemoryCache;
  private static final int DISK_CACHE_SIZE = 1024 * 1024 * 20; // 10MB
  private static final String DISK_CACHE_SUBDIR = "thumbnails";
  public DiskLruCache mDiskCache;
  private static MyApplication myapp;
  
  private Stack<ImageRef> mImageQueue = new Stack<ImageRef>();
  
  private Queue<ImageRef> mRequestQueue = new LinkedList<ImageRef>();
  
  private Handler mImageLoaderHandler;
  
  private boolean mImageLoaderIdle = true;
  
  private static final int MSG_REQUEST = 1;
  
  private static final int MSG_REPLY = 2;
  
  private static final int MSG_STOP = 3;
  
  private boolean isFromNet = true;
  
  public static ImageManager2 from(Context context) {
    // 如果不在ui线程中，则抛出异常
    if (Looper.myLooper() != Looper.getMainLooper()) {
      throw new RuntimeException("Cannot instantiate outside UI thread.");
    }
    if (myapp == null) {
      myapp = (MyApplication) context.getApplicationContext();
    }
    if (imageManager == null) {
      imageManager = new ImageManager2(myapp);
    }
    return imageManager;
  }
  
  private ImageManager2(Context context) {
    int memClass = ((ActivityManager) context
        .getSystemService(Context.ACTIVITY_SERVICE)).getMemoryClass();
    memClass = memClass > 32 ? 32 : memClass;
    // 使用可用内存的1/8作为图片缓存
    final int cacheSize = 1024 * 1024 * memClass / 8;
    mMemoryCache = new LruCache<String, Bitmap>(cacheSize) {
      protected int sizeOf(String key, Bitmap bitmap) {
        return bitmap.getRowBytes() * bitmap.getHeight();
      }
    };
    File cacheDir = DiskLruCache
        .getDiskCacheDir(context, DISK_CACHE_SUBDIR);
    mDiskCache = DiskLruCache.openCache(context, cacheDir, DISK_CACHE_SIZE);
  }
  
  class ImageRef {
    
    ImageView imageView;
    
    String url;
    
    String filePath;
    
    int resId;
    int width = 0;
    int height = 0;
    
    ImageRef(ImageView imageView, String url, String filePath, int resId) {
      this.imageView = imageView;
      this.url = url;
      this.filePath = filePath;
      this.resId = resId;
    }
    ImageRef(ImageView imageView, String url, String filePath, int resId,
        int width, int height) {
      this.imageView = imageView;
      this.url = url;
      this.filePath = filePath;
      this.resId = resId;
      this.width = width;
      this.height = height;
    }
  }
  
  public void displayImage(ImageView imageView, String url, int resId) {
    if (imageView == null) {
      return;
    }
    if (imageView.getTag() != null
        && imageView.getTag().toString().equals(url)) {
      return;
    }
    if (resId >= 0) {
      if (imageView.getBackground() == null) {
        imageView.setBackgroundResource(resId);
      }
      imageView.setImageDrawable(null);
    }
    if (url == null || url.equals("")) {
      return;
    }
    // 添加url tag
    imageView.setTag(url);
    // 读取map缓存
    Bitmap bitmap = mMemoryCache.get(url);
    if (bitmap != null) {
      setImageBitmap(imageView, bitmap, false);
      return;
    }
    // 生成文件名
    String filePath = urlToFilePath(url);
    if (filePath == null) {
      return;
    }
    queueImage(new ImageRef(imageView, url, filePath, resId));
  }
  
  public void displayImage(ImageView imageView, String url, int resId,
      int width, int height) {
    if (imageView == null) {
      return;
    }
    if (resId >= 0) {
      if (imageView.getBackground() == null) {
        imageView.setBackgroundResource(resId);
      }
      imageView.setImageDrawable(null);
    }
    if (url == null || url.equals("")) {
      return;
    }
    // 添加url tag
    imageView.setTag(url);
    // 读取map缓存
    Bitmap bitmap = mMemoryCache.get(url + width + height);
    if (bitmap != null) {
      setImageBitmap(imageView, bitmap, false);
      return;
    }
    // 生成文件名
    String filePath = urlToFilePath(url);
    if (filePath == null) {
      return;
    }
    queueImage(new ImageRef(imageView, url, filePath, resId, width, height));
  }
  
  public void queueImage(ImageRef imageRef) {
    // 删除已有ImageView
    Iterator<ImageRef> iterator = mImageQueue.iterator();
    while (iterator.hasNext()) {
      if (iterator.next().imageView == imageRef.imageView) {
        iterator.remove();
      }
    }
    // 添加请求
    mImageQueue.push(imageRef);
    sendRequest();
  }
  
  private void sendRequest() {
    // 开启图片加载线程
    if (mImageLoaderHandler == null) {
      HandlerThread imageLoader = new HandlerThread("image_loader");
      imageLoader.start();
      mImageLoaderHandler = new ImageLoaderHandler(
          imageLoader.getLooper());
    }
    // 发送请求
    if (mImageLoaderIdle && mImageQueue.size() > 0) {
      ImageRef imageRef = mImageQueue.pop();
      Message message = mImageLoaderHandler.obtainMessage(MSG_REQUEST,
          imageRef);
      mImageLoaderHandler.sendMessage(message);
      mImageLoaderIdle = false;
      mRequestQueue.add(imageRef);
    }
  }
  
  class ImageLoaderHandler extends Handler {
    public ImageLoaderHandler(Looper looper) {
      super(looper);
    }
    public void handleMessage(Message msg) {
      if (msg == null)
        return;
      switch (msg.what) {
      case MSG_REQUEST: // 收到请求
        Bitmap bitmap = null;
        Bitmap tBitmap = null;
        if (msg.obj != null && msg.obj instanceof ImageRef) {
          ImageRef imageRef = (ImageRef) msg.obj;
          String url = imageRef.url;
          if (url == null)
            return;
          // 如果本地url即读取sd相册图片，则直接读取，不用经过DiskCache
          if (url.toLowerCase().contains("dcim")) {
            tBitmap = null;
            BitmapFactory.Options opt = new BitmapFactory.Options();
            opt.inSampleSize = 1;
            opt.inJustDecodeBounds = true;
            BitmapFactory.decodeFile(url, opt);
            int bitmapSize = opt.outHeight * opt.outWidth * 4;
            opt.inSampleSize = bitmapSize / (1000 * 2000);
            opt.inJustDecodeBounds = false;
            tBitmap = BitmapFactory.decodeFile(url, opt);
            if (imageRef.width != 0 && imageRef.height != 0) {
              bitmap = ThumbnailUtils.extractThumbnail(tBitmap,
                  imageRef.width, imageRef.height,
                  ThumbnailUtils.OPTIONS_RECYCLE_INPUT);
              isFromNet = true;
            } else {
              bitmap = tBitmap;
              tBitmap = null;
            }
          } else
            bitmap = mDiskCache.get(url);
          if (bitmap != null) {
            // ToolUtil.log("从disk缓存读取");
            // 写入map缓存
            if (imageRef.width != 0 && imageRef.height != 0) {
              if (mMemoryCache.get(url + imageRef.width
                  + imageRef.height) == null)
                mMemoryCache.put(url + imageRef.width
                    + imageRef.height, bitmap);
            } else {
              if (mMemoryCache.get(url) == null)
                mMemoryCache.put(url, bitmap);
            }
          } else {
            try {
              byte[] data = loadByteArrayFromNetwork(url);
              if (data != null) {
                BitmapFactory.Options opt = new BitmapFactory.Options();
                opt.inSampleSize = 1;
                opt.inJustDecodeBounds = true;
                BitmapFactory.decodeByteArray(data, 0,
                    data.length, opt);
                int bitmapSize = opt.outHeight * opt.outWidth
                    * 4;// pixels*3 if it's RGB and pixels*4
                      // if it's ARGB
                if (bitmapSize > 1000 * 1200)
                  opt.inSampleSize = 2;
                opt.inJustDecodeBounds = false;
                tBitmap = BitmapFactory.decodeByteArray(data,
                    0, data.length, opt);
                if (imageRef.width != 0 && imageRef.height != 0) {
                  bitmap = ThumbnailUtils
                      .extractThumbnail(
                          tBitmap,
                          imageRef.width,
                          imageRef.height,
                          ThumbnailUtils.OPTIONS_RECYCLE_INPUT);
                } else {
                  bitmap = tBitmap;
                  tBitmap = null;
                }
                if (bitmap != null && url != null) {
                  // 写入SD卡
                  if (imageRef.width != 0
                      && imageRef.height != 0) {
                    mDiskCache.put(url + imageRef.width
                        + imageRef.height, bitmap);
                    mMemoryCache.put(url + imageRef.width
                        + imageRef.height, bitmap);
                  } else {
                    mDiskCache.put(url, bitmap);
                    mMemoryCache.put(url, bitmap);
                  }
                  isFromNet = true;
                }
              }
            } catch (OutOfMemoryError e) {
            }
          }
        }
        if (mImageManagerHandler != null) {
          Message message = mImageManagerHandler.obtainMessage(
              MSG_REPLY, bitmap);
          mImageManagerHandler.sendMessage(message);
        }
        break;
      case MSG_STOP: // 收到终止指令
        Looper.myLooper().quit();
        break;
      }
    }
  }
  
  private Handler mImageManagerHandler = new Handler() {
    @Override
    public void handleMessage(Message msg) {
      if (msg != null) {
        switch (msg.what) {
        case MSG_REPLY: // 收到应答
          do {
            ImageRef imageRef = mRequestQueue.remove();
            if (imageRef == null)
              break;
            if (imageRef.imageView == null
                || imageRef.imageView.getTag() == null
                || imageRef.url == null)
              break;
            if (!(msg.obj instanceof Bitmap) || msg.obj == null) {
              break;
            }
            Bitmap bitmap = (Bitmap) msg.obj;
            // 非同一ImageView
            if (!(imageRef.url).equals((String) imageRef.imageView
                .getTag())) {
              break;
            }
            setImageBitmap(imageRef.imageView, bitmap, isFromNet);
            isFromNet = false;
          } while (false);
          break;
        }
      }
      // 设置闲置标志
      mImageLoaderIdle = true;
      // 若服务未关闭，则发送下一个请求。
      if (mImageLoaderHandler != null) {
        sendRequest();
      }
    }
  };
  
  private void setImageBitmap(ImageView imageView, Bitmap bitmap,
      boolean isTran) {
    if (isTran) {
      final TransitionDrawable td = new TransitionDrawable(
          new Drawable[] {
              new ColorDrawable(android.R.color.transparent),
              new BitmapDrawable(bitmap) });
      td.setCrossFadeEnabled(true);
      imageView.setImageDrawable(td);
      td.startTransition(300);
    } else {
      imageView.setImageBitmap(bitmap);
    }
  }
  
  private byte[] loadByteArrayFromNetwork(String url) {
    try {
      HttpGet method = new HttpGet(url);
      HttpResponse response = myapp.getHttpClient().execute(method);
      HttpEntity entity = response.getEntity();
      return EntityUtils.toByteArray(entity);
    } catch (Exception e) {
      return null;
    }
  }
  
  public String urlToFilePath(String url) {
    // 扩展名位置
    int index = url.lastIndexOf('.');
    if (index == -1) {
      return null;
    }
    StringBuilder filePath = new StringBuilder();
    // 图片存取路径
    filePath.append(myapp.getCacheDir().toString()).append('/');
    // 图片文件名 
    filePath.append(MD5.Md5(url)).append(url.substring(index));
    return filePath.toString();
  }
  
  public void stop() {
    // 清空请求队列
    mImageQueue.clear();
  }
}

这里就是给出了异步加载、内存缓存和硬盘缓存的解决方案，希望对大家的学习有所帮助。

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