1、采用线程池
2、内存缓存+文件缓存
3、内存缓存中网上很多是采用SoftReference来防止堆溢出,这儿严格限制只能使用最大JVM内存的1/4
4、对下载的图片进行按比例缩放,以减少内存的消耗
具体的代码里面说明。先放上内存缓存类的代码MemoryCache.java:
public class MemoryCache { private static final String TAG = "MemoryCache"; // 放入缓存时是个同步操作 // LinkedHashMap构造方法的最后一个参数true代表这个map里的元素将按照最近使用次数由少到多排列,即LRU // 这样的好处是如果要将缓存中的元素替换,则先遍历出最近最少使用的元素来替换以提高效率 private Map<String, Bitmap> cache = Collections .synchronizedMap(new LinkedHashMap<String, Bitmap>(10, 1.5f, true)); // 缓存中图片所占用的字节,初始0,将通过此变量严格控制缓存所占用的堆内存 private long size = 0;// current allocated size // 缓存只能占用的最大堆内存 private long limit = 1000000;// max memory in bytes public MemoryCache() { // use 25% of available heap size setLimit(Runtime.getRuntime().maxMemory() / 4); } public void setLimit(long new_limit) { limit = new_limit; Log.i(TAG, "MemoryCache will use up to " + limit / 1024. / 1024. + "MB"); } public Bitmap get(String id) { try { if (!cache.containsKey(id)) return null; return cache.get(id); } catch (NullPointerException ex) { return null; } } public void put(String id, Bitmap bitmap) { try { if (cache.containsKey(id)) size -= getSizeInBytes(cache.get(id)); cache.put(id, bitmap); size += getSizeInBytes(bitmap); checkSize(); } catch (Throwable th) { th.printStackTrace(); } } /** * 严格控制堆内存,如果超过将首先替换最近最少使用的那个图片缓存 * */ private void checkSize() { Log.i(TAG, "cache size=" + size + " length=" + cache.size()); if (size > limit) { // 先遍历最近最少使用的元素 Iterator<Entry<String, Bitmap>> iter = cache.entrySet().iterator(); while (iter.hasNext()) { Entry<String, Bitmap> entry = iter.next(); size -= getSizeInBytes(entry.getValue()); iter.remove(); if (size <= limit) break; } Log.i(TAG, "Clean cache. New size " + cache.size()); } } public void clear() { cache.clear(); } /** * 图片占用的内存 * * @param bitmap * @return */ long getSizeInBytes(Bitmap bitmap) { if (bitmap == null) return 0; return bitmap.getRowBytes() * bitmap.getHeight(); } }
也可以使用SoftReference,代码会简单很多,但是我推荐上面的方法。
public class MemoryCache { private Map<String, SoftReference<Bitmap>> cache = Collections .synchronizedMap(new HashMap<String, SoftReference<Bitmap>>()); public Bitmap get(String id) { if (!cache.containsKey(id)) return null; SoftReference<Bitmap> ref = cache.get(id); return ref.get(); } public void put(String id, Bitmap bitmap) { cache.put(id, new SoftReference<Bitmap>(bitmap)); } public void clear() { cache.clear(); } }
下面是文件缓存类的代码FileCache.java:
public class FileCache { private File cacheDir; public FileCache(Context context) { // 如果有SD卡则在SD卡中建一个LazyList的目录存放缓存的图片 // 没有SD卡就放在系统的缓存目录中 if (android.os.Environment.getExternalStorageState().equals( android.os.Environment.MEDIA_MOUNTED)) cacheDir = new File( android.os.Environment.getExternalStorageDirectory(), "LazyList"); else cacheDir = context.getCacheDir(); if (!cacheDir.exists()) cacheDir.mkdirs(); } public File getFile(String url) { // 将url的hashCode作为缓存的文件名 String filename = String.valueOf(url.hashCode()); // Another possible solution // String filename = URLEncoder.encode(url); File f = new File(cacheDir, filename); return f; } public void clear() { File[] files = cacheDir.listFiles(); if (files == null) return; for (File f : files) f.delete(); } }
最后最重要的加载图片的类,ImageLoader.java:
public class ImageLoader { MemoryCache memoryCache = new MemoryCache(); FileCache fileCache; private Map<ImageView, String> imageViews = Collections .synchronizedMap(new WeakHashMap<ImageView, String>()); // 线程池 ExecutorService executorService; public ImageLoader(Context context) { fileCache = new FileCache(context); executorService = Executors.newFixedThreadPool(5); } // 当进入listview时默认的图片,可换成你自己的默认图片 final int stub_id = R.drawable.stub; // 最主要的方法 public void DisplayImage(String url, ImageView imageView) { imageViews.put(imageView, url); // 先从内存缓存中查找 Bitmap bitmap = memoryCache.get(url); if (bitmap != null) imageView.setImageBitmap(bitmap); else { // 若没有的话则开启新线程加载图片 queuePhoto(url, imageView); imageView.setImageResource(stub_id); } } private void queuePhoto(String url, ImageView imageView) { PhotoToLoad p = new PhotoToLoad(url, imageView); executorService.submit(new PhotosLoader(p)); } private Bitmap getBitmap(String url) { File f = fileCache.getFile(url); // 先从文件缓存中查找是否有 Bitmap b = decodeFile(f); if (b != null) return b; // 最后从指定的url中下载图片 try { Bitmap bitmap = null; URL imageUrl = new URL(url); HttpURLConnection conn = (HttpURLConnection) imageUrl .openConnection(); conn.setConnectTimeout(30000); conn.setReadTimeout(30000); conn.setInstanceFollowRedirects(true); InputStream is = conn.getInputStream(); OutputStream os = new FileOutputStream(f); CopyStream(is, os); os.close(); bitmap = decodeFile(f); return bitmap; } catch (Exception ex) { ex.printStackTrace(); return null; } } // decode这个图片并且按比例缩放以减少内存消耗,虚拟机对每张图片的缓存大小也是有限制的 private Bitmap decodeFile(File f) { try { // decode image size BitmapFactory.Options o = new BitmapFactory.Options(); o.inJustDecodeBounds = true; BitmapFactory.decodeStream(new FileInputStream(f), null, o); // Find the correct scale value. It should be the power of 2. final int REQUIRED_SIZE = 70; int width_tmp = o.outWidth, height_tmp = o.outHeight; int scale = 1; while (true) { if (width_tmp / 2 < REQUIRED_SIZE || height_tmp / 2 < REQUIRED_SIZE) break; width_tmp /= 2; height_tmp /= 2; scale *= 2; } // decode with inSampleSize BitmapFactory.Options o2 = new BitmapFactory.Options(); o2.inSampleSize = scale; return BitmapFactory.decodeStream(new FileInputStream(f), null, o2); } catch (FileNotFoundException e) { } return null; } // Task for the queue private class PhotoToLoad { public String url; public ImageView imageView; public PhotoToLoad(String u, ImageView i) { url = u; imageView = i; } } class PhotosLoader implements Runnable { PhotoToLoad photoToLoad; PhotosLoader(PhotoToLoad photoToLoad) { this.photoToLoad = photoToLoad; } @Override public void run() { if (imageViewReused(photoToLoad)) return; Bitmap bmp = getBitmap(photoToLoad.url); memoryCache.put(photoToLoad.url, bmp); if (imageViewReused(photoToLoad)) return; BitmapDisplayer bd = new BitmapDisplayer(bmp, photoToLoad); // 更新的操作放在UI线程中 Activity a = (Activity) photoToLoad.imageView.getContext(); a.runOnUiThread(bd); } } /** * 防止图片错位 * * @param photoToLoad * @return */ boolean imageViewReused(PhotoToLoad photoToLoad) { String tag = imageViews.get(photoToLoad.imageView); if (tag == null || !tag.equals(photoToLoad.url)) return true; return false; } // 用于在UI线程中更新界面 class BitmapDisplayer implements Runnable { Bitmap bitmap; PhotoToLoad photoToLoad; public BitmapDisplayer(Bitmap b, PhotoToLoad p) { bitmap = b; photoToLoad = p; } public void run() { if (imageViewReused(photoToLoad)) return; if (bitmap != null) photoToLoad.imageView.setImageBitmap(bitmap); else photoToLoad.imageView.setImageResource(stub_id); } } public void clearCache() { memoryCache.clear(); fileCache.clear(); } public static void CopyStream(InputStream is, OutputStream os) { final int buffer_size = 1024; try { byte[] bytes = new byte[buffer_size]; for (;;) { int count = is.read(bytes, 0, buffer_size); if (count == -1) break; os.write(bytes, 0, count); } } catch (Exception ex) { } } }
主要流程是先从内存缓存中查找,若没有再开线程,从文件缓存中查找都没有则从指定的url中查找,并对bitmap进行处理,最后通过下面方法对UI进行更新操作。
a.runOnUiThread(...);
在你的程序中的基本用法:
ImageLoader imageLoader=new ImageLoader(context); ... imageLoader.DisplayImage(url, imageView);
转自:http://www.jb51.net/article/37793.htm
时间: 2024-11-10 01:12:29