Android远程图片获取和本地缓存

核心提示：先定义A缓存： private final HashMap<String, Bitmap>mHardBitmapCache = new LinkedHashMap<String, Bitmap>(HARD_CACHE_CAPACITY/ 2, 0.75f, true) { @Over

1. private final HashMap<String, Bitmap>mHardBitmapCache = new LinkedHashMap<String, Bitmap>(HARD_CACHE_CAPACITY/ 2, 0.75f, true) {
2. @Override
3. protected booleanremoveEldestEntry(LinkedHashMap.Entry<String, Bitmap> eldest) {
4. if (size() >HARD_CACHE_CAPACITY) {
5. //当map的size大于30时，把最近不常用的key放到mSoftBitmapCache中，从而保证mHardBitmapCache的效率
6. mSoftBitmapCache.put(eldest.getKey(), newSoftReference<Bitmap>(eldest.getValue()));
7. return true;
8. } else
9. return false;
10. }
11. };

再定于B缓存：

1. /**
2. *当mHardBitmapCache的key大于30的时候，会根据LRU算法把最近没有被使用的key放入到这个缓存中。
3. *Bitmap使用了SoftReference，当内存空间不足时，此cache中的bitmap会被垃圾回收掉
4. */
5. private final staticConcurrentHashMap<String, SoftReference<Bitmap>> mSoftBitmapCache =new ConcurrentHashMap<String,SoftReference<Bitmap>>(HARD_CACHE_CAPACITY / 2);

从缓存中获取数据：

1. /**
2. * 从缓存中获取图片
3. */
4. private Bitmap getBitmapFromCache(Stringurl) {
5. // 先从mHardBitmapCache缓存中获取
6. synchronized (mHardBitmapCache) {
7. final Bitmap bitmap =mHardBitmapCache.get(url);
8. if (bitmap != null) {
9. //如果找到的话，把元素移到linkedhashmap的最前面，从而保证在LRU算法中是最后被删除
10. mHardBitmapCache.remove(url);
11. mHardBitmapCache.put(url,bitmap);
12. return bitmap;
13. }
14. }
15. //如果mHardBitmapCache中找不到，到mSoftBitmapCache中找
16. SoftReference<Bitmap>bitmapReference = mSoftBitmapCache.get(url);
17. if (bitmapReference != null) {
18. final Bitmap bitmap =bitmapReference.get();
19. if (bitmap != null) {
20. return bitmap;
21. } else {
22. mSoftBitmapCache.remove(url);
23. }
24. }
25. return null;
26. }

如果缓存中不存在，那么就只能去服务器端去下载：

1. /**
2. * 异步下载图片
3. */
4. class ImageDownloaderTask extendsAsyncTask<String, Void, Bitmap> {
5. private static final int IO_BUFFER_SIZE= 4 * 1024;
6. private String url;
7. private finalWeakReference<ImageView> imageViewReference;
8. public ImageDownloaderTask(ImageViewimageView) {
9. imageViewReference = newWeakReference<ImageView>(imageView);
10. }
11. @Override
12. protected BitmapdoInBackground(String... params) {
13. final AndroidHttpClient client =AndroidHttpClient.newInstance("Android");
14. url = params[0];
15. final HttpGet getRequest = newHttpGet(url);
16. try {
17. HttpResponse response =client.execute(getRequest);
18. final int statusCode =response.getStatusLine().getStatusCode();
19. if (statusCode !=HttpStatus.SC_OK) {
20. Log.w(TAG, "从" +url + "中下载图片时出错!,错误码:" + statusCode);
21. return null;
22. }
23. final HttpEntity entity =response.getEntity();
24. if (entity != null) {
25. InputStream inputStream =null;
26. OutputStream outputStream =null;
27. try {
28. inputStream =entity.getContent();
29. finalByteArrayOutputStream dataStream = new ByteArrayOutputStream();
30. outputStream = newBufferedOutputStream(dataStream, IO_BUFFER_SIZE);
31. copy(inputStream,outputStream);
32. outputStream.flush();
33. final byte[] data =dataStream.toByteArray();
34. final Bitmap bitmap =BitmapFactory.decodeByteArray(data, 0, data.length);
35. return bitmap;
36. } finally {
37. if (inputStream !=null) {
38. inputStream.close();
39. }
40. if (outputStream !=null) {
41. outputStream.close();
42. }
43. entity.consumeContent();
44. }
45. }
46. } catch (IOException e) {
47. getRequest.abort();
48. Log.w(TAG, "I/O errorwhile retrieving bitmap from " + url, e);
49. } catch (IllegalStateException e) {
50. getRequest.abort();
51. Log.w(TAG, "Incorrect URL:" + url);
52. } catch (Exception e) {
53. getRequest.abort();
54. Log.w(TAG, "Error whileretrieving bitmap from " + url, e);
55. } finally {
56. if (client != null) {
57. client.close();
58. }
59. }
60. return null;
61. }

这是两种做法，还有一些应用在下载的时候使用了线程池和消息队列MQ，对于图片下载的效率要更好一些。有兴趣的同学可以看下。