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如果每次加载同一张图片都要从网络获取，那代价实在太大了。所以同一张图片只要从网络获取一次就够了，然后在本地缓存起来，之后加载同一张图片时就从缓存中加载就可以了。从内存缓存读取图片是最快的，但是因为内存容量有限，所以最好再加上文件缓存。文件缓存空间也不是无限大的，容量越大读取效率越低，因此可以设置一个限定大小比如10M，或者限定保存时间比如一天。
因此，加载图片的流程应该是：
1、先从内存缓存中获取，取到则返回，取不到则进行下一步；
2、从文件缓存中获取，取到则返回并更新到内存缓存，取不到则进行下一步；
3、从网络下载图片，并更新到内存缓存和文件缓存。
接下来看内存缓存类：ImageMemoryCache
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 | public class ImageMemoryCache { /** * 从内存读取数据速度是最快的，为了更大限度使用内存，这里使用了两层缓存。 * 硬引用缓存不会轻易被回收，用来保存常用数据，不常用的转入软引用缓存。 */ private static final int SOFT_CACHE_SIZE = 15 ; //软引用缓存容量 private static LruCache<String, Bitmap> mLruCache; //硬引用缓存 private static LinkedHashMap<String, SoftReference<Bitmap>> mSoftCache; //软引用缓存 public ImageMemoryCache(Context context) { int memClass = ((ActivityManager)context.getSystemService(Context.ACTIVITY_SERVICE)).getMemoryClass(); int cacheSize = 1024 * 1024 * memClass / 4 ; //硬引用缓存容量，为系统可用内存的1/4 mLruCache = new LruCache<String, Bitmap>(cacheSize) { @Override protected int sizeOf(String key, Bitmap value) { if (value != null ) return value.getRowBytes() * value.getHeight(); else return 0 ; } @Override protected void entryRemoved( boolean evicted, String key, Bitmap oldValue, Bitmap newValue) { if (oldValue != null ) // 硬引用缓存容量满的时候，会根据LRU算法把最近没有被使用的图片转入此软引用缓存 mSoftCache.put(key, new SoftReference<Bitmap>(oldValue)); } }; mSoftCache = new LinkedHashMap<String, SoftReference<Bitmap>>(SOFT_CACHE_SIZE, 0 .75f, true ) { private static final long serialVersionUID = 6040103833179403725L; @Override protected boolean removeEldestEntry(Entry<String, SoftReference<Bitmap>> eldest) { if (size() > SOFT_CACHE_SIZE){ return true ; } return false ; } }; } /** * 从缓存中获取图片 */ public Bitmap getBitmapFromCache(String url) { Bitmap bitmap; //先从硬引用缓存中获取 synchronized (mLruCache) { bitmap = mLruCache.get(url); if (bitmap != null ) { //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 mLruCache.remove(url); mLruCache.put(url, bitmap); return bitmap; } } //如果硬引用缓存中找不到，到软引用缓存中找 synchronized (mSoftCache) { SoftReference<Bitmap> bitmapReference = mSoftCache.get(url); if (bitmapReference != null ) { bitmap = bitmapReference.get(); if (bitmap != null ) { //将图片移回硬缓存 mLruCache.put(url, bitmap); mSoftCache.remove(url); return bitmap; } else { mSoftCache.remove(url); } } } return null ; } /** * 添加图片到缓存 */ public void addBitmapToCache(String url, Bitmap bitmap) { if (bitmap != null ) { synchronized (mLruCache) { mLruCache.put(url, bitmap); } } } public void clearCache() { mSoftCache.clear(); } } |
文件缓存类：ImageFileCache
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 | public class ImageFileCache { private static final String CACHDIR = "ImgCach" ; private static final String WHOLESALE_CONV = ".cach" ; private static final int MB = 1024 * 1024 ; private static final int CACHE_SIZE = 10 ; private static final int FREE_SD_SPACE_NEEDED_TO_CACHE = 10 ; public ImageFileCache() { //清理文件缓存 removeCache(getDirectory()); } /** 从缓存中获取图片 **/ public Bitmap getImage( final String url) { final String path = getDirectory() + "/" + convertUrlToFileName(url); File file = new File(path); if (file.exists()) { Bitmap bmp = BitmapFactory.decodeFile(path); if (bmp == null ) { file.delete(); } else { updateFileTime(path); return bmp; } } return null ; } /** 将图片存入文件缓存 **/ public void saveBitmap(Bitmap bm, String url) { if (bm == null ) { return ; } //判断sdcard上的空间 if (FREE_SD_SPACE_NEEDED_TO_CACHE > freeSpaceOnSd()) { //SD空间不足 return ; } String filename = convertUrlToFileName(url); String dir = getDirectory(); File dirFile = new File(dir); if (!dirFile.exists()) dirFile.mkdirs(); File file = new File(dir + "/" + filename); try { file.createNewFile(); OutputStream outStream = new FileOutputStream(file); bm.compress(Bitmap.CompressFormat.JPEG, 100 , outStream); outStream.flush(); outStream.close(); } catch (FileNotFoundException e) { Log.w( "ImageFileCache" , "FileNotFoundException" ); } catch (IOException e) { Log.w( "ImageFileCache" , "IOException" ); } } /** * 计算存储目录下的文件大小， * 当文件总大小大于规定的CACHE_SIZE或者sdcard剩余空间小于FREE_SD_SPACE_NEEDED_TO_CACHE的规定 * 那么删除40%最近没有被使用的文件 */ private boolean removeCache(String dirPath) { File dir = new File(dirPath); File[] files = dir.listFiles(); if (files == null ) { return true ; } if (!android.os.Environment.getExternalStorageState().equals( android.os.Environment.MEDIA_MOUNTED)) { return false ; } int dirSize = 0 ; for ( int i = 0 ; i < files.length; i++) { if (files[i].getName().contains(WHOLESALE_CONV)) { dirSize += files[i].length(); } } if (dirSize > CACHE_SIZE * MB || FREE_SD_SPACE_NEEDED_TO_CACHE > freeSpaceOnSd()) { int removeFactor = ( int ) (( 0.4 * files.length) + 1 ); Arrays.sort(files, new FileLastModifSort()); for ( int i = 0 ; i < removeFactor; i++) { if (files[i].getName().contains(WHOLESALE_CONV)) { files[i].delete(); } } } if (freeSpaceOnSd() <= CACHE_SIZE) { return false ; } return true ; } /** 修改文件的最后修改时间 **/ public void updateFileTime(String path) { File file = new File(path); long newModifiedTime = System.currentTimeMillis(); file.setLastModified(newModifiedTime); } /** 计算sdcard上的剩余空间 **/ private int freeSpaceOnSd() { StatFs stat = new StatFs(Environment.getExternalStorageDirectory().getPath()); double sdFreeMB = (( double )stat.getAvailableBlocks() * ( double ) stat.getBlockSize()) / MB; return ( int ) sdFreeMB; } /** 将url转成文件名 **/ private String convertUrlToFileName(String url) { String[] strs = url.split( "/" ); return strs[strs.length - 1 ] + WHOLESALE_CONV; } /** 获得缓存目录 **/ private String getDirectory() { String dir = getSDPath() + "/" + CACHDIR; return dir; } /** 取SD卡路径 **/ private String getSDPath() { File sdDir = null ; boolean sdCardExist = Environment.getExternalStorageState().equals( android.os.Environment.MEDIA_MOUNTED); //判断sd卡是否存在 if (sdCardExist) { sdDir = Environment.getExternalStorageDirectory(); //获取根目录 } if (sdDir != null ) { return sdDir.toString(); } else { return "" ; } } /** * 根据文件的最后修改时间进行排序 */ private class FileLastModifSort implements Comparator<File> { public int compare(File arg0, File arg1) { if (arg0.lastModified() > arg1.lastModified()) { return 1 ; } else if (arg0.lastModified() == arg1.lastModified()) { return 0 ; } else { return - 1 ; } } } } |
从网络获取图片：
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 | public class ImageGetFromHttp { private static final String LOG_TAG = "ImageGetFromHttp" ; public static Bitmap downloadBitmap(String url) { final HttpClient client = new DefaultHttpClient(); final HttpGet getRequest = new HttpGet(url); try { HttpResponse response = client.execute(getRequest); final int statusCode = response.getStatusLine().getStatusCode(); if (statusCode != HttpStatus.SC_OK) { Log.w(LOG_TAG, "Error " + statusCode + " while retrieving bitmap from " + url); return null ; } final HttpEntity entity = response.getEntity(); if (entity != null ) { InputStream inputStream = null ; try { inputStream = entity.getContent(); FilterInputStream fit = new FlushedInputStream(inputStream); return BitmapFactory.decodeStream(fit); } finally { if (inputStream != null ) { inputStream.close(); inputStream = null ; } entity.consumeContent(); } } } catch (IOException e) { getRequest.abort(); Log.w(LOG_TAG, "I/O error while retrieving bitmap from " + url, e); } catch (IllegalStateException e) { getRequest.abort(); Log.w(LOG_TAG, "Incorrect URL: " + url); } catch (Exception e) { getRequest.abort(); Log.w(LOG_TAG, "Error while retrieving bitmap from " + url, e); } finally { client.getConnectionManager().shutdown(); } return null ; } /* * An InputStream that skips the exact number of bytes provided, unless it reaches EOF. */ static class FlushedInputStream extends FilterInputStream { public FlushedInputStream(InputStream inputStream) { super (inputStream); } @Override public long skip( long n) throws IOException { long totalBytesSkipped = 0L; while (totalBytesSkipped < n) { long bytesSkipped = in.skip(n - totalBytesSkipped); if (bytesSkipped == 0L) { int b = read(); if (b < 0 ) { break ; // we reached EOF } else { bytesSkipped = 1 ; // we read one byte } } totalBytesSkipped += bytesSkipped; } return totalBytesSkipped; } } } |
最后，获取一张图片的流程就如下代码所示：
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 | /*** 获得一张图片,从三个地方获取,首先是内存缓存,然后是文件缓存,最后从网络获取 ***/ public Bitmap getBitmap(String url) { // 从内存缓存中获取图片 Bitmap result = memoryCache.getBitmapFromCache(url); if (result == null ) { // 文件缓存中获取 result = fileCache.getImage(url); if (result == null ) { // 从网络获取 result = ImageGetFromHttp.downloadBitmap(url); if (result != null ) { fileCache.saveBitmap(result, url); memoryCache.addBitmapToCache(url, result); } } else { // 添加到内存缓存 memoryCache.addBitmapToCache(url, result); } } return result; } |