Android.异步加载图片.使用LruCache和SD卡或手机缓存

亦凉 2022-06-04 08:52 243阅读 0赞

#### **技术概述:** ####

1.使用了线程池来管理下载任务  
2.使用LruCache来缓存图片  
3.使用手机来缓存图片  
4.GridView滑动的时候取消下载任务，静止的时候进行下载，GridView滑动更加的流畅  
5.降低了代码的耦合性，结构更加的清晰，便于以后重用

接下来我们先来看看项目的结构:  
![这里写图片描述][SouthEast]

**FileUtils 文件操作的工具类，提供保存图片，获取图片，判断图片是否存在，删除图片的一些方法，这个类比较简单**

package com.example.asyncimageloader;
    
    import java.io.File;
    import java.io.FileOutputStream;
    import java.io.IOException;
    
    import android.content.Context;
    import android.graphics.Bitmap;
    import android.graphics.Bitmap.CompressFormat;
    import android.graphics.BitmapFactory;
    import android.os.Environment;
    
    public class FileUtils { 
        /** * sd卡的根目录 */
        private static String mSdRootPath = Environment.getExternalStorageDirectory().getPath();
        /** * 手机的缓存根目录 */
        private static String mDataRootPath = null;
        /** * 保存Image的目录名 */
        private final static String FOLDER_NAME = "/AndroidImage";
    
    
        public FileUtils(Context context){
            mDataRootPath = context.getCacheDir().getPath();
        }
    
    
        /** * 获取储存Image的目录 * @return */
        private String getStorageDirectory(){
            return Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED) ?
                    mSdRootPath + FOLDER_NAME : mDataRootPath + FOLDER_NAME;
        }
    
        /** * 保存Image的方法，有sd卡存储到sd卡，没有就存储到手机目录 * @param fileName * @param bitmap * @throws IOException */
        public void savaBitmap(String fileName, Bitmap bitmap) throws IOException{
            if(bitmap == null){
                return;
            }
            String path = getStorageDirectory();
            File folderFile = new File(path);
            if(!folderFile.exists()){
                folderFile.mkdir();
            }
            File file = new File(path + File.separator + fileName);
            file.createNewFile();
            FileOutputStream fos = new FileOutputStream(file);
            bitmap.compress(CompressFormat.JPEG, 100, fos);
            fos.flush();
            fos.close();
        }
    
        /** * 从手机或者sd卡获取Bitmap * @param fileName * @return */
        public Bitmap getBitmap(String fileName){
            return BitmapFactory.decodeFile(getStorageDirectory() + File.separator + fileName);
        }
    
        /** * 判断文件是否存在 * @param fileName * @return */
        public boolean isFileExists(String fileName){
            return new File(getStorageDirectory() + File.separator + fileName).exists();
        }
    
        /** * 获取文件的大小 * @param fileName * @return */
        public long getFileSize(String fileName) {
            return new File(getStorageDirectory() + File.separator + fileName).length();
        }
    
    
        /** * 删除SD卡或者手机的缓存图片和目录 */
        public void deleteFile() {
            File dirFile = new File(getStorageDirectory());
            if(! dirFile.exists()){
                return;
            }
            if (dirFile.isDirectory()) {
                String[] children = dirFile.list();
                for (int i = 0; i < children.length; i++) {
                    new File(dirFile, children[i]).delete();
                }
            }
    
            dirFile.delete();
        }
    }

**Images类，提供图片资源的类，来自Android文档中BitmapFun中[http://developer.android.com/training/displaying-bitmaps/index.html][http_developer.android.com_training_displaying-bitmaps_index.html]，里面一个静态的图片Url数组，这里面比较坑爹，因为是图片是google官网提供的，有时候会下载超时**

package com.example.asyncimageloader;
    
    public class Images {
    
        public final static String[] imageThumbUrls = new String[] {
                "https://lh6.googleusercontent.com/-55osAWw3x0Q/URquUtcFr5I/AAAAAAAAAbs/rWlj1RUKrYI/s160-c/A%252520Photographer.jpg",
                "https://lh4.googleusercontent.com/--dq8niRp7W4/URquVgmXvgI/AAAAAAAAAbs/-gnuLQfNnBA/s160-c/A%252520Song%252520of%252520Ice%252520and%252520Fire.jpg",
                "https://lh5.googleusercontent.com/-7qZeDtRKFKc/URquWZT1gOI/AAAAAAAAAbs/hqWgteyNXsg/s160-c/Another%252520Rockaway%252520Sunset.jpg",
                "https://lh3.googleusercontent.com/--L0Km39l5J8/URquXHGcdNI/AAAAAAAAAbs/3ZrSJNrSomQ/s160-c/Antelope%252520Butte.jpg",
                "https://lh6.googleusercontent.com/-8HO-4vIFnlw/URquZnsFgtI/AAAAAAAAAbs/WT8jViTF7vw/s160-c/Antelope%252520Hallway.jpg",
                "https://lh4.googleusercontent.com/-WIuWgVcU3Qw/URqubRVcj4I/AAAAAAAAAbs/YvbwgGjwdIQ/s160-c/Antelope%252520Walls.jpg",
                "https://lh6.googleusercontent.com/-UBmLbPELvoQ/URqucCdv0kI/AAAAAAAAAbs/IdNhr2VQoQs/s160-c/Apre%2525CC%252580s%252520la%252520Pluie.jpg",
                "https://lh3.googleusercontent.com/-s-AFpvgSeew/URquc6dF-JI/AAAAAAAAAbs/Mt3xNGRUd68/s160-c/Backlit%252520Cloud.jpg",
                "https://lh5.googleusercontent.com/-bvmif9a9YOQ/URquea3heHI/AAAAAAAAAbs/rcr6wyeQtAo/s160-c/Bee%252520and%252520Flower.jpg",
                "https://lh5.googleusercontent.com/-n7mdm7I7FGs/URqueT_BT-I/AAAAAAAAAbs/9MYmXlmpSAo/s160-c/Bonzai%252520Rock%252520Sunset.jpg",
                "https://lh6.googleusercontent.com/-4CN4X4t0M1k/URqufPozWzI/AAAAAAAAAbs/8wK41lg1KPs/s160-c/Caterpillar.jpg",
                "https://lh3.googleusercontent.com/-rrFnVC8xQEg/URqufdrLBaI/AAAAAAAAAbs/s69WYy_fl1E/s160-c/Chess.jpg",
                "https://lh5.googleusercontent.com/-WVpRptWH8Yw/URqugh-QmDI/AAAAAAAAAbs/E-MgBgtlUWU/s160-c/Chihuly.jpg",
                "https://lh5.googleusercontent.com/-0BDXkYmckbo/URquhKFW84I/AAAAAAAAAbs/ogQtHCTk2JQ/s160-c/Closed%252520Door.jpg",
                "https://lh3.googleusercontent.com/-PyggXXZRykM/URquh-kVvoI/AAAAAAAAAbs/hFtDwhtrHHQ/s160-c/Colorado%252520River%252520Sunset.jpg",
                "https://lh3.googleusercontent.com/-ZAs4dNZtALc/URquikvOCWI/AAAAAAAAAbs/DXz4h3dll1Y/s160-c/Colors%252520of%252520Autumn.jpg",
                "https://lh4.googleusercontent.com/-GztnWEIiMz8/URqukVCU7bI/AAAAAAAAAbs/jo2Hjv6MZ6M/s160-c/Countryside.jpg",
                "https://lh4.googleusercontent.com/-bEg9EZ9QoiM/URquklz3FGI/AAAAAAAAAbs/UUuv8Ac2BaE/s160-c/Death%252520Valley%252520-%252520Dunes.jpg",
                "https://lh6.googleusercontent.com/-ijQJ8W68tEE/URqulGkvFEI/AAAAAAAAAbs/zPXvIwi_rFw/s160-c/Delicate%252520Arch.jpg",
                "https://lh5.googleusercontent.com/-Oh8mMy2ieng/URqullDwehI/AAAAAAAAAbs/TbdeEfsaIZY/s160-c/Despair.jpg",
                "https://lh5.googleusercontent.com/-gl0y4UiAOlk/URqumC_KjBI/AAAAAAAAAbs/PM1eT7dn4oo/s160-c/Eagle%252520Fall%252520Sunrise.jpg",
                "https://lh3.googleusercontent.com/-hYYHd2_vXPQ/URqumtJa9eI/AAAAAAAAAbs/wAalXVkbSh0/s160-c/Electric%252520Storm.jpg",
                "https://lh5.googleusercontent.com/-PyY_yiyjPTo/URqunUOhHFI/AAAAAAAAAbs/azZoULNuJXc/s160-c/False%252520Kiva.jpg",
                "https://lh6.googleusercontent.com/-PYvLVdvXywk/URqunwd8hfI/AAAAAAAAAbs/qiMwgkFvf6I/s160-c/Fitzgerald%252520Streaks.jpg",
                "https://lh4.googleusercontent.com/-KIR_UobIIqY/URquoCZ9SlI/AAAAAAAAAbs/Y4d4q8sXu4c/s160-c/Foggy%252520Sunset.jpg",
                "https://lh6.googleusercontent.com/-9lzOk_OWZH0/URquoo4xYoI/AAAAAAAAAbs/AwgzHtNVCwU/s160-c/Frantic.jpg",
                "https://lh3.googleusercontent.com/-0X3JNaKaz48/URqupH78wpI/AAAAAAAAAbs/lHXxu_zbH8s/s160-c/Golden%252520Gate%252520Afternoon.jpg",
                "https://lh6.googleusercontent.com/-95sb5ag7ABc/URqupl95RDI/AAAAAAAAAbs/g73R20iVTRA/s160-c/Golden%252520Gate%252520Fog.jpg",
                "https://lh3.googleusercontent.com/-JB9v6rtgHhk/URqup21F-zI/AAAAAAAAAbs/64Fb8qMZWXk/s160-c/Golden%252520Grass.jpg",
                "https://lh4.googleusercontent.com/-EIBGfnuLtII/URquqVHwaRI/AAAAAAAAAbs/FA4McV2u8VE/s160-c/Grand%252520Teton.jpg",
                "https://lh4.googleusercontent.com/-WoMxZvmN9nY/URquq1v2AoI/AAAAAAAAAbs/grj5uMhL6NA/s160-c/Grass%252520Closeup.jpg",
                "https://lh3.googleusercontent.com/-6hZiEHXx64Q/URqurxvNdqI/AAAAAAAAAbs/kWMXM3o5OVI/s160-c/Green%252520Grass.jpg",
                "https://lh5.googleusercontent.com/-6LVb9OXtQ60/URquteBFuKI/AAAAAAAAAbs/4F4kRgecwFs/s160-c/Hanging%252520Leaf.jpg",
                "https://lh4.googleusercontent.com/-zAvf__52ONk/URqutT_IuxI/AAAAAAAAAbs/D_bcuc0thoU/s160-c/Highway%2525201.jpg",
                "https://lh6.googleusercontent.com/-H4SrUg615rA/URquuL27fXI/AAAAAAAAAbs/4aEqJfiMsOU/s160-c/Horseshoe%252520Bend%252520Sunset.jpg",
                "https://lh4.googleusercontent.com/-JhFi4fb_Pqw/URquuX-QXbI/AAAAAAAAAbs/IXpYUxuweYM/s160-c/Horseshoe%252520Bend.jpg",
                "https://lh5.googleusercontent.com/-UGgssvFRJ7g/URquueyJzGI/AAAAAAAAAbs/yYIBlLT0toM/s160-c/Into%252520the%252520Blue.jpg",
                "https://lh3.googleusercontent.com/-CH7KoupI7uI/URquu0FF__I/AAAAAAAAAbs/R7GDmI7v_G0/s160-c/Jelly%252520Fish%2525202.jpg",
                "https://lh4.googleusercontent.com/-pwuuw6yhg8U/URquvPxR3FI/AAAAAAAAAbs/VNGk6f-tsGE/s160-c/Jelly%252520Fish%2525203.jpg",
                "https://lh5.googleusercontent.com/-GoUQVw1fnFw/URquv6xbC0I/AAAAAAAAAbs/zEUVTQQ43Zc/s160-c/Kauai.jpg",
                "https://lh6.googleusercontent.com/-8QdYYQEpYjw/URquwvdh88I/AAAAAAAAAbs/cktDy-ysfHo/s160-c/Kyoto%252520Sunset.jpg",
                "https://lh4.googleusercontent.com/-vPeekyDjOE0/URquwzJ28qI/AAAAAAAAAbs/qxcyXULsZrg/s160-c/Lake%252520Tahoe%252520Colors.jpg",
                "https://lh4.googleusercontent.com/-xBPxWpD4yxU/URquxWHk8AI/AAAAAAAAAbs/ARDPeDYPiMY/s160-c/Lava%252520from%252520the%252520Sky.jpg",
                "https://lh3.googleusercontent.com/-897VXrJB6RE/URquxxxd-5I/AAAAAAAAAbs/j-Cz4T4YvIw/s160-c/Leica%25252050mm%252520Summilux.jpg",
                "https://lh5.googleusercontent.com/-qSJ4D4iXzGo/URquyDWiJ1I/AAAAAAAAAbs/k2pBXeWehOA/s160-c/Leica%25252050mm%252520Summilux.jpg",
                "https://lh6.googleusercontent.com/-dwlPg83vzLg/URquylTVuFI/AAAAAAAAAbs/G6SyQ8b4YsI/s160-c/Leica%252520M8%252520%252528Front%252529.jpg",
                "https://lh3.googleusercontent.com/-R3_EYAyJvfk/URquzQBv8eI/AAAAAAAAAbs/b9xhpUM3pEI/s160-c/Light%252520to%252520Sand.jpg",
                "https://lh3.googleusercontent.com/-fHY5h67QPi0/URqu0Cp4J1I/AAAAAAAAAbs/0lG6m94Z6vM/s160-c/Little%252520Bit%252520of%252520Paradise.jpg",
                "https://lh5.googleusercontent.com/-TzF_LwrCnRM/URqu0RddPOI/AAAAAAAAAbs/gaj2dLiuX0s/s160-c/Lone%252520Pine%252520Sunset.jpg",
                "https://lh3.googleusercontent.com/-4HdpJ4_DXU4/URqu046dJ9I/AAAAAAAAAbs/eBOodtk2_uk/s160-c/Lonely%252520Rock.jpg",
                "https://lh6.googleusercontent.com/-erbF--z-W4s/URqu1ajSLkI/AAAAAAAAAbs/xjDCDO1INzM/s160-c/Longue%252520Vue.jpg",
                "https://lh6.googleusercontent.com/-0CXJRdJaqvc/URqu1opNZNI/AAAAAAAAAbs/PFB2oPUU7Lk/s160-c/Look%252520Me%252520in%252520the%252520Eye.jpg",
                "https://lh3.googleusercontent.com/-D_5lNxnDN6g/URqu2Tk7HVI/AAAAAAAAAbs/p0ddca9W__Y/s160-c/Lost%252520in%252520a%252520Field.jpg",
                "https://lh6.googleusercontent.com/-flsqwMrIk2Q/URqu24PcmjI/AAAAAAAAAbs/5ocIH85XofM/s160-c/Marshall%252520Beach%252520Sunset.jpg",
                "https://lh4.googleusercontent.com/-Y4lgryEVTmU/URqu28kG3gI/AAAAAAAAAbs/OjXpekqtbJ4/s160-c/Mono%252520Lake%252520Blue.jpg",
                "https://lh4.googleusercontent.com/-AaHAJPmcGYA/URqu3PIldHI/AAAAAAAAAbs/lcTqk1SIcRs/s160-c/Monument%252520Valley%252520Overlook.jpg",
                "https://lh4.googleusercontent.com/-vKxfdQ83dQA/URqu31Yq_BI/AAAAAAAAAbs/OUoGk_2AyfM/s160-c/Moving%252520Rock.jpg",
                "https://lh5.googleusercontent.com/-CG62QiPpWXg/URqu4ia4vRI/AAAAAAAAAbs/0YOdqLAlcAc/s160-c/Napali%252520Coast.jpg",
                "https://lh6.googleusercontent.com/-wdGrP5PMmJQ/URqu5PZvn7I/AAAAAAAAAbs/m0abEcdPXe4/s160-c/One%252520Wheel.jpg",
                "https://lh6.googleusercontent.com/-6WS5DoCGuOA/URqu5qx1UgI/AAAAAAAAAbs/giMw2ixPvrY/s160-c/Open%252520Sky.jpg",
                "https://lh6.googleusercontent.com/-u8EHKj8G8GQ/URqu55sM6yI/AAAAAAAAAbs/lIXX_GlTdmI/s160-c/Orange%252520Sunset.jpg",
                "https://lh6.googleusercontent.com/-74Z5qj4bTDE/URqu6LSrJrI/AAAAAAAAAbs/XzmVkw90szQ/s160-c/Orchid.jpg",
                "https://lh6.googleusercontent.com/-lEQE4h6TePE/URqu6t_lSkI/AAAAAAAAAbs/zvGYKOea_qY/s160-c/Over%252520there.jpg",
                "https://lh5.googleusercontent.com/-cauH-53JH2M/URqu66v_USI/AAAAAAAAAbs/EucwwqclfKQ/s160-c/Plumes.jpg",
                "https://lh3.googleusercontent.com/-eDLT2jHDoy4/URqu7axzkAI/AAAAAAAAAbs/iVZE-xJ7lZs/s160-c/Rainbokeh.jpg",
                "https://lh5.googleusercontent.com/-j1NLqEFIyco/URqu8L1CGcI/AAAAAAAAAbs/aqZkgX66zlI/s160-c/Rainbow.jpg",
                "https://lh5.googleusercontent.com/-DRnqmK0t4VU/URqu8XYN9yI/AAAAAAAAAbs/LgvF_592WLU/s160-c/Rice%252520Fields.jpg",
                "https://lh3.googleusercontent.com/-hwh1v3EOGcQ/URqu8qOaKwI/AAAAAAAAAbs/IljRJRnbJGw/s160-c/Rockaway%252520Fire%252520Sky.jpg",
                "https://lh5.googleusercontent.com/-wjV6FQk7tlk/URqu9jCQ8sI/AAAAAAAAAbs/RyYUpdo-c9o/s160-c/Rockaway%252520Flow.jpg",
                "https://lh6.googleusercontent.com/-6cAXNfo7D20/URqu-BdzgPI/AAAAAAAAAbs/OmsYllzJqwo/s160-c/Rockaway%252520Sunset%252520Sky.jpg",
                "https://lh3.googleusercontent.com/-sl8fpGPS-RE/URqu_BOkfgI/AAAAAAAAAbs/Dg2Fv-JxOeg/s160-c/Russian%252520Ridge%252520Sunset.jpg",
                "https://lh6.googleusercontent.com/-gVtY36mMBIg/URqu_q91lkI/AAAAAAAAAbs/3CiFMBcy5MA/s160-c/Rust%252520Knot.jpg",
                "https://lh6.googleusercontent.com/-GHeImuHqJBE/URqu_FKfVLI/AAAAAAAAAbs/axuEJeqam7Q/s160-c/Sailing%252520Stones.jpg",
                "https://lh3.googleusercontent.com/-hBbYZjTOwGc/URqu_ycpIrI/AAAAAAAAAbs/nAdJUXnGJYE/s160-c/Seahorse.jpg",
                "https://lh3.googleusercontent.com/-Iwi6-i6IexY/URqvAYZHsVI/AAAAAAAAAbs/5ETWl4qXsFE/s160-c/Shinjuku%252520Street.jpg",
                "https://lh6.googleusercontent.com/-amhnySTM_MY/URqvAlb5KoI/AAAAAAAAAbs/pFCFgzlKsn0/s160-c/Sierra%252520Heavens.jpg",
                "https://lh5.googleusercontent.com/-dJgjepFrYSo/URqvBVJZrAI/AAAAAAAAAbs/v-F5QWpYO6s/s160-c/Sierra%252520Sunset.jpg",
                "https://lh4.googleusercontent.com/-Z4zGiC5nWdc/URqvBdEwivI/AAAAAAAAAbs/ZRZR1VJ84QA/s160-c/Sin%252520Lights.jpg",
                "https://lh4.googleusercontent.com/-_0cYiWW8ccY/URqvBz3iM4I/AAAAAAAAAbs/9N_Wq8MhLTY/s160-c/Starry%252520Lake.jpg",
                "https://lh3.googleusercontent.com/-A9LMoRyuQUA/URqvCYx_JoI/AAAAAAAAAbs/s7sde1Bz9cI/s160-c/Starry%252520Night.jpg",
                "https://lh3.googleusercontent.com/-KtLJ3k858eY/URqvC_2h_bI/AAAAAAAAAbs/zzEBImwDA_g/s160-c/Stream.jpg",
                "https://lh5.googleusercontent.com/-dFB7Lad6RcA/URqvDUftwWI/AAAAAAAAAbs/BrhoUtXTN7o/s160-c/Strip%252520Sunset.jpg",
                "https://lh5.googleusercontent.com/-at6apgFiN20/URqvDyffUZI/AAAAAAAAAbs/clABCx171bE/s160-c/Sunset%252520Hills.jpg",
                "https://lh4.googleusercontent.com/-7-EHhtQthII/URqvEYTk4vI/AAAAAAAAAbs/QSJZoB3YjVg/s160-c/Tenaya%252520Lake%2525202.jpg",
                "https://lh6.googleusercontent.com/-8MrjV_a-Pok/URqvFC5repI/AAAAAAAAAbs/9inKTg9fbCE/s160-c/Tenaya%252520Lake.jpg",
                "https://lh5.googleusercontent.com/-B1HW-z4zwao/URqvFWYRwUI/AAAAAAAAAbs/8Peli53Bs8I/s160-c/The%252520Cave%252520BW.jpg",
                "https://lh3.googleusercontent.com/-PO4E-xZKAnQ/URqvGRqjYkI/AAAAAAAAAbs/42nyADFsXag/s160-c/The%252520Fisherman.jpg",
                "https://lh4.googleusercontent.com/-iLyZlzfdy7s/URqvG0YScdI/AAAAAAAAAbs/1J9eDKmkXtk/s160-c/The%252520Night%252520is%252520Coming.jpg",
                "https://lh6.googleusercontent.com/-G-k7YkkUco0/URqvHhah6fI/AAAAAAAAAbs/_taQQG7t0vo/s160-c/The%252520Road.jpg",
                "https://lh6.googleusercontent.com/-h-ALJt7kSus/URqvIThqYfI/AAAAAAAAAbs/ejiv35olWS8/s160-c/Tokyo%252520Heights.jpg",
                "https://lh5.googleusercontent.com/-Hy9k-TbS7xg/URqvIjQMOxI/AAAAAAAAAbs/RSpmmOATSkg/s160-c/Tokyo%252520Highway.jpg",
                "https://lh6.googleusercontent.com/-83oOvMb4OZs/URqvJL0T7lI/AAAAAAAAAbs/c5TECZ6RONM/s160-c/Tokyo%252520Smog.jpg",
                "https://lh3.googleusercontent.com/-FB-jfgREEfI/URqvJI3EXAI/AAAAAAAAAbs/XfyweiRF4v8/s160-c/Tufa%252520at%252520Night.jpg",
                "https://lh4.googleusercontent.com/-vngKD5Z1U8w/URqvJUCEgPI/AAAAAAAAAbs/ulxCMVcU6EU/s160-c/Valley%252520Sunset.jpg",
                "https://lh6.googleusercontent.com/-DOz5I2E2oMQ/URqvKMND1kI/AAAAAAAAAbs/Iqf0IsInleo/s160-c/Windmill%252520Sunrise.jpg",
                "https://lh5.googleusercontent.com/-biyiyWcJ9MU/URqvKculiAI/AAAAAAAAAbs/jyPsCplJOpE/s160-c/Windmill.jpg",
                "https://lh4.googleusercontent.com/-PDT167_xRdA/URqvK36mLcI/AAAAAAAAAbs/oi2ik9QseMI/s160-c/Windmills.jpg",
                "https://lh5.googleusercontent.com/-kI_QdYx7VlU/URqvLXCB6gI/AAAAAAAAAbs/N31vlZ6u89o/s160-c/Yet%252520Another%252520Rockaway%252520Sunset.jpg",
                "https://lh4.googleusercontent.com/-e9NHZ5k5MSs/URqvMIBZjtI/AAAAAAAAAbs/1fV810rDNfQ/s160-c/Yosemite%252520Tree.jpg", };
    
    }

**ImageDownLoader类，异步下载的核心类，保存图片到手机缓存，将图片加入LruCache中等等**

package com.example.asyncimageloader;
    
    import java.io.IOException;
    import java.net.HttpURLConnection;
    import java.net.URL;
    import java.util.concurrent.ExecutorService;
    import java.util.concurrent.Executors;
    
    import android.content.Context;
    import android.graphics.Bitmap;
    import android.graphics.BitmapFactory;
    import android.os.Handler;
    import android.os.Message;
    import android.support.v4.util.LruCache;
    
    public class ImageDownLoader { 
        /** * 缓存Image的类，当存储Image的大小大于LruCache设定的值，系统自动释放内存 */
        private LruCache<String, Bitmap> mMemoryCache;
        /** * 操作文件相关类对象的引用 */
        private FileUtils fileUtils;
        /** * 下载Image的线程池 */
        private ExecutorService mImageThreadPool = null;
    
    
        public ImageDownLoader(Context context){
            //获取系统分配给每个应用程序的最大内存，每个应用系统分配32M
            int maxMemory = (int) Runtime.getRuntime().maxMemory();  
            int mCacheSize = maxMemory / 8;
            //给LruCache分配1/8 4M
            mMemoryCache = new LruCache<String, Bitmap>(mCacheSize){
    
                //必须重写此方法，来测量Bitmap的大小
                @Override
                protected int sizeOf(String key, Bitmap value) {
                    return value.getRowBytes() * value.getHeight();
                }
    
            };
    
            fileUtils = new FileUtils(context);
        }
    
    
        /** * 获取线程池的方法，因为涉及到并发的问题，我们加上同步锁 * @return */
        public ExecutorService getThreadPool(){
            if(mImageThreadPool == null){
                synchronized(ExecutorService.class){
                    if(mImageThreadPool == null){
                        //为了下载图片更加的流畅，我们用了2个线程来下载图片
                        mImageThreadPool = Executors.newFixedThreadPool(2);
                    }
                }
            }
    
            return mImageThreadPool;
    
        }
    
        /** * 添加Bitmap到内存缓存 * @param key * @param bitmap */
        public void addBitmapToMemoryCache(String key, Bitmap bitmap) {  
            if (getBitmapFromMemCache(key) == null && bitmap != null) {  
                mMemoryCache.put(key, bitmap);  
            }  
        }  
    
        /** * 从内存缓存中获取一个Bitmap * @param key * @return */
        public Bitmap getBitmapFromMemCache(String key) {  
            return mMemoryCache.get(key);  
        } 
    
        /** * 先从内存缓存中获取Bitmap,如果没有就从SD卡或者手机缓存中获取，SD卡或者手机缓存 * 没有就去下载 * @param url * @param listener * @return */
        public Bitmap downloadImage(final String url, final onImageLoaderListener listener){
            //替换Url中非字母和非数字的字符，这里比较重要，因为我们用Url作为文件名，比如我们的Url
            //是Http://xiaanming/abc.jpg;用这个作为图片名称，系统会认为xiaanming为一个目录，
            //我们没有创建此目录保存文件就会报错
            final String subUrl = url.replaceAll("[^\\w]", "");
            Bitmap bitmap = showCacheBitmap(subUrl);
            if(bitmap != null){
                return bitmap;
            }else{
    
                final Handler handler = new Handler(){
                    @Override
                    public void handleMessage(Message msg) {
                        super.handleMessage(msg);
                        listener.onImageLoader((Bitmap)msg.obj, url);
                    }
                };
    
                getThreadPool().execute(new Runnable() {
    
                    @Override
                    public void run() {
                        Bitmap bitmap = getBitmapFormUrl(url);
                        Message msg = handler.obtainMessage();
                        msg.obj = bitmap;
                        handler.sendMessage(msg);
    
                        try {
                            //保存在SD卡或者手机目录
                            fileUtils.savaBitmap(subUrl, bitmap);
                        } catch (IOException e) {
                            e.printStackTrace();
                        }
    
                        //将Bitmap 加入内存缓存
                        addBitmapToMemoryCache(subUrl, bitmap);
                    }
                });
            }
    
            return null;
        }
    
        /** * 获取Bitmap, 内存中没有就去手机或者sd卡中获取，这一步在getView中会调用，比较关键的一步 * @param url * @return */
        public Bitmap showCacheBitmap(String url){
            if(getBitmapFromMemCache(url) != null){
                return getBitmapFromMemCache(url);
            }else if(fileUtils.isFileExists(url) && fileUtils.getFileSize(url) != 0){
                //从SD卡获取手机里面获取Bitmap
                Bitmap bitmap = fileUtils.getBitmap(url);
    
                //将Bitmap 加入内存缓存
                addBitmapToMemoryCache(url, bitmap);
                return bitmap;
            }
    
            return null;
        }
    
    
        /** * 从Url中获取Bitmap * @param url * @return */
        private Bitmap getBitmapFormUrl(String url) {
            Bitmap bitmap = null;
            HttpURLConnection con = null;
            try {
                URL mImageUrl = new URL(url);
                con = (HttpURLConnection) mImageUrl.openConnection();
                con.setConnectTimeout(10 * 1000);
                con.setReadTimeout(10 * 1000);
                con.setDoInput(true);
                con.setDoOutput(true);
                bitmap = BitmapFactory.decodeStream(con.getInputStream());
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                if (con != null) {
                    con.disconnect();
                }
            }
            return bitmap;
        }
    
        /** * 取消正在下载的任务 */
        public synchronized void cancelTask() {
            if(mImageThreadPool != null){
                mImageThreadPool.shutdownNow();
                mImageThreadPool = null;
            }
        }
    
    
        /** * 异步下载图片的回调接口 * @author len * */
        public interface onImageLoaderListener{ 
            void onImageLoader(Bitmap bitmap, String url);
        }
    
    }

**ImageDownLoader中有几个方法比较重要:**  
1.首先我们需要重写**sizeOf**(String key, Bitmap value)来计算图片的大小，默认返回图片的数量

2.**downloadImage**(final String url, final onImageLoaderListener listener)先去LruCache查看Image,没有再去手机缓存中查看，在没有则开启线程下载，这里我们提供了一个回调接口，回调方法中我们将Bitmap和图片Url作为参数，String subUrl = url.replaceAll(“\[^\\w\]”, “”) 我在代码中注释写的比较清楚

3.**showCacheBitmap**(String url)方法，此方法在Adapter中的getView()当中调用，如果getView()中不调用此方法试试你就知道效果了

**ImageAdapter GridView的适配器类，主要是GridView滑动的时候取消下载任务，静止的时候去下载当前显示的item的图片，其他也没什么不同了**

package com.example.asyncimageloader;
    
    import android.content.Context;
    import android.graphics.Bitmap;
    import android.view.View;
    import android.view.ViewGroup;
    import android.widget.AbsListView;
    import android.widget.AbsListView.OnScrollListener;
    import android.widget.BaseAdapter;
    import android.widget.GridView;
    import android.widget.ImageView;
    
    import com.example.asyncimageloader.ImageDownLoader.onImageLoaderListener;
    
    public class ImageAdapter extends BaseAdapter implements OnScrollListener{ 
        /** * 上下文对象的引用 */
        private Context context;
    
        /** * Image Url的数组 */
        private String [] imageThumbUrls;
    
        /** * GridView对象的应用 */
        private GridView mGridView;
    
        /** * Image 下载器 */
        private ImageDownLoader mImageDownLoader;
    
        /** * 记录是否刚打开程序，用于解决进入程序不滚动屏幕，不会下载图片的问题。 * 参考http://blog.csdn.net/guolin_blog/article/details/9526203#comments */
        private boolean isFirstEnter = true;
    
        /** * 一屏中第一个item的位置 */
        private int mFirstVisibleItem;
    
        /** * 一屏中所有item的个数 */
        private int mVisibleItemCount;
    
    
        public ImageAdapter(Context context, GridView mGridView, String [] imageThumbUrls){
            this.context = context;
            this.mGridView = mGridView;
            this.imageThumbUrls = imageThumbUrls;
            mImageDownLoader = new ImageDownLoader(context);
            mGridView.setOnScrollListener(this);
        }
    
        @Override
        public void onScrollStateChanged(AbsListView view, int scrollState) {
            //仅当GridView静止时才去下载图片，GridView滑动时取消所有正在下载的任务 
            if(scrollState == AbsListView.OnScrollListener.SCROLL_STATE_IDLE){
                showImage(mFirstVisibleItem, mVisibleItemCount);
            }else{
                cancelTask();
            }
    
        }
    
    
        /** * GridView滚动的时候调用的方法，刚开始显示GridView也会调用此方法 */
        @Override
        public void onScroll(AbsListView view, int firstVisibleItem,
                int visibleItemCount, int totalItemCount) {
            mFirstVisibleItem = firstVisibleItem;
            mVisibleItemCount = visibleItemCount;
            // 因此在这里为首次进入程序开启下载任务。 
            if(isFirstEnter && visibleItemCount > 0){
                showImage(mFirstVisibleItem, mVisibleItemCount);
                isFirstEnter = false;
            }
        }
    
    
        @Override
        public int getCount() {
            return imageThumbUrls.length;
        }
    
        @Override
        public Object getItem(int position) {
            return imageThumbUrls[position];
        }
    
        @Override
        public long getItemId(int position) {
            return position;
        }
    
        @Override
        public View getView(int position, View convertView, ViewGroup parent) {
            ImageView mImageView;
            final String mImageUrl = imageThumbUrls[position];
            if(convertView == null){
                mImageView = new ImageView(context);
            }else{
                mImageView = (ImageView) convertView;
            }
    
            mImageView.setLayoutParams(new GridView.LayoutParams(150, 150));
            mImageView.setScaleType(ImageView.ScaleType.CENTER_INSIDE);
    
            //给ImageView设置Tag,这里已经是司空见惯了
            mImageView.setTag(mImageUrl);
    
    
            /*******************************去掉下面这几行试试是什么效果****************************/
            Bitmap bitmap = mImageDownLoader.showCacheBitmap(mImageUrl.replaceAll("[^\\w]", ""));
            if(bitmap != null){
                mImageView.setImageBitmap(bitmap);
            }else{
                mImageView.setImageDrawable(context.getResources().getDrawable(R.drawable.ic_empty));
            }
            /**********************************************************************************/
    
    
            return mImageView;
        }
    
        /** * 显示当前屏幕的图片，先会去查找LruCache，LruCache没有就去sd卡或者手机目录查找，在没有就开启线程去下载 * @param firstVisibleItem * @param visibleItemCount */
        private void showImage(int firstVisibleItem, int visibleItemCount){
            Bitmap bitmap = null;
            for(int i=firstVisibleItem; i<firstVisibleItem + visibleItemCount; i++){
                String mImageUrl = imageThumbUrls[i];
                final ImageView mImageView = (ImageView) mGridView.findViewWithTag(mImageUrl);
                bitmap = mImageDownLoader.downloadImage(mImageUrl, new onImageLoaderListener() {
    
                    @Override
                    public void onImageLoader(Bitmap bitmap, String url) {
                        if(mImageView != null && bitmap != null){
                            mImageView.setImageBitmap(bitmap);
                        }
    
                    }
                });
    
                //if(bitmap != null){ 
                // mImageView.setImageBitmap(bitmap);
                //}else{ 
                // mImageView.setImageDrawable(context.getResources().getDrawable(R.drawable.ic_empty));
                //}
            }
        }
    
        /** * 取消下载任务 */
        public void cancelTask(){
            mImageDownLoader.cancelTask();
        }
    
    
    }

**MainActivity 里面一个GridView，然后提供一个系统菜单来删除手机上的缓存图片，直接上代码，比较简单所以里面也没有注释**

package com.example.asyncimageloader;
    
    import android.app.Activity;
    import android.os.Bundle;
    import android.view.Menu;
    import android.view.MenuItem;
    import android.widget.GridView;
    import android.widget.Toast;
    
    public class MainActivity extends Activity { 
        private GridView mGridView;
        private String [] imageThumbUrls = Images.imageThumbUrls; 
        private ImageAdapter mImageAdapter;
        private FileUtils fileUtils;
    
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
            fileUtils = new FileUtils(this);
            mGridView = (GridView) findViewById(R.id.gridView);
            mImageAdapter = new ImageAdapter(this, mGridView, imageThumbUrls);
            mGridView.setAdapter(mImageAdapter);
        }
    
        @Override
        protected void onDestroy() {
            mImageAdapter.cancelTask();
            super.onDestroy();
        }
    
        @Override
        public boolean onCreateOptionsMenu(Menu menu) {
            super.onCreateOptionsMenu(menu);
            menu.add("删除手机中图片缓存");
            return super.onCreateOptionsMenu(menu);
        }
    
        @Override
        public boolean onOptionsItemSelected(MenuItem item) {
            switch (item.getItemId()) {
            case 0:
                fileUtils.deleteFile();
                Toast.makeText(getApplication(), "清空缓存成功", Toast.LENGTH_SHORT).show();
                break;
            }
            return super.onOptionsItemSelected(item);
        }
    
    }

**MainActivity的布局文件**

**因为有网络操作和对sd卡的操作，所以我们必须加上相对应的权限**

<uses-permission android:name="android.permission.INTERNET" /> 
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

好了，代码就全部介绍完了，我们运行下程序看看效果  
![这里写图片描述][SouthEast 1]

可以看到，滚动照片墙，会异步加载图片到相应的ImageView上。随着加载图片的增多，会释放掉一些之前加载过的图片，你多滚动几次就可以看得出了。另外为了能让大家明显看出图片的释放情况，我在这个程序中没有使用本地缓存，所有被释放掉的图片再次显示需要从网络上再下载一遍。在实际的项目中配合适当的本地缓存效果会更好。

打开DDMS，我们可以发现，由于有LruCache帮我们管理图片缓存，不管如何滚动照片墙，程序内存始终会保持在一个合理的范围内。

![这里写图片描述][SouthEast 2]

[PhotoWallFallsDemo][]  
[大图加载][Link 1]

**推荐阅读：**  
[Android照片墙应用实现，再多的图片也不怕崩溃][Android]  
[Android高效加载大图、多图解决方案，有效避免程序OOM][Android_OOM]

[SouthEast]: /images/20220604/5d2de570742c4db58d160f3ba5f38262.png
[http_developer.android.com_training_displaying-bitmaps_index.html]: http://developer.android.com/training/displaying-bitmaps/index.html
[SouthEast 1]: /images/20220604/b1aec6f6b2ca4d32a8154fb8c9b210be.png
[SouthEast 2]: /images/20220604/25bbcd8798ca4725b8f1aa5574cd7b07.png
[PhotoWallFallsDemo]: http://download.csdn.net/download/lwzhang1101/10152075
[Link 1]: http://download.csdn.net/download/lwzhang1101/10152083
[Android]: http://blog.csdn.net/guolin_blog/article/details/9526203#comments
[Android_OOM]: http://blog.csdn.net/guolin_blog/article/details/9316683