Android 异步加载图片，使用LruCache和SD卡或手机缓存(使用线程池下载图片）

转载请注明出处http://blog.csdn.net/xiaanming/article/details/9825113

异步加载图片的例子，网上也比较多，大部分用了HashMap<String, SoftReference<Drawable>> imageCache ，但是现在已经不再推荐使用这种方式了，因为从 Android 2.3 (API Level 9)开始，垃圾回收器会更倾向于回收持有软引用或弱引用的对象，这让软引用和弱引用变得不再可靠。另外，Android 3.0 (API Level 11)中，图片的数据会存储在本地的内存当中，因而无法用一种可预见的方式将其释放，这就有潜在的风险造成应用程序的内存溢出并崩溃，所以我这里用得是LruCache来缓存图片，当存储Image的大小大于LruCache设定的值，系统自动释放内存，这个类是3.1版本中提供的，如果你是在更早的Android版本中开发，则需要导入android-support-v4的jar包（这里要注意咯）

因为我之前做的项目中，也有异步加载图片，那时候用得是Thread去下载图片，每次下载图片都要new Thread去下载，而且还是并发去下载，每次都new 一个线程浪费内存，老板说服务器承受不起这么多的连接，叫我改成先获取一张图片之后再去获取下一张，这样子保存与服务器的连接为一个，服务器压力小了，然后楼主就想到线程池，线程池很好的帮我们管理并发的问题，并发的问题解决了，可是后面又出问题了，图片多了就出现OOM(OutOfMemory)异常，之后用了SoftReference，先用SoftReference中获取图片，SoftReference没有就开线程去下载，老板说你为什么不把图片在手机上做个缓存呢，于是我用了手机缓存，大概思路就是先从SoftReference中获取图片，如果SoftReference没有就去手机缓存中获取，手机缓存中没有就开启先从去下载，然后成功的解决了OOM的问题，前些天老板要我重构下代码，我也觉得之前写的代码耦合性太强，早就想改，然后之前看到guolin的Android照片墙应用实现，再多的图片也不怕崩溃的这篇文章，LruCache和滑动过程中取消下载任务，停下来的时候才去下载这2点比较好，值得我学习，然后我就将我的项目异步加载这一块改了下，发到这里做个记录吧，以后类似的异步加载图片直接拷贝代码，提交开发的效率

这篇文章做了哪些方面的优化

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

• 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，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32Mint maxMemory = (int) Runtime.getRuntime().maxMemory();  int mCacheSize = maxMemory / 8;//给LruCache分配1/8 4MmMemoryCache = new LruCache<String, Bitmap>(mCacheSize){//必须重写此方法，来测量Bitmap的大小@Overrideprotected 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(){@Overridepublic void handleMessage(Message msg) {super.handleMessage(msg);listener.onImageLoader((Bitmap)msg.obj, url);}};getThreadPool().execute(new Runnable() {@Overridepublic 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BitmapBitmap 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);}}

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);}@Overridepublic 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*/@Overridepublic 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;}}@Overridepublic int getCount() {return imageThumbUrls.length;}@Overridepublic Object getItem(int position) {return imageThumbUrls[position];}@Overridepublic long getItemId(int position) {return position;}@Overridepublic 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() {@Overridepublic 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;@Overrideprotected 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);}@Overrideprotected void onDestroy() {mImageAdapter.cancelTask();super.onDestroy();}@Overridepublic boolean onCreateOptionsMenu(Menu menu) {super.onCreateOptionsMenu(menu);menu.add("删除手机中图片缓存");return super.onCreateOptionsMenu(menu);}@Overridepublic 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

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"xmlns:tools="http://schemas.android.com/tools"android:layout_width="match_parent"android:layout_height="match_parent"tools:context=".MainActivity" ><GridViewandroid:id="@+id/gridView"android:layout_width="match_parent"android:layout_height="match_parent"android:stretchMode="columnWidth" android:columnWidth="90dip" android:verticalSpacing="10dip"android:horizontalSpacing="10dip"android:cacheColorHint="@android:color/transparent"android:numColumns="auto_fit" ></GridView></RelativeLayout>

• 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" /> 

效果图：

源码下载，点击这里