Android 异步加载图片

异步加载图片的例子，网上也比较多，大部分用了> ，但是现在已经不再推荐使用这种方式了，因为从 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， 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，所以我这里用得是来缓存图片，当存储Image的大小大于设定的值，系统自动释放内存，这个类是3.1版本中提供的，如果你是在更早的版本中开发，则需要导入--v4的jar包（这里要注意咯）

为什么写这篇文章呢？

因为我之前做的项目中，也有异步加载图片，那时候用得是去下载图片，每次下载图片都要new 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()异常，之后用了，先用中获取图片，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，大概思路就是先从中获取图片，如果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，然后之前看到的照片墙应用实现，再多的图片也不怕崩溃的这篇文章，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

使用了线程池来管理下载任务使用来缓存图片使用手机来缓存图片滑动的时候取消下载任务，静止的时候进行下载，滑动更加的流畅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

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();}
}

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:/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};}

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 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(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);}}

中有几个方法比较重要首先我们需要重写( key, value)来计算图片的大小，默认返回图片的数量(final url, final r )先去查看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，回调方法中我们将和图片Url作为参数， = url.("[^\\w]", "") 我在代码中注释写的比较清楚( url)方法，此方法在中的()当中调用，如果()中不调用此方法试试你就知道效果了

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

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);}}