• 1.命中率；
• 2.合理分配占用的空间；
• 3.合理的缓存层级。

（四）、内存保存：

在内存中保存的话，只能保存一定的量，而不能一直往里面放，需要设置数据的过期时间、LRU等算法。这里有一个方法是把常用的数据放到一个缓存中（A），不常用的放到另外一个缓存中（B）。当要获取数据时先从A中去获取，如果A中不存在那么再去B中获取。B中的数据主要是A中LRU出来的数据，这里的内存回收主要针对B内存，从而保持A中的数据可以有效的被命中。

要写带有缓存机制的异步图片批量加载首先要有数据源

Json字符串：

{"data":{"BookList":[{"BookId":"87","CoverStoryName":"JNC 8存争议，我国高血压实践以中国指南为准","CoverStoryName2":"小儿便秘多由饮食和排便习惯引起","IssueDate":"2014年01月10日","Issue":"584","IssueYear":"2014年第2期","CoverStoryImage":"http://192.168.125.125:8080/AndroidServer/one.jpg"},{"BookId":"86","CoverStoryName":"道阻且长，溯洄从之","CoverStoryName2":"对慢性荨麻疹病因治疗的思考","IssueDate":"2014年01月01日","Issue":"583","IssueYear":"2014年第1期","CoverStoryImage":"http://192.168.125.125:8080/AndroidServer/two.jpg"},{"BookId":"85","CoverStoryName":"急性胰腺炎诊断：上腹部疼痛结合辅助检查","CoverStoryName2":"急性胰腺炎一经诊断，应作严重程度评估","IssueDate":"2013年12月27日","Issue":"582","IssueYear":"2013年第48期","CoverStoryImage":"http://192.168.125.125:8080/AndroidServer/three.jpg"},{"BookId":"84","CoverStoryName":"绩效考核分配机制引争议","CoverStoryName2":"护肝药物知多少","IssueDate":"2013年12月20日","Issue":"581","IssueYear":"2013年第47期","CoverStoryImage":"http://192.168.125.125:8080/AndroidServer/four.jpg"},{"BookId":"83","CoverStoryName":"脑卒中后运动功能障碍的康复治疗","CoverStoryName2":"脑卒中后感觉、认知、言语、吞咽功能障碍的康复","IssueDate":"2013年12月13日","Issue":"580","IssueYear":"2013年第46期","CoverStoryImage":"http://192.168.125.125:8080/AndroidServer/five.jpg"},{"BookId":"82","CoverStoryName":"契合基层定位，有的放矢谈糖尿病的药物应用","CoverStoryName2":"导致不孕的因素具有多样性和复杂性","IssueDate":"2013年12月06日","Issue":"579","IssueYear":"2013年第45期","CoverStoryImage":"http://192.168.125.125:8080/AndroidServer/six.jpg"},{"BookId":"81","CoverStoryName":"川崎病：容易与感冒混淆的皮肤黏膜淋巴结综合征","CoverStoryName2":"川崎病的治疗措施","IssueDate":"2013年10月18日","Issue":"573","IssueYear":"2013年第38期","CoverStoryImage":"http://192.168.125.125:8080/AndroidServer/seven.jpg"},{"BookId":"80","CoverStoryName":"人文关怀与服务意识——好医生的两大法宝","CoverStoryName2":"特发性震颤的再认识：并非总是良性过程","IssueDate":"2013年10月04日","Issue":"571","IssueYear":"2013年第37期","CoverStoryImage":"http://192.168.125.125:8080/AndroidServer/eight.jpg"},{"BookId":"79","CoverStoryName":"中西药复方制剂花甲之年的哀愁","CoverStoryName2":"白癜风，受遗传基因与环境的多因素影响的疾病","IssueDate":"2013年08月23日","Issue":"566","IssueYear":"2013年第32期","CoverStoryImage":"http://192.168.125.125:8080/AndroidServer/nine.jpg"},{"BookId":"78","CoverStoryName":"聚焦基本药物制度：破浪前行需要多助力","CoverStoryName2":"认识小儿特应性皮炎","IssueDate":"2013年08月16日","Issue":"565","IssueYear":"2013年第31期","CoverStoryImage":"http://192.168.125.125:8080/AndroidServer/ten.jpg"}]}}

通过这个json字符串我们来模拟一下

首先创建布局文件

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"android:layout_width="fill_parent"android:layout_height="fill_parent"android:orientation="vertical" ><ListViewandroid:id="@+id/listView_main"android:layout_width="match_parent"android:layout_height="match_parent" ></ListView></LinearLayout>

item的布局文件

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"android:layout_width="match_parent"android:layout_height="match_parent"><ImageViewandroid:id="@+id/imageView_main_cover"android:adjustViewBounds="true"android:layout_width="190dp"android:layout_height="220dp"android:contentDescription="@string/app_name"android:background="@drawable/magazine_bg"android:paddingTop="8dp"android:paddingBottom="8dp"android:paddingLeft="6dp"android:paddingRight="6dp"/><TextViewandroid:id="@+id/CoverStoryName"android:layout_width="match_parent"android:layout_height="wrap_content"android:layout_alignParentTop="true"android:layout_marginTop="5dp"android:layout_toRightOf="@id/imageView_main_cover"android:text="标题"android:textSize="18sp"android:textColor="#69f"/><TextViewandroid:id="@+id/CoverStoryName2"android:layout_width="match_parent"android:layout_height="wrap_content"android:layout_below="@id/CoverStoryName"android:layout_marginTop="5dp"android:layout_toRightOf="@id/imageView_main_cover"android:text="标题2"android:textSize="18sp"android:textColor="#700"/><TextViewandroid:id="@+id/IssueDate"android:layout_width="match_parent"android:layout_height="wrap_content"android:layout_below="@id/CoverStoryName2"android:layout_toRightOf="@id/imageView_main_cover"android:layout_marginTop="5dp"android:text="日期"android:textSize="18sp"android:textColor="#8cf"/><TextViewandroid:id="@+id/Issue"android:layout_width="match_parent"android:layout_height="wrap_content"android:layout_toRightOf="@id/imageView_main_cover"android:layout_below="@id/IssueDate"android:layout_marginTop="5dp"android:text="期刊号"android:textSize="18sp"android:textColor="#8cf"/><TextViewandroid:id="@+id/IssueYear"android:layout_width="match_parent"android:layout_height="wrap_content"android:layout_toRightOf="@id/imageView_main_cover"android:layout_below="@id/Issue"android:text="发行日期"android:textSize="18sp"android:textColor="#8cf"/>
</RelativeLayout>

MainActivity类的相关操作

package com.hht.homework_asynclistviewimage;import java.io.IOException;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;import org.apache.http.HttpResponse;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;
import org.apache.http.util.EntityUtils;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.view.Menu;
import android.widget.ListView;public class MainActivity extends Activity {private ListView listView_main;private static final String uriString = "http://192.168.125.125:8080/AndroidServer/MyJsonServlet";private Handler handler = new Handler() {@Overridepublic void handleMessage(Message msg) {// TODO Auto-generated method stubsuper.handleMessage(msg);switch (msg.what) {case 1:List<Map<String, Object>> list = (List<Map<String, Object>>) msg.obj;listView_main.setAdapter(new MyAdapter(MainActivity.this, list));break;default:break;}}};@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);listView_main = (ListView) findViewById(R.id.listView_main);new Thread(new Mythread()).start();}@Overridepublic boolean onCreateOptionsMenu(Menu menu) {// Inflate the menu; this adds items to the action bar if it is present.getMenuInflater().inflate(R.menu.main, menu);return true;}class Mythread implements Runnable {@Overridepublic void run() {handler.sendEmptyMessage(0);HttpGet get = new HttpGet(uriString);HttpClient client = new DefaultHttpClient();HttpResponse response;try {response = client.execute(get);Message message = Message.obtain();if (response.getStatusLine().getStatusCode() == 200) {String data = EntityUtils.toString(response.getEntity());try {// json字符串的解析List<Map<String, Object>> list = new ArrayList<Map<String, Object>>();JSONObject jsonObject = new JSONObject(data);JSONObject jsonObject2 = jsonObject.getJSONObject("data");JSONArray array = jsonObject2.getJSONArray("BookList");for (int i = 0; i < array.length(); i++) {Map<String, Object> map = new HashMap<String, Object>();JSONObject jsonObject3 = array.getJSONObject(i);map.put("BookId", jsonObject3.get("BookId"));map.put("CoverStoryName",jsonObject3.get("CoverStoryName"));map.put("CoverStoryName2",jsonObject3.get("CoverStoryName2"));map.put("IssueDate", jsonObject3.get("IssueDate"));map.put("Issue", jsonObject3.get("Issue"));map.put("IssueYear", jsonObject3.get("IssueYear"));map.put("CoverStoryImage",jsonObject3.get("CoverStoryImage"));list.add(map);}// 将得到的list返回到主线程,进行操作message.what = 1;message.obj = list;handler.sendMessage(message);} catch (JSONException e) {// TODO Auto-generated catch blocke.printStackTrace();}}} catch (ClientProtocolException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (IOException e) {// TODO Auto-generated catch blocke.printStackTrace();}}}}

自定义适配器

package com.hht.homework_asynclistviewimage;import java.util.List;
import java.util.Map;import android.content.Context;
import android.graphics.Bitmap;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.ImageView;
import android.widget.TextView;import com.hht.homework_asynclistviewimage.ImageDownloadHelper.OnImageDownloadListener;public class MyAdapter extends BaseAdapter {private Context context;private List<Map<String, Object>> list;private View view = null;private ImageDownloadHelper downloadHelper = null;public MyAdapter(Context context, List<Map<String, Object>> list) {super();this.context = context;this.list = list;}@Overridepublic int getCount() {// TODO Auto-generated method stubreturn list.size();}@Overridepublic Object getItem(int position) {// TODO Auto-generated method stubreturn list.get(position);}@Overridepublic long getItemId(int position) {// TODO Auto-generated method stubreturn position;}@Overridepublic View getView(int position, View convertView, ViewGroup parent) {// TODO Auto-generated method stub// 重写getView方法，并进行相关优化，合理利用converViewfinal ViewHolder holder;if (convertView == null) {holder = new ViewHolder();convertView = LayoutInflater.from(context).inflate(R.layout.item_listview_main, null);holder.imageView_main_cover = (ImageView) convertView.findViewById(R.id.imageView_main_cover);holder.CoverStoryName = (TextView) convertView.findViewById(R.id.CoverStoryName);holder.CoverStoryName2 = (TextView) convertView.findViewById(R.id.CoverStoryName2);holder.IssueDate = (TextView) convertView.findViewById(R.id.IssueDate);holder.Issue = (TextView) convertView.findViewById(R.id.Issue);holder.IssueYear = (TextView) convertView.findViewById(R.id.IssueYear);// 打标签convertView.setTag(holder);} else {// 通过标签获得holderholder = (ViewHolder) convertView.getTag();}view = convertView;String uriString = list.get(position).get("CoverStoryImage").toString();holder.imageView_main_cover.setTag(uriString);holder.imageView_main_cover.setImageResource(R.drawable.defaultcovers);holder.CoverStoryName.setText(list.get(position).get("CoverStoryName").toString());holder.CoverStoryName2.setText(list.get(position).get("CoverStoryName2").toString());holder.IssueDate.setText(list.get(position).get("IssueDate").toString());holder.Issue.setText(list.get(position).get("Issue").toString());holder.IssueYear.setText(list.get(position).get("IssueYear").toString());if (downloadHelper == null) {downloadHelper = new ImageDownloadHelper();}if (downloadHelper != null) {// 图片下载利用了缓存的机制强缓存，软缓存，以及文件缓存downloadHelper.imageDownload(context, uriString,holder.imageView_main_cover, "hht",new OnImageDownloadListener() {// 利用接口回调的方法使得view控件能与工作线程进行数据交换@Overridepublic void onImageDownload(Bitmap bitmap, String imgUrl) {// TODO Auto-generated method stubImageView imageView = (ImageView) view.findViewWithTag(imgUrl);if (imageView != null) {imageView.setImageBitmap(bitmap);imageView.setTag(null);}}});}return convertView;}class ViewHolder {private ImageView imageView_main_cover;private TextView CoverStoryName;private TextView CoverStoryName2;private TextView IssueDate;private TextView Issue;private TextView IssueYear;}}

其实主要的操作在一个异步下载图片的工具类中，里面融合了异步加载，缓存技术，IO流，网络操作，SDCard文件操作等知识点

package com.hht.homework_asynclistviewimage;import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.InputStream;
import java.lang.ref.SoftReference;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.Map;import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.os.Environment;
import android.util.Log;
import android.util.LruCache;
import android.widget.ImageView;public class ImageDownloadHelper {private static final String TAG = "ImageDownloaderHelper";private HashMap<String, MyAsyncTask> map = new HashMap<String, MyAsyncTask>();private Map<String, SoftReference<Bitmap>> softCaches = new LinkedHashMap<String, SoftReference<Bitmap>>();private LruCache<String, Bitmap> lruCache = null;public ImageDownloadHelper() {int memoryAmount = (int) Runtime.getRuntime().maxMemory();// 获取剩余内存的8分之一作为缓存int cacheSize = memoryAmount / 8;if (lruCache == null) {lruCache = new MyLruCache(cacheSize);}Log.i(TAG, "==LruCache尺寸：" + cacheSize);}/*** * @param context* @param url*            该mImageView对应的url* @param mImageView* @param path*            文件存储路径* @param downloadListener*            OnImageDownload回调接口，在onPostExecute()中被调用*/public void imageDownload(Context context, String url,ImageView mImageView, String path,OnImageDownloadListener downloadListener) {Bitmap bitmap = null;// 先从强引用中拿数据if (lruCache != null) {bitmap = lruCache.get(url);}if (bitmap != null && url.equals(mImageView.getTag())) {Log.i(TAG, "==从强引用中找到数据");mImageView.setImageBitmap(bitmap);} else {SoftReference<Bitmap> softReference = softCaches.get(url);if (softReference != null) {bitmap = softReference.get();}// 从软引用中拿数据if (bitmap != null && url.equals(mImageView.getTag())) {Log.i(TAG, "==从软引用中找到数据");// 添加到强引用中lruCache.put(url, bitmap);Log.i(TAG, "==添加到强引用中");// 从软引用集合中移除softCaches.remove(url);mImageView.setImageBitmap(bitmap);} else {// 从文件缓存中拿数据String imageName = "";if (url != null) {imageName = ImageDownloaderUtil.getInstance().getImageName(url);}bitmap = getBitmapFromFile(context, imageName, path);if (bitmap != null && url.equals(mImageView.getTag())) {Log.i(TAG, "==从文件缓存中找到数据");// 放入强缓存lruCache.put(url, bitmap);mImageView.setImageBitmap(bitmap);} else {// 从网络中拿数据if (url != null && needCreateNewTask(mImageView)) {MyAsyncTask task = new MyAsyncTask(context, url,mImageView, path, downloadListener);Log.i(TAG, "==从网络中拿数据");if (mImageView != null) {task.execute();// 将对应的url对应的任务存起来map.put(url, task);}}}}}}/*** 判断是否需要重新创建线程下载图片，如果需要，返回值为true。* * @param url* @param mImageView* @return*/private boolean needCreateNewTask(ImageView mImageView) {boolean b = true;if (mImageView != null) {String curr_task_url = (String) mImageView.getTag();if (isTasksContains(curr_task_url)) {b = false;}}return b;}/*** 检查该url（最终反映的是当前的ImageView的tag，tag会根据position的不同而不同）对应的task是否存在* * @param url* @return*/private boolean isTasksContains(String url) {boolean b = false;if (map != null && map.get(url) != null) {b = true;}return b;}/*** 删除map中该url的信息，这一步很重要，不然MyAsyncTask的引用会“一直”存在于map中* * @param url*/private void removeTaskFromMap(String url) {if (url != null && map != null && map.get(url) != null) {map.remove(url);Log.i(TAG, "当前map的大小==" + map.size());}}/*** 从文件中拿图片* * @param mActivity* @param imageName*            图片名字* @param path*            图片路径* @return*/private Bitmap getBitmapFromFile(Context context, String imageName,String path) {Bitmap bitmap = null;if (imageName != null) {File file = null;String real_path = "";try {if (ImageDownloaderUtil.getInstance().hasSDCard()) {real_path = ImageDownloaderUtil.getInstance().getExtPath()+ (path != null && path.startsWith("/") ? path: "/" + path);} else {real_path = ImageDownloaderUtil.getInstance().getPackagePath(context)+ (path != null && path.startsWith("/") ? path: "/" + path);}file = new File(real_path, imageName);if (file.exists())bitmap = BitmapFactory.decodeStream(new FileInputStream(file));} catch (Exception e) {e.printStackTrace();bitmap = null;}}return bitmap;}/*** 将下载好的图片存放到文件中* * @param path*            图片路径* @param mActivity* @param imageName*            图片名字* @param bitmap*            图片* @return*/private boolean setBitmapToFile(String path, Context mActivity,String imageName, Bitmap bitmap) {File file = null;String real_path = "";try {if (ImageDownloaderUtil.getInstance().hasSDCard()) {real_path = ImageDownloaderUtil.getInstance().getExtPath()+ (path != null && path.startsWith("/") ? path : "/"+ path);} else {real_path = ImageDownloaderUtil.getInstance().getPackagePath(mActivity)+ (path != null && path.startsWith("/") ? path : "/"+ path);}file = new File(real_path, imageName);if (!file.exists()) {File file2 = new File(real_path + "/");file2.mkdirs();}file.createNewFile();FileOutputStream fos = null;if (ImageDownloaderUtil.getInstance().hasSDCard()) {fos = new FileOutputStream(file);} else {fos = mActivity.openFileOutput(imageName, Context.MODE_PRIVATE);}if (imageName != null&& (imageName.contains(".png") || imageName.contains(".PNG"))) {bitmap.compress(Bitmap.CompressFormat.PNG, 90, fos);} else {bitmap.compress(Bitmap.CompressFormat.JPEG, 90, fos);}fos.flush();if (fos != null) {fos.close();}return true;} catch (Exception e) {e.printStackTrace();return false;}}/*** 辅助方法，一般不调用* * @param path* @param mActivity* @param imageName*/private void removeBitmapFromFile(String path, Context mActivity,String imageName) {File file = null;String real_path = "";try {if (ImageDownloaderUtil.getInstance().hasSDCard()) {real_path = ImageDownloaderUtil.getInstance().getExtPath()+ (path != null && path.startsWith("/") ? path : "/"+ path);} else {real_path = ImageDownloaderUtil.getInstance().getPackagePath(mActivity)+ (path != null && path.startsWith("/") ? path : "/"+ path);}file = new File(real_path, imageName);if (file != null)file.delete();} catch (Exception e) {e.printStackTrace();}}/*** 异步下载图片的方法* * @author* */private class MyAsyncTask extends AsyncTask<String, Void, Bitmap> {private Context context;private ImageView mImageView;private String url;private OnImageDownloadListener downloadListener;private String path;public MyAsyncTask(Context context, String url, ImageView mImageView,String path, OnImageDownloadListener downloadListener) {this.context = context;this.url = url;this.mImageView = mImageView;this.path = path;this.downloadListener = downloadListener;}@Overrideprotected Bitmap doInBackground(String... params) {Bitmap bm = null;if (url != null) {try {URL urlObj = new URL(url);HttpURLConnection httpConn = (HttpURLConnection) urlObj.openConnection();httpConn.setDoInput(true);httpConn.setRequestMethod("GET");httpConn.connect();if (httpConn.getResponseCode() == 200) {InputStream is = httpConn.getInputStream();bm = BitmapFactory.decodeStream(is);}String imageName = ImageDownloaderUtil.getInstance().getImageName(url);if (!setBitmapToFile(path, context, imageName, bm)) {removeBitmapFromFile(path, context, imageName);}// 放入强缓存lruCache.put(url, bm);Log.i(TAG, "==放入强缓存ok");} catch (Exception e) {e.printStackTrace();}}return bm;}@Overrideprotected void onPostExecute(Bitmap result) {// 回调设置图片if (downloadListener != null) {downloadListener.onImageDownload(result, url);// 该url对应的task已经下载完成，从map中将其删除removeTaskFromMap(url);}super.onPostExecute(result);}}public interface OnImageDownloadListener {void onImageDownload(Bitmap bitmap, String imgUrl);}// 定义强引用缓存class MyLruCache extends LruCache<String, Bitmap> {public MyLruCache(int maxSize) {super(maxSize);}@Overrideprotected int sizeOf(String key, Bitmap value) {// return value.getHeight() * value.getWidth() * 4;// Bitmap图片的一个像素是4个字节return value.getByteCount();}@Overrideprotected void entryRemoved(boolean evicted, String key,Bitmap oldValue, Bitmap newValue) {if (evicted) {SoftReference<Bitmap> softReference = new SoftReference<Bitmap>(oldValue);softCaches.put(key, softReference);}}}static class ImageDownloaderUtil {private static ImageDownloaderUtil util;private ImageDownloaderUtil() {}public static ImageDownloaderUtil getInstance() {if (util == null) {util = new ImageDownloaderUtil();}return util;}/*** 判断是否有sdcard* * @return*/public boolean hasSDCard() {boolean b = false;if (Environment.MEDIA_MOUNTED.equals(Environment.getExternalStorageState())) {b = true;}return b;}/*** 得到sdcard路径* * @return*/public String getExtPath() {String path = "";if (hasSDCard()) {path = Environment.getExternalStorageDirectory().getAbsolutePath();}return path;}/*** 得到包目录* * @param mActivity* @return*/public String getPackagePath(Context mActivity) {return mActivity.getFilesDir().toString();}/*** 根据url得到图片名* * @param url* @return*/public String getImageName(String url) {String imageName = "";if (url != null) {imageName = url.substring(url.lastIndexOf("/") + 1);}return imageName;}}
}

看几张效果图

初始的加载页面，此时从网络加载的图片还没加载完全

从网络加载的图片加载完全显示出来

我们看一下图片加载的日志文件以及第二次启动程序是图片加载的日志

这个例子可以很好的练习网络，异步加载，Handler，looper，IO操作，SDCard操作，缓存技术等相关知识点

我是菜鸟，如有不妥之处，多谢指出。。。。。