实现二级缓存加载图片的功能，在使用DiskLruCache时，需先在工程中添加名为libcore.io的包，并将DiskLruCache.Java文件放进去。DiskLruCache直接百度下载即可。
在GridView的适配器中，为ImageView添加图片时，先从内存缓存中加载，内存中无缓存的话则在磁盘缓存中加载，磁盘缓存也没有的话开启线程下载，然后将下载的图片缓存到磁盘，内存中。下载的图片最好先进行压缩，文章最后给出了压缩代码，但本例中并未实现压缩。
/*二级缓存实现图片墙功能，先在内存中加载缓存，内存中无缓存的话到磁盘缓存中加载，仍然没有的话开启线程下载图片，下载后缓存到磁盘中，然后缓存到内存中*/
public class ErJiHuanCun extends ArrayAdapter<String> {
/**
* 记录所有正在下载或等待下载的任务。
*/
private Set<BitmapWorkerTask> taskCollection;
/**
* 图片缓存技术的核心类，用于缓存所有下载好的图片，在程序内存达到设定值时会将最少最近使用的图片移除掉。
*/
private LruCache<String, Bitmap> mMemoryCache;
/**
* 图片硬盘缓存核心类。
*/
private DiskLruCache mDiskLruCache;
/**
* GridView的实例
*/
private GridView mPhotoWall;
/**
* 记录每个子项的高度。
*/
private int mItemHeight = 0;
public ErJiHuanCun(Context context, int textViewResourceId, String[] objects,
GridView photoWall) {
super(context, textViewResourceId, objects);
mPhotoWall = photoWall;
taskCollection = new HashSet<BitmapWorkerTask>();
// 获取应用程序最大可用内存
int maxMemory = (int) Runtime.getRuntime().maxMemory();
int cacheSize = maxMemory / 8;
// 设置图片缓存大小为程序最大可用内存的1/8
mMemoryCache = new LruCache<String, Bitmap>(cacheSize) {
@Override
protected int sizeOf(String key, Bitmap bitmap) {
return bitmap.getByteCount();
}
};
try {
// 获取图片缓存路径
File cacheDir = getDiskCacheDir(context, "thumb");
if (!cacheDir.exists()) {
cacheDir.mkdirs();
}
// 创建DiskLruCache实例，初始化缓存数据
mDiskLruCache = DiskLruCache
.open(cacheDir, getAppVersion(context), 1, 10 * 1024 * 1024);
} catch (IOException e) {
e.printStackTrace();
}
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
final String url = getItem(position);
View view;
if (convertView == null) {
view = LayoutInflater.from(getContext()).inflate(R.layout.photo_layout, null);
} else {
view = convertView;
}
final ImageView imageView = (ImageView) view.findViewById(R.id.photo);
if (imageView.getLayoutParams().height != mItemHeight) {
imageView.getLayoutParams().height = mItemHeight;
}
// 给ImageView设置一个Tag，保证异步加载图片时不会乱序
imageView.setTag(url);
imageView.setImageResource(R.drawable.ic_launcher);
loadBitmaps(imageView, url);
return view;
}
/**
* 将一张图片存储到LruCache中。
*
* @param key
* LruCache的键，这里传入图片的URL地址。
* @param bitmap
* LruCache的键，这里传入从网络上下载的Bitmap对象。
*/
public void addBitmapToMemoryCache(String key, Bitmap bitmap) {
if (getBitmapFromMemoryCache(key) == null) {
mMemoryCache.put(key, bitmap);
}
}
/**
* 从LruCache中获取一张图片，如果不存在就返回null。
*
* @param key
* LruCache的键，这里传入图片的URL地址。
* @return 对应传入键的Bitmap对象，或者null。
*/
public Bitmap getBitmapFromMemoryCache(String key) {
return mMemoryCache.get(key);
}
/**
* 加载Bitmap对象。此方法会在LruCache中检查所有屏幕中可见的ImageView的Bitmap对象，
* 如果发现任何一个ImageView的Bitmap对象不在缓存中，就会开启异步线程去下载图片。
*/
public void loadBitmaps(ImageView imageView, String imageUrl) {
try {
Bitmap bitmap = getBitmapFromMemoryCache(imageUrl);
if (bitmap == null) {
BitmapWorkerTask task = new BitmapWorkerTask();
taskCollection.add(task);
task.execute(imageUrl);
} else {
if (imageView != null && bitmap != null) {
imageView.setImageBitmap(bitmap);
}
}
} catch (Exception e) {
e.printStackTrace();
}
}
/**
* 取消所有正在下载或等待下载的任务。
*/
public void cancelAllTasks() {
if (taskCollection != null) {
for (BitmapWorkerTask task : taskCollection) {
task.cancel(false);
}
}
}
/**
* 根据传入的uniqueName获取硬盘缓存的路径地址。
*/
public File getDiskCacheDir(Context context, String uniqueName) {
String cachePath;
if (Environment.MEDIA_MOUNTED.equals(Environment.getExternalStorageState())
|| !Environment.isExternalStorageRemovable()) {
cachePath = context.getExternalCacheDir().getPath();
} else {
cachePath = context.getCacheDir().getPath();
}
return new File(cachePath + File.separator + uniqueName);
}
/**
* 获取当前应用程序的版本号。
*/
public int getAppVersion(Context context) {
try {
PackageInfo info = context.getPackageManager().getPackageInfo(context.getPackageName(),
0);
return info.versionCode;
} catch (NameNotFoundException e) {
e.printStackTrace();
}
return 1;
}
/**
* 设置item子项的高度。
*/
public void setItemHeight(int height) {
if (height == mItemHeight) {
return;
}
mItemHeight = height;
notifyDataSetChanged();
}
/**
* 使用MD5算法对传入的key进行加密并返回。
*/
public String hashKeyForDisk(String key) {
String cacheKey;
try {
final MessageDigest mDigest = MessageDigest.getInstance("MD5");
mDigest.update(key.getBytes());
cacheKey = bytesToHexString(mDigest.digest());
} catch (NoSuchAlgorithmException e) {
cacheKey = String.valueOf(key.hashCode());
}
return cacheKey;
}
/**
* 将缓存记录同步到journal文件中。
*/
public void flushCache() {
if (mDiskLruCache != null) {
try {
mDiskLruCache.flush();
} catch (IOException e) {
e.printStackTrace();
}
}
}
private String bytesToHexString(byte[] bytes) {
StringBuilder sb = new StringBuilder();
for (int i = 0; i < bytes.length; i++) {
String hex = Integer.toHexString(0xFF & bytes[i]);
if (hex.length() == 1) {
sb.append('0');
}
sb.append(hex);
}
return sb.toString();
}
/**
* 异步下载图片的任务。
*
*
*/
class BitmapWorkerTask extends AsyncTask<String, Void, Bitmap> {
/**
* 图片的URL地址
*/
private String imageUrl;
@Override
protected Bitmap doInBackground(String... params) {
imageUrl = params[0];
FileDescriptor fileDescriptor = null;
FileInputStream fileInputStream = null;
Snapshot snapShot = null;
try {
// 生成图片URL对应的key
final String key = hashKeyForDisk(imageUrl);
// 查找key对应的缓存
snapShot = mDiskLruCache.get(key);
if (snapShot == null) {
// 如果没有找到对应的缓存，则准备从网络上请求数据，并写入缓存
DiskLruCache.Editor editor = mDiskLruCache.edit(key);
if (editor != null) {
OutputStream outputStream = editor.newOutputStream(0);
if (downloadUrlToStream(imageUrl, outputStream)) {
editor.commit();
} else {
editor.abort();
}
}
// 缓存被写入后，再次查找key对应的缓存
snapShot = mDiskLruCache.get(key);
}
if (snapShot != null) {
fileInputStream = (FileInputStream) snapShot.getInputStream(0);
fileDescriptor = fileInputStream.getFD();
}
// 将缓存数据解析成Bitmap对象
Bitmap bitmap = null;
if (fileDescriptor != null) {
// bitmap = BitmapFactory.decodeFileDescriptor(fileDescriptor);
WindowManager wm= (WindowManager)getContext().getSystemService(Context.WINDOW_SERVICE);
int width=wm.getDefaultDisplay().getWidth();
bitmap= ImageResizer.decodeSampleBithmapFromFileDescriptor(fileDescriptor,width/3,width/3);
}
if (bitmap != null) {
// 将Bitmap对象添加到内存缓存当中
addBitmapToMemoryCache(params[0], bitmap);
}
return bitmap;
} catch (IOException e) {
e.printStackTrace();
} finally {
if (fileDescriptor == null && fileInputStream != null) {
try {
fileInputStream.close();
} catch (IOException e) {
}
}
}
return null;
}
@Override
protected void onPostExecute(Bitmap bitmap) {
super.onPostExecute(bitmap);
// 根据Tag找到相应的ImageView控件，将下载好的图片显示出来。
ImageView imageView = (ImageView) mPhotoWall.findViewWithTag(imageUrl);
if (imageView != null && bitmap != null) {
imageView.setImageBitmap(bitmap);
}
taskCollection.remove(this);
}
/**
* 建立HTTP请求，并获取Bitmap对象。
*
* @param imageUrl
* 图片的URL地址
* @return 解析后的Bitmap对象
*/
private boolean downloadUrlToStream(String urlString, OutputStream outputStream) {
HttpURLConnection urlConnection = null;
BufferedOutputStream out = null;
BufferedInputStream in = null;
try {
final URL url = new URL(urlString);
urlConnection = (HttpURLConnection) url.openConnection();
in = new BufferedInputStream(urlConnection.getInputStream(), 8 * 1024);
out = new BufferedOutputStream(outputStream, 8 * 1024);
int b;
while ((b = in.read()) != -1) {
out.write(b);
}
return true;
} catch (final IOException e) {
e.printStackTrace();
} finally {
if (urlConnection != null) {
urlConnection.disconnect();
}
try {
if (out != null) {
out.close();
}
if (in != null) {
in.close();
}
} catch (final IOException e) {
e.printStackTrace();
}
}
return false;
}
}
}
MainActivity
/**
* 照片墙主活动，使用GridView展示照片墙。
*
*
*/
public class MainActivity extends Activity {
/**
* 用于展示照片墙的GridView
*/
private GridView mPhotoWall;
/**
* GridView的适配器
*/
private ErJiHuanCun mAdapter;
private int mImageThumbSize;
private int mImageThumbSpacing;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
//mImageThumbSize = getResources().getDimensionPixelSize(
// R.dimen.image_thumbnail_size);
//mImageThumbSpacing = getResources().getDimensionPixelSize(
// R.dimen.image_thumbnail_spacing);
mPhotoWall = (GridView) findViewById(R.id.photo_wall);
mAdapter = new ErJiHuanCun(this, 0, Images.imageThumbUrls,
mPhotoWall);
mPhotoWall.setAdapter(mAdapter);
/* mPhotoWall.getViewTreeObserver().addOnGlobalLayoutListener(
new ViewTreeObserver.OnGlobalLayoutListener() {
@Override
public void onGlobalLayout() {
final int numColumns = (int) Math.floor(mPhotoWall
.getWidth()
/ (mImageThumbSize + mImageThumbSpacing));
if (numColumns > 0) {
int columnWidth = (mPhotoWall.getWidth() / numColumns)
- mImageThumbSpacing;
mAdapter.setItemHeight(columnWidth);
mPhotoWall.getViewTreeObserver()
.removeGlobalOnLayoutListener(this);
}
}
});*/
}
@Override
protected void onPause() {
super.onPause();
//将缓存记录同步到journal文件中
mAdapter.flushCache();
}
@Override
protected void onDestroy() {
super.onDestroy();
// // 退出程序时结束所有的下载任务
mAdapter.cancelAllTasks();
}
}
/**
* 自定义正方形的ImageView
*
*/
public class MyImageView extends ImageView {
public MyImageView(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
// TODO Auto-generated constructor stub
}
public MyImageView(Context context, AttributeSet attrs) {
super(context, attrs);
// TODO Auto-generated constructor stub
}
public MyImageView(Context context) {
super(context);
// TODO Auto-generated constructor stub
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
// TODO Auto-generated method stub
//将高度信息改成宽度即可
super.onMeasure(widthMeasureSpec, widthMeasureSpec);
}
}
主Activity的layout
<LinearLayout 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"
android:orientation="vertical"
android:padding="5dp"
>
<GridView
android:id="@+id/photo_wall"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:gravity="center"
android:horizontalSpacing="5dp"
android:verticalSpacing="5dp"
android:numColumns="3"
android:stretchMode="columnWidth"
/>
</LinearLayout>
GridView中的Item ImageView
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >
<com.example.imageloader.MyImageView
android:layout_width="match_parent"
android:layout_height="0dp"
android:id="@+id/photo"
/>
</LinearLayout>
图片压缩实现
public class ImageResizer {
private static final String TAG="ImageResizer";
public static Bitmap decodeSampledBitmapFromResource(Resources res,
int resId,int reqWidth,int reqHeight){
final BitmapFactory.Options options=new BitmapFactory.Options();
options.inJustDecodeBounds=true;
BitmapFactory.decodeResource(res,resId,options);
options.inSampleSize=calculateInSampleSize(options,reqWidth,reqHeight);
options.inJustDecodeBounds=false;
return BitmapFactory.decodeResource(res, resId, options);
}
public static Bitmap decodeSampleBithmapFromFileDescriptor(FileDescriptor fd,
int reqWidth,int reqHeight){
final BitmapFactory.Options options=new BitmapFactory.Options();
options.inJustDecodeBounds=true;
BitmapFactory.decodeFileDescriptor(fd, null,options);
options.inSampleSize=calculateInSampleSize(options,reqWidth,reqHeight);
options.inJustDecodeBounds=false;
return BitmapFactory.decodeFileDescriptor(fd, null,options);
}
public static int calculateInSampleSize(BitmapFactory.Options options,
int reqWidth,int reqHeight){
if(reqWidth==0||reqHeight==0)
return 1;
final int width=options.outWidth;
final int height=options.outHeight;
int inSampleSize=1;
if(height>reqHeight||width>reqWidth ){
final int halfHeight=height/2;
final int halfWidth=width/2;
//尽最大限度的压缩图片，不能让图片的宽高比ImageView的宽高小，否则在将
//图片显示到ImageView时，图片会放大导致图片失真
while(halfHeight/inSampleSize>reqHeight&&halfWidth/inSampleSize>reqWidth){
inSampleSize*=2;
}
}
return inSampleSize;
}
}
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