Android图片文件缓存

基本设计框架：

ImageFileCache构造函数中清理缓存空间。
设计get方法，注意添加时间戳更新，LRU确保删除时最后删除
设计save方法，可以采取压缩方法存放
清理缓存是根据文件修改时间排序后再清理40%空间，清除的都是.cash文件

public class ImageFileCache {
private static final String CACHDIR = "wantToGoImgCache";
private static final String WHOLESALE_CONV = ".cach";

private static final int MB = 1024*1024;
private static final int CACHE_SIZE = 10;
private static final int FREE_SD_SPACE_NEEDED_TO_CACHE = 10;

public ImageFileCache() {
//清理文件缓存
removeCache(getDirectory());
}

/** 从文件缓存中获取图片 **/
public Bitmap getImage(final String url) {
File file = new File(getDirectory(),MD5Util.md5(url)+WHOLESALE_CONV);
if (file.exists()) {
//Log.d("ImageFileCache","绝对路径："+file.
4000
getAbsolutePath());
Bitmap bmp = BitmapFactory.decodeFile(file.getAbsolutePath());
if (bmp == null) {
//如果图片文件无法转化为bitmap，则删除该文件
file.delete();
} else {
updateFileTime(file.getAbsolutePath());
return bmp;
}
}else{
//Log.d("ImageFileCache","文件不存在");
}
return null;
}

/** 将图片存入文件缓存 **/
public void saveBitmap(Bitmap bm, String url) {
if (bm == null) {
return;
}
//判断sdcard上的空间
if (FREE_SD_SPACE_NEEDED_TO_CACHE > freeSpaceOnSd()) {
//SD空间不足
return;
}
//进行映射
String filename = MD5Util.md5(url);
//获取缓存目录
File dirFile = getDirectory();
//缓存目录不存在，建立缓存目录
if (!dirFile.exists())
dirFile.mkdirs();
//新建文件对象
File file = new File(dirFile,filename);
try {
//在缓存文件下创建文件
file.createNewFile();

OutputStream outStream = new FileOutputStream(file);
bm.compress(Bitmap.CompressFormat.JPEG, 100, outStream);
outStream.flush();
outStream.close();
} catch (FileNotFoundException e) {
Log.w("ImageFileCache", "FileNotFoundException");
} catch (IOException e) {
Log.w("ImageFileCache", "IOException");
}
}

/**
* 计算存储目录下的文件大小，
* 当文件总大小大于规定的CACHE_SIZE或者sdcard剩余空间小于FREE_SD_SPACE_NEEDED_TO_CACHE的规定
* 那么删除40%最近没有被使用的文件
*/
private boolean removeCache(File dirFile) {
File[] files = dirFile.listFiles();
if (files == null) {
return true;
}
//判断读写权限
if (!Environment.getExternalStorageState().equals(
Environment.MEDIA_MOUNTED)) {
return false;
}

int dirSize = 0;
for (int i = 0; i < files.length; i++) {
if (files[i].getName().contains(WHOLESALE_CONV)) {
dirSize += files[i].length();
}
}

if (dirSize > CACHE_SIZE * MB || FREE_SD_SPACE_NEEDED_TO_CACHE > freeSpaceOnSd()) {
int removeFactor = (int) ((0.4 * files.length) + 1);
Arrays.sort(files, new FileLastModifSort());
for (int i = 0; i < removeFactor; i++) {
if (files[i].getName().contains(WHOLESALE_CONV)) {
files[i].delete();
}
}
}

if (freeSpaceOnSd() <= CACHE_SIZE) {
return false;
}

return true;
}

/** 修改文件的最后修改时间 **/
public void updateFileTime(String path) {
File file = new File(path);
long newModifiedTime = System.currentTimeMillis();
file.setLastModified(newModifiedTime);
}

/** 计算sdcard上的剩余空间 **/
private int freeSpaceOnSd() {
StatFs stat = new StatFs(Environment.getExternalStorageDirectory().getPath());
//block大小和空闲的block的数量
double sdFreeMB = ((double)stat.getAvailableBlocks() * (double) stat.getBlockSize()) / MB;
return (int) sdFreeMB;
}

/** 获得缓存目录 **/
private File getDirectory() {
File dir = new File(getSDPathFile(),CACHDIR);
return dir;
}

/** 取SD卡路径 **/
private File getSDPathFile() {
File storageDirectory = null;
boolean sdCardExist = Environment.getExternalStorageState().equals(
Environment.MEDIA_MOUNTED);  //判断sd卡是否存在
if (sdCardExist) {
storageDirectory = Environment.getExternalStorageDirectory();  //获取根目录
}
return storageDirectory;
}

/**
* 根据文件的最后修改时间进行排序
*/
private class FileLastModifSort implements Comparator<File> {
public int compare(File arg0, File arg1) {
if (arg0.lastModified() > arg1.lastModified()) {
return 1;
} else if (arg0.lastModified() == arg1.lastModified()) {
return 0;
} else {
return -1;
}
}
}

}