首先，使用图片尽量用bitmap，因为drawable创建的对象不会自动适应分辨率和密度，所以，最好不用。
File file = new File(imagePath);
FileInputStream fileInputStream=null;
if (file.exists()) {
try {
fileInputStream=new FileInputStream(imagePath);
} catch (FileNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
方法一：
/*
TypedValue value = new TypedValue();
Bitmap bitmap = BitmapFactory.decodeResourceStream(res, value,
fileInputStream, null, null);
*/
方法二：
/*
TypedValue value = new TypedValue();
BitmapFactory.Options opts = new BitmapFactory.Options();
int density = value.density;
if (density == TypedValue.DENSITY_DEFAULT) {
opts.inDensity = DisplayMetrics.DENSITY_DEFAULT;
} else if (density != TypedValue.DENSITY_NONE) {
opts.inDensity = density;
}
opts.inTargetDensity = res.getDisplayMetrics().densityDpi;
Bitmap bitmap = BitmapFactory.decodeStream(fileInputStream,null,opts);
*/
方法三：
BitmapFactory.Options options = new BitmapFactory.Options();
DisplayMetrics metrics = res.getDisplayMetrics();
options.inScreenDensity = metrics.densityDpi;
options.inTargetDensity = metrics.densityDpi;
options.inDensity = DisplayMetrics.DENSITY_DEFAULT;
Bitmap bitmap = BitmapFactory.decodeStream(fileInputStream, null, options);
方法四：
Bitmap bitmap = BitmapFactory.decodeFile(imagePath,options);
file.setLastModified(System.currentTimeMillis());
return bitmap;
}