把assets目录下的db文件拷贝进来

public void copyDb(String filename) {
File file = new File(getFilesDir(), filename);
if (file.exists() && file.length() > 0) {
Log.i(TAG, "DB 已经存在不需要拷贝");
} else {
try {
InputStream is = getAssets().open(filename);
FileOutputStream fos = new FileOutputStream(file);
byte[] buffer = new byte[1024];
int len = -1;
while ((len = is.read(buffer)) != -1) {
fos.write(buffer, 0, len);
}
fos.close();
is.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}