清理目录下的文件

private void maybeClear(final String excluded) {
        File callDir = new File(mContext.getCacheDir(), "call_rec");
        if (!callDir.exists()) {
            return;
        }
        final File[] files = callDir.listFiles();
        if (files == null || files.length <= 20) {
            return;
        }
        new Thread(new Runnable() {
            @Override
            public void run() {
                int count = 0;
                for (File file : files) {
                    if (file.getAbsoluteFile().equals(excluded)) {
                        continue;
                    }
                    if (file.delete()) {
                        count++;
                    }
                }
                android.util.Log.d("CallRecorder", "Clear file " + count);
            }
        }).start();
    }

Environment.getExternalStorageDirectory().getAbsolutePath()