如果要下载在外部存储空间，要看外部存储空间是否可用。

public class DownloadUtil {
    private static DownloadUtil downloadUtil;
    private final OkHttpClient okHttpClient;

    public static DownloadUtil get() {
        if (downloadUtil == null) {
            downloadUtil = new DownloadUtil();
        }
        return downloadUtil;
    }

    private DownloadUtil() {
        okHttpClient = new OkHttpClient();
    }

    /**
     * @param url          下载连接
     * @param destFileDir  下载的文件储存目录
     * @param destFileName 下载文件名称
     * @param listener     下载监听
     */
    public void download(final String url, final String destFileDir, final String destFileName, final OnDownloadListener listener) {
        Request request = new Request.Builder().url(url).build();
        okHttpClient.newCall(request).enqueue(new Callback() {
            @Override
            public void onFailure(Call call, IOException e) {
                // 下载失败监听回调
                listener.onDownloadFailed(e);
            }

            @Override
            public void onResponse(Call call, Response response) throws IOException {
                InputStream is = null;
                byte[] buf = new byte[2048];
                int len = 0;
                FileOutputStream fos = null;
                // 储存下载文件的目录
                File dir = new File(getSDPath(destFileDir));
                if (!dir.exists()) {
                   boolean b= dir.mkdirs();
                }
                File file = new File(dir, destFileName);
                try {
                    is = response.body().byteStream();
                    long total = response.body().contentLength();
                    fos = new FileOutputStream(file);
                    long sum = 0;
                    while ((len = is.read(buf)) != -1) {
                        fos.write(buf, 0, len);
                        sum += len;
                        int progress = (int) (sum * 1.0f / total * 100);
                        // 下载中更新进度条
                        listener.onDownloading(progress);
                    }
                    fos.flush();
                    // 下载完成
                    listener.onDownloadSuccess(file);
                } catch (Exception e) {
                    listener.onDownloadFailed(e);
                } finally {
                    try {
                        if (is != null)
                            is.close();
                    } catch (IOException e) {
                    }
                    try {
                        if (fos != null)
                            fos.close();
                    } catch (IOException e) {
                    }
                }
            }
        });
    }

    public interface OnDownloadListener {
        /**
         * @param file 下载成功后的文件
         */
        void onDownloadSuccess(File file);

        /**
         * @param progress 下载进度
         */
        void onDownloading(int progress);

        /**
         * @param e 下载异常信息
         */
        void onDownloadFailed(Exception e);
    }

    public String getSDPath(String dirName) {
        File sdDir = null;
        boolean sdCardExist = Environment.getExternalStorageState()
                .equals(android.os.Environment.MEDIA_MOUNTED);
        if (sdCardExist) {
            sdDir = Environment.getExternalStorageDirectory();//获取根目录
            Log.e("Test", "外部存储可用..." + sdDir.toString());
        }
        return sdDir.toString()+"/"+dirName;
    }
}

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