申请权限

<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.READ_PHONE_STATE" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

API大于6.0时 主动申请权限

public static void ApplyPermission(Context context) {
    ActivityCompat.requestPermissions((Activity) context, new String[]{
            Manifest.permission.CAMERA,
            Manifest.permission.READ_EXTERNAL_STORAGE,
            Manifest.permission.WRITE_EXTERNAL_STORAGE,
            Manifest.permission.INTERNET
    }, 1);
}

添加okhttp依赖

implementation 'com.squareup.okhttp3:okhttp:3.10.0'

下载文件工具类

public class DownloadUtil {

    private static DownloadUtil downloadUtil;
    private final OkHttpClient okHttpClient;

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

        return downloadUtil;
    }

    public 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 client = new OkHttpClient();

        try {
            Response response = client.newCall(request).execute();
        } catch (IOException e) {
            e.printStackTrace();
        }

        //异步请求
        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(destFileDir);
                if (!dir.exists()) {
                    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();
                        }
                        if (fos != null) {
                            fos.close();
                        }
                    } catch (IOException e) {

                    }
                }
            }
        });
    }

    public interface OnDownloadListener{
        //下载成功之后的文件
        void onDownloadSuccess(File file);
        //下载进度
        void onDownloading(int progress);
        //下载异常信息
        void onDownloadFailed(Exception e);
    }

}

使用

DownloadUtil.get().download("http://47.112.96.171:8081/in/test.html", "/mnt/sdcard/", "test.html",

        new  DownloadUtil.OnDownloadListener() {
            @Override
            public void onDownloadSuccess(File file) {
                Log.e("---------", "onDownloadSuccess");
            }
            @Override
            public void onDownloading(int progress) {
                Log.e("---------", "onDownloading");
            }
            @Override
            public void onDownloadFailed(Exception e) {
                Log.e("---------", "onDownloadFailed" + e.getMessage());

            }
        });

NetworkOnMainThreadException异常
解决：网络耗时请求要放在子线程中

Handler handler = new Handler() {
    @Override
    public void handleMessage(Message msg) {
        super.handleMessage(msg);
        Bundle data = msg.getData();
        String val = data.getString("value");
        Log.i("mylog", "请求结果为-->" + val);
        // TODO
        // UI界面的更新等相关操作
    }
};

/**
 * 网络操作相关的子线程
 */
Runnable networkTask = new Runnable() {
    @Override
    public void run() {
        // TODO
        // 在这里进行 http request.网络请求相关操作
        DownloadUtil.get().download("http://47.112.96.171:8081/in/test.html", "/mnt/sdcard/", "test.html",
                new  DownloadUtil.OnDownloadListener() {
                    @Override
                    public void onDownloadSuccess(File file) {
                        Log.e("---------", "onDownloadSuccess");
                    }
                    @Override
                    public void onDownloading(int progress) {
                        Log.e("---------", "onDownloading");
                    }
                    @Override
                    public void onDownloadFailed(Exception e) {
                        Log.e("---------", "onDownloadFailed" + e.getMessage());
                    }
                });
    }

};

//调用
new Thread(networkTask).start();

communication to … not permitted by network security policy异常
网络安全策略不允许与 www…com之间进行明文通信
原因：Android P以后限制了非加密的流量请求

解决方案：

1. 在res目录下创建xml文件夹，新建 文件network_security_config.xml

2. 在文件中输入以下内容：

   
   

3. 在AndroidManifest中的application里添加你新建的 xml文件

   <application
   ****
   android:networkSecurityConfig="@xml/network_security_config"
   ****