参考:
https://developer.android.com/guide/topics/connectivity/usb/host.html
https://blog.csdn.net/qq_29924041/article/details/80141514

本文介绍Android手机通过OTG数据线读写USB存储设备(U盘,移动硬盘,存储卡)的两种方法

方法一: 直接和USB设备建立连接，借助第三方库libaums识别U盘的文件系统

由于libaums只支持FAT32文件系统，所以U盘的格式化必须采用FAT32！
该库的GitHub地址: https://github.com/magnusja/libaums

1.权限

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" /> 
// 手机必须支持USB主机特性(OTG)
<uses-feature android:name="android.hardware.usb.host" />

2.监听USB插入/拔出

private static final String ACTION_USB_PERMISSION = "com.demo.otgusb.USB_PERMISSION";
private UsbManager mUsbManager;
private PendingIntent mPermissionIntent;    
private BroadcastReceiver mUsbReceiver = new BroadcastReceiver() {
    public void onReceive(Context context, Intent intent) {
        Log.d(TAG, "onReceive: " + intent);
        String action = intent.getAction();
        if (action == null)
            return;
        switch (action) {
            case ACTION_USB_PERMISSION://用户授权广播
                synchronized (this) {
                    if (intent.getBooleanExtra(UsbManager.EXTRA_PERMISSION_GRANTED, false)) { //允许权限申请
                        test();
                    } else {
                        logShow("用户未授权，访问USB设备失败");
                    }
                }
                break;
            case UsbManager.ACTION_USB_DEVICE_ATTACHED://USB设备插入广播
                logShow("USB设备插入");
                break;
            case UsbManager.ACTION_USB_DEVICE_DETACHED://USB设备拔出广播
                logShow("USB设备拔出");
                break;
        }
    }
};

private void init() {   
    //USB管理器
    mUsbManager = (UsbManager) getSystemService(Context.USB_SERVICE);
    mPermissionIntent = PendingIntent.getBroadcast(this, 0, new Intent(ACTION_USB_PERMISSION), 0);
    
    //注册广播,监听USB插入和拔出
    IntentFilter intentFilter = new IntentFilter();
    intentFilter.addAction(UsbManager.ACTION_USB_DEVICE_ATTACHED);
    intentFilter.addAction(UsbManager.ACTION_USB_DEVICE_DETACHED);
    intentFilter.addAction(ACTION_USB_PERMISSION);
    registerReceiver(mUsbReceiver, intentFilter);

    //读写权限
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
        requestPermissions(new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE,
                Manifest.permission.READ_EXTERNAL_STORAGE}, 111);
    }
}

3.使用libaums库读写U盘文件

private void test() {
    try {
        UsbMassStorageDevice[] storageDevices = UsbMassStorageDevice.getMassStorageDevices(this);
        for (UsbMassStorageDevice storageDevice : storageDevices) { //一般手机只有一个USB设备
            // 申请USB权限
            if (!mUsbManager.hasPermission(storageDevice.getUsbDevice())) {
                mUsbManager.requestPermission(storageDevice.getUsbDevice(), mPermissionIntent);
                break;
            }
            // 初始化
            storageDevice.init();
            // 获取分区
            List<Partition> partitions = storageDevice.getPartitions();
            if (partitions.size() == 0) {
                logShow("错误: 读取分区失败");
                return;
            }
            // 仅使用第一分区
            FileSystem fileSystem = partitions.get(0).getFileSystem();
            logShow("Volume Label: " + fileSystem.getVolumeLabel());
            logShow("Capacity: " + fSize(fileSystem.getCapacity()));
            logShow("Occupied Space: " + fSize(fileSystem.getOccupiedSpace()));
            logShow("Free Space: " + fSize(fileSystem.getFreeSpace()));
            logShow("Chunk size: " + fSize(fileSystem.getChunkSize()));

            UsbFile root = fileSystem.getRootDirectory();
            UsbFile[] files = root.listFiles();
            for (UsbFile file : files)
                logShow("文件: " + file.getName());

            // 新建文件
            UsbFile newFile = root.createFile("hello_" + System.currentTimeMillis() + ".txt");
            logShow("新建文件: " + newFile.getName());

            // 写文件
            // OutputStream os = new UsbFileOutputStream(newFile);
            OutputStream os = UsbFileStreamFactory.createBufferedOutputStream(newFile, fileSystem);
            os.write(("hi_" + System.currentTimeMillis()).getBytes());
            os.close();
            logShow("写文件: " + newFile.getName());

            // 读文件
            // InputStream is = new UsbFileInputStream(newFile);
            InputStream is = UsbFileStreamFactory.createBufferedInputStream(newFile, fileSystem);
            byte[] buffer = new byte[fileSystem.getChunkSize()];
            int len;
            File sdFile = new File("/sdcard/111");
            sdFile.mkdirs();
            FileOutputStream sdOut = new FileOutputStream(sdFile.getAbsolutePath() + "/" + newFile.getName());
            while ((len = is.read(buffer)) != -1) {
                sdOut.write(buffer, 0, len);
            }
            is.close();
            sdOut.close();
            logShow("读文件: " + newFile.getName() + " ->复制到/sdcard/111/");

            storageDevice.close();
        }
    } catch (Exception e) {
        logShow("错误: " + e);
    }
}

public static String fSize(long sizeInByte) {
    if (sizeInByte < 1024)
        return String.format("%s", sizeInByte);
    else if (sizeInByte < 1024 * 1024)
        return String.format(Locale.CANADA, "%.2fKB", sizeInByte / 1024.);
    else if (sizeInByte < 1024 * 1024 * 1024)
        return String.format(Locale.CANADA, "%.2fMB", sizeInByte / 1024. / 1024);
    else
        return String.format(Locale.CANADA, "%.2fGB", sizeInByte / 1024. / 1024 / 1024);
}

方法二: 获取U盘的挂载路径,直接读写U盘(就像挂载sdcard读写文件)

对于U盘的文件系统，只依赖于手机系统是否支持，无需我们做额外工作(所有Android手机都支持FAT32，有个别手机还支持NTFS)
但是有些手机无法获取挂载路径（如小米等，就算通过mount命令找到挂载路径也没有权限读写），所以该方法通用性其实不如方法一！

1.通过MEDIA广播获取挂载路径

// 注册系统广播
<receiver android:name=".MediaReceiver">
    <intent-filter>
        <action android:name="android.intent.action.MEDIA_CHECKING" />
        <action android:name="android.intent.action.MEDIA_MOUNTED" />
        <action android:name="android.intent.action.MEDIA_EJECT" />
        <action android:name="android.intent.action.MEDIA_UNMOUNTED" />

        <data android:scheme="file" />
    </intent-filter>
</receiver>

// 获取USB挂载路径
public class MediaReceiver extends BroadcastReceiver {      
    @Override
    public void onReceive(Context context, Intent intent) {
        switch (intent.getAction()) {
            case Intent.ACTION_MEDIA_CHECKING:
                break;
            case Intent.ACTION_MEDIA_MOUNTED:
                // 获取挂载路径, 读取U盘文件
                Uri uri = intent.getData();
                if (uri != null) {
                    String filePath = uri.getPath();
                    File rootFile = new File(filePath);
                    for (File file : rootFile.listFiles()) {
                        // 文件列表...
                    }
                }
                break;
            case Intent.ACTION_MEDIA_EJECT:
                break;
            case Intent.ACTION_MEDIA_UNMOUNTED:
                break;
        }
    }
}

2.通过反射系统方法获取挂载路径

public static List<String> getUsbPaths(Context cxt) {
    List<String> usbPaths = new ArrayList<>();
    try {
        StorageManager srgMgr = (StorageManager) cxt.getSystemService(Context.STORAGE_SERVICE);
        Class<StorageManager> srgMgrClass = StorageManager.class;
        String[] paths = (String[]) srgMgrClass.getMethod("getVolumePaths").invoke(srgMgr);
        for (String path : paths) {
            Object volumeState = srgMgrClass.getMethod("getVolumeState", String.class).invoke(srgMgr, path);
            if (!path.contains("emulated") && Environment.MEDIA_MOUNTED.equals(volumeState))
                usbPaths.add(path);
        }
    } catch (Exception e) {
        e.printStackTrace();
    }
    return usbPaths;
}

简书: https://www.jianshu.com/p/a32e376ea70e
CSDN: https://blog.csdn.net/qq_32115439/article/details/80918046
GitHub博客: http://lioil.win/2018/07/04/Android-USB-OTG.html
Coding博客: http://c.lioil.win/2018/07/04/Android-USB-OTG.html