Android6.0+系统挂载U盘（外部存储）后，无法读写_android6 第三方应用无法读写u盘

要解决在外部存储，挂载地址为/mnt/media_rw/B85B-FF8D下无法创建文件问题
6.0、7.0可用，8.0以上未测。
最后创建成功地址为/storage/B85B-FF8D/Android/data/com.xxb.test/files/1.txt

第一、静态权限问题。

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.xxb.test" >
+    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
+    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>
    <application
        android:allowBackup="true"
        android:icon="@mipmap/logo"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme" >

        <activity android:name=".MainActivity" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>
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第二、动态权限问题
摘自：https://blog.csdn.net/LosingCarryJie/article/details/80889154
原谅我的搬砖行为。

    private static final int NOT_NOTICE = 2;//如果勾选了不再询问
    private AlertDialog alertDialog;
    private AlertDialog mDialog;

private void myRequetPermission() {
        if (ContextCompat.checkSelfPermission(this, Manifest.permission.WRITE_EXTERNAL_STORAGE) != PERMISSION_GRANTED) {
            ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE}, 1);
        }else {
            Toast.makeText(this,"您已经申请了权限!",Toast.LENGTH_SHORT).show();
        }
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, String[] permissions, int[] grantResults) {
        super.onRequestPermissionsResult(requestCode, permissions, grantResults);

        if (requestCode == 1) {
            for (int i = 0; i < permissions.length; i++) {
                if (grantResults[i] == PERMISSION_GRANTED) {//选择了“始终允许”
                    Toast.makeText(this, "" + "权限" + permissions[i] + "申请成功", Toast.LENGTH_SHORT).show();
                } else {
                    if (!ActivityCompat.shouldShowRequestPermissionRationale(this, permissions[i])) {//用户选择了禁止不再询问

                        AlertDialog.Builder builder = new AlertDialog.Builder(this);
                        builder.setTitle("permission")
                                .setMessage("点击允许才可以使用我们的app哦")
                                .setPositiveButton("去允许", new DialogInterface.OnClickListener() {
                                    public void onClick(DialogInterface dialog, int id) {
                                        if (mDialog != null && mDialog.isShowing()) {
                                            mDialog.dismiss();
                                        }
                                        Intent intent = new Intent(Settings.ACTION_APPLICATION_DETAILS_SETTINGS);
                                        Uri uri = Uri.fromParts("package", getPackageName(), null);//注意就是"package",不用改成自己的包名
                                        intent.setData(uri);
                                        startActivityForResult(intent, NOT_NOTICE);
                                    }
                                });
                        mDialog = builder.create();
                        mDialog.setCanceledOnTouchOutside(false);
                        mDialog.show();


                    } else {//选择禁止
                        AlertDialog.Builder builder = new AlertDialog.Builder(this);
                        builder.setTitle("permission")
                                .setMessage("点击允许才可以使用我们的app哦")
                                .setPositiveButton("去允许", new DialogInterface.OnClickListener() {
                                    public void onClick(DialogInterface dialog, int id) {
                                        if (alertDialog != null && alertDialog.isShowing()) {
                                            alertDialog.dismiss();
                                        }
                                        ActivityCompat.requestPermissions(MainActivity.this,
                                                new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE}, 1);
                                    }
                                });
                        alertDialog = builder.create();
                        alertDialog.setCanceledOnTouchOutside(false);
                        alertDialog.show();
                    }

                }
            }
        }
    }
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第三步、比较坑的一步。私有化。
摘自：https://blog.csdn.net/marketandtechnology/article/details/73275284

    public void MakedirFile() {
        this.getExternalFilesDir(null).getAbsolutePath();//这一步重要
        File file = new File(getStoragePath(this,true)+"/Android/data/"+this.getPackageName()+"/files/1.txt");
        try {
            file.createNewFile();
            Log.i(TAG, "创建成功 -1 "+file);
        } catch (IOException e) {
            e.printStackTrace();
            Log.i(TAG, "创建失败 -2 " +file);
        }
    }

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其他、U盘地址，
摘自：https://blog.csdn.net/u010838555/article/details/51783384

private static String getStoragePath(Context mContext, boolean is_removale) {

        StorageManager mStorageManager = (StorageManager) mContext.getSystemService(Context.STORAGE_SERVICE);
        Class<?> storageVolumeClazz = null;
        try {
            storageVolumeClazz = Class.forName("android.os.storage.StorageVolume");
            Method getVolumeList = mStorageManager.getClass().getMethod("getVolumeList");
            Method getPath = storageVolumeClazz.getMethod("getPath");
            Method isRemovable = storageVolumeClazz.getMethod("isRemovable");
            Object result = getVolumeList.invoke(mStorageManager);
            final int length = Array.getLength(result);
            for (int i = 0; i < length; i++) {
                Object storageVolumeElement = Array.get(result, i);
                String path = (String) getPath.invoke(storageVolumeElement);
                boolean removable = (Boolean) isRemovable.invoke(storageVolumeElement);
                if (is_removale == removable) {
                    return path;
                }
            }
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            e.printStackTrace();
        } catch (NoSuchMethodException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }
        return null;
    }
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