安卓6.0以上系统写入SD卡权限失败问题

如图所示，在mainfest文件中声明了SD卡的读和写权限，仍旧报错：

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

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安卓23.0版本以上，不仅仅要设置上面的权限，还要在对SD卡有读写操作的地方授权，下面是公共类：

import android.Manifest;
import android.app.Activity;
import android.content.pm.PackageManager;
import android.support.v4.app.ActivityCompat;

public class PermisionUtils {

    // Storage Permissions
    private static final int REQUEST_EXTERNAL_STORAGE = 1;
    private static String[] PERMISSIONS_STORAGE = {
            Manifest.permission.READ_EXTERNAL_STORAGE,
            Manifest.permission.WRITE_EXTERNAL_STORAGE};

    /**
     * Checks if the app has permission to write to device storage
     * If the app does not has permission then the user will be prompted to
     * grant permissions
     *
     * @param activity
     */
    public static void verifyStoragePermissions(Activity activity) {
        // Check if we have write permission
        int permission = ActivityCompat.checkSelfPermission(activity,
                Manifest.permission.WRITE_EXTERNAL_STORAGE);

        if (permission != PackageManager.PERMISSION_GRANTED) {
            // We don't have permission so prompt the user
            ActivityCompat.requestPermissions(activity, PERMISSIONS_STORAGE,
                    REQUEST_EXTERNAL_STORAGE);
        }
    }
}

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然后直接在需要授权的地方调用：

verifyStoragePermissions(this);

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this指的是当前的Activity，程序运行会出现以下结果：

点击ALLOW之后，程序获取SD卡读写权限，就算你把代码里的
verifyStoragePermissions(this);语句删了，只要不卸载应用程序，权限就仍然存在。

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