android9 对sdcard，usb无写入权限，可以通过Do

manifest无需配置权限
代码如下：
activity:

package io.wuzz.myapplication;

import androidx.appcompat.app.AppCompatActivity;
import androidx.lifecycle.Observer;
import androidx.lifecycle.ViewModelProvider;

import android.app.Activity;
import android.content.Context;
import android.content.Intent;
import android.net.Uri;
import android.os.Bundle;
import android.os.Environment;
import android.os.storage.StorageManager;
import android.os.storage.StorageVolume;
import android.provider.DocumentsContract;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.CompoundButton;
import android.widget.RadioGroup;
import android.widget.Switch;
import android.widget.Toast;

import java.io.File;

public class MainActivity extends AppCompatActivity {

    Vm myViewModel1;
    Button but1, but2;
    Switch aSwitch;
    String ROOT = getUSBDiskPath();
    String PATH = ROOT + "/testfolder";

    Context context;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        but1 = findViewById(R.id.button);
        but2 = findViewById(R.id.button2);
        aSwitch = findViewById(R.id.switch1);
        context = this;

        myViewModel1 = new ViewModelProvider(this, new ViewModelProvider.NewInstanceFactory()).get(Vm.class);

        but1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                File f = new File(ROOT + "/testabcdef");
                boolean b = DocumentsUtils.mkdirs(context,f);
                Log.d("====1",b+""); // truue
                File f1 = new File(ROOT+"/testxyz");
                boolean b1 = f1.mkdirs();
                Log.d("====2",b1+"");  // false
            }
        });

        but2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
            }
        });

    }

    @Override
    protected void onResume() {
        super.onResume();
    }


    @Override
    protected void onStart() {
        super.onStart();
        // 加载权限，并建立PATH目录
        if (DocumentsUtils.checkWritableRootPath(this, PATH)) {
            showOpenDocumentTree();
        }
    }

    private void showOpenDocumentTree() {
        Intent intent = null;
        if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.N) {
            StorageManager sm = getSystemService(StorageManager.class);
            StorageVolume volume = sm.getStorageVolume(new File(ROOT));
            if (volume != null) {
                intent = volume.createAccessIntent(null);
            }
        }
        if (intent == null) {
            intent = new Intent(Intent.ACTION_OPEN_DOCUMENT_TREE);
        }
        startActivityForResult(intent, DocumentsUtils.OPEN_DOCUMENT_TREE_CODE_USB);
    }

    @Override
    public void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        switch (requestCode) {
            case DocumentsUtils.OPEN_DOCUMENT_TREE_CODE_USB:
                if (data != null && data.getData() != null) {
                    Uri uri = data.getData();
                    DocumentsUtils.saveTreeUri(this, ROOT, uri);
                }
                break;
            default:
                break;
        }
    }

    public static String getUSBDiskPath(){
        String path = null;
        File f = new File("/storage");
        File[] files = f.listFiles();
        for(File file : files){
            if(file.getAbsolutePath().contains("Udisk")){
                path = file.getAbsolutePath();
                break;
            }
        }
        return path;
    }

}

DocumentUtils代码：

package io.wuzz.myapplication;

import android.annotation.TargetApi;
import android.content.Context;
import android.content.SharedPreferences;
import android.net.Uri;
import android.os.Build;
import android.preference.PreferenceManager;
import android.provider.DocumentsContract;
import android.util.Log;

import androidx.documentfile.provider.DocumentFile;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.util.ArrayList;
import java.util.List;

public class DocumentsUtils {


    private static final String TAG = "app";
    public static final int OPEN_DOCUMENT_TREE_CODE_SDCARD = 8000;
    public static final int OPEN_DOCUMENT_TREE_CODE_USB = 8001;

    private static List<String> sExtSdCardPaths = new ArrayList<>();
 
    private DocumentsUtils() {
 
    }
 
    public static void cleanCache() {
        sExtSdCardPaths.clear();
    }
 
    /**
     * Get a list of external SD card paths. (Kitkat or higher.)
     *
     * @return A list of external SD card paths.
     */
    @TargetApi(Build.VERSION_CODES.KITKAT)
    private static String[] getExtSdCardPaths(Context context) {
        if (sExtSdCardPaths.size() > 0) {
            return sExtSdCardPaths.toArray(new String[0]);
        }
        for (File file : context.getExternalFilesDirs("external")) {
            if (file != null && !file.equals(context.getExternalFilesDir("external"))) {
                int index = file.getAbsolutePath().lastIndexOf("/Android/data");
                if (index < 0) {
                    Log.w(TAG, "Unexpected external file dir: " + file.getAbsolutePath());
                } else {
                    String path = file.getAbsolutePath().substring(0, index);
                    try {
                        path = new File(path).getCanonicalPath();
                    } catch (IOException e) {
                        // Keep non-canonical path.
                    }
                    sExtSdCardPaths.add(path);
                }
            }
        }
        if (sExtSdCardPaths.isEmpty()) sExtSdCardPaths.add("/storage/sdcard1");
        return sExtSdCardPaths.toArray(new String[0]);
    }
 
    /**
     * Determine the main folder of the external SD card containing the given file.
     *
     * @param file the file.
     * @return The main folder of the external SD card containing this file, if the file is on an SD
     * card. Otherwise,
     * null is returned.
     */
    @TargetApi(Build.VERSION_CODES.KITKAT)
    private static String getExtSdCardFolder(final File file, Context context) {
        String[] extSdPaths = getExtSdCardPaths(context);
        try {
            for (int i = 0; i < extSdPaths.length; i++) {
                if (file.getCanonicalPath().startsWith(extSdPaths[i])) {
                    return extSdPaths[i];
                }
            }
        } catch (IOException e) {
            return null;
        }
        return null;
    }
 
    /**
     * Determine if a file is on external sd card. (Kitkat or higher.)
     *
     * @param file The file.
     * @return true if on external sd card.
     */
    @TargetApi(Build.VERSION_CODES.KITKAT)
    public static boolean isOnExtSdCard(final File file, Context c) {
        return getExtSdCardFolder(file, c) != null;
    }
 
    /**
     * Get a DocumentFile corresponding to the given file (for writing on ExtSdCard on Android 5).
     * If the file is not
     * existing, it is created.
     *
     * @param file        The file.
     * @param isDirectory flag indicating if the file should be a directory.
     * @return The DocumentFile
     */
    public static DocumentFile getDocumentFile(final File file, final boolean isDirectory,
                                               Context context) {
 
        if (Build.VERSION.SDK_INT <= Build.VERSION_CODES.KITKAT) {
            return DocumentFile.fromFile(file);
        }
 
        String baseFolder = getExtSdCardFolder(file, context);
        boolean originalDirectory = false;
        if (baseFolder == null) {
            return null;
        }
 
        String relativePath = null;
        try {
            String fullPath = file.getCanonicalPath();
            if (!baseFolder.equals(fullPath)) {
                relativePath = fullPath.substring(baseFolder.length() + 1);
            } else {
                originalDirectory = true;
            }
        } catch (IOException e) {
            return null;
        } catch (Exception f) {
            originalDirectory = true;
            //continue
        }
        String as = PreferenceManager.getDefaultSharedPreferences(context).getString(baseFolder,
                null);
 
        Uri treeUri = null;
        if (as != null) treeUri = Uri.parse(as);
        if (treeUri == null) {
            return null;
        }
 
        // start with root of SD card and then parse through document tree.
        DocumentFile document = DocumentFile.fromTreeUri(context, treeUri);
        if (originalDirectory) return document;
        String[] parts = relativePath.split("/");
        for (int i = 0; i < parts.length; i++) {
            DocumentFile nextDocument = document.findFile(parts[i]);
 
            if (nextDocument == null) {
                if ((i < parts.length - 1) || isDirectory) {
                    nextDocument = document.createDirectory(parts[i]);
                } else {
                    nextDocument = document.createFile("image", parts[i]);
                }
            }
            document = nextDocument;
        }
 
        return document;
    }
 
    public static boolean mkdirs(Context context, File dir) {
        boolean res = dir.mkdirs();
        if (!res) {
            if (isOnExtSdCard(dir, context)) {
                DocumentFile documentFile = getDocumentFile(dir, true, context);
                res = documentFile != null && documentFile.canWrite();
            }
        }
        return res;
    }
 
    public static boolean delete(Context context, File file) {
        boolean ret = file.delete();
 
        if (!ret && isOnExtSdCard(file, context)) {
            DocumentFile f = getDocumentFile(file, false, context);
            if (f != null) {
                ret = f.delete();
            }
        }
        return ret;
    }
 
    public static boolean canWrite(File file) {
        boolean res = file.exists() && file.canWrite();
 
        if (!res && !file.exists()) {
            try {
                if (!file.isDirectory()) {
                    res = file.createNewFile() && file.delete();
                } else {
                    res = file.mkdirs() && file.delete();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return res;
    }
 
    public static boolean canWrite(Context context, File file) {
        boolean res = canWrite(file);
 
        if (!res && isOnExtSdCard(file, context)) {
            DocumentFile documentFile = getDocumentFile(file, true, context);
            res = documentFile != null && documentFile.canWrite();
        }
        return res;
    }
 
    public static boolean renameTo(Context context, File src, File dest) {
        boolean res = src.renameTo(dest);
 
        if (!res && isOnExtSdCard(dest, context)) {
            DocumentFile srcDoc;
            if (isOnExtSdCard(src, context)) {
                srcDoc = getDocumentFile(src, false, context);
            } else {
                srcDoc = DocumentFile.fromFile(src);
            }
            DocumentFile destDoc = getDocumentFile(dest.getParentFile(), true, context);
            if (srcDoc != null && destDoc != null) {
                try {
                    if (src.getParent().equals(dest.getParent())) {
                        res = srcDoc.renameTo(dest.getName());
                    } else if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
                        res = DocumentsContract.moveDocument(context.getContentResolver(),
                                srcDoc.getUri(),
                                srcDoc.getParentFile().getUri(),
                                destDoc.getUri()) != null;
                    }
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        }
 
        return res;
    }
 
    public static InputStream getInputStream(Context context, File destFile) {
        InputStream in = null;
        try {
            if (!canWrite(destFile) && isOnExtSdCard(destFile, context)) {
                DocumentFile file = getDocumentFile(destFile, false, context);
                if (file != null && file.canWrite()) {
                    in = context.getContentResolver().openInputStream(file.getUri());
                }
            } else {
                in = new FileInputStream(destFile);
 
            }
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }
        return in;
    }
 
    public static OutputStream getOutputStream(Context context, File destFile) {
        OutputStream out = null;
        try {
            if (!canWrite(destFile) && isOnExtSdCard(destFile, context)) {
                DocumentFile file = getDocumentFile(destFile, false, context);
                if (file != null && file.canWrite()) {
                    out = context.getContentResolver().openOutputStream(file.getUri());
                }
            } else {
                out = new FileOutputStream(destFile);
 
            }
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }
        return out;
    }
 
    public static boolean saveTreeUri(Context context, String rootPath, Uri uri) {
        DocumentFile file = DocumentFile.fromTreeUri(context, uri);
        if (file != null && file.canWrite()) {
            SharedPreferences perf = PreferenceManager.getDefaultSharedPreferences(context);
            perf.edit().putString(rootPath, uri.toString()).apply();
            return true;
        } else {
            Log.e(TAG, "no write permission: " + rootPath);
        }
        return false;
    }
 
    public static boolean checkWritableRootPath(Context context, String rootPath) {
        File root = new File(rootPath);
        if (!root.canWrite()) {
 
            if (isOnExtSdCard(root, context)) {
                DocumentFile documentFile = getDocumentFile(root, true, context);
                return documentFile == null || !documentFile.canWrite();
            } else {
                SharedPreferences perf = PreferenceManager.getDefaultSharedPreferences(context);
 
                String documentUri = perf.getString(rootPath, "");
 
                if (documentUri == null || documentUri.isEmpty()) {
                    return true;
                } else {
                    DocumentFile file = DocumentFile.fromTreeUri(context, Uri.parse(documentUri));
                    return !(file != null && file.canWrite());
                }
            }
        }
        return false;
    }
}