Android 8.0 API 24.0 以上版本对SD卡的读写权限无效的问题

与Android 5.0不同，除了要在AndroidManifest.xml中设置读写权限外：

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

还要在java代码中申请相关权限。Android6.0以后都要有动态申请权限。
参考快速解决设置Android 23.0以上版本对SD卡的读写权限无效的问题这篇博客，我在MainActivity.java中写了一个公共类，并在SDCardDemoActivity.java中进行授权调用。Android不同版本的Java代码弃用更改的比较频繁，我根据AS的提示做了一些修改。

package com.narziss.qqlogindemo2;

import android.Manifest;
import android.app.Activity;
import android.content.pm.PackageManager;
import android.os.Bundle;
import android.text.TextUtils;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.InputStreamReader;

import androidx.appcompat.app.AppCompatActivity;
import androidx.core.app.ActivityCompat;

import static android.Manifest.permission;

public class MainActivity extends AppCompatActivity {

    private static final String TAG = "MainActivity";
    private EditText mAccountText;
    private EditText mPasswordText;
    private Button mLogin;

    public static class permisionUtils {

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

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        initView();

        initListener();

    }

    private void initListener() {
        mLogin.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Log.d(TAG, "onClick: 点击了登录按钮...");
                handlerLoginEvent(v);
            }
        });
    }

    /**
     * 打开info.text文件，使其可见
     */
    @Override
    protected void onResume() {
        super.onResume();
//        File fileDir = this.getFilesDir();
//        File saveFile = new File(fileDir, "info.text");

        try {
            FileInputStream fileInputStream = this.openFileInput("info.text");
            BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(fileInputStream));
            String info = bufferedReader.readLine();

//            fos.write((accountText + "***" + passwordText).getBytes());
            String[] splits = info.split("\\*\\*\\*");
            String account = splits[0];
            String password = splits[1];

            mAccountText.setText(account);
            mPasswordText.setText(password);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    /**
     * 处理登录事件
     *
     * @param v
     */
    private void handlerLoginEvent(View v) {

        String accountText = mAccountText.getText().toString();
        //getText()返回值是Editable(可编辑)所以要toString()
        String passwordText = mPasswordText.getText().toString();

        //对账号密码进行检查，判空
//        if (accountText.length() == 0) {
//            Toast.makeText(this, "账号不可以为空..", Toast.LENGTH_SHORT).show();
//            return;
//        }
//
//        if (passwordText.length() == 0) {
//            Toast.makeText(this, "密码不可以为空..", Toast.LENGTH_SHORT).show();
//        }

        if (TextUtils.isEmpty(accountText)) {
            Toast.makeText(this, "账号不能为空..", Toast.LENGTH_SHORT).show();
        }
        if (TextUtils.isEmpty(passwordText)) {
            Toast.makeText(this, "密码不能为空..", Toast.LENGTH_SHORT).show();
        }

        saveUserInfo(accountText, passwordText);
    }

    private void saveUserInfo(String accountText, String passwordText) {
        Log.d(TAG, "saveUserInfo: 保存用户信息...");

        File cacheDir = this.getCacheDir();
        Log.d(TAG, "cache dir ==  " + cacheDir.toString());

        File filesDir = this.getFilesDir();
        File saveFile = new File(filesDir, "info.text");
        Log.d(TAG, "files dir == " + filesDir.toString());

//        File file = new File("/data/data/com.narziss.qqlogindemo2/info.text");
        try {
            if (!saveFile.exists()) {
                saveFile.createNewFile();
            }
            FileOutputStream fos = new FileOutputStream(saveFile);
            fos.write((accountText + "***" + passwordText).getBytes());

            fos.close();
            Toast.makeText(this, "数据保存成功...", Toast.LENGTH_SHORT).show();
        } catch (Exception e) {
            e.printStackTrace();
        }

    }


    private void initView() {
        mAccountText = this.findViewById(R.id.et_account);
        mPasswordText = this.findViewById(R.id.et_password);
        mLogin = this.findViewById(R.id.bt_login);

    }
}

package com.narziss.qqlogindemo2;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;

import java.io.File;
import java.io.FileOutputStream;

import androidx.annotation.Nullable;

public class SDCardDemoActivity extends Activity implements View.OnClickListener {

    private Button mWriteDataBtn;



    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_sd_card);
        mWriteDataBtn = this.findViewById(R.id.write_data_to_sd_btn);
        mWriteDataBtn.setOnClickListener(this);

    }

    @Override
    public void onClick(View v) {
        MainActivity.permisionUtils.verifyStoragePermissions(this);
        if (v == mWriteDataBtn) {
            File filepath = new File("/storage/self/primary");
            File file = new File(filepath, "info.txt");

            try {
                //检测读写权限

                FileOutputStream fos = new FileOutputStream(file);
                fos.write("com.narziss.com".getBytes());
                fos.close();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }


}

同时也参考了Android学习之请求外部存储sd卡读写权限（适配Android8.0以上包含Android10.0）少花时间少踩坑！这篇博客的内容

本文地址：https://blog.csdn.net/weixin_44145648/article/details/109647821