AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools">
    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
    <application
        android:allowBackup="true"
        android:dataExtractionRules="@xml/data_extraction_rules"
        android:fullBackupContent="@xml/backup_rules"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/Theme.Logindemo"
        tools:targetApi="31">
        <activity
            android:name=".MainActivity"
            android:exported="true">
        </activity>
        <activity
            android:name=".SDCardDemoActivity"
            android:exported="true">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

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

</manifest>

SDCardDemoActivity.java

package com.java.logindemo;

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

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

public class SDCardDemoActivity extends Activity implements View.OnClickListener {

    private Button mwritedata;
    private static final String TAG ="externaStorageDirectory";

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_sd_card);
         mwritedata =(Button) this.findViewById(R.id.write_data);
         mwritedata.setOnClickListener(this);
    }
    @Override
    public void onClick(View view) {
        File externaStorageDirectory= Environment.getExternalStorageDirectory();
        Log.d(TAG,"ext_FilePath == "+externaStorageDirectory);
        if (view == mwritedata){
            //写数据到sd卡
            //File filePath=new File("/storage/emulated/0");
            File file=new File(externaStorageDirectory,"info.text");
            try {
                FileOutputStream fos=new FileOutputStream(file);
                fos.write("bbs.sunofbeache.com".getBytes());
                fos.close();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }

    }
}