外部存储，个人理解是在app作用域之外存储，就是数据没有和app做关联，app卸载后，它依旧存在

而不再是之前手机本身存储空间和sd卡的区别，现在安卓手机已经不携带可拆卸的SD卡

androidManifest.xml 需要写入读写权限

简单布局:

java代码:  需要动态权限，android8以后，真机都需要，模拟器可能不需要

package com.example.handleproject;

import androidx.annotation.NonNull;

import androidx.appcompat.app.AppCompatActivity;

import androidx.core.app.ActivityCompat;

import androidx.core.content.ContextCompat;

import android.Manifest;

import android.content.pm.PackageManager;

import android.os.Bundle;

import android.os.Environment;

import android.util.Log;

import android.view.View;

import android.widget.Button;

import android.widget.TextView;

import java.io.File;

import java.io.FileInputStream;

import java.io.FileNotFoundException;

import java.io.FileOutputStream;

import java.io.IOException;

public class externalActivity extends AppCompatActivity {

private TextView mTextView;

private Button save;

private Button read;

private TextView showText;

@Override

protected void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.activity_external);

mTextView=findViewById(R.id.text);

save=findViewById(R.id.save);

read=findViewById(R.id.read);

showText=findViewById(R.id.showTxt);

int permisson= ContextCompat.checkSelfPermission(this, Manifest.permission.WRITE_EXTERNAL_STORAGE);

if(permisson!= PackageManager.PERMISSION_GRANTED){

// TODO: 2020-08-14 动态申请权限

ActivityCompat.requestPermissions(this,

new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE},

1);

}

}

public void oprerate(View v){

String path= Environment.getExternalStorageDirectory().getAbsolutePath()+"/imooc.txt";

Log.d("External", path);

switch (v.getId()){

case R.id.save:

File f=new File(path);

try{

if(!f.exists()){

f.createNewFile();

}

//用追加的模式写入

FileOutputStream fos=new FileOutputStream(path,true);

String str=mTextView.getText().toString();

fos.write(str.getBytes());

}catch (IOException E){

Log.d("External err", E.getMessage());

}

break;

case R.id.read:

try{

FileInputStream fis=new FileInputStream(path);

byte[] b=new byte[1024];

int len=fis.read(b);

String str2=new String(b,0,len);

showText.setText(str2);

fos.close();//不要忘记关闭写入

}catch (IOException E){

E.printStackTrace();

}

break;

}

}

@Override

public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {

super.onRequestPermissionsResult(requestCode, permissions, grantResults);

}

}

存储地址在mnt/sdcard(androidStudio上面查看) 的更目录下面，在手机上看时候其实是在根目录下(手机文件管理器显示是"内部存储",也可能是其他的)

更详细的说明