介绍

这篇文章主要关于我学习SD卡的动态获取权限时的一些问题。

Android的目录结构

参考自解析Android内部存储、外部存储的区别
图：目录结构
以我的手机为例，可以看到：
手机目录
与adb shell的ls命令相结合：
shell ls命令
可见，一般的手机用户（即非程序员用户）在手机的文件管理所能看到、操纵的文件都是手机的/storage/emulated/0目录下文件。

数据的主要存储方式

在疯狂Android讲义–第四版中，主要提到的几种存储方式中：

使用数据文件夹。即内存储
- 使用SharedPreferences与Editor
  数据总保持在/data/data/< package name>/shared_prefs中
- 使用openFileOutput和openFileInput
  数据总保持在/data/data/< package name>/files中
读取SD卡上文件
数据总保存在/storage/emulated/0目录下。
SQLite数据库
嗯，存储的位置大概是比较随意。
而这篇文章主要是关于SD卡的

疑惑

在学习教科书时，我发现课本的8.2.2节中，用Genymotion模拟器（google nexus 5，API 23）可以基本还原实验效果，但用我本人的真机（红米k30）以及Android Studio（Pixel_XL_API_30）自带的就不行了。
同时每次访问SD卡，都要来个：
在这里插入图片描述
你说这谁顶得住啊？
我搜了一下，说什么Android 6.0+需要动态申请。尤其是我发现，平时的文件也是放在所谓的SD卡目录下的，为什么其他应用就不用说什么动态申请？

原来的代码：MainActivity.java

package com.example.android.filetest;import android.Manifest;
import android.app.Activity;
import android.content.pm.PackageManager;
import android.os.Bundle;
import android.os.Environment;
import android.util.Log;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;import androidx.core.app.ActivityCompat;
import androidx.core.content.ContextCompat;import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.RandomAccessFile;/*** Description:<br>* 网站: <a href="http://www.crazyit.org">疯狂Java联盟</a><br>* Copyright (C), 2001-2020, Yeeku.H.Lee<br>* This program is protected by copyright laws.<br>* Program Name:<br>* Date:<br>** @author Yeeku.H.Lee kongyeeku@163.com<br>* @version 1.0*/
public class MainActivity extends Activity {private static final String FILE_NAME = "/crazy.bin";private EditText edit1;private TextView edit2;File f1;File f2;@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);// 获取两个按钮Button read = findViewById(R.id.read);Button write = findViewById(R.id.write);// 获取两个文本框edit1 = findViewById(R.id.edit1);edit2 = findViewById(R.id.edit2);// 为write按钮绑定事件监听器write.setOnClickListener(view ->// 运行时请求获取写入SD卡的权限requestPermissions(new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE}, 0x123)); // ①read.setOnClickListener(view ->requestPermissions(new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE}, 0x456));}private String read() {
//        for (int i = 0; i < 10; i++) {
//            System.out.println(f2.canRead() + ", " + f2.canWrite());
//        }// 如果手机插入了SD卡，而且应用程序具有访问SD卡的权限if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {System.out.println(Environment.getExternalStorageState());// 获取SD卡对应的存储目录File sdCardDir = Environment.getExternalStorageDirectory();try (// 获取指定文件对应的输入流FileInputStream fis = new FileInputStream(sdCardDir.getCanonicalPath() + FILE_NAME);// 将指定输入流包装成BufferedReaderBufferedReader br = new BufferedReader(new InputStreamReader(fis))) {StringBuilder sb = new StringBuilder();String line = null;// 循环读取文件内容while ((line = br.readLine()) != null) {sb.append(line);}return sb.toString();} catch (IOException e) {e.printStackTrace();}}return null;}private void write(String content) {// 获取SD卡的目录File sdCardDir = Environment.getExternalStorageDirectory();try {System.out.println(sdCardDir.getCanonicalPath() + FILE_NAME);File targetFile = new File(sdCardDir.getCanonicalPath() + FILE_NAME);System.out.println("file is exist: " + targetFile.exists());// 以指定文件创建 RandomAccessFile对象RandomAccessFile raf = new RandomAccessFile(targetFile, "rw");raf.seek(targetFile.length());// 将文件记录指针移动到最后// 输出文件内容raf.write(content.getBytes());// 关闭RandomAccessFileraf.close();} catch (IOException e) {e.printStackTrace();}}@Overridepublic void onRequestPermissionsResult(int requestCode,String[] permissions, int[] grantResults) {if (requestCode == 0x123) {// 如果用户同意授权访问if (grantResults != null && grantResults[0] ==PackageManager.PERMISSION_GRANTED) {write(edit1.getText().toString());edit1.setText("");} else {// 提示用户必须允许写入SD卡的权限Toast.makeText(this, R.string.writesd_tip, Toast.LENGTH_LONG).show();}}if (requestCode == 0x456) {// 如果用户同意授权访问if (grantResults != null && grantResults[0] ==PackageManager.PERMISSION_GRANTED) {// 读取指定文件中的内容，并显示出来edit2.setText(read());} else {// 提示用户必须允许写入SD卡的权限Toast.makeText(this, R.string.writesd_tip, Toast.LENGTH_LONG).show();}}}
}

我觉得说可能是教材没说，就找了找，最终找了个参考：Android无法在SD卡写入文件（Android6.0+需要动态申请
解决了。

修改后代码：

package com.example.android.sdtest_my_1;import android.Manifest;
import android.app.Activity;
import android.content.pm.PackageManager;
import android.os.Bundle;
import android.os.Environment;
import android.util.Log;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;import androidx.core.app.ActivityCompat;
import androidx.core.content.ContextCompat;import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.RandomAccessFile;/*** Description:<br>* 网站: <a href="http://www.crazyit.org">疯狂Java联盟</a><br>* Copyright (C), 2001-2020, Yeeku.H.Lee<br>* This program is protected by copyright laws.<br>* Program Name:<br>* Date:<br>** @author Yeeku.H.Lee kongyeeku@163.com<br>* @version 1.0*/
public class MainActivity extends Activity {private static final String FILE_NAME = "/crazy.bin";private EditText edit1;private TextView edit2;File f1;File f2;//动态申请sd卡读写权限private static final int REQUEST_EXTERNAL_STORAGE = 1;private static String[] PERMISSIONS_STORAGE = {"android.permission.READ_EXTERNAL_STORAGE","android.permission.WRITE_EXTERNAL_STORAGE" };public static void verifyStoragePermissions(Activity activity) {try {//检测是否有写的权限int permission = ActivityCompat.checkSelfPermission(activity,"android.permission.WRITE_EXTERNAL_STORAGE");if (permission != PackageManager.PERMISSION_GRANTED) {// 没有写的权限，去申请写的权限，会弹出对话框ActivityCompat.requestPermissions(activity, PERMISSIONS_STORAGE,REQUEST_EXTERNAL_STORAGE);}} catch (Exception e) {e.printStackTrace();}}@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);// 获取两个按钮Button read = findViewById(R.id.read);Button write = findViewById(R.id.write);// 获取两个文本框edit1 = findViewById(R.id.edit1);edit2 = findViewById(R.id.edit2);// 为write按钮绑定事件监听器write.setOnClickListener(view ->write());read.setOnClickListener(view ->read());verifyStoragePermissions(this);}private void read() {
//        for (int i = 0; i < 10; i++) {
//            System.out.println(f2.canRead() + ", " + f2.canWrite());
//        }// 如果手机插入了SD卡，而且应用程序具有访问SD卡的权限if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {System.out.println(Environment.getExternalStorageState());// 获取SD卡对应的存储目录File sdCardDir = Environment.getExternalStorageDirectory();try (// 获取指定文件对应的输入流FileInputStream fis = new FileInputStream(sdCardDir.getCanonicalPath() + FILE_NAME);// 将指定输入流包装成BufferedReaderBufferedReader br = new BufferedReader(new InputStreamReader(fis))) {StringBuilder sb = new StringBuilder();String line = null;// 循环读取文件内容while ((line = br.readLine()) != null) {sb.append(line);}edit2.setText(sb.toString());} catch (IOException e) {e.printStackTrace();}} else {edit2.setText(null);}}private void write() {String content = edit1.getText().toString();edit1.setText("");// 获取SD卡的目录File sdCardDir = Environment.getExternalStorageDirectory();try {System.out.println(sdCardDir.getCanonicalPath() + FILE_NAME);   // /storage/emulated/0/crazy.binFile targetFile = new File(sdCardDir.getCanonicalPath() + FILE_NAME);System.out.println("file is exist: " + targetFile.exists());// 以指定文件创建 RandomAccessFile对象RandomAccessFile raf = new RandomAccessFile(targetFile, "rw");raf.seek(targetFile.length());// 将文件记录指针移动到最后// 输出文件内容raf.write(content.getBytes());// 关闭RandomAccessFileraf.close();System.out.println("ok, writedc");} catch (IOException e) {e.printStackTrace();}}}