Android 10 中读写SD卡权限的问题

记一次在android的学习中遇到的SD卡权限问题。最开始题主本来是想做一个小的demo来操作SD卡。如图所示：

没想到在开发的过程中遇到了权限问题。
首先贴一下activity的代码。其实就是三个按钮，绑定三个事件。

package com.example.demo;

import android.Manifest;
import android.app.Activity;
import android.content.Context;
import android.content.pm.PackageManager;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

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

import java.io.IOException;

/**
 * @Author godv
 * Date on 2020/4/14  22:02
 */
public class FileActivity extends AppCompatActivity {
    private EditText editname;
    private EditText editdetail;
    private Button btnsave;
    private Button btnclean;
    private Button btnread;
    private Context mContext;

    private static final int REQUEST_EXTERNAL_STORAGE = 1;
    private static String[] PERMISSIONS_STORAGE = {
            "android.permission.READ_EXTERNAL_STORAGE",
            "android.permission.WRITE_EXTERNAL_STORAGE" };
    /*
     * android 动态权限申请
     * */
    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();
        }
    }




    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.file_layout);
        verifyStoragePermissions(this);

        mContext = getApplication();
        editdetail = (EditText) findViewById(R.id.editdetail);
        editname = (EditText) findViewById(R.id.editname);
        btnclean = (Button) findViewById(R.id.btnclean);
        btnsave = (Button) findViewById(R.id.btnsave);
        btnread = (Button) findViewById(R.id.btnread);
        //清空
        btnclean.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                editdetail.setText("");
                editname.setText("");
            }
        });
        //保存
        btnsave.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                FileHelper fHelper = new FileHelper(mContext);
                String filename = editname.getText().toString();
                String filedetail = editdetail.getText().toString();
                try {
                    fHelper.savFileToSD(filename, filedetail);
                    Toast.makeText(getApplicationContext(), "数据写入成功", Toast.LENGTH_SHORT).show();
                } catch (Exception e) {
                    e.printStackTrace();
                    Toast.makeText(getApplicationContext(), "数据写入失败", Toast.LENGTH_SHORT).show();
                }
            }
        });
        btnread.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String detail = "";
                FileHelper fHelper2 = new FileHelper(getApplicationContext());
                String fname = editname.getText().toString();
                try {
                    detail = fHelper2.readFromSD(fname);
                } catch (IOException e) {
                    e.printStackTrace();
                }
                Toast.makeText(getApplicationContext(), detail, Toast.LENGTH_SHORT).show();
            }
        });
    }
}

其中FileHelper的代码如下：

package com.example.demo;

import android.content.Context;
import android.content.SharedPreferences;
import android.os.Environment;
import android.widget.Toast;


import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;

/**
 * @Author godv
 * Date on 2020/4/14  22:13
 */
public class FileHelper {

    private Context mContext;

    public FileHelper() {
    }

    public FileHelper(Context mContext) {
        super();
        this.mContext = mContext;
    }


    /**
     * @param filename
     * @param fileContent
     * @throws Exception
     */
    public void savFileToSD(String filename, String fileContent) throws Exception {
        //如果手机已插入sd卡,且app具有读写sd卡的权限
        if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
            filename = Environment.getExternalStorageDirectory().getCanonicalPath() + "/" + filename;
            File file=new File(filename);
            FileOutputStream output = new FileOutputStream(file);
             output.write(fileContent.getBytes());
            //将String字符串以字节流的形式写入到输出流中
            output.close();
            //关闭输出流
        } else{
            Toast.makeText(mContext, "SD卡不存在或者不可读写", Toast.LENGTH_SHORT).show();
        }
    }

    //读取SD卡中文件的方法
    //定义读取文件的方法:
    public String readFromSD(String filename) throws IOException {
        StringBuilder sb = new StringBuilder("");
        if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
            filename = Environment.getExternalStorageDirectory().getCanonicalPath() + "/" + filename;
            //打开文件输入流
            File file=new File(filename);
            FileInputStream input = new FileInputStream(file);
            byte[] temp = new byte[1024];

            int len = 0;
            //读取文件内容:
            while ((len = input.read(temp)) > 0) {
                sb.append(new String(temp, 0, len));
            }
            //关闭输入流
            input.close();
        }
        return sb.toString();
    }

}

但是在android 6+后操作SD卡有动态权限问题。我们需要在Android中添加动态权限 也就是上述activity中的verifyStoragePermissions方法。当然在Androidmanifast中配置也是必不可少的。

<!--    sd读写权限-->
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>

但是问题还是没有解决。在我们获取操作流的时候。
FileOutputStream output = new FileOutputStream(file);
还是会出现报错。
出现报错的原因是由于android 10中文件读写新特性。还需要在Androidmanifast的application节点中加入android:requestLegacyExternalStorage=“true”。最后问题得到解决。
参考文档：
适配策略
解决方案