folder.listfiles() 在 android 11 中返回 null

问题描述

我正在创建一个应用程序来测试 listfiles() 方法是否正常工作，以检查这一点 这是我的完整代码，请帮忙，我已授予 android 11 的所有权限

MainActivity.java

package com.rajkumarcreations.file;

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

import android.content.Intent;
import android.content.pm.PackageManager;
import android.net.Uri;
import android.os.Build;
import android.os.Bundle;
import android.os.Environment;
import android.provider.Settings;
import android.widget.TextView;
import android.widget.Toast;

import org.w3c.dom.Text;

import java.io.File;

import static android.Manifest.permission.READ_EXTERNAL_STORAGE;
import static android.Manifest.permission.WRITE_EXTERNAL_STORAGE;
import static android.os.Build.VERSION.SDK_INT;

public class MainActivity extends AppCompatActivity {
TextView tv;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        tv = (TextView) findViewById(R.id.btn);
        if(!checkPermission()){
            requestPermission();
        }else{
            File file = new File(Environment.getExternalStorageDirectory()+"/Download/");
            File[] allfiles = null;
            allfiles = file.listFiles();
            if(file.exists()){
                tv.setText("Exist");
            }
            if(allfiles!=null){
                Toast.makeText(this,"length is "+allfiles.length,Toast.LENGTH_SHORT).show();
            }else{
                Toast.makeText(this,"Array is null",Toast.LENGTH_SHORT).show();
            }
        }
    }

    private boolean checkPermission() {
        if (SDK_INT >= Build.VERSION_CODES.R) {
            return Environment.isExternalStorageManager();
        } else {
            int write = ContextCompat.checkSelfPermission(MainActivity.this,WRITE_EXTERNAL_STORAGE);
            int read = ContextCompat.checkSelfPermission(MainActivity.this,READ_EXTERNAL_STORAGE);
            return write == PackageManager.PERMISSION_GRANTED && read == PackageManager.PERMISSION_GRANTED;
        }
    }

    private void requestPermission() {
        if (SDK_INT >= Build.VERSION_CODES.R) {
            try {
                Intent intent = new Intent(Settings.ACTION_MANAGE_ALL_FILES_ACCESS_PERMISSION);
                intent.addCategory("android.intent.category.DEFAULT");
                intent.setData(Uri.parse(String.format("package:%s",new Object[]{getApplicationContext().getPackageName()})));
                startActivityForResult(intent,2000);
            } catch (Exception e) {
                Intent intent = new Intent();
                intent.setAction(Settings.ACTION_MANAGE_ALL_FILES_ACCESS_PERMISSION);
                startActivityForResult(intent,2000);
            }
        } else {
            ActivityCompat.requestPermissions(MainActivity.this,new String[]{WRITE_EXTERNAL_STORAGE,READ_EXTERNAL_STORAGE},333);
        }
    }

    @Override
    protected void onActivityResult(int requestCode,int resultCode,@Nullable Intent data) {
        super.onActivityResult(requestCode,resultCode,data);
        if (requestCode == 2000) {
            if (SDK_INT >= Build.VERSION_CODES.R) {
                if (Environment.isExternalStorageManager()) {

                    Toast.makeText(this,"Allow permissions granted",Toast.LENGTH_SHORT).show();

                } else {
                    Toast.makeText(this,"Allow permission for storage access!",Toast.LENGTH_SHORT).show();
                }
            }
        }
    }

    @Override
    public void onRequestPermissionsResult(int requestCode,String[] permissions,int[] grantResults) {
        super.onRequestPermissionsResult(requestCode,permissions,grantResults);
        if (requestCode==333){
            if (grantResults.length > 0) {
                boolean WRITE_EXTERNAL_STORAGE = grantResults[0] == PackageManager.PERMISSION_GRANTED;
                boolean READ_EXTERNAL_STORAGE = grantResults[1] == PackageManager.PERMISSION_GRANTED;
                if (READ_EXTERNAL_STORAGE && WRITE_EXTERNAL_STORAGE) {

                    Toast.makeText(this,"All permissions granted",Toast.LENGTH_SHORT).show();
                }
            }
        }
    }
}

清单文件

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.rajkumarcreations.file">
    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
    <uses-permission android:name="android.permission.MANAGE_EXTERNAL_STORAGE"/>

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:requestLegacyExternalStorage="true"
        android:theme="@style/AppTheme">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

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

</manifest>

XML 文件

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!"
        android:id="@+id/btn"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

解决方法

试试下面的代码，它可以工作。

 //Add below permission in your manifest file
 <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
 <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />


//Add below attribute in manifest application tag
 android:requestLegacyExternalStorage="true"

//Activity code,ask file read/write runtime permission
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main2);
    requestForPermission();
    getFile();
}

private void getFile() {
    File file;
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.R) {
        file = new File(Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS).toString());
    } else {
        file = new File(Environment.getExternalStorageDirectory().toString() + "/Download/");
    }
    File[] allfiles = null;
    allfiles = file.listFiles();
    if (file.exists()) {
        tv.setText("Exist");
    }
    if (allfiles != null) {
        Toast.makeText(this,"length is " + allfiles.length,Toast.LENGTH_SHORT).show();
    } else {
        Toast.makeText(this,"Array is null",Toast.LENGTH_SHORT).show();
    }
}

private void requestForPermission() {
    ActivityCompat.requestPermissions(
            this,new String[]{
                    Manifest.permission.READ_EXTERNAL_STORAGE,Manifest.permission.WRITE_EXTERNAL_STORAGE
            },101
    );
}


@Override
public void onRequestPermissionsResult(int requestCode,@NonNull String[] permissions,@NonNull int[] grantResults) {
    super.onRequestPermissionsResult(requestCode,permissions,grantResults);
    switch (requestCode) {
        case 101:
            if (grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                getFile();
            } else {
                //not granted
            }
            break;
        default:
            break;
    }
}
}