我正在创建一个应用程序来测试 File.listFiles()方法是否有效。为了检查这一点，我创建了一个应用程序并在那里使用了它，但是这个返回 null 代替了一个数组。
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();
} else {
Toast.makeText(this, "Allow permission for storage access!", Toast.LENGTH_SHORT).show();
}
}
}
}
}
<?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 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>
最佳答案
According to your requirement you were trying to access list of files under a directory by using listfiles() method. And want check it is working or not. But this returning null.
首先，在 list 中声明 MANAGE_EXTERNAL_STORAGE 权限。
// For 30 and after
<uses-permission
android:name="android.permission.MANAGE_EXTERNAL_STORAGE"
tools:ignore="ScopedStorage" />
// For before 30
<uses-permission
android:name="android.permission.READ_EXTERNAL_STORAGE"
tools:node="merge" />
<uses-permission
android:name="android.permission.STORAGE"
tools:node="merge" />
Second, Use the ACTION_MANAGE_ALL_FILES_ACCESS_PERMISSION intent action to direct users to a system settings page where they can enable the following option for your app: Allow access to manage all files.
在您的 onCreate 方法上编写代码:
if (Build.VERSION.SDK_INT < 30) {
if (!checkBefore30()) {
requestBefore30();
} else {
// User granted file permission, Access your file
readFiles();
}
} else if (Build.VERSION.SDK_INT >= 30) {
check30AndAfter();
} else {
// User already has file access permission
readFiles();
}
private boolean checkBefore30() {
return ContextCompat.checkSelfPermission(YourActivity.this,
Manifest.permission.READ_EXTERNAL_STORAGE) == PackageManager.PERMISSION_GRANTED;
}
private void requestBefore30() {
if(ActivityCompat.shouldShowRequestPermissionRationale(YourActivity.this, Manifest.permission.READ_EXTERNAL_STORAGE)) {
Toast.makeText(LoginActivity.this, "Storage permission required. Please allow this permission", Toast.LENGTH_LONG).show();
ActivityCompat.requestPermissions(YourActivity.this,
new String[]{android.Manifest.permission.READ_EXTERNAL_STORAGE}, 100);
} else {
ActivityCompat.requestPermissions(YourActivity.this,
new String[]{android.Manifest.permission.READ_EXTERNAL_STORAGE}, 100);
}
}
@Override
public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
switch (requestCode) {
case 100:
if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
// Permission for storage access successful!
// Read your files now
} else {
// Allow permission for storage access!
}
break;
}
}
@RequiresApi(api = Build.VERSION_CODES.R)
private void check30AndAfter() {
if (!Environment.isExternalStorageManager()) {
try {
Intent intent = new Intent(Settings.ACTION_MANAGE_APP_ALL_FILES_ACCESS_PERMISSION);
intent.addCategory("android.intent.category.DEFAULT");
intent.setData(Uri.parse(String.format("package:%s", getApplicationContext().getPackageName())));
startActivityForResult(intent, 200);
} catch (Exception e) {
Intent intent = new Intent();
intent.setAction(Settings.ACTION_MANAGE_ALL_FILES_ACCESS_PERMISSION);
startActivityForResult(intent, 200);
}
}
}
@RequiresApi(api = Build.VERSION_CODES.R)
@Override
protected void onActivityResult(int requestCode, int resultCode, @Nullable Intent data) {
super.onActivityResult(requestCode, resultCode, data);
if (requestCode == 200) {
if (30 >= Build.VERSION_CODES.R) {
if (Environment.isExternalStorageManager()) {
// Permission for storage access successful!
// Read your files now
} else {
// Allow permission for storage access!
}
}
}
}
private void readFiles() {
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[] files= null;
files= file.listFiles();
if (file.exists()) {
// files exist
}
if (allfiles != null) {
Toast.makeText(this, " file length is" + files.length, Toast.LENGTH_SHORT).show();
} else {
Toast.makeText(this, "No files", Toast.LENGTH_SHORT).show();
}
}
关于java - File.listFiles() 在 android 11 中返回 null，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/68032841/