即使文件夾存在，file.listFiles() 也返回 null

Question

我在 onCreate 中有這個活動類我正在檢查文件是否存在，我已經編寫了代碼來請求外部讀取權限，我什至在清單中添加了權限但是 onCreate 中的代碼使應用程序崩潰files.length 為空，在我向用戶授予權限后，應用程序崩潰，因為執行了獲取文件列表的代碼。

package com.example.instantshare.UI.Activities;

import android.Manifest;
import android.content.Context;
import android.content.pm.PackageManager;
import android.graphics.Typeface;
import android.os.Bundle;
import android.os.Environment;
import android.util.Log;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.appcompat.app.AppCompatActivity;
import androidx.core.app.ActivityCompat;
import androidx.core.content.ContextCompat;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import com.example.instantshare.R;
import com.example.instantshare.UI.Helpers.FileExplorerHelper;
import com.example.instantshare.UI.Models.FileItem;
import com.example.instantshare.UI.UIComponents.FileListRecyclerViewAdapter;
import com.faraji.environment3.Device;
import com.faraji.environment3.Environment3;

import java.io.File;
import java.util.ArrayList;
import java.util.Date;
import java.util.HashSet;
import java.util.Iterator;

public class FilePickerActivity extends AppCompatActivity {
    private static final String TAG = "FilePickerActivity";
    private final int REQUEST_CODE = 1;
    private static final ArrayList<FileItem> list = new ArrayList<>();
    private static Context context;


    // View variables
    static  RecyclerView fileListRecyclerView;


    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_file_picker);
        init();
        final String rawExternalStorage = System.getenv("EXTERNAL_STORAGE");
        File file = new File("/sdcard/Android");
        Log.d(TAG, "onCreate: " + file.isDirectory() + " " + file.exists());
        File[] files = file.listFiles();
        Log.d(TAG, "onCreate: " + files.length);
    }



    private void init(){
        fileListRecyclerView = findViewById(R.id.file_list_recycler_view);
        context = FilePickerActivity.this;

        if(ContextCompat.checkSelfPermission(this, Manifest.permission.READ_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED || ActivityCompat.checkSelfPermission(this,Manifest.permission.WRITE_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED){
            ActivityCompat.requestPermissions(this,new String[]{Manifest.permission.READ_EXTERNAL_STORAGE,Manifest.permission.WRITE_EXTERNAL_STORAGE},REQUEST_CODE);
        }
        else{
            readFileInit();
        }
    }

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

        switch (requestCode){
            case REQUEST_CODE:
            {
                if(grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED){
                    readFileInit();
                }
                else{
                    finish();
                }
            }
        }
    }


    private void readFileInit(){
        FileItem internalStorage = new FileItem("Internal Storage","","internal_storage","internal_storage", Environment.getExternalStorageDirectory().getAbsolutePath(),"",true,false,false);
        list.add(internalStorage);


        try {
            File storage = Environment3.getSecondaryExternalStorage().getFile();
            FileItem externalStorage = new FileItem(storage.getName(),"","external_storage","external_storage", storage.getAbsolutePath(),"",true,false,false);
            Log.d(TAG, "readFileInit: " + externalStorage.getPath());
            list.add(externalStorage);

        }
        catch (Exception e){
            e.printStackTrace();

        }





        FileListRecyclerViewAdapter fileListRecyclerViewAdapter = new FileListRecyclerViewAdapter(this,list);
        fileListRecyclerView.setLayoutManager(new LinearLayoutManager(this));
        fileListRecyclerView.setAdapter(fileListRecyclerViewAdapter);



    }


    public static void clickedItem(FileItem fileItem){
        if (fileItem.getStorage() ){
            File directory = new File(fileItem.getPath());

            File[] files = directory.listFiles();
            Log.d(TAG, "clickedItem: " + files.length);

            for(int i = 0; i < files.length; i++){
                FileItem list_item = new FileItem();

                File file = files[i];

                list_item.setName(file.getName());
                list_item.setDirectory(file.isDirectory());
                list_item.setFile(file.isFile());
                list_item.setStorage(false);
                list_item.setLastModified(new Date(file.lastModified()).toString());
                list_item.setPath(file.getAbsolutePath());
                list_item.setExtension(file.getName().split(".")[1]);
                list_item.setType(getFileType());

                list.add(list_item);
            }

            FileListRecyclerViewAdapter fileListRecyclerViewAdapter = new FileListRecyclerViewAdapter(context,list);
            fileListRecyclerView.setAdapter(fileListRecyclerViewAdapter);

        }
        else if(fileItem.getDirectory()){
            File directory = new File(fileItem.getPath());

            File[] files = directory.listFiles();


            for(int i = 0; i < files.length; i++){
                FileItem list_item = new FileItem();

                File file = files[i];

                list_item.setName(file.getName());
                list_item.setDirectory(file.isDirectory());
                list_item.setFile(file.isFile());
                list_item.setStorage(false);
                list_item.setLastModified(new Date(file.lastModified()).toString());
                list_item.setPath(file.getAbsolutePath());
                list_item.setExtension(file.getName().split(".")[1]);
                list_item.setType(getFileType());

                list.add(list_item);
            }

            FileListRecyclerViewAdapter fileListRecyclerViewAdapter = new FileListRecyclerViewAdapter(context,list);
            fileListRecyclerView.setAdapter(fileListRecyclerViewAdapter);
        }
        else{
            return;
        }

    }


    private static String getFileType(){
        return "";
    }
}

堆棧跟蹤

2020-02-24 20:39:33.934 10270-10270/com.example.instantshare D/FilePickerActivity: onCreate: true true

2020-02-24 20:39:33.936 10270-10270/com.example.instantshare D/AndroidRuntime: Shutting down VM
2020-02-24 20:39:33.944 10270-10270/com.example.instantshare E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.instantshare, PID: 10270
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.instantshare/com.example.instantshare.UI.Activities.FilePickerActivity}: java.lang.NullPointerException: Attempt to get length of null array
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3344)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3488)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2049)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:216)
        at android.app.ActivityThread.main(ActivityThread.java:7506)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.an
2020-02-24 20:39:34.060 10270-10270/com.example.instantshare I/Process: Sending signal. PID: 10270 SIG: 9

Answer 1

為了訪問文件，必須在清單文件中授予權限。

<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />

在您的 MainActivity.java 中添加它（不在同一個類中，因為安裝應用程序時它不會第一次工作，因為不是異步的）

checkPermissionReadStorage(this);

嘗試這個：

String path = Environment.getExternalStorageDirectory().getAbsolutePath()+"/Android";
Log.d("Files", "Path: " + path);
File directory = new File(path);
File[] files = directory.listFiles();
if (files != null)
{
   Log.d("Files", "Size: "+ files.length);
   for (int i = 0; i < files.length; i++)
   {
       Log.d("Files", "FileName:" + files[i].getName());
   }
}

將此方法添加到您的課程中，我遇到了與您所說的相同的崩潰，但添加此方法后，我得到了正確的結果 -

D/文件：文件名：數據

public static void checkPermissionReadStorage(Activity activity) {
        if (ContextCompat.checkSelfPermission(activity,
                Manifest.permission.READ_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED) {

            if (!ActivityCompat.shouldShowRequestPermissionRationale(activity,
                    Manifest.permission.READ_EXTERNAL_STORAGE)) {
                ActivityCompat.requestPermissions(activity,
                        new String[]{Manifest.permission.READ_EXTERNAL_STORAGE},
                        1);
            }
        }
    }

如果您對此有任何問題，請告訴我。