权限请求

在 Android 11 及以上版本，需要请求 MANAGE_EXTERNAL_STORAGE 权限；在 Android 10 及以下版本，请求 READ_EXTERNAL_STORAGE 权限。

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools">
    <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"
        tools:ignore="ScopedStorage" />

    <application
        android:allowBackup="true"
        ...
    </application>

文件读取

使用 FileReader 和 BufferedReader 从 /sdcard 下读取 test.txt 文件的内容。

import android.Manifest;
import android.content.pm.PackageManager;
import android.os.Build;
import android.os.Bundle;
import android.os.Environment;
import android.widget.TextView;
import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.core.app.ActivityCompat;
import androidx.core.content.ContextCompat;
import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.IOException;
import android.Manifest;
import android.content.Intent;

public class MainActivity extends AppCompatActivity {

    private static final int PERMISSION_REQUEST_CODE = 1;
    private TextView textView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        textView = findViewById(R.id.textView);

        if (checkPermissions()) {
            readFileFromExternalStorage();
        } else {
            requestPermissions();
        }
    }

    private boolean checkPermissions() {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.R) {
            return Environment.isExternalStorageManager();
        } else {
            int result = ContextCompat.checkSelfPermission(this, Manifest.permission.READ_EXTERNAL_STORAGE);
            return result == PackageManager.PERMISSION_GRANTED;
        }
    }

    private void requestPermissions() {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.R) {
            // 对于 Android 11 及以上版本，需要请求管理外部存储权限
            Intent intent = new Intent();
            intent.setAction(android.provider.Settings.ACTION_MANAGE_ALL_FILES_ACCESS_PERMISSION);
            startActivityForResult(intent, PERMISSION_REQUEST_CODE);
        } else {
            // 对于 Android 10 及以下版本，请求读取外部存储权限
            ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.READ_EXTERNAL_STORAGE}, PERMISSION_REQUEST_CODE);
        }
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        super.onRequestPermissionsResult(requestCode, permissions, grantResults);
        if (requestCode == PERMISSION_REQUEST_CODE) {
            if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                readFileFromExternalStorage();
            } else {
                textView.setText("权限被拒绝");
            }
        }
    }

    private void readFileFromExternalStorage() {
        File file = new File(Environment.getExternalStorageDirectory(), "t1.txt");
        StringBuilder stringBuilder = new StringBuilder();
        try {
            BufferedReader reader = new BufferedReader(new FileReader(file));
            String line;
            while ((line = reader.readLine()) != null) {
                stringBuilder.append(line).append("\n");
            }
            reader.close();
            textView.setText(stringBuilder.toString());
        } catch (IOException e) {
            e.printStackTrace();
            textView.setText("读取文件时出错");
        }
    }
}

布局文件

使用 TextView 显示读取到的文件内容。

<?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:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="174dp"
        android:layout_marginTop="268dp"
        android:layout_marginEnd="140dp"
        android:text="Hello World!"
        android:textSize="18sp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>