Android入門之讀寫本地檔案的實現

2022-12-21 14:01:22

簡介

為了這個系列，我的程式碼已經準備到了第150天了。接下來的內容會越來越精彩，我們也越來越開始進入Android的一些高階功能上的程式設計了。今天我們就要講Android中對本地檔案進行讀寫的全過程。

課程目標

輸入檔名、輸入檔案內容後按【儲存到SD卡】，可以把檔案儲存到SD卡根目錄；
輸入檔名，按【讀取SD卡中的檔案】，可以根據輸入的檔名把檔案內容顯示成Toast；
搞清Android中對於SD卡讀寫時所需要的靜態許可權申請、動態許可權申請；

以上一共我們有3個目標，根據目標下面開始教學。

UI端

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/LinearLayout1"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".MainActivity">
 
    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="清輸入檔名" />
 
    <EditText
        android:id="@+id/editFileName"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="檔名" />
 
    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="清輸入檔案內容" />
 
    <EditText
        android:id="@+id/editFileContents"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="檔案內容" />
 
    <Button
        android:id="@+id/buttonSave"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="儲存到SD卡" />
 
    <Button
        android:id="@+id/buttonClean"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="清空" />
 
    <Button
        android:id="@+id/buttonRead"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="讀取sd卡中的檔案" />
 
</LinearLayout>

我們的UI端很簡單，用LinearLayout從上到下依次把一系列元素都設定好。接著我們來看我們的後端程式碼。

靜態授權-AndroidManifest.xml檔案內容

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools">
    <!-- 在SDCard中建立與刪除檔案許可權 -->
    <uses-permission android:name="android.permission.MOUNT_UNMOUNT_FILESYSTEMS"
        tools:ignore="ProtectedPermissions" />
    <!-- 往SDCard寫入資料許可權 -->
    <uses-permission android:name="android.permission.ACCESS_MEDIA_LOCATION"/>
    <uses-permission android:name="android.permission.MANAGE_EXTERNAL_STORAGE"
        tools:ignore="ScopedStorage" />
    <!--外部儲存的寫許可權-->
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
    <!--外部儲存的讀許可權-->
    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.WAKE_LOCK" />
    <application
        android:requestLegacyExternalStorage="true"
        android:allowBackup="true"
        android:dataExtractionRules="@xml/data_extraction_rules"
        android:fullBackupContent="@xml/backup_rules"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/Theme.DemoSimpleFile"
        tools:targetApi="31">
        <activity
            android:name=".MainActivity"
            android:exported="true">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
 
            <meta-data
                android:name="android.app.lib_name"
                android:value="" />
            <meta-data
                android:name="ScopedStorage"
                android:value="true" />
        </activity>
    </application>
 
</manifest>

注意以上的4行<uses-permission>。

後端程式碼

檔案讀寫幫助類-SDFileUtility.java

package org.mk.android.demo;
 
import android.content.Context;
import android.os.Environment;
import android.util.Log;
import android.widget.Toast;
 
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
 
public class SDFileUtility {
    private final static String TAG = "DemoSimpleFile";
    private Context context;
 
    public SDFileUtility() {
    }
 
    public SDFileUtility(Context context) {
        super();
        this.context = context;
    }
 
    //往SD卡寫入檔案的方法
    public void savaFileToSD(String fileName, String fileContents) throws Exception {
        //如果手機已插入sd卡,且app具有讀寫sd卡的許可權
        if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
            fileName = Environment.getExternalStorageDirectory().getCanonicalPath() + "/" + fileName;
            //這裡就不要用openFileOutput了,那個是往手機記憶體中寫資料的
            FileOutputStream output = null;
            try {
                output = new FileOutputStream(fileName);
                output.write(fileContents.getBytes());
                //將String字串以位元組流的形式寫入到輸出流中
            } catch (Exception e) {
                Log.e(TAG, "saveFileTOSD error: " + e.getMessage(), e);
            } finally {
                try {
                    output.close();
                    //關閉輸出流
                } catch (Exception e) {
                }
            }
 
        } else Toast.makeText(context, "SD卡不存在或者不可讀寫", Toast.LENGTH_SHORT).show();
    }
 
    //讀取SD卡中檔案的方法
    //定義讀取檔案的方法:
    public String readFromSD(String fileName) throws IOException {
        StringBuilder sb = new StringBuilder("");
        if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
            fileName = Environment.getExternalStorageDirectory().getCanonicalPath() + "/" + fileName;
            FileInputStream input = null;
            try {
                //開啟檔案輸入流
                input = new FileInputStream(fileName);
                byte[] temp = new byte[1024];
 
                int len = 0;
                //讀取檔案內容:
                while ((len = input.read(temp)) > 0) {
                    sb.append(new String(temp, 0, len));
                }
            } catch (Exception e) {
                Log.e(TAG, "readFromSD error: " + e.getMessage(), e);
            } finally {
                try {
                    //關閉輸入流
                    input.close();
                } catch (Exception e) {
                }
            }
 
        }
        return sb.toString();
    }
}

後端主互動類-MainActivity.java

package org.mk.android.demo;
 
import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.core.app.ActivityCompat;
import androidx.core.content.ContextCompat;
 
import android.Manifest;
import android.app.AlertDialog;
import android.content.Context;
import android.content.DialogInterface;
import android.content.Intent;
import android.content.pm.PackageManager;
import android.os.Build;
import android.os.Bundle;
import android.os.Environment;
import android.provider.Settings;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;
 
public class MainActivity extends AppCompatActivity implements View.OnClickListener {
    private EditText editFileName;
    private EditText editContents;
    private Button buttonSave;
    private Button buttonClean;
    private Button buttonRead;
    private Context mContext;
    private final static String TAG = "DemoSimpleFile";
 
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mContext = getApplicationContext();
        bindViews();
    }
 
    private void bindViews() {
        editFileName = (EditText) findViewById(R.id.editFileName);
        editContents = (EditText) findViewById(R.id.editFileContents);
        buttonSave = (Button) findViewById(R.id.buttonSave);
        buttonClean = (Button) findViewById(R.id.buttonClean);
        buttonRead = (Button) findViewById(R.id.buttonRead);
 
        buttonSave.setOnClickListener(this);
        buttonClean.setOnClickListener(this);
        buttonRead.setOnClickListener(this);
    }
 
    @Override
    public void onClick(View v) {
        switch (v.getId()) {
            case R.id.buttonClean:
                editContents.setText("");
                editFileName.setText("");
                break;
            case R.id.buttonSave:
                if (Build.VERSION.SDK_INT>=Build.VERSION_CODES.R) {
                    Log.i(TAG,">>>>>>version.SDK->"+Build.VERSION.SDK_INT);
                    if (!Environment.isExternalStorageManager()) {
                        Intent intent = new Intent(Settings.ACTION_MANAGE_ALL_FILES_ACCESS_PERMISSION);
                        this.startActivity(intent);
                        return;
                    }
                }
                Log.i(TAG,">>>>>>start to writeFile");
                writeFile();
                Log.i(TAG,">>>>>>write success");
 
                break;
            case R.id.buttonRead:
                if (Build.VERSION.SDK_INT>=Build.VERSION_CODES.R) {
                    Log.i(TAG,">>>>>>version.SDK->"+Build.VERSION.SDK_INT);
                    if (!Environment.isExternalStorageManager()) {
                        Intent intent = new Intent(Settings.ACTION_MANAGE_ALL_FILES_ACCESS_PERMISSION);
                        this.startActivity(intent);
                        return;
                    }
                }
                Log.i(TAG,">>>>>>start to readFile");
                readFile();
                Log.i(TAG,">>>>>>read success");
 
                break;
        }
    }
 
    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        super.onRequestPermissionsResult(requestCode, permissions, grantResults);
        if (requestCode == 1 && (grantResults[0] == PackageManager.PERMISSION_GRANTED)) {
            //writeFile();
            Log.i(TAG,">>>>>>onRequestPermissionsResult");
        }
    }
 
    private void writeFile() {
        String fileName = editFileName.getText().toString();
        String fileContents = editContents.getText().toString();
        SDFileUtility sdHelper = new SDFileUtility(mContext);
        try {
            sdHelper.savaFileToSD(fileName, fileContents);
            Toast.makeText(getApplicationContext(), "資料寫入成功", Toast.LENGTH_SHORT).show();
        } catch (Exception e) {
            Log.e(TAG, "save contents into file has errors: " + e.getMessage(), e);
            Toast.makeText(getApplicationContext(), "資料寫入失敗", Toast.LENGTH_SHORT).show();
        }
    }
 
    private void readFile() {
        String detail = "";
        SDFileUtility sdHelper2 = new SDFileUtility(mContext);
        try {
            String fileName2 = editFileName.getText().toString();
            detail = sdHelper2.readFromSD(fileName2);
        } catch (Exception e) {
            Log.e(TAG, "read contents from file has errors: " + e.getMessage(), e);
        }
        Toast.makeText(getApplicationContext(), detail, Toast.LENGTH_SHORT).show();
    }
}

核心程式碼導讀

讀寫手機SD卡，我們除了在AndroidManifest.xml檔案中靜態申請許可權外還需要使用程式碼動態申請許可權，這是Android6後的許可權限制帶來的問題。

case R.id.buttonSave:
     if (Build.VERSION.SDK_INT>=Build.VERSION_CODES.R) {
       Log.i(TAG,">>>>>>version.SDK->"+Build.VERSION.SDK_INT);
       if (!Environment.isExternalStorageManager()) {
         Intent intent = new Intent(Settings.ACTION_MANAGE_ALL_FILES_ACCESS_PERMISSION);
         his.startActivity(intent);
         return;
       }
     }

這一段程式碼就是使用程式碼在寫檔案前動態申請許可權用的，當這段程式碼執行後會彈出以下這樣的一個對話方塊