o7planning

Android External Storage Tutorial with Examples

  1. Android External Storage
  2. Read and write files on the external storage example
  3. Appendix: Interface design

1. Android External Storage

Android External Storage: a place to store addition data of Android, the files that you store here is not applied the security system.
Usually there are two types of external storage:
  • Fixed external storage: Commonly understood as the hard drive of the device.
  • Removable storage: Such as SD Card.
Use the static method of Environment class you can get the information on the directory of the external storage.
Here is the results table when running on the emulator:
Method
Returns
getDataDirectory()
/data
getDownloadCacheDirectory()
/cache
getExternalStorageState()
mounted
getExternalStoragePublicDirectory(Environment.Music):
/storage/emulated/0/Music
getDownloadCacheDirectory()
/cache
getRootDirectory()
/system
To read/write the data on the external storage requires you to configure AndroidManifest.xml, added:
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>
With Android Level> = 23 to read and write data on the in external storage device, you need to ask the user by using code. (See in the example).
Android API Level < 29
With Android API Level < 29, apps can store its data on external storage, specifically in the folder below (or subfolders).
if (android.os.Build.VERSION.SDK_INT < 29) {

     // ==>  /storage/emulated/0    (Emulator)
     File dir = Environment.getExternalStorageDirectory();

}
The storage method mentioned above can cause conflicts between different applications, since all of these applications can store its data in the same directory (or subfolders), further if you uninstall an application, the data created by it still exists on the external storage.
Android API Level 29+
With Android API 29+, each application stores its created data in a different folder on the external storage. When users uninstall application all data created by it will also be deleted.
if (android.os.Build.VERSION.SDK_INT >= 29) {

     // ==> /storage/emulated/0/Android/data/org.o7planning.externalstoragedemo/files
     File dir = this.getExternalFilesDir(null);

}
This class may be useful to you when working with external storage:
ExternalStorageUtils
package org.o7planning.externalstoragedemo.utils;

import android.content.Context;
import android.os.Build;
import android.os.Environment;
import android.os.StatFs;

import androidx.annotation.RequiresApi;

import java.io.File;

public class ExternalStorageUtils {

    // Check whether the external storage is mounted or not.
    public static boolean isExternalStorageMounted() {

        String dirState = Environment.getExternalStorageState();
        if(Environment.MEDIA_MOUNTED.equals(dirState))
        {
            return true;
        }else
        {
            return false;
        }
    }

    // Check whether the external storage is read only or not.
    public static boolean isExternalStorageReadOnly() {

        String dirState = Environment.getExternalStorageState();
        if(Environment.MEDIA_MOUNTED_READ_ONLY.equals(dirState))
        {
            return true;
        }else
        {
            return false;
        }
    }


    // Get private external storage base directory.
    public static String getPrivateExternalStorageBaseDir(Context context, String dirType)
    {
        String ret = "";
        if(isExternalStorageMounted()) {
            File file = context.getExternalFilesDir(dirType);
            ret = file.getAbsolutePath();
        }
        return ret;
    }

    // Get private cache external storage base directory.
    public static String getPrivateCacheExternalStorageBaseDir(Context context)
    {
        String ret = "";
        if(isExternalStorageMounted()) {
            File file = context.getExternalCacheDir();
            ret = file.getAbsolutePath();
        }
        return ret;
    }


    // Get public external storage base directory.
    public static String getPublicExternalStorageBaseDir()
    {
        String ret = "";
        if(isExternalStorageMounted()) {
            File file = Environment.getExternalStorageDirectory();
            ret = file.getAbsolutePath();
        }
        return ret;
    }

    // Get public external storage base directory.
    public static String getPublicExternalStorageBaseDir(String dirType)
    {
        String ret = "";
        if(isExternalStorageMounted()) {
            File file = Environment.getExternalStoragePublicDirectory(dirType);
            ret = file.getAbsolutePath();
        }
        return ret;
    }

    // Get external storage disk space, return MB
    @RequiresApi(api = Build.VERSION_CODES.JELLY_BEAN_MR2)
    public static long getExternalStorageSpace() {
        long ret = 0;
        if (isExternalStorageMounted()) {
            StatFs fileState = new StatFs(getPublicExternalStorageBaseDir());

            // Get total block count.
            long count = fileState.getBlockCountLong();

            // Get each block size.
            long size = fileState.getBlockSizeLong();

            // Calculate total space size
            ret = count * size / 1024 / 1024;
        }
        return ret;
    }

    // Get external storage left free disk space, return MB
    @RequiresApi(api = Build.VERSION_CODES.JELLY_BEAN_MR2)
    public static long getExternalStorageLeftSpace() {
        long ret = 0;
        if (isExternalStorageMounted()) {
            StatFs fileState = new StatFs(getPublicExternalStorageBaseDir());

            // Get free block count.
            long count = fileState.getFreeBlocksLong();

            // Get each block size.
            long size = fileState.getBlockSizeLong();

            // Calculate free space size
            ret = count * size / 1024 / 1024;
        }
        return ret;
    }

    // Get external storage available disk space, return MB
    @RequiresApi(api = Build.VERSION_CODES.JELLY_BEAN_MR2)
    public static long getExternalStorageAvailableSpace() {
        long ret = 0;
        if (isExternalStorageMounted()) {
            StatFs fileState = new StatFs(getPublicExternalStorageBaseDir());

            // Get available block count.
            long count = fileState.getAvailableBlocksLong();

            // Get each block size.
            long size = fileState.getBlockSizeLong();

            // Calculate available space size
            ret = count * size / 1024 / 1024;
        }
        return ret;
    }
}

2. Read and write files on the external storage example

Create a project named ExternalStorageDemo.
  • File > New > New Project > Empty Activity
    • Name: ExternalStorageDemo
    • Package name: org.o7planning.externalstoragedemo
    • Language: Java
AndroidManifest.xml configuration allows read and write data on the external storage memory.
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>
Full content of AndroidManifest.xml:
AndroidManifest.xml
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="org.o7planning.externalstoragedemo">

    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
    <uses-permission android:name="android.permission.READ_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: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>
The application interface:
If you are interested in the interface design steps of this application, please see the appendix at the end of the article.
activity_main.xml
<?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">

    <EditText
        android:id="@+id/editText"
        android:layout_width="0dp"
        android:layout_height="48dp"
        android:layout_marginStart="16dp"
        android:layout_marginLeft="16dp"
        android:layout_marginTop="19dp"
        android:layout_marginEnd="16dp"
        android:layout_marginRight="16dp"
        android:ems="10"
        android:inputType="textPersonName"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <LinearLayout
        android:id="@+id/linearLayout"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="16dp"
        android:layout_marginLeft="16dp"
        android:layout_marginTop="16dp"
        android:layout_marginEnd="16dp"
        android:layout_marginRight="16dp"
        android:orientation="horizontal"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/editText">

        <Button
            android:id="@+id/button_save"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="0"
            android:text="Save" />

        <Button
            android:id="@+id/button_read"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="0"
            android:text="Read" />

        <Button
            android:id="@+id/button_list"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="0"
            android:text="List Dirs" />
    </LinearLayout>

    <TextView
        android:id="@+id/textView"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:layout_marginStart="16dp"
        android:layout_marginLeft="16dp"
        android:layout_marginTop="16dp"
        android:layout_marginEnd="16dp"
        android:layout_marginRight="16dp"
        android:layout_marginBottom="16dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/linearLayout" />
</androidx.constraintlayout.widget.ConstraintLayout>
MainActivity.java
package org.o7planning.externalstoragedemo;

import androidx.appcompat.app.AppCompatActivity;
import androidx.core.app.ActivityCompat;

import android.Manifest;
import android.content.pm.PackageManager;
import android.os.Bundle;
import android.os.Environment;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;

public class MainActivity extends AppCompatActivity {
    private EditText editText;
    private TextView textView;
    private Button saveButton;
    private Button readButton;
    private Button listButton;

    private static final String LOG_TAG = "ExternalStorageDemo";


    private static final int REQUEST_ID_READ_PERMISSION = 100;
    private static final int REQUEST_ID_WRITE_PERMISSION = 200;

    private final String fileName = "note.txt";

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

        editText = (EditText) findViewById(R.id.editText);
        textView = (TextView) findViewById(R.id.textView);

        saveButton = (Button) findViewById(R.id.button_save);
        readButton = (Button) findViewById(R.id.button_read);
        listButton = (Button) findViewById(R.id.button_list);

        saveButton.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View arg0) {
                askPermissionAndWriteFile();
            }

        });

        readButton.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View arg0) {
                askPermissionAndReadFile();
            }

        });

        listButton.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View arg0) {
                listExternalStorages();
            }

        });
    }

    private void askPermissionAndWriteFile() {
        boolean canWrite = this.askPermission(REQUEST_ID_WRITE_PERMISSION,
                Manifest.permission.WRITE_EXTERNAL_STORAGE);
        if(!canWrite)  {
            Toast.makeText(getApplicationContext(),
                    "You do not allow this app to write files.", Toast.LENGTH_LONG).show();
            return;
        }
        //
         this.writeFile();
    }

    private void askPermissionAndReadFile() {
        boolean canRead = this.askPermission(REQUEST_ID_READ_PERMISSION,
                Manifest.permission.READ_EXTERNAL_STORAGE);
        //
        if (!canRead) {
            Toast.makeText(getApplicationContext(),
                    "You do not allow this app to read files.", Toast.LENGTH_LONG).show();
            return;
        }
        //
        this.readFile();
    }

    // With Android Level >= 23, you have to ask the user
    // for permission with device (For example read/write data on the device).
    private boolean askPermission(int requestId, String permissionName) {


        Log.i(LOG_TAG, "Ask for Permission: " + permissionName);
        Log.i(LOG_TAG, "Build.VERSION.SDK_INT: " + android.os.Build.VERSION.SDK_INT);

        if (android.os.Build.VERSION.SDK_INT >= 23) {

            // Check if we have permission
            int permission = ActivityCompat.checkSelfPermission(this, permissionName);

            Log.i(LOG_TAG, "permission: " + permission);
            Log.i(LOG_TAG, "PackageManager.PERMISSION_GRANTED: " + PackageManager.PERMISSION_GRANTED);

            if (permission != PackageManager.PERMISSION_GRANTED) {
                // If don't have permission so prompt the user.
                this.requestPermissions(
                        new String[]{permissionName},
                        requestId
                );
                return false;
            }
        }
        return true;
    }


    // As soon as the user decides, allows or doesn't allow.
    @Override
    public void onRequestPermissionsResult(int requestCode,
                                           String permissions[], int[] grantResults) {

        super.onRequestPermissionsResult(requestCode, permissions, grantResults);
        //
        // Note: If request is cancelled, the result arrays are empty.
        if (grantResults.length > 0) {
            switch (requestCode) {
                case REQUEST_ID_READ_PERMISSION: {
                    if (grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                        readFile();
                    }
                }
                case REQUEST_ID_WRITE_PERMISSION: {
                    if (grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                        writeFile();
                    }
                }
            }
        } else {
            Toast.makeText(getApplicationContext(), "Permission Cancelled!", Toast.LENGTH_SHORT).show();
        }
    }

    // IMPORTANT!!
    public File getAppExternalFilesDir()  {
        if (android.os.Build.VERSION.SDK_INT >= 29) {
            // /storage/emulated/0/Android/data/org.o7planning.externalstoragedemo/files
            return this.getExternalFilesDir(null);
        } else {
            // @Deprecated in API 29.
            // /storage/emulated/0
            return Environment.getExternalStorageDirectory();
        }
    }


    private void writeFile() {
        try {
            File extStore = this.getAppExternalFilesDir( );

            boolean canWrite = extStore.canWrite();
            Log.i(LOG_TAG, "Can write: " + extStore.getAbsolutePath()+" : " + canWrite);

            // ==> /storage/emulated/0/note.txt  (API < 29)
            // ==> /storage/emulated/0/Android/data/org.o7planning.externalstoragedemo/files/note.txt (API >=29)
            String path = extStore.getAbsolutePath() + "/" + fileName;
            Log.i(LOG_TAG, "Save to: " + path);

            String data = editText.getText().toString();
            Log.i(LOG_TAG, "Data: " + data);


            File myFile = new File(path);
            FileOutputStream fOut = new FileOutputStream(myFile);
            fOut.write(data.getBytes("UTF-8"));
            fOut.close();

            Toast.makeText(getApplicationContext(), fileName + " saved", Toast.LENGTH_LONG).show();
        } catch (Exception e) {
            Toast.makeText(getApplicationContext(), "Write Error:" + e.getMessage(), Toast.LENGTH_LONG).show();
            Log.e(LOG_TAG, "Write Error: " + e.getMessage());
            e.printStackTrace();
        }
    }

    private void readFile() {

        File extStore = this.getAppExternalFilesDir();

        // ==> /storage/emulated/0/note.txt  (API < 29)
        // ==> /storage/emulated/0/Android/data/org.o7planning.externalstoragedemo/note.txt (API >=29)
        String path = extStore.getAbsolutePath() + "/" + fileName;
        Log.i(LOG_TAG, "Read file: " + path);

        String s = "";
        String fileContent = "";
        try {
            File myFile = new File(path);
            FileInputStream fIn = new FileInputStream(myFile);
            BufferedReader myReader = new BufferedReader(
                    new InputStreamReader(fIn));

            while ((s = myReader.readLine()) != null) {
                fileContent += s + "\n";
            }
            myReader.close();

            this.textView.setText(fileContent);
        } catch (IOException e) {
            Toast.makeText(getApplicationContext(), "Read Error:" + e.getMessage(), Toast.LENGTH_LONG).show();
            Log.e(LOG_TAG, "Read Error: " + e.getMessage());
            e.printStackTrace();
        }
        Toast.makeText(getApplicationContext(), fileContent, Toast.LENGTH_LONG).show();
    }

    private void listExternalStorages() {
        StringBuilder sb = new StringBuilder();

        sb.append("Data Directory: ").append("\n - ")
                .append(Environment.getDataDirectory().toString()).append("\n");

        sb.append("Download Cache Directory: ").append("\n - ")
                .append(Environment.getDownloadCacheDirectory().toString()).append("\n");

        sb.append("External Storage State: ").append("\n - ")
                .append(Environment.getExternalStorageState().toString()).append("\n");

        sb.append("External Storage Directory: ").append("\n - ")
                .append(Environment.getExternalStorageDirectory().toString()).append("\n");

        sb.append("Is External Storage Emulated?: ").append("\n - ")
                .append(Environment.isExternalStorageEmulated()).append("\n");

        sb.append("Is External Storage Removable?: ").append("\n - ")
                .append(Environment.isExternalStorageRemovable()).append("\n");

        sb.append("External Storage Public Directory (Music): ").append("\n - ")
                .append(Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_MUSIC).toString()).append("\n");

        sb.append("Download Cache Directory: ").append("\n - ")
                .append(Environment.getDownloadCacheDirectory().toString()).append("\n");

        sb.append("Root Directory: ").append("\n - ")
                .append(Environment.getRootDirectory().toString()).append("\n");

        Log.i(LOG_TAG, sb.toString());
        this.textView.setText(sb.toString());
    }

}
The app will save a file to the SD Card of the Emulator, so make sure you have your SD Card set up.
Running app:
See more about "Device File Explorer".

3. Appendix: Interface design

Add EditText and Button(s) to the interface.
Add a TextView to the interface.
Set ID, Text for components on the interface:

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