Android external storage can be used to write and save data, read configuration files etc. This article is continuation of the Android Internal Storage tutorial in the series of tutorials on structured data storage in android.
Android External Storage
External storage such as SD card can also store application data, there’s no security enforced upon files you save to the external storage.
In general there are two types of External Storage:
- Primary External Storage: In built shared storage which is “accessible by the user by plugging in a USB cable and mounting it as a drive on a host computer”. Example: When we say Nexus 5 32 GB.
- Secondary External Storage: Removable storage. Example: SD Card
All applications can read and write files placed on the external storage and the user can remove them. We need to check if the SD card is available and if we can write to it. Once we’ve checked that the external storage is available only then we can write to it else the save button would be disabled.
Android External Storage Example Project Structure
Firstly, we need to make sure that the application has permission to read and write data to the users SD card, so lets open up the AndroidManifest.xml and add the following permissions:
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<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/> <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/> |
Also, external storage may be tied up by the user having mounted it as a USB storage device. So we need to check if the external storage is available and is not read only.
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if (!isExternalStorageAvailable() || isExternalStorageReadOnly()) { saveButton.setEnabled(false); } private static boolean isExternalStorageReadOnly() { String extStorageState = Environment.getExternalStorageState(); if (Environment.MEDIA_MOUNTED_READ_ONLY.equals(extStorageState)) { return true; } return false; } private static boolean isExternalStorageAvailable() { String extStorageState = Environment.getExternalStorageState(); if (Environment.MEDIA_MOUNTED.equals(extStorageState)) { return true; } return false; } |
getExternalStorageState() is a static method of Environment to determine if external storage is presently available or not. As you can see if the condition is false we’ve disabled the save button.
Android External Storage Example Code
The activity_main.xml layout is defined as follows:
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<?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="https://schemas.android.com/apk/res/android" android:layout_width="fill_parent" android:layout_height="fill_parent" android:orientation="vertical"> <TextView android:layout_width="fill_parent" android:layout_height="wrap_content" android:text="Reading and Writing to External Storage" android:textSize="24sp"/> <EditText android:id="@+id/myInputText" android:layout_width="match_parent" android:layout_height="wrap_content" android:ems="10" android:lines="5" android:minLines="3" android:gravity="top|left" android:inputType="textMultiLine"> <requestFocus /> </EditText> <LinearLayout android:layout_width="match_parent" android:layout_height="wrap_content" android:orientation="horizontal" android:weightSum="1.0" android:layout_marginTop="20dp"> <Button android:id="@+id/saveExternalStorage" android:layout_width="match_parent" android:layout_height="wrap_content" android:text="SAVE" android:layout_weight="0.5"/> <Button android:id="@+id/getExternalStorage" android:layout_width="match_parent" android:layout_height="wrap_content" android:layout_weight="0.5" android:text="READ" /> </LinearLayout> <TextView android:id="@+id/response" android:layout_width="wrap_content" android:layout_height="wrap_content" android:padding="5dp" android:text="" android:textAppearance="?android:attr/textAppearanceMedium" /> </LinearLayout> |
Here apart from the save and read from external storage buttons we display the response of saving/reading to/from an external storage in a textview unlike in the previous tutorial where android toast was displayed.
The MainActivity.java class is given below:
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package com.journaldev.externalstorage; import java.io.BufferedReader; import java.io.DataInputStream; import java.io.File; import java.io.FileInputStream; import java.io.FileOutputStream; import java.io.IOException; import java.io.InputStreamReader; import android.os.Bundle; import android.app.Activity; import android.os.Environment; import android.view.View; import android.view.View.OnClickListener; import android.widget.Button; import android.widget.EditText; import android.widget.TextView; public class MainActivity extends Activity { EditText inputText; TextView response; Button saveButton,readButton; private String filename = "SampleFile.txt"; private String filepath = "MyFileStorage"; File myExternalFile; String myData = ""; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); inputText = (EditText) findViewById(R.id.myInputText); response = (TextView) findViewById(R.id.response); saveButton = (Button) findViewById(R.id.saveExternalStorage); saveButton.setOnClickListener(new OnClickListener() { @Override public void onClick(View v) { try { FileOutputStream fos = new FileOutputStream(myExternalFile); fos.write(inputText.getText().toString().getBytes()); fos.close(); } catch (IOException e) { e.printStackTrace(); } inputText.setText(""); response.setText("SampleFile.txt saved to External Storage..."); } }); readButton = (Button) findViewById(R.id.getExternalStorage); readButton.setOnClickListener(new OnClickListener() { @Override public void onClick(View v) { try { FileInputStream fis = new FileInputStream(myExternalFile); DataInputStream in = new DataInputStream(fis); BufferedReader br = new BufferedReader(new InputStreamReader(in)); String strLine; while ((strLine = br.readLine()) != null) { myData = myData + strLine; } in.close(); } catch (IOException e) { e.printStackTrace(); } inputText.setText(myData); response.setText("SampleFile.txt data retrieved from Internal Storage..."); } }); if (!isExternalStorageAvailable() || isExternalStorageReadOnly()) { saveButton.setEnabled(false); } else { myExternalFile = new File(getExternalFilesDir(filepath), filename); } } private static boolean isExternalStorageReadOnly() { String extStorageState = Environment.getExternalStorageState(); if (Environment.MEDIA_MOUNTED_READ_ONLY.equals(extStorageState)) { return true; } return false; } private static boolean isExternalStorageAvailable() { String extStorageState = Environment.getExternalStorageState(); if (Environment.MEDIA_MOUNTED.equals(extStorageState)) { return true; } return false; } } |
- Environment.getExternalStorageState(): returns path to internal SD mount point like “/mnt/sdcard”
- getExternalFilesDir(): It returns the path to files folder inside Android/data/data/application_package/ on the SD card. It is used to store any required files for your app (like images downloaded from web or cache files). Once the app is uninstalled, any data stored in this folder is gone too.
Also if the external storage is not available we disable the save button using the if condition that was discussed earlier in this tutorial.
Below is our application running in android emulator, where we are writing data to file and then reading it.
Note: Make sure your Android Emulator is configured such that it has a SD card as shown in the image dialog from AVD below. Go to Tools->Android->Android Virtual Device, edit configurations->Show Advance Settings.
This brings an end to this tutorial. We’ll discuss storage using Shared Preferences in the next tutorial. You can download the final Android External Storage Project from the below link.