This is a follow-up question for [Android APP connect to FTP server in Java](https://codereview.stackexchange.com/q/260576/231235) and [Android APP FTP uploading file implementation in Java](https://codereview.stackexchange.com/q/261514/231235). I am attempting to perform the download operation to specified FTP server in Android APP. The public class `FTPconnection` has been implemented for performing the download details and the class `FtpDownloadTask` is to make the download operation separate from main thread.
**The experimental implementation**
- Project name: FTPDownloadFile
- `FTPconnection` class implementation
```Java
package com.example.ftpdownloadfile;
import android.util.Log;
import org.apache.commons.net.ftp.FTP;
import org.apache.commons.net.ftp.FTPClient;
import org.apache.commons.net.ftp.FTPReply;
import java.io.BufferedOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.net.SocketException;
import java.net.UnknownHostException;
public class FTPconnection {
private native String getHostname();
private native String getUsername();
private native String getPassword();
private static String FTPHostName = "";
private static final int FTPPort = 21;
private static String FTPUsername = "";
private static String FTPPassword = "";
public FTPconnection()
{
FTPHostName = getHostname();
FTPUsername = getUsername();
FTPPassword = getPassword();
}
// Reference: https://stackoverflow.com/a/24605713/6667035
public Boolean downloadAndSaveFile(String filename, File localFile)
throws IOException {
String LOG_TAG = "FTPconnection_download";
FTPClient ftp = null;
try {
ftp = new FTPClient();
ftp.connect(FTPHostName, FTPPort);
Log.d(LOG_TAG, "Connected. Reply: " + ftp.getReplyString());
ftp.login(FTPUsername, FTPPassword);
Log.d(LOG_TAG, "Logged in");
ftp.setFileType(FTP.BINARY_FILE_TYPE);
Log.d(LOG_TAG, "Downloading");
ftp.enterLocalPassiveMode();
OutputStream outputStream = null;
boolean success = false;
try {
outputStream = new BufferedOutputStream(new FileOutputStream(
localFile));
success = ftp.retrieveFile(filename, outputStream);
} finally {
if (outputStream != null) {
outputStream.close();
}
}
return success;
} finally {
if (ftp != null) {
ftp.logout();
ftp.disconnect();
}
}
}
}
```
- `ftpdownloadfile.cpp`
```C++
#include <jni.h>
#include <string>
extern "C"
JNIEXPORT jstring JNICALL
Java_com_example_ftpdownloadfile_FTPconnection_getHostname(JNIEnv *env, jobject thiz) {
std::string output = "Hostname";
return env->NewStringUTF(output.c_str());
}
extern "C"
JNIEXPORT jstring JNICALL
Java_com_example_ftpdownloadfile_FTPconnection_getUsername(JNIEnv *env, jobject thiz) {
std::string output = "Username";
return env->NewStringUTF(output.c_str());
}
extern "C"
JNIEXPORT jstring JNICALL
Java_com_example_ftpdownloadfile_FTPconnection_getPassword(JNIEnv *env, jobject thiz) {
std::string output = "Password";
return env->NewStringUTF(output.c_str());
}
```
- [`ExecutorService`](https://developer.android.com/reference/java/util/concurrent/Executor) for new thread
```Java
ExecutorService executor = Executors.newSingleThreadExecutor();
executor.submit(() -> {
String threadName = Thread.currentThread().getName();
showToast(threadName, Toast.LENGTH_SHORT);
FTPconnection ftpConnect = new FTPconnection();
final String LOG_TAG = "executor";
try {
String filename = "test.txt";
Log.v(LOG_TAG,"Preparing to download " + filename);
var result = ftpConnect.downloadAndSaveFile(filename, Paths.get(getApplicationInfo().dataDir + "/" + filename).toFile());
if (result == false) {
Log.v(LOG_TAG,"Fail to download " + filename + "!");
} else {
Log.v(LOG_TAG,"Download " + filename + " successfully!");
}
} catch (Exception ex) {
ex.printStackTrace();
}
});
```
- User permission setting
```
<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<uses-permission android:name="android.permission.ACCESS_WIFI_STATE" />
```
- `AndroidManifest.xml`
```XML
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.ftpdownloadfile">
<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/Theme.FTPDownloadFile">
<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>
</activity>
</application>
<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<uses-permission android:name="android.permission.ACCESS_WIFI_STATE" />
</manifest>
```
- `build.gradle`
```Gradle
plugins {
id 'com.android.application'
}
android {
compileSdk 30
defaultConfig {
applicationId "com.example.ftpdownloadfile"
minSdk 26
targetSdk 30
versionCode 1
versionName "1.0"
testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
externalNativeBuild {
cmake {
cppFlags ''
}
}
}
buildTypes {
release {
minifyEnabled false
proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
}
}
compileOptions {
sourceCompatibility JavaVersion.VERSION_11
targetCompatibility JavaVersion.VERSION_11
}
externalNativeBuild {
cmake {
path file('src/main/cpp/CMakeLists.txt')
version '3.18.1'
}
}
}
dependencies {
implementation fileTree(include: ['*.jar'], dir: 'libs')
// https://mvnrepository.com/artifact/commons-net/commons-net
implementation group: 'commons-net', name: 'commons-net', version: '20030805.205232'
implementation 'androidx.appcompat:appcompat:1.3.0'
implementation 'com.google.android.material:material:1.4.0'
implementation 'androidx.constraintlayout:constraintlayout:2.0.4'
testImplementation 'junit:junit:4.13.2'
androidTestImplementation 'androidx.test.ext:junit:1.1.3'
androidTestImplementation 'androidx.test.espresso:espresso-core:3.4.0'
}
```
**Full Testing Code**
`MainActivity.java` implementation:
```Java
package com.example.ftpdownloadfile;
import androidx.appcompat.app.AppCompatActivity;
import androidx.core.app.ActivityCompat;
import androidx.core.content.ContextCompat;
import android.Manifest;
import android.content.Context;
import android.content.pm.PackageManager;
import android.os.AsyncTask;
import android.os.Bundle;
import android.util.Log;
import android.widget.Toast;
import org.apache.commons.net.ftp.FTPClient;
import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.nio.file.Paths;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class MainActivity extends AppCompatActivity {
static {
System.loadLibrary("ftpdownloadfile");
}
private static final int READ_EXTERNAL_STORAGE_CODE = 1;
private static final int WRITE_EXTERNAL_STORAGE_CODE = 2;
private static final int ACCESS_NETWORK_STATE_CODE = 3;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
checkPermission(Manifest.permission.WRITE_EXTERNAL_STORAGE, WRITE_EXTERNAL_STORAGE_CODE);
checkPermission(Manifest.permission.READ_EXTERNAL_STORAGE, READ_EXTERNAL_STORAGE_CODE);
checkPermission(Manifest.permission.ACCESS_NETWORK_STATE, ACCESS_NETWORK_STATE_CODE);
ExecutorService executor = Executors.newSingleThreadExecutor();
executor.submit(() -> {
String threadName = Thread.currentThread().getName();
showToast(threadName, Toast.LENGTH_SHORT);
FTPconnection ftpConnect = new FTPconnection();
final String LOG_TAG = "executor";
try {
String filename = "test.txt";
Log.v(LOG_TAG,"Preparing to download " + filename);
var result = ftpConnect.downloadAndSaveFile(filename, Paths.get(getApplicationInfo().dataDir + "/" + filename).toFile());
if (result == false) {
Log.v(LOG_TAG,"Fail to download " + filename + "!");
} else {
Log.v(LOG_TAG,"Download " + filename + " successfully!");
}
} catch (Exception ex) {
ex.printStackTrace();
}
});
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
try {
BufferedReader fileReader = new BufferedReader(new FileReader(this.getApplicationInfo().dataDir + "/test.txt"));
String row;
while ((row = fileReader.readLine()) != null) {
showToast(row, Toast.LENGTH_SHORT);
}
fileReader.close();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
void showToast(String textInput, int duration)
{
Context context = getApplicationContext();
CharSequence text = textInput;
// Reference: https://stackoverflow.com/a/12897386/6667035
runOnUiThread(() -> Toast.makeText(context, text, duration).show());
}
// Function to check and request permission.
public void checkPermission(String permission, int requestCode) {
if (ContextCompat.checkSelfPermission(MainActivity.this, permission) == PackageManager.PERMISSION_DENIED) {
// Requesting the permission
ActivityCompat.requestPermissions(MainActivity.this, new String[] { permission }, requestCode);
}
else {
Toast.makeText(MainActivity.this, "Permission already granted", Toast.LENGTH_SHORT).show();
}
}
}
```
All suggestions are welcome.
The summary information:
- Which question it is a follow-up to?
[Android APP connect to FTP server in Java](https://codereview.stackexchange.com/q/260576/231235) and
[Android APP FTP uploading file implementation in Java](https://codereview.stackexchange.com/q/261514/231235)
- What changes has been made in the code since last question?
I am attempting to perform the download operation to specified FTP server in Android APP in this post.
- Why a new review is being asked for?
If there is any possible improvement, please let me know.