How to enable password recovery functionality through email in Android applications involves several key steps including server-side email configuration and client-side implementation.

Configuring Email Services

To implement email-based password recovery, you first need to configure you're email service provider. For instance, if using QQ email, enable IMAP access in your account settings. This allows third-party clients like Foxmail to connect and manage emails programmatically.

Email Utility Class Implementation

The follwoing Java class demonstrates how to send an email using SMTP with SSL encryption:

import javax.mail.*;
import javax.mail.internet.*;
import java.util.*;

public class EmailSender {
    private static final String SENDER_EMAIL = "your_email@qq.com";
    private static final String SENDER_PASSWORD = "your_app_password";
    private static final String SMTP_HOST = "smtp.qq.com";

    public void sendRecoveryEmail(String recipient, String password) {
        Properties props = new Properties();
        props.put("mail.transport.protocol", "smtp");
        props.put("mail.smtp.host", SMTP_HOST);
        props.put("mail.smtp.auth", "true");
        props.put("mail.smtp.port", "465");
        props.put("mail.smtp.socketFactory.class", "javax.net.ssl.SSLSocketFactory");
        props.put("mail.smtp.socketFactory.fallback", "false");
        props.put("mail.smtp.socketFactory.port", "465");

        Session session = Session.getInstance(props, new Authenticator() {
            @Override
            protected PasswordAuthentication getPasswordAuthentication() {
                return new PasswordAuthentication(SENDER_EMAIL, SENDER_PASSWORD);
            }
        });

        try {
            Message message = createMessage(session, SENDER_EMAIL, recipient, password);
            Transport.send(message);
        } catch (MessagingException e) {
            e.printStackTrace();
        }
    }

    private Message createMessage(Session session, String sender, String recipient, String password) 
            throws MessagingException {
        Message message = new MimeMessage(session);
        message.setFrom(new InternetAddress(sender, "System", "UTF-8"));
        message.setRecipients(Message.RecipientType.TO, InternetAddress.parse(recipient));
        message.setSubject("Password Recovery", "UTF-8");
        message.setText("Your password is: " + password + "\nPlease change it immediately after login.");
        return message;
    }
}

Server-Side Servlet Implementation

The servlet handles requests for password recovery:

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
import java.util.*;
import net.sf.json.JSONObject;

public class PasswordRecoveryServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("application/json;charset=UTF-8");
        request.setCharacterEncoding("UTF-8");

        String username = request.getParameter("username");
        String email = request.getParameter("email");

        int verification = verifyCredentials(username, email);
        Map<String, String> result = new HashMap<>();
        JSONObject jsonResponse = new JSONObject();

        if (verification == -1) {
            result.put("status", "user_not_found");
        } else if (verification == 0) {
            result.put("status", "email_mismatch");
        } else {
            result.put("status", "success");
            // Send email with password
            EmailSender sender = new EmailSender();
            sender.sendRecoveryEmail(email, getUserPassword(username));
        }

        jsonResponse.put("data", result);
        response.getWriter().write(jsonResponse.toString());
    }

    private int verifyCredentials(String username, String email) {
        // Check if user exists and email matches
        return UserDAO.validateUser(username, email) ? 1 : (UserDAO.userExists(username) ? 0 : -1);
    }

    private String getUserPassword(String username) {
        return UserDAO.getUserByUsername(username).getPassword();
    }
}

Android Client Implementation

The Android activity implements the UI and network communication:

import android.os.Bundle;
import android.view.View;
import android.widget.*;
import com.android.volley.*;
import org.json.JSONObject;
import java.util.HashMap;
import java.util.Map;

public class PasswordRecoveryActivity extends AppCompatActivity {
    private EditText usernameField, emailField;
    private RequestQueue requestQueue;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_password_recovery);
        
        requestQueue = Volley.newRequestQueue(this);
        usernameField = findViewById(R.id.username_input);
        emailField = findViewById(R.id.email_input);
        Button submitButton = findViewById(R.id.submit_button);

        submitButton.setOnClickListener(v -> {
            String username = usernameField.getText().toString().trim();
            String email = emailField.getText().toString().trim();
            
            // Encrypt credentials before sending
            String encryptedUsername = encryptWithPublicKey(username);
            String encryptedEmail = encryptWithPublicKey(email);
            
            sendRecoveryRequest(encryptedUsername, encryptedEmail);
        });
    }

    private void sendRecoveryRequest(String username, String email) {
        String url = "http://yourserver.com/recovery";
        StringRequest request = new StringRequest(Request.Method.POST, url,
                response -> {
                    try {
                        JSONObject jsonObject = new JSONObject(response);
                        String status = jsonObject.getJSONObject("data").getString("status");
                        
                        if ("user_not_found".equals(status)) {
                            Toast.makeText(this, "User not found", Toast.LENGTH_SHORT).show();
                        } else if ("email_mismatch".equals(status)) {
                            Toast.makeText(this, "Email does not match", Toast.LENGTH_SHORT).show();
                        } else {
                            Toast.makeText(this, "Password sent to email", Toast.LENGTH_LONG).show();
                        }
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                },
                error -> Toast.makeText(this, "Network error", Toast.LENGTH_SHORT).show()) {
            @Override
            protected Map<String, String> getParams() {
                Map<String, String> params = new HashMap<>();
                params.put("username", username);
                params.put("email", email);
                return params;
            }
        };
        
        requestQueue.add(request);
    }
}

Layout File

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <EditText
        android:id="@+id/username_input"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="Enter username" />

    <EditText
        android:id="@+id/email_input"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="Enter registered email" />

    <Button
        android:id="@+id/submit_button"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Recover Password" />

</LinearLayout>