Android登录验证教程：正确获取用户输入与安全实践

<EditText
    android:id="@+id/etUsername"
    android:layout_width="0dp"
    android:layout_height="wrap_content"
    android:layout_marginTop="48dp"
    android:ems="10"
    android:hint="@string/username"
    android:inputType="textPersonName"
    app:layout_constraintEnd_toStartOf="@+id/guideline2"
    app:layout_constraintStart_toStartOf="@+id/guideline"
    app:layout_constraintTop_toBottomOf="@+id/imageView" />

<EditText
    android:id="@+id/etPassword"
    android:layout_width="0dp"
    android:layout_height="wrap_content"
    android:layout_marginTop="15dp"
    android:ems="10"
    android:hint="@string/password"
    android:inputType="textPassword"
    app:layout_constraintEnd_toStartOf="@+id/guideline2"
    app:layout_constraintStart_toStartOf="@+id/guideline"
    app:layout_constraintTop_toBottomOf="@+id/etUsername" />

<Button
    android:id="@+id/btnLogin"
    android:layout_width="0dp"
    android:layout_height="wrap_content"
    android:layout_marginTop="36dp"
    android:background="@color/yellow_700"
    android:text="@string/enterApp"
    app:layout_constraintEnd_toStartOf="@+id/guideline2"
    app:layout_constraintHorizontal_bias="0.495"
    app:layout_constraintStart_toStartOf="@+id/guideline"
    app:layout_constraintTop_toBottomOf="@+id/etPassword" />

public class MainActivity extends AppCompatActivity {

    private Button btnLogin;
    private EditText etUsername;
    private EditText etPassword;
    String username = "bartul"; // 预设用户名
    String password = "kalinic"; // 预设密码

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

        btnLogin = findViewById(R.id.btnLogin);
        etUsername = findViewById(R.id.etUsername);
        etPassword = findViewById(R.id.etPassword);

        // 错误：在onCreate中过早地获取输入
        // 此时EditText通常是空的，因此user和pass变量会是空字符串
        String user = etUsername.getText().toString();
        String pass = etPassword.getText().toString();

        btnLogin.setOnClickListener(new View.OnClickListener(){
            @Override
            public void onClick(View view) {
                // 这里的验证逻辑将使用onCreate中获取到的空字符串user和pass
                // 导致即使输入正确，也会被判断为错误
                if(!user.equals(username) || !pass.equals(password)) {
                    Toast.makeText(getApplicationContext(), "Wrong username or password entered!", Toast.LENGTH_SHORT).show();
                    etUsername.setText("");
                    etPassword.setText("");
                }
                else {
                    Intent intent = new Intent(MainActivity.this, MiNoteMenuActivity.class);
                    intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK | Intent.FLAG_ACTIVITY_NEW_TASK);
                    startActivity(intent);
                }
            }
        });
    }
}

import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

import androidx.appcompat.app.AppCompatActivity;

public class MainActivity extends AppCompatActivity {

    private Button btnLogin;
    private EditText etUsername;
    private EditText etPassword;
    // 预设的用户名和密码，仅用于演示。
    // 在实际生产环境中，请勿将敏感信息硬编码。
    String predefinedUsername = "bartul";
    String predefinedPassword = "kalinic";

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

        // 初始化UI组件
        btnLogin = findViewById(R.id.btnLogin);
        etUsername = findViewById(R.id.etUsername);
        etPassword = findViewById(R.id.etPassword);

        // 设置登录按钮的点击监听器
        btnLogin.setOnClickListener(new View.OnClickListener(){
            @Override
            public void onClick(View view) {
                // 正确：在点击按钮时获取用户输入，确保获取的是最新内容
                String enteredUsername = etUsername.getText().toString();
                String enteredPassword = etPassword.getText().toString();

                // 执行验证逻辑
                if(!enteredUsername.equals(predefinedUsername) || !enteredPassword.equals(predefinedPassword)) {
                    // 验证失败：显示Toast提示，并清空输入框
                    Toast.makeText(getApplicationContext(), "用户名或密码错误！", Toast.LENGTH_SHORT).show();
                    etUsername.setText("");
                    etPassword.setText("");
                }
                else {
                    // 验证成功：显示Toast提示，并跳转到下一个Activity
                    Toast.makeText(getApplicationContext(), "登录成功！", Toast.LENGTH_SHORT).show();
                    Intent intent = new Intent(MainActivity.this, MiNoteMenuActivity.class);
                    // 设置Intent标志，清除任务栈中所有Activity并启动新Activity
                    // 这样用户在下一个Activity按返回键时不会回到登录界面
                    intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK | Intent.FLAG_ACTIVITY_NEW_TASK);
                    startActivity(intent);
                    // 如果希望彻底销毁当前Activity，可以调用finish()
                    // finish();
                }
            }
        });
    }
}