Android程序未启动即崩溃，请求SQL Server数据库连接调试帮助

Hey there, let's break down why your app is crashing before it even runs and fix those issues step by step:

Key Issues Causing the Crash #

• Early View Initialization: You're calling findViewById directly in the Activity's member variable section, but the layout hasn't been loaded yet (this happens in onCreate). This means edtMed is null right away, triggering a NullPointerException—the main reason your app crashes on launch.
• Stale Input Value: ConsulMed is set once when the Activity is created, not when the user actually enters text. It won't reflect any input the user types later.
• Main Thread Network Operation: You're running SQL Server connection and query code directly on the main thread. Android blocks this to keep the UI responsive, which will throw a NetworkOnMainThreadException even if the initial crash is fixed.
• Missing Connection Details: Your DriverManager.getConnection call doesn't include SQL Server credentials (username/password), and you haven't explicitly loaded the JDBC driver, which is required for most setups.
• Unclosed Resources: You don't close Connection, Statement, or ResultSet after use, leading to resource leaks over time.

Corrected Code Example #

Here's a revised version of your code that fixes all these issues, using AsyncTask to handle the database work off the main thread:

import android.os.AsyncTask;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.Statement;

public class Farmaciapp extends AppCompatActivity {

    // Declare views as member variables, but don't initialize them yet
    private EditText edtMed;
    private TextView mltExplic;

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

        // Initialize views AFTER setting the layout
        edtMed = findViewById(R.id.edtMedicamento);
        mltExplic = findViewById(R.id.mltExplic);
    }

    public void Meds(View v) {
        // Get the input text WHEN the button is clicked, not on Activity creation
        String consulMed = edtMed.getText().toString().trim();

        if (consulMed.isEmpty()) {
            Toast.makeText(this, "Please enter a medicine name", Toast.LENGTH_SHORT).show();
            return;
        }

        // Run database operations in a background thread
        new DatabaseQueryTask().execute(consulMed);
    }

    // AsyncTask to handle background database work
    private class DatabaseQueryTask extends AsyncTask<String, Void, String> {

        @Override
        protected String doInBackground(String... params) {
            String medicamentoDescripcion = null;
            Connection conexion = null;
            Statement state = null;
            ResultSet resultado = null;

            try {
                // Load the JDBC driver explicitly
                Class.forName("net.sourceforge.jtds.jdbc.Driver");

                // Update URL with your SQL Server username and password
                String url = "jdbc:jtds:sqlserver://192.168.1.69;databaseName=Medicamentos;user=YOUR_USERNAME;password=YOUR_PASSWORD;";
                String query = "SELECT Descripcion FROM Medicina WHERE Nombre = '" + params[0] + "'";

                conexion = DriverManager.getConnection(url);
                state = conexion.createStatement();
                resultado = state.executeQuery(query);

                if (resultado.next()) {
                    medicamentoDescripcion = resultado.getString("Descripcion");
                }

            } catch (Exception e) {
                e.printStackTrace();
                return "Error: " + e.getMessage();
            } finally {
                // Close all resources to prevent leaks
                try {
                    if (resultado != null) resultado.close();
                    if (state != null) state.close();
                    if (conexion != null) conexion.close();
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
            return medicamentoDescripcion;
        }

        @Override
        protected void onPostExecute(String result) {
            super.onPostExecute(result);
            if (result != null && !result.startsWith("Error")) {
                mltExplic.setText(result);
            } else {
                Toast.makeText(Farmaciapp.this, result != null ? result : "No medicine found", Toast.LENGTH_SHORT).show();
            }
        }
    }
}

Additional Notes #

• Replace YOUR_USERNAME and YOUR_PASSWORD with your actual SQL Server credentials.
• Make sure you've added the JTDS JDBC driver dependency to your project (check your build.gradle file for implementation 'net.sourceforge.jtds:jtds:1.3.1' or similar).
• Ensure your Android device and SQL Server are on the same local network, and that SQL Server is configured to allow remote connections and uses the correct port (default is 1433).
• For newer Android versions, consider using Coroutines instead of AsyncTask (since AsyncTask is deprecated), but AsyncTask is simpler for beginners to understand.

内容的提问来源于stack exchange，提问作者Daniel Urbina