使用jdbc驱动程序将android与postgresql链接的问题

Question

我所做的是尝试两种不同的方式。第一个是在主活动类中编写所有代码。但是我在某个地方读到你需要把它放在一个不同的线程或其他东西中它建议使用asynctask，这是我尝试的第二种方式。

这是我的代码：

import android.app.FragmentManager;
import android.content.Intent;
import android.os.AsyncTask;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.util.Log;
import android.view.View;
import android.support.design.widget.NavigationView;
import android.support.v4.view.GravityCompat;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarDrawerToggle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.Toast;

import java.sql.*;

public class MainActivity extends AppCompatActivity
    implements NavigationView.OnNavigationItemSelectedListener {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    /*String sql;
    sql = "SELECT * FROM users";
    new DatabaseConnectivity().execute(sql);*/

    try {
        Class.forName("org.postgresql.Driver");
    }
    catch (ClassNotFoundException e) {
        e.printStackTrace();
    }

    try{
        String url;
        url = "jdbc:postgresql://localhost:5432/nikhildb";
        Connection conn = DriverManager.getConnection(url,"postgres","5438");
        String sql;
        sql = "SELECT * FROM users";
        Statement st = conn.createStatement();
        ResultSet rs = st.executeQuery(sql);

        while (rs.next()) {
            String relname = rs.getString("name");
            Toast.makeText(MainActivity.this,relname,Toast.LENGTH_LONG).show();
            //Log.e("Worked", relname);
        }

        //rs.close();
        //st.close();
        conn.close();
    }
    catch(SQLException e){
        //Log.e("Error","Error");
        Toast.makeText(this,"Error",Toast.LENGTH_LONG).show();
        e.printStackTrace();
    }

    FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
    fab.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                    .setAction("Action", null).show();
        }
    });

    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
            this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
    drawer.setDrawerListener(toggle);
    toggle.syncState();

    NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
    navigationView.setNavigationItemSelectedListener(this);
}

@Override
public void onBackPressed() {
    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    if (drawer.isDrawerOpen(GravityCompat.START)) {
        drawer.closeDrawer(GravityCompat.START);
    } else {
        super.onBackPressed();
    }
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}

@SuppressWarnings("StatementWithEmptyBody")
@Override
public boolean onNavigationItemSelected(MenuItem item) {
    // Handle navigation view item clicks here.
    int id = item.getItemId();

    Intent intent;
    FragmentManager fragmentManager = getFragmentManager();

    if (id == R.id.nav_piano) {
        intent = new Intent(this,PianoActivity.class);
        startActivity(intent);
    } else if (id == R.id.nav_guitar) {
        intent = new Intent(this,GuitarActivity.class);
        startActivity(intent);
    } else if (id == R.id.nav_synth) {
        FragmentSynth fs = new FragmentSynth();
        fragmentManager.beginTransaction().replace(R.id.relView, fs).commit();
    } else if (id == R.id.nav_cart) {
        FragmentCart fc = new FragmentCart();
        fragmentManager.beginTransaction().replace(android.R.id.content, fc).commit();
    } else if (id == R.id.nav_orders) {
        FragmentOrders fo = new FragmentOrders();
        fragmentManager.beginTransaction().replace(android.R.id.content, fo).commit();
    } else if (id == R.id.nav_home) {
        FragmentHome fh = new FragmentHome();
        fragmentManager.beginTransaction().replace(android.R.id.content, fh).commit();
    }

    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    drawer.closeDrawer(GravityCompat.START);
    return true;
  }
}

/*class DatabaseConnectivity extends AsyncTask<String,Void,ResultSet> {
public void onPreExecute(){
    try {
        Class.forName("org.postgresql.Driver");
    }
    catch (ClassNotFoundException e) {
        e.printStackTrace();
    }
}

public ResultSet doInBackground(String... sql){
    ResultSet rs=null;
    try{
        String url;
        url = "jdbc:postgresql://localhost:5432/nikhildb";
        Connection conn = DriverManager.getConnection(url,"postgres","5438");

        Statement st = conn.createStatement();
        rs = st.executeQuery(sql[0]);

        //rs.close();
        //st.close();
        conn.close();
    }
    catch(SQLException e){
        Log.e("Error","Error");
        //Toast.makeText(this,"Error",Toast.LENGTH_LONG).show();
        e.printStackTrace();
    }
    return rs;
}

public void onPostExecute(ResultSet rs){
    try {
        while (rs.next()) {
            String relname = rs.getString("name");
            //Toast.makeText(MainActivity.this,relname,Toast.LENGTH_LONG).show();
            Log.e("Worked",relname);
        }
    }catch (SQLException e){
        Log.e("Error","Showed error here");
        e.printStackTrace();
    }
  }
}

这是错误：

beginning of crash
07-05 23:00:25.159 3493-3493/com.example.nikhil.soundofmusic
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.nikhil.soundofmusic, PID: 3493
java.lang.NullPointerException: Attempt to invoke interface method 'boolean
java.sql.ResultSet.next()' on a null object reference
at com.example.nikhil.soundofmusic.DatabaseConnectivity.onPostExecute(MainActivity.java:157)
at com.example.nikhil.soundofmusic.DatabaseConnectivity.onPostExecute(MainActivity.java:123)
at android.os.AsyncTask.finish(AsyncTask.java:636)
at android.os.AsyncTask.access$500(AsyncTask.java:177)
at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:653)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5254)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
07-05 23:00:31.890 3493-3503/com.example.nikhil.soundofmusic E/System: Uncaught exception thrown by finalizer
07-05 23:00:31.890 3493-3503/com.example.nikhil.soundofmusic E/System: java.lang.NullPointerException: Attempt to invoke interface method 'void org.postgresql.core.ProtocolConnection.close()' on a null object reference
at org.postgresql.jdbc.PgConnection.close(PgConnection.java:690)
at org.postgresql.jdbc.PgConnection.finalize(PgConnection.java:924)
at java.lang.Daemons$FinalizerDaemon.doFinalize(Daemons.java:191)
at java.lang.Daemons$FinalizerDaemon.run(Daemons.java:174)
at java.lang.Thread.run(Thread.java:818)
07-05 23:05:35.610 8049-8049/com.example.nikhil.soundofmusic W/art: Failed to find OatDexFile for DexFile /data/data/com.example.nikhil.soundofmusic/files/instant-run/dex/slice-slice_1-classes.dex ( canonical path /data/data/com.example.nikhil.soundofmusic/files/instant-run/dex/slice-slice_1-classes.dex) with checksum 0x373843a4 in OatFile /data/data/com.example.nikhil.soundofmusic/cache/slice-slice_1-classes.dex
07-05 23:05:36.150 8049-8062/com.example.nikhil.soundofmusic I/art: Background sticky concurrent mark sweep GC freed 1899(738KB) AllocSpace objects, 0(0B) LOS objects, 43% free, 967KB/1698KB, paused 8.436ms total 25.104ms
07-05 23:05:36.416 8049-8049/com.example.nikhil.soundofmusic W/art: Before Android 4.1, method android.graphics.PorterDuffColorFilter android.support.graphics.drawable.VectorDrawableCompat.updateTintFilter(android.graphics.PorterDuffColorFilter, android.content.res.ColorStateList, android.graphics.PorterDuff$Mode) would have incorrectly overridden the package-private method in android.graphics.drawable.Drawable
07-05 23:05:36.677 8049-8059/com.example.nikhil.soundofmusic E/System: Uncaught exception thrown by finalizer
07-05 23:05:36.677 8049-8059/com.example.nikhil.soundofmusic E/System: java.lang.NullPointerException: Attempt to invoke interface method 'void org.postgresql.core.ProtocolConnection.close()' on a null object reference
at org.postgresql.jdbc.PgConnection.close(PgConnection.java:690)
at org.postgresql.jdbc.PgConnection.finalize(PgConnection.java:924)
at java.lang.Daemons$FinalizerDaemon.doFinalize(Daemons.java:191)
at java.lang.Daemons$FinalizerDaemon.run(Daemons.java:174)
at java.lang.Thread.run(Thread.java:818)

提前感谢任何帮助