Je sais que ce sujet à été traiter mille fois. Mais je n'arrive vraiment pas a faire une connection avec la db .

Je me suis fais un boutton qui permet de tester et ça marche jamais.

Voici mon code

final Button button = findViewById(R.id.button);
        button.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                try {
                    Class.forName("org.postgresql.Driver");
                    System.out.println("Driver O.K.");
                    String url = "jdbc:postgresql://192.168.0.21:5432/test"; // IP d'exemple
                    String user = "postgres";
                    String passwd = "mdp";
                    Connection conn = DriverManager.getConnection(url, user, passwd);
                    System.out.println("Connexion effective !");
                } catch (Exception e) {
                    e.printStackTrace();
                }

            }

Comme erreur j'ai plusieurs fois 

org.postgresql.util.PSQLException: Connection to localhost:5432 refused. Check that the hostname and port are correct and that the postmaster is accepting TCP/IP connections.

pourtant dans le manifest j'ai bien mis

    <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" />

j'ai fais une autre classe

import java.sql.Connection;
import java.sql.DriverManager;

public class Database {

    private Connection connection;

    // private final String host = "ssprojectinstance.csv2nbvvgbcb.us-east-2.rds.amazonaws.com"  // For Amazon Postgresql
    private final String host = "192.168.0.21";  
    private final String database = "test";
    private final int port = 5432;
    private final String user = "postgres";
    private final String pass = "pass";
    private String url = "jdbc:postgresql://%s:%d/%s";
    private boolean status;

    public Database() {
        this.url = String.format(this.url, this.host, this.port, this.database);
        connect();
        //this.disconnect();
        System.out.println("connection status:" + status);
    }

    private void connect() {
        Thread thread = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    Class.forName("org.postgresql.Driver");
                    connection = DriverManager.getConnection(url, user, pass);
                    status = true;
                    System.out.println("connected:" + status);
                } catch (Exception e) {
                    status = false;
                    System.out.print(e.getMessage());
                    e.printStackTrace();
                }
            }
        });
        thread.start();
        try {
            thread.join();
        } catch (Exception e) {
            e.printStackTrace();
            this.status = false;
        }
    }

    public Connection getExtraConnection(){
        Connection c = null;
        try {
            Class.forName("org.postgresql.Driver");
            c = DriverManager.getConnection(url, user, pass);
        } catch (Exception e) {
            e.printStackTrace();
        }

        return c;
    }
}

qui ne fonctionne pas non plus.

Caused by: java.lang.ClassNotFoundException: Didn't find class "java.lang.management.ManagementFactory" on path: DexPathList[[zip file "/data/app/~~jjxh5PFUiZMDogrDDFgwqw==/com.example.toolpss-RvMuPOZeIwf8zttg7YuZOg==/base.apk"],nativeLibraryDirectories=[/data/app/~~jjxh5PFUiZMDogrDDFgwqw==/com.example.toolpss-RvMuPOZeIwf8zttg7YuZOg==/lib/x86, /system/lib, /system_ext/lib]]

Je me dis que je me trompe quelque part mais je trouve pas

Si vous avez des idée