How do I get asynchronous / event-driven LISTEN/NOTIFY support in Java using a Postgres database?

Use the pgjdbc-ng driver.

http://impossibl.github.io/pgjdbc-ng/

It supports async notifications, without polling. I have used it successfully.

See https://database-patterns.blogspot.com/2014/04/postgresql-nofify-websocket-spring-mvc.html

Oleg has a nice example answer as well

Here's an asynchronous pattern using com.impossibl.postgres.api (pgjdbc-ng-0.6-complete.jar) with JDK 1.8:

import com.impossibl.postgres.api.jdbc.PGConnection;
import com.impossibl.postgres.api.jdbc.PGNotificationListener;
import com.impossibl.postgres.jdbc.PGDataSource;    
import java.sql.Statement;

public static void listenToNotifyMessage(){
    PGDataSource dataSource = new PGDataSource();
    dataSource.setHost("localhost");
    dataSource.setPort(5432);
    dataSource.setDatabase("database_name");
    dataSource.setUser("postgres");
    dataSource.setPassword("password");

    PGNotificationListener listener = (int processId, String channelName, String payload) 
        -> System.out.println("notification = " + payload);

    try (PGConnection connection = (PGConnection) dataSource.getConnection()){
        Statement statement = connection.createStatement();
        statement.execute("LISTEN test");
        statement.close();
        connection.addNotificationListener(listener);
        while (true){ }
    } catch (Exception e) {
        System.err.println(e);
    }
}

Create a trigger function for your database:

CREATE OR REPLACE FUNCTION notify_change() RETURNS TRIGGER AS $$
    BEGIN
        SELECT pg_notify('test', TG_TABLE_NAME);
        RETURN NEW;
    END;
$$ LANGUAGE plpgsql;

Assign a trigger for every table you want to track:

CREATE TRIGGER table_change 
    AFTER INSERT OR UPDATE OR DELETE ON table_name
    FOR EACH ROW EXECUTE PROCEDURE notify_change();