Java and Postgres

I wanted to get Java working with PostgreSQL to test some GUI interfaces on Linux. Figuring out the necessary JAR file for the JDBC was my first hurdle. I found it was postgreSQL-42-2.5.jar file.

You can download it with the following command line:

wget https://jdbc.postgresql.org/download/postgresql-42.2.5.jar

I downloaded it to a Java directory off the home/student directory. Then, I added the following CLASSPATH to local java.env environment file.

#!/usr/bin/bash

# Set the Java CLASSPATH environment variable to include the JDBC jar file.
export set CLASSPATH=/home/student/Java/postgresql-42.2.5.jar:.

I sourced the postgresql-42.2.5.jar file and I wrote the following JavaTest.java program:

/* Import classes. */
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.logging.Level;
import java.util.logging.Logger;

public class JavaTest { 
  public static void main(String[] args) {
    /* Set default strings for the connection. */
    String url = "jdbc:postgresql://localhost:5432/videodb";
    String user = "student";
    String password = "student";
    
    /* Try the connection and statement. */
    try {
      Connection conn = DriverManager.getConnection(url, user, password);
      Statement st = conn.createStatement();
      ResultSet rs = st.executeQuery("SELECT VERSION()");

      if (rs.next()) {
        System.out.println(rs.getString(1));
      }

      /* Close the result set and statement. */
      rs.close();
      st.close();
    }
    catch (SQLException e) {
      Logger logger = Logger.getLogger(JavaTest.class.getName());
      logger.log(Level.SEVERE, e.getMessage(), e);
    }
  }
}

I compiled the JavaTest.java program and tested it. It failed with the following error:

FATAL: Ident authentication failed for user - Unable to connect to PostgreSQL

The failure occurred because I hadn’t allowed the connection in PostgreSQL’s pg_hba.conf file. I changed the following line in my pg_hba.conf file:

host    all             all             127.0.0.1/32            ident

to

host    all             all             127.0.0.1/32            trust

Then, I restarted the postgresql-11 service, like this:

systemctl restart postgresql-11.service

The JavaTest program ran successfully and returned:

PostgreSQL 11.5 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 9.1.1 20190503 (Red Hat 9.1.1-1), 64-bit

You can extend the logic to output a comma-separated value file by leveraging the JDBC help page, like this:

/* Import classes. */
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.ResultSetMetaData;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.logging.Level;
import java.util.logging.Logger;

/* Create class withonly a static method for runtime testing. */
public class JavaReturnSet { 
  public static void main(String[] args) {
    /* Set default strings for the connection. */
    String url = "jdbc:postgresql://localhost:5432/videodb";
    String user = "student";
    String password = "student";
    
    /* Try the connection and statement. */
    try {
      /* Set connection, statement, and result set. */
      Connection conn = DriverManager.getConnection(url, user, password);
      Statement st = conn.createStatement();

      /* Use the + to concatenate lines for query clarity. */
      ResultSet rs = st.executeQuery("SELECT m.account_number\n" +
                                     ",      CONCAT(c.last_name,', ',c.first_name)\n" +
                                     "FROM   member m JOIN contact c\n" +
                                     "ON     m.member_id = c.member_id");

      /* Get query metadata for subsequent management of results. */
      ResultSetMetaData rsmd = rs.getMetaData();
      String line;

      while (rs.next()) {
        /* Initialize the line output for each row. */
        line = "";

        /* Process the columns. */
        for (int i = 0; i < rsmd.getColumnCount(); i++) {
          if (rsmd.getColumnType(i + 1) == 12) {
            line = line + "\"" + rs.getString(i + 1) + "\"";
          }
          else {
            line = line + rs.getInt(i + 1);
          }

          /* Put a comma between output columns. */
          if (i < rsmd.getColumnCount() - 1) {
            line = line + ",";
          }
        }
        System.out.println(line);
      }

      /* Close the result set and statement. */
      rs.close();
      st.close();
    }
    catch (SQLException e) {
      Logger logger = Logger.getLogger(JavaReturnSet.class.getName());
      logger.log(Level.SEVERE, e.getMessage(), e);
    }
  }
}

As always, I hope this helps those looking for a clean solution.