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PostgreSQL Java

The majority of information to write this post comes form knowing how Java works and where to find the PostgreSQL JDBC Java archive (.jar) file and the standard documentation. Here are the URLs:

The rest of the example is simply demonstrating how to create a fully working program to return one or more rows from a static query. After you download the latest PostgreSQL JDBC archive, with a command like:

wget https://jdbc.postgresql.org/download/postgresql-42.3.7.jar

Assuming you put it in test directory, like /home/student/java, you would add it to your Java $CLASSPATH environment variable, like this:

export set CLASSPATH="/home/student/Code/java/postgresql-42.3.7.jar:."

If you’re new to Java and Linux, the . (dot) represents the present working directory and is required in the Java $CLASSPATH to avoid raising a java.lang.ClassNotFoundException when you test your code. For example, the sample program name is PostgreSQLDriver.java and if you failed to include the present working directory in the $CLASSPATH it would raise the following error message when you try to run the compiled class file:

Error: Could not find or load main class PSQL
Caused by: java.lang.ClassNotFoundException: PSQL

Now that you’ve set your Java $CLASSPATH correctly, you can copy or type this PostgreSQLDriver.java Java program into a file.

// 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.Properties;
 
/* You can't include the following on Linux without raising an exception. */
// import com.mysql.jdbc.Driver;
 
public class PostgreSQLDriver {
  public PostgreSQLDriver() {
    /* Declare variables that require explicit assignments because
       they're addressed in the finally block. */
    Connection conn = null;
    Statement stmt = null;
    ResultSet rset = null;
 
    /* Declare other variables. */
    String url;
    String username = "student";
    String password = "student";
    String database = "videodb";
    String hostname = "[::1]";
    String port = "5432";
    String sql;
 
    /* Attempt a connection. */
    try {
      // Set URL.
      url = "jdbc:postgresql://" + hostname + ":" + port + "/" + database;
 
      // Create instance of MySQLDriver.
      conn = DriverManager.getConnection (url, username, password);
 
      // Query the version of the database.
      sql = "SELECT version()";
      stmt = conn.createStatement();
      rset = stmt.executeQuery(sql);
 
      System.out.println ("Database connection established");
 
      // Read row returns for one column.
      while (rset.next()) {
        System.out.println("PostgreSQL Connected to the [" + rset.getString(1) + "] database."); }
 
    }
    catch (SQLException e) {
      System.err.println ("Cannot connect to database server:");
      System.out.println(e.getMessage());
    }
    finally {
      if (conn != null) {
        try {
          rset.close();
          stmt.close();
          conn.close();
          System.out.println ("Database connection terminated");
        }
        catch (Exception e) { /* ignore close errors */ }
      }
    }
  }
  /* Unit test. */
  public static void main(String args[]) {
    new PostgreSQLDriver();
  }
}

Now, you compile the program from the present working directory with this syntax:

javac PostgreSQLDriver.java

It creates a PostgreSQLDriver.class file, which you can run with this syntax:

java PostgreSQLDriver

It will return the following, which verifies you’ve connected to a valid database in the PostgreSQL instance. You should note that the IPV6 syntax is used in the example on line #25 but you could substitute localhost, an assigned host name, or an IP address.

Written by maclochlainn

August 8th, 2023 at 8:12 pm

Posted in Java,Java 17,Linux,Postgres,PostgreSQL,PostgreSQL 15

Tagged with Java Development, PostgreSQL Developer

AlmaLinxu + javac

without comments

I neglected to show how to install and configure the Java SDK on my earlier post without relying on including the $JAVA_HOME in the $PATH. This post shows how to install the Java and Java SDK on Alma Linux without layering it with the $JAVA_HOME environment variable.

You install the Java runtime and development kit with the following command:

sudo dnf install -y java-17-openjdk java-17-openjdk-devel

Then, you need to add it to the $PATH environment variable with the following syntax in your .bashrc file or environment file of your choice.

export set PATH=$PATH:/usr/lib/jvm/java-17-openjdk-17.0.8.0.7-2.el9.x86_64/bin/

After you source the new $PATH environment value, you can then write a file like this Hello.java file to test it.

// Define the Java program.
public class HelloWorld {
  public static void main(String[] args) {
    System.out.println("Hello World!");
  }
}

You compile the Hello.java file with this syntax:

javac Hello.java

Then, you run the compiled program with the following syntax:

java Hello

It’ll print:

Hello World!

Written by maclochlainn

August 8th, 2023 at 2:59 pm

Posted in AlmaLinux,Java,Java 17,Linux

Tagged with Java Development, Linux Administration

MySQL on Ubuntu

without comments

Working with my students to create an Ubuntu virtual environment for Python development with the MySQL database. After completing the general provisioning covered in this older post, I’d recommend you create a python symbolic link before installing the MySQL-Python driver.

sudo ln -s /usr/bin/python3 /usr/bin/python

You install the Python development driver with the following:

sudo apt-get -y install python3-mysql.connector

Create a python_connect.py file to test your Python deployment’s ability to connect to the MySQL database:

#!/usr/bin/python
 
# Import the library.
import mysql.connector
from mysql.connector import errorcode
 
try:
  # Open connection.
  cnx = mysql.connector.connect(user='student', password='student',
                                host='localhost',
                                database='sakila')
 
  # Print the value.
  print("Database connection resolved.")
 
# Handle exception and close connection.
except mysql.connector.Error as e:
  if e.errno == errorcode.ER_ACCESS_DENIED_ERROR:
    print("Something is wrong with your user name or password")
  elif e.errno == errorcode.ER_BAD_DB_ERROR:
    print("Database does not exist")
  else:
    print(e)
 
# Close the connection when the try block completes.
else:
  cnx.close()

You should change the file permissions of the python_connect.py to read-write-execute as owner, and read-execute as group and other with the following command:

chmod 755 python_connect.py

Then, you can test your python_connect.py program from the local directory with the following command:

./python_connect.py

You see the following output if it works:

Database connection resolved.

As alway, I hope those looking for a code complete solutionk

Written by maclochlainn

July 14th, 2023 at 2:29 pm

Posted in Linux,MySQL,MySQL 8,MySQL Connect,Python 3.x,Ubuntu

Tagged with MySQL DBA Techniques, MySQL Developer

User/Group Setups

without comments

The following are samples of creating, changing, and removing users and groups in Linux. These are the command-line options in the event you don’t have access to the GUI tools.

Managing Users:

Adding a user:

The prototype is:

# useradd [-u uid] [-g initial_group] [-G group[,...]] \
 > [-d home_directory] [-s shell] [-c comment] \
 > [-m [-k skeleton_directory]] [-f inactive_time] \
 > [-e expire_date] -n username