How do I get timezone ids by their milliseconds offset?

By Wayan in Core API, Util Package 0 Comment 
package org.kodejava.util;

import java.util.TimeZone;

public class TimeZoneByOffset {
    public static void main(String[] args) {
        // Gets the available IDs according to the given time zone
        // offset in milliseconds.
        int offset = 8 * 60 * 60 * 1000;
        String[] timezones = TimeZone.getAvailableIDs(offset);

        System.out.println("List of available IDs for GMT:+08:00");
        System.out.println("====================================");
        for (String id : timezones) {
            System.out.println(id);
        }
    }
}

Here are the timezone ids in the GMT+8 offset:

List of available IDs for GMT:+08:00
====================================
Asia/Brunei
Asia/Choibalsan
Asia/Chongqing
Asia/Chungking
Asia/Harbin
Asia/Hong_Kong
Asia/Irkutsk
Asia/Kuala_Lumpur
Asia/Kuching
Asia/Macao
Asia/Macau
Asia/Makassar
Asia/Manila
Asia/Shanghai
Asia/Singapore
Asia/Taipei
Asia/Ujung_Pandang
Asia/Ulaanbaatar
Asia/Ulan_Bator
Australia/Perth
Australia/West
CTT
Etc/GMT-8
Hongkong
PRC
Singapore

How do I retrieve available catalogs in database?

By Wayan in JDBC 0 Comment

The examples below show you how to get the available catalog names in MySQL database. The available catalog names can be obtained from the DatabaseMetaData object by calling the getCatalogs() method. This method returns a ResultSet object. We iterate it and get the schema name by calling the getString() method and pass TABLE_CAT as the column name.

package org.kodejava.jdbc;

import java.sql.Connection;
import java.sql.DatabaseMetaData;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;

public class MetadataGetCatalog {
    private static final String URL = "jdbc:mysql://localhost";
    private static final String USERNAME = "kodejava";
    private static final String PASSWORD = "s3cr*t";

    public static void main(String[] args) {
        try (Connection connection =
                     DriverManager.getConnection(URL, USERNAME, PASSWORD)) {

            // Gets DatabaseMetaData
            DatabaseMetaData metadata = connection.getMetaData();

            // Retrieves the schema names available in this database
            ResultSet rs = metadata.getCatalogs();

            while (rs.next()) {
                String catalog = rs.getString("TABLE_CAT");
                System.out.println("Catalog: " + catalog);
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

Here is the list of catalog names available in the MySQL database:

Catalog: kodejava
Catalog: musicdb
Catalog: mysql
Catalog: performance_schema
Catalog: sys

Maven Dependencies

<dependency>
    <groupId>com.mysql</groupId>
    <artifactId>mysql-connector-j</artifactId>
    <version>8.1.0</version>
</dependency>

Maven Central

How do I create port scanner program?

By Wayan in Core API, Networking 0 Comment

In this example you’ll see how to create a simple port scanner program to check the open ports for the specified host name.

package org.kodejava.net;

import java.io.IOException;
import java.net.InetAddress;
import java.net.Socket;

public class PortScanner {
    public static void main(String[] args) throws Exception {
        String host = "localhost";
        InetAddress inetAddress = InetAddress.getByName(host);

        String hostName = inetAddress.getHostName();
        for (int port = 0; port <= 65535; port++) {
            try {
                Socket socket = new Socket(hostName, port);
                String text = hostName + " is listening on port " + port;
                System.out.println(text);
                socket.close();
            } catch (IOException e) {
                // empty catch block
            }
        }
    }
}

The result of the code snippet above are:

localhost is listening on port 21
localhost is listening on port 80
localhost is listening on port 135
localhost is listening on port 445
localhost is listening on port 3000
...
...
localhost is listening on port 63342
localhost is listening on port 63467
localhost is listening on port 64891
localhost is listening on port 64921
localhost is listening on port 65001

How do I get the maximum number of concurrent connections?

By Wayan in JDBC 0 Comment

This example shows you how to get the maximum number of concurrent connections to a database that are possible.
To get this information, we use the DatabaseMetaData.getMaxConnections() method call. If the return value is zero, it means that there is no limit or the limit is unknown.

package org.kodejava.jdbc;

import java.sql.Connection;
import java.sql.DatabaseMetaData;
import java.sql.SQLException;
import java.sql.DriverManager;

public class MaxConnections {
    private static final String URL = "jdbc:mysql://localhost/kodejava";
    private static final String USERNAME = "kodejava";
    private static final String PASSWORD = "s3cr*t";

    public static void main(String[] args) {
        try (Connection connection =
                     DriverManager.getConnection(URL, USERNAME, PASSWORD)) {

            // Get database meta data.
            DatabaseMetaData metaData = connection.getMetaData();

            // Retrieves the maximum number of concurrent
            // connections to this database that are possible.
            // A result of zero means that there is no limit or
            // the limit is not known.
            int max = metaData.getMaxConnections();
            System.out.println("Max concurrent connections: " + max);
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

Maven Dependencies

<dependency>
    <groupId>com.mysql</groupId>
    <artifactId>mysql-connector-j</artifactId>
    <version>8.4.0</version>
</dependency>

Maven Central