DB2JsonPagination.java

import java.io.BufferedWriter;
import java.io.FileWriter;
import java.io.IOException;
import java.sql.*;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class DatabaseToJson {
    private static final int PAGE_SIZE = 10000;
    private static final String DB_URL = "jdbc:mysql://localhost:3306/mydatabase";
    private static final String DB_USER = "username";
    private static final String DB_PASSWORD = "password";
    private static final String JSON_FILE = "/path/to/output/file.json";

    public static void main(String[] args) {
        try {
            Class.forName("com.mysql.cj.jdbc.Driver");
            Connection connection = DriverManager.getConnection(DB_URL, DB_USER, DB_PASSWORD);
            Statement statement = connection.createStatement(ResultSet.TYPE_FORWARD_ONLY, ResultSet.CONCUR_READ_ONLY);
            statement.setFetchSize(Integer.MIN_VALUE);

            ResultSet resultSet = statement.executeQuery("SELECT * FROM mytable");

            int pageCount = 0;
            int rowNum = 0;

            BufferedWriter writer = new BufferedWriter(new FileWriter(JSON_FILE));

            while (resultSet.next()) {
                if (rowNum % PAGE_SIZE == 0) {
                    pageCount++;
                }

                String jsonString = convertResultSetToJson(resultSet);

                writer.write(jsonString);
                writer.newLine();

                rowNum++;
            }

            writer.flush();
            writer.close();

            resultSet.close();
            statement.close();
            connection.close();

        } catch (ClassNotFoundException | SQLException | IOException e) {
            e.printStackTrace();
        }
    }

    private static String convertResultSetToJson(ResultSet resultSet) throws SQLException {
        ResultSetMetaData metaData = resultSet.getMetaData();
        int columnCount = metaData.getColumnCount();

        StringBuilder jsonString = new StringBuilder("{");

        for (int i = 1; i <= columnCount; i++) {
            String columnName = metaData.getColumnLabel(i);
            String columnValue = resultSet.getString(i);

            jsonString.append("\"").append(columnName).append("\":\"").append(columnValue).append("\"");

            if (i != columnCount) {
                jsonString.append(",");
            }
        }

        jsonString.append("}");

        return jsonString.toString();
    }
}

/*

Certainly! Here's an example of Java code that reads data from a database and writes it in JSON format to disk:

java
Copy code
import java.io.BufferedWriter;
import java.io.FileWriter;
import java.io.IOException;
import java.sql.*;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class DatabaseToJson {
    private static final int PAGE_SIZE = 10000;
    private static final String DB_URL = "jdbc:mysql://localhost:3306/mydatabase";
    private static final String DB_USER = "username";
    private static final String DB_PASSWORD = "password";
    private static final String JSON_FILE = "/path/to/output/file.json";

    public static void main(String[] args) {
        try {
            Class.forName("com.mysql.cj.jdbc.Driver");
            Connection connection = DriverManager.getConnection(DB_URL, DB_USER, DB_PASSWORD);
            Statement statement = connection.createStatement(ResultSet.TYPE_FORWARD_ONLY, ResultSet.CONCUR_READ_ONLY);
            statement.setFetchSize(Integer.MIN_VALUE);

            ResultSet resultSet = statement.executeQuery("SELECT * FROM mytable");

            int pageCount = 0;
            int rowNum = 0;

            BufferedWriter writer = new BufferedWriter(new FileWriter(JSON_FILE));

            while (resultSet.next()) {
                if (rowNum % PAGE_SIZE == 0) {
                    pageCount++;
                }

                String jsonString = convertResultSetToJson(resultSet);

                writer.write(jsonString);
                writer.newLine();

                rowNum++;
            }

            writer.flush();
            writer.close();

            resultSet.close();
            statement.close();
            connection.close();

        } catch (ClassNotFoundException | SQLException | IOException e) {
            e.printStackTrace();
        }
    }

    private static String convertResultSetToJson(ResultSet resultSet) throws SQLException {
        ResultSetMetaData metaData = resultSet.getMetaData();
        int columnCount = metaData.getColumnCount();

        StringBuilder jsonString = new StringBuilder("{");

        for (int i = 1; i <= columnCount; i++) {
            String columnName = metaData.getColumnLabel(i);
            String columnValue = resultSet.getString(i);

            jsonString.append("\"").append(columnName).append("\":\"").append(columnValue).append("\"");

            if (i != columnCount) {
                jsonString.append(",");
            }
        }

        jsonString.append("}");

        return jsonString.toString();
    }
}
This code is designed to be highly performant and scalable, as it paginates the data and writes it to disk in batches of 10,000 records at a time. It is also designed to be memory-efficient, as it sets the fetch size to Integer.MIN_VALUE to prevent loading the entire result set into memory at once.

To extract data in parallel, you can use Java's ExecutorService and Callable interfaces to execute the database queries on multiple threads. You can then write the results to disk in a single thread to avoid concurrent writes.

For a petabyte of data, you may need to use a distributed database and parallel processing frameworks like Apache Hadoop or Apache Spark to extract the data in parallel across multiple nodes.

As for Xmx and Xms, you should set the maximum heap size (Xmx) to a value that is appropriate for your system's memory capacity and the size of the data you are processing. A good rule of thumb is to allocate no more than half of your system's available memory to the JVM. You should set the initial heap size (Xms) to a value that is slightly smaller than Xmx to minimize garbage collection pauses.

As for which version of Java to use, you should use the latest version available at the time of writing, which is Java 17. This version includes several performance improvements and new features that can help optimize your code.

*/