DB2Json.java

import com.fasterxml.jackson.databind.ObjectMapper;
import java.io.File;
import java.sql.*;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class DatabaseToJson {
    private static final String DB_URL = "jdbc:mysql://localhost:3306/mydatabase";
    private static final String DB_USERNAME = "root";
    private static final String DB_PASSWORD = "password";

    private static final String SQL_QUERY = "SELECT * FROM mytable";

    private static final String JSON_FILE_PATH = "/tmp/data.json";

    private static final int CHUNK_SIZE = 1000;

    public static void main(String[] args) throws Exception {
        Class.forName("com.mysql.jdbc.Driver");
        Connection connection = DriverManager.getConnection(DB_URL, DB_USERNAME, DB_PASSWORD);

        PreparedStatement statement = connection.prepareStatement(SQL_QUERY, ResultSet.TYPE_FORWARD_ONLY, ResultSet.CONCUR_READ_ONLY);
        statement.setFetchSize(Integer.MIN_VALUE);

        ResultSet resultSet = statement.executeQuery();

        ObjectMapper objectMapper = new ObjectMapper();
        File jsonFile = new File(JSON_FILE_PATH);

        ExecutorService executorService = Executors.newFixedThreadPool(10);

        List<List<Object>> chunks = chunkResultSet(resultSet, CHUNK_SIZE);

        for (int i = 0; i < chunks.size(); i++) {
            executorService.execute(() -> {
                try {
                    List<Object> chunk = chunks.get(i);
                    List<String> jsonStrings = new ArrayList<>();

                    while (chunk.size() > 0) {
                        Object[] row = chunk.remove(0).toArray();
                        jsonStrings.add(objectMapper.writeValueAsString(row));
                    }

                    synchronized (jsonFile) {
                        objectMapper.writeValue(jsonFile, jsonStrings);
                    }
                } catch (Exception e) {
                    e.printStackTrace();
                }
            });
        }

        executorService.shutdown();
    }

    private static List<List<Object>> chunkResultSet(ResultSet resultSet, int chunkSize) throws SQLException {
        List<List<Object>> chunks = new ArrayList<>();
        List<Object> chunk = new ArrayList<>();

        while (resultSet.next()) {
            if (chunk.size() >= chunkSize) {
                chunks.add(chunk);
                chunk = new ArrayList<>();
            }

            ResultSetMetaData metaData = resultSet.getMetaData();
            int columnCount = metaData.getColumnCount();

            List<Object> row = new ArrayList<>();
            for (int i = 1; i <= columnCount; i++) {
                Object value = resultSet.getObject(i);
                row.add(value);
            }

            chunk.add(row);
        }

        if (chunk.size() > 0) {
            chunks.add(chunk);
        }

        return chunks;
    }
}