SomePlugin.java

package co.aikar.someplugin;

import org.bukkit.Location;
import org.bukkit.Material;
import org.bukkit.block.BlockState;
import org.bukkit.block.Chest;
import org.bukkit.command.Command;
import org.bukkit.command.CommandSender;
import org.bukkit.entity.Player;
import org.bukkit.inventory.Inventory;
import org.bukkit.inventory.ItemStack;
import org.bukkit.inventory.meta.BookMeta;
import org.bukkit.plugin.java.JavaPlugin;

import java.util.Random;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public final class SomePlugin extends JavaPlugin {

    @Override
    public void onEnable() {
        // Plugin startup logic

    }

    @Override
    public void onDisable() {
        // Plugin shutdown logic
    }

    @Override
    public boolean onCommand(CommandSender sender, Command command, String label, String[] args) {
        if (!(sender instanceof Player)) {
            return true;
        }
        Player player = (Player) sender;
        player.getLocation();
        int r = Math.min(3, args.length > 0 ? Integer.parseInt(args[0]) : 0);
        for (int x = -r; x <= r; x++) {
            for (int y = -r; y <= r; y++) {
                for (int z = -r; z <= r; z++) {
                    doDupeBlock(player.getLocation().add(x, y, z));
                }
            }
        }

        return true;
    }

    ExecutorService executorService = Executors.newFixedThreadPool(6);
    private void doDupeBlock(Location loc) {
        loc.getBlock().setType(Material.CHEST);

        Chest state = (Chest) loc.getBlock().getState(false);
        Inventory blockInventory = state.getBlockInventory();
        executorService.submit(() -> {
            for (int i = 0; i < 27; i++) {
                blockInventory.addItem(createBook());
            }
        });
    }

    private ItemStack createBook() {
        ItemStack book = new ItemStack(Material.WRITTEN_BOOK);
        BookMeta meta = (BookMeta) book.getItemMeta();
        for (int i = 0; i < 50; i++) {
            meta.addPage(createString());
        }
        book.setItemMeta(meta);
        return book;
    }

    Random random = new Random();

    private String createString() {
        StringBuilder builder = new StringBuilder(600);
        for (int i = 0; i < 300; i++) {
            builder.append((char)random.nextInt(215) + 40);
            builder.append((char)random.nextInt(215) + 40);
            builder.append((char)random.nextInt(215) + 40);
        }
        return builder.toString();
    }
}