JsonDatabase.java

//Setting up gson
        GsonBuilder builder = new GsonBuilder();
        builder.setPrettyPrinting();
        builder.enableComplexMapKeySerialization();

        //fix for map key that is null
        //gson does UUID.fromString even if the object is null, and it does not accept nulls
        builder.registerTypeAdapter(UUID.class, (JsonDeserializer<UUID>) (element, type, context) -> {
            if (element.isJsonNull() || element.getAsString().equals("null")) {
                return null;
            }
            return UUID.fromString(element.getAsString());
        });
        gson = builder.create();

        //Loading database
        File file = new File(getDataFolder(), "data.json");
        if (file.exists()) {
            getLogger().info("Database exists, reading data...");
            try (JsonReader reader = new JsonReader(new FileReader(file))) {
                database = gson.fromJson(reader, JsonDataPool.class);
            } catch (IOException e) {
                e.printStackTrace();
            }
        } else {
            getLogger().fine("Database does not exist, it will be created on server shutdown");
            database = new JsonDataPool();
        }


        //Database save task
        getLogger().info(String.format("The database will be saved every %s minutes", SAVE_INTERVAL));
        new BukkitRunnable() {
            @Override
            public void run() {
                getLogger().info("Periodically saving database...");
                saveDatabase();
            }
        }.runTaskTimerAsynchronously(this, SAVE_INTERVAL * 60 * 20, SAVE_INTERVAL * 60 * 20);

@Override
    public void onDisable() {
        getServer().getScheduler().cancelTasks(this);
        if (database != null) {
            getLogger().info("Saving database...");
            saveDatabase();
        } else {
            getLogger().info("Database is null, not saving database...");
        }
    }

    private void saveDatabase() {
        try (Writer writer = new FileWriter(new File(getDataFolder(), "data.json"))) {
            String output = gson.toJson(database, JsonDataPool.class);
            writer.write(output);
        } catch (IOException e) {
            getLogger().severe("Something went terribly wrong, couldn't save the database");
            e.printStackTrace();
        }
    }