paper-mc/paper-api/src/main/java/org/bukkit/Bukkit.java

package org.bukkit;

/**
 * Represents the Bukkit core, for version and Server singleton handling
 */
public final class Bukkit {
    private static Server server;

    /**
     * Static class cannot be initialized.
     */
    private Bukkit() {
    }

    /**
     * Gets the current {@link Server} singleton
     *
     * @return Server instance being ran
     */
    public static Server getServer() {
        return server;
    }

    /**
     * Attempts to set the {@link Server} singleton.
     *
     * This cannot be done if the Server is already set.
     *
     * @param server Server instance
     */
    public static void setServer(Server server) {
        if (Bukkit.server != null) {
            throw new UnsupportedOperationException("Cannot redefine singleton Server");
        }

        Bukkit.server = server;
    }
}