A simple option for enabling debug logging in your Bukkit plugin,

DebugLogHandler.java

/**
 * Copyright 2019 dumptruckman
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated
 * documentation files (the "Software"), to deal in the Software without restriction, including without limitation the
 * rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to
 * permit persons to whom the Software is furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all copies or substantial portions of
 * the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO
 * THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
 * TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */
import org.bukkit.plugin.Plugin;

import java.util.logging.Handler;
import java.util.logging.Level;
import java.util.logging.LogRecord;

/**
 * A logger handler for a bukkit plugin that will cause messages below {@link Level#INFO} to be logged normally when
 * the plugin's logger is set to the level of the message or lower.
 * <p/>
 * To enable logging of messages below {@link Level#INFO}, the plugin's logger must be set to an appropriately lower
 * level. For example, to log all message levels: <code>plugin.getLogger().setLevel(Level.ALL);</code>
 */
public class DebugLogHandler extends Handler {

    private static final String DEFAULT_DEBUG_PREFIX_FORMAT = "[%1$s-%2$s] %3$s";
    private static final String DEFAULT_DEBUG_LOG_PREFIX = "DEBUG";

    /**
     * Attaches a DebugLogHandler to the given plugin's logger.
     *
     * @param plugin the plugin to add debug logging for.
     */
    public static void attachDebugLogger(Plugin plugin) {
        plugin.getLogger().addHandler(new DebugLogHandler(plugin));
    }

    private final Plugin plugin;
    private final String pluginName;

    private DebugLogHandler(Plugin plugin) {
        this.plugin = plugin;
        String prefix = plugin.getDescription().getPrefix();
        this.pluginName = prefix != null ? prefix : plugin.getDescription().getName();
    }

    @Override
    public void publish(LogRecord record) {
        if (plugin.getLogger().getLevel().intValue() <= record.getLevel().intValue()
                && record.getLevel().intValue() < Level.INFO.intValue()) {
            record.setLevel(Level.INFO);
            record.setMessage(String.format(DEFAULT_DEBUG_PREFIX_FORMAT,
                                            pluginName,
                                            DEFAULT_DEBUG_LOG_PREFIX,
                                            record.getMessage().substring(pluginName.length() + 3)));
        }
    }

    @Override
    public void flush() { }

    @Override
    public void close() throws SecurityException { }
}