Convert a Log4j PatternLayout Extension to Log4j2

Since PatternLayout is final itself you can instead extend the parent class of PatternLayout and get things done.

package com.example;


import java.nio.charset.Charset;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.HashMap;
import org.apache.logging.log4j.Level;
import org.apache.logging.log4j.core.LogEvent;
import org.apache.logging.log4j.core.config.Configuration;
import org.apache.logging.log4j.core.layout.AbstractStringLayout;

public class HelloPattern extends AbstractStringLayout {

    private static final SimpleDateFormat DATE_FORMAT;
    /**
     * ANSI-Format for the level
     */
    private static final HashMap<Level, CliColor> LEVELS;

    static {
        LEVELS = new HashMap<Level, CliColor>();
        LEVELS.put(Level.TRACE, CliColor.BLUE);
        LEVELS.put(Level.DEBUG, CliColor.CYAN);
        LEVELS.put(Level.INFO, CliColor.GREEN);
        LEVELS.put(Level.WARN, CliColor.YELLOW);
        LEVELS.put(Level.ERROR, CliColor.RED);
        LEVELS.put(Level.FATAL, CliColor.RED);
        DATE_FORMAT = new SimpleDateFormat("HH:mm:ss,SSS");
    }

    protected HelloPattern(Charset charset) {
        super(charset);
    }

    protected HelloPattern(Charset aCharset, byte[] header, byte[] footer) {
        super(aCharset, header, footer);
    }

    protected HelloPattern(Configuration config,
        Charset aCharset,
        Serializer headerSerializer,
        Serializer footerSerializer) {
        super(config, aCharset, headerSerializer, footerSerializer);
    }

    /**
     * Pads a string until it has the desired length. The space is filled with spaces.
     *
     * @param str    The string to pad.
     * @param length The desired length of the string.
     * @return A padded string.
     */
    public static String leftPad(String str, int length) {
        if (str.length() > length) {
            return str.substring(0, length);
        }

        if (str.length() < length) {
            StringBuilder stringBuilder = new StringBuilder(str);
            for (int i = str.length(); i < length; ++i) {
                stringBuilder.append(' ');
            }
            return stringBuilder.toString();
        }

        return str;
    }

    @Override
    public boolean requiresLocation() {
        return true;
    }

    @Override
    public String toSerializable(LogEvent event) {
        return String.format("%s %s %s %s\n", DATE_FORMAT.format(new Date()),
            formatLevel(event), formatLocationInfo(event),
            formatMessage(event));
    }

    /**
     * @param event The event that gets formatted
     * @return A string representing the log level.
     */
    private String formatLevel(LogEvent event) {
        Level level = event.getLevel();
        String levelName = leftPad(level.toString(), 5);
        CliColor ansiConfig = LEVELS.get(level);

        if (ansiConfig == null) {
            return CliColor.color(levelName, CliColor.BOLD);
        }

        return CliColor.color(levelName, CliColor.BOLD, ansiConfig);
    }

    /**
     * @param event The event that gets formatted
     * @return A formatted location info.
     */
    private String formatLocationInfo(LogEvent event) {
        Throwable thrown = event.getThrown();
        final int line = thrown.getStackTrace()[0].getLineNumber();
        String className = thrown.getStackTrace()[0].getClassName();
        className = className.substring(className.lastIndexOf('.') + 1);

        return "[\u001b[0;33m" + className + ":" + line + "\u001B[m]";
    }

    //Tried best to get it right

    /**
     * @param event The event that gets formatted
     * @return Formatted message.
     */
    private String formatMessage(LogEvent event) {
        if (event.getMessage() == null) {
            return "";
        }

        return event.getMessage().toString();
    }

    enum CliColor {
        BLUE, CYAN, GREEN, YELLOW, RED, BOLD;

        public static String color(String level, CliColor color) {
            return "stringBasedonyourimplementation";
        }

        public static String color(String levelName, CliColor bold, CliColor ansiConfig) {
            return "stringBasedonyourimplementation";
        }
    }
}

Note: Since I did not have enough info on CliColor, added mock impl to give you idea on that. Plus for locationInfo as well.