A custom log appender for log4j that allows one to catch log messages generated by log4j from within the application.

CachingSingletonAppender .java

import java.util.ArrayList;
import java.util.Date;
import java.util.List;
import org.apache.log4j.spi.LoggingEvent;

public class CachingSingletonAppender {

    private static final CachingSingletonAppender theInstance = new CachingSingletonAppender();
    private List<LogEventListener> listeners;
    private List<LogEvent> eventCache;

    private CachingSingletonAppender() {
        listeners = new ArrayList<>();
        eventCache = new ArrayList<>();
    }

    public static CachingSingletonAppender getInstance() {
        return theInstance;
    }

    public void append(LoggingEvent le) {
        LogEvent event = new LogEvent(le.getLevel().toString(), le.getMessage().toString(), new Date(le.getTimeStamp()));
        if (!listeners.isEmpty()) {
            for (LogEventListener listener : listeners) {
                listener.handle(event);
            }
        } else {
            eventCache.add(event);
        }
    }

    public void addLoggingEventListener(LogEventListener listener) {
        listeners.add(listener);
        if (listeners.size() == 1 && !eventCache.isEmpty()) {
            sendAndClearCache();
        }
    }

    public void removeLoggingEventListener(LogEventListener listener) {
        listeners.remove(listener);
    }

    synchronized private void sendAndClearCache() {
        for (LogEventListener listener : listeners) {
            for (LogEvent event : eventCache) {
                listener.handle(event);
            }
        }
        eventCache.clear();
    }
}

InternalAppenderWrapper.java

import org.apache.log4j.AppenderSkeleton;
import org.apache.log4j.spi.LoggingEvent;

public class InternalAppenderWrapper extends AppenderSkeleton {

    private CachingSingletonAppender appender = CachingSingletonAppender.getInstance();
    
    @Override
    protected void append(LoggingEvent le) {
        appender.append(le);
    }

    @Override
    public void close() {
    }

    @Override
    public boolean requiresLayout() {
        return false;
    }
    
}

log4j.properties

log4j.rootLogger=INFO, internal
log4j.appender.internal=InternalAppenderWrapper

LogEvent.java

import java.util.Date;

public class LogEvent {

    private String level;
    private String message;
    private Date timestamp;

    public LogEvent() {
    }

    public LogEvent(String level, String message, Date timestamp) {
        this.level = level;
        this.message = message;
        this.timestamp = timestamp;
    }

    public String getLevel() {
        return level;
    }

    public void setLevel(String level) {
        this.level = level;
    }

    public String getMessage() {
        return message;
    }

    public void setMessage(String message) {
        this.message = message;
    }

    public Date getTimestamp() {
        return timestamp;
    }

    public void setTimestamp(Date timestamp) {
        this.timestamp = timestamp;
    }
}

LogEventListener.java

public interface LogEventListener {
    void handle(LogEvent event);
}

is it designed for log4jv1 or log4j2?

@gaurav9822
basing on package name "org.apache.log4j" it's log4jv1