Frontend Logging with JavaFX and Log4j 2

2015-10-11

Although I’m developing Java EE applications with HTML5, I sometimes have to build Java SE applications with a Frontend. I’ve used Swing or AWT in the past. But since JavaFX 2.0 get lost of that awful JavaFX Script, JavaFX is my weapon of choice. Recently I wrote a multithreaded applications that needs to inform the user of some working results. I guess I was some kind of blue-eyed when I wrote this “log” function:

public static void guiLog(String logstring){
        logTextArea.appendText(logstring);
        }

and used it like that:

Platform.runLater(()->Controller.guiLog("Hello World :)"));

I still don’t know if that would’ve worked reliable in a single thread application, but in my case the GUI hangs after 30-45 seconds while using 8 threads. And to be honest: That kind of logging didn’t feel correct…

So I took a look at Log4j 2 appenders and I was not disappointed. Implementing your own appender, in my case a TextArea appender is quite easy.

import javafx.application.Platform;
import javafx.scene.control.TextArea;
import org.apache.logging.log4j.core.Filter;
import org.apache.logging.log4j.core.Layout;
import org.apache.logging.log4j.core.LogEvent;
import org.apache.logging.log4j.core.appender.AbstractAppender;
import org.apache.logging.log4j.core.config.plugins.Plugin;
import org.apache.logging.log4j.core.config.plugins.PluginAttribute;
import org.apache.logging.log4j.core.config.plugins.PluginElement;
import org.apache.logging.log4j.core.config.plugins.PluginFactory;
import org.apache.logging.log4j.core.layout.PatternLayout;

import java.io.Serializable;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

/**
 * TextAreaAppender for Log4j 2
 */
@Plugin(
        name = "TextAreaAppender",
        category = "Core",
        elementType = "appender",
        printObject = true)
public final class TextAreaAppender extends AbstractAppender {

    private static TextArea textArea;


    private final ReadWriteLock rwLock = new ReentrantReadWriteLock();
    private final Lock readLock = rwLock.readLock();


    protected TextAreaAppender(String name, Filter filter,
                               Layout<? extends Serializable> layout,
                               final boolean ignoreExceptions) {
        super(name, filter, layout, ignoreExceptions);
    }

    /**
     * This method is where the appender does the work.
     *
     * @param event Log event with log data
     */
    @Override
    public void append(LogEvent event) {
        readLock.lock();

        final String message = new String(getLayout().toByteArray(event));

        // append log text to TextArea
        try {
            Platform.runLater(() -> {
                try {
                    if (textArea != null) {
                        if (textArea.getText().length() == 0) {
                            textArea.setText(message);
                        } else {
                            textArea.selectEnd();
                            textArea.insertText(textArea.getText().length(),
                                    message);
                        }
                    }
                } catch (final Throwable t) {
                    System.out.println("Error while append to TextArea: "
                                       + t.getMessage());
                }
            });
        } catch (final IllegalStateException ex) {
            ex.printStackTrace();

        } finally {
            readLock.unlock();
        }
    }

    /**
     * Factory method. Log4j will parse the configuration and call this factory 
     * method to construct the appender with
     * the configured attributes.
     *
     * @param name   Name of appender
     * @param layout Log layout of appender
     * @param filter Filter for appender
     * @return The TextAreaAppender
     */
    @PluginFactory
    public static TextAreaAppender createAppender(
            @PluginAttribute("name") String name,
            @PluginElement("Layout") Layout<? extends Serializable> layout,
            @PluginElement("Filter") final Filter filter) {
        if (name == null) {
            LOGGER.error("No name provided for TextAreaAppender");
            return null;
        }
        if (layout == null) {
            layout = PatternLayout.createDefaultLayout();
        }
        return new TextAreaAppender(name, filter, layout, true);
    }


    /**
     * Set TextArea to append
     *
     * @param textArea TextArea to append
     */
    public static void setTextArea(TextArea textArea) {
        TextAreaAppender.textArea = textArea;
    }
}

The only thing left is to announce the appender in the Log4j configuration

<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="INFO">
    <Appenders>
        <TextAreaAppender name="JavaFXLogger">
            <PatternLayout pattern="%d{yyyy-MM-dd HH:mm:ss} %c{1}:%L - %m%n"/>
        </TextAreaAppender>
    </Appenders>
    <Loggers>
        <Root level="debug">
            <AppenderRef ref="JavaFXLogger"/>
        </Root>
    </Loggers>
</Configuration>

Happy logging ;) !