How to Create a Custom Appender in log4j2?

Question

As disscussed in this link : How to create a own Appender in log4j?

For creating a custom appender in log4j 1.x we have to extend the AppenderSkeleton class and implements its append method.

Similarly How we can create a custom appender in log4j2 as we dont have AppenderSkelton class to extend and all other appender extend AppenderBase class .

Answer 1

This works quite differently in log4j2 than in log4j-1.2.

In log4j2, you would create a plugin for this. The manual has an explanation with an example for a custom appender here: http://logging.apache.org/log4j/2.x/manual/extending.html#Appenders

It may be convenient to extend org.apache.logging.log4j.core.appender.AbstractAppender, but this is not required.

When you annotate your custom Appender class with @Plugin(name="MyCustomAppender", ...., the plugin name becomes the configuration element name, so a configuration with your custom appender would then look like this:

<Configuration packages="com.yourcompany.yourcustomappenderpackage">
  <Appenders>
    <MyCustomAppender name="ABC" otherAttribute="...">
    ...
  </Appenders>
  <Loggers><Root><AppenderRef ref="ABC" /></Root></Loggers>
</Configuration>

Note that the packages attribute on the configuration is a comma-separated list of all the packages with custom log4j2 plugins. Log4j2 will search these packages in the classpath for classes annotated with @Plugin.

Here is a sample custom appender that prints to the console:

package com.yourcompany.yourcustomappenderpackage;

import java.io.Serializable;
import java.util.concurrent.locks.*;
import org.apache.logging.log4j.core.*;
import org.apache.logging.log4j.core.config.plugins.*;
import org.apache.logging.log4j.core.layout.PatternLayout;

// note: class name need not match the @Plugin name.
@Plugin(name="MyCustomAppender", category="Core", elementType="appender", printObject=true)
public final class MyCustomAppenderImpl extends AbstractAppender {

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

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

    // The append method is where the appender does the work.
    // Given a log event, you are free to do with it what you want.
    // This example demonstrates:
    // 1. Concurrency: this method may be called by multiple threads concurrently
    // 2. How to use layouts
    // 3. Error handling
    @Override
    public void append(LogEvent event) {
        readLock.lock();
        try {
            final byte[] bytes = getLayout().toByteArray(event);
            System.out.write(bytes);
        } catch (Exception ex) {
            if (!ignoreExceptions()) {
                throw new AppenderLoggingException(ex);
            }
        } finally {
            readLock.unlock();
        }
    }

    // Your custom appender needs to declare a factory method
    // annotated with `@PluginFactory`. Log4j will parse the configuration
    // and call this factory method to construct an appender instance with
    // the configured attributes.
    @PluginFactory
    public static MyCustomAppenderImpl createAppender(
            @PluginAttribute("name") String name,
            @PluginElement("Layout") Layout<? extends Serializable> layout,
            @PluginElement("Filter") final Filter filter,
            @PluginAttribute("otherAttribute") String otherAttribute) {
        if (name == null) {
            LOGGER.error("No name provided for MyCustomAppenderImpl");
            return null;
        }
        if (layout == null) {
            layout = PatternLayout.createDefaultLayout();
        }
        return new MyCustomAppenderImpl(name, filter, layout, true);
    }
}

For more details on plugins: http://logging.apache.org/log4j/2.x/manual/plugins.html

If the manual is not enough, it may be useful to look at the source code for the built-in appenders in log4j-core.