a custom logger for Dropwizard apps to output correlation ids in request logs

CustomRequestLogFactory.java

package com.filippomortari;

import ch.qos.logback.classic.*;
import ch.qos.logback.classic.spi.*;
import ch.qos.logback.core.*;
import ch.qos.logback.core.spi.*;
import com.google.common.collect.ImmutableList;
import io.dropwizard.jetty.*;
import io.dropwizard.jetty.Slf4jRequestLog;
import io.dropwizard.logging.*;
import io.dropwizard.server.AbstractServerFactory;
import io.dropwizard.server.DefaultServerFactory;
import org.eclipse.jetty.http.HttpHeader;
import org.eclipse.jetty.server.*;
import org.slf4j.LoggerFactory;
import org.slf4j.MDC;

import javax.validation.Valid;
import javax.validation.constraints.NotNull;
import java.io.IOException;
import java.util.Enumeration;
import java.util.TimeZone;

public class CustomRequestLogFactory extends RequestLogFactory {

    private final ApplicationConfiguration applicationConfiguration;

    @NotNull
    private TimeZone timeZone = TimeZone.getTimeZone("UTC");

    @Valid
    @NotNull
    private ImmutableList<AppenderFactory> appenders; 

    public CustomRequestLogFactory(ApplicationConfiguration configuration) {
        this.applicationConfiguration = configuration;
        this.appenders = ((AbstractServerFactory) applicationConfiguration.getServerFactory()).getRequestLogFactory().getAppenders();
    }

    private static class RequestLogLayout extends LayoutBase<ILoggingEvent> {
        @Override
        public String doLayout(ILoggingEvent event) {
            return event.getFormattedMessage() + CoreConstants.LINE_SEPARATOR;
        }
    }

    private static class CustomRequestLog extends Slf4jRequestLog {

        private static Logger logger = (Logger) LoggerFactory.getLogger(CustomRequestLog.class);
        private static ThreadLocal<StringBuilder> _buffers = new ThreadLocal<StringBuilder>() {
            @Override
            protected StringBuilder initialValue() {
                return new StringBuilder(256);
            }
        };

        public CustomRequestLog(AppenderAttachableImpl<ILoggingEvent> appenders,
                                TimeZone timeZone) {
            super(appenders, timeZone);
            this.setExtended(true);
        }

        @Override
        protected void logExtended(Request request,
                                   Response response,
                                   StringBuilder b) throws IOException
        {
            
           super.logExtended(request,response, b);
           
           // it violates the NCSA log standard, YMMV
           b.append(" ["+response.getHeader("X-CORRELATION-ID")+"]");
        }
    }

    public RequestLog build(String name) {
        final Logger logger = (Logger) LoggerFactory.getLogger("http.request");
        logger.setAdditive(false);
        final LoggerContext context = logger.getLoggerContext();
        final RequestLogLayout layout = new RequestLogLayout();
        layout.start();
        final AppenderAttachableImpl<ILoggingEvent> attachable =
                new AppenderAttachableImpl<>();
        for (AppenderFactory output : this.appenders) {
            attachable.addAppender(output.build(context, name, layout));
        }
        return new CustomRequestLog(attachable, timeZone);
    }
}

DropwizardApplication.java

package com.filippomortari;

import com.yammer.tenacity.core.bundle.TenacityBundleBuilder;
import io.dropwizard.Application;
import io.dropwizard.server.DefaultServerFactory;
import io.dropwizard.setup.Bootstrap;
import io.dropwizard.setup.Environment;
import io.dropwizard.views.ViewBundle;

import static java.lang.String.format;

public class DropwizardApplication extends Application<ApplicationConfiguration> {

    public static void main(final String[] args) throws Exception {
        new DropwizardApplication().run(args);
    }

    @Override
    public String getName() {
        return "bla-bla-bla";
    }

    @Override
    public void initialize(final Bootstrap<ApplicationConfiguration> bootstrap) {
        bootstrap.addBundle(new ViewBundle());
    }

    @Override
    public void run(final ApplicationConfiguration configuration, final Environment environment) throws Exception {
        ((DefaultServerFactory) configuration.getServerFactory())
                .setRequestLogFactory(new CustomRequestLogFactory(configuration));
    }
}