A log4j2 rewrite policy that maps the log level to StackDriver severity and stores the value inside the context.

StackDriverRewritePolicy.java

package com.github.mnuessler.log4j;

import org.apache.logging.log4j.Level;
import org.apache.logging.log4j.core.Core;
import org.apache.logging.log4j.core.LogEvent;
import org.apache.logging.log4j.core.appender.rewrite.RewritePolicy;
import org.apache.logging.log4j.core.config.plugins.Plugin;
import org.apache.logging.log4j.core.config.plugins.PluginFactory;
import org.apache.logging.log4j.core.impl.ContextDataFactory;
import org.apache.logging.log4j.core.impl.Log4jLogEvent;
import org.apache.logging.log4j.util.StringMap;

/**
 * Maps the log4j level to StackDriver severity and stores the value inside the context, so that it can
 * later be added as an additional field in the JSON appender.
 */
@Plugin(name = "StackDriverRewritePolicy", category = Core.CATEGORY_NAME, elementType = "rewritePolicy")
public class StackDriverRewritePolicy implements RewritePolicy {

    private StackDriverRewritePolicy() {
    }

    @Override
    public LogEvent rewrite(final LogEvent source) {
        final StringMap newContextData = ContextDataFactory.createContextData(source.getContextData());
        newContextData.putValue("severity", severityForLevel(source.getLevel()));
        return new Log4jLogEvent.Builder(source).setContextData(newContextData).build();
    }

    private static String severityForLevel(final Level level) {
        switch (level.name()) {
            case "TRACE":
            case "DEBUG":
            case "ALL":
                return "DEBUG";
            case "INFO":
                return "INFO";
            case "WARN":
                return "WARNING";
            case "ERROR":
                return "ERROR";
            case "FATAL":
                return "EMERGENCY";
            default:
                return "DEFAULT";
        }
    }

    @PluginFactory
    public static StackDriverRewritePolicy createPolicy() {
        return new StackDriverRewritePolicy();
    }
}