src/site/asciidoc/manual/eventlogging.adoc

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= Log4j 2 API
Ralph Goers <rgoers@apache.org>

[#EventLogging]
== Event Logging

The `EventLogger` class provides a simple mechanism for logging events
that occur in an application. While the `EventLogger` is useful as a way
of initiating events that should be processed by an audit Logging
system, by itself it does not implement any of the features an audit
logging system would require such as guaranteed delivery.

The recommended way of using the `EventLogger` in a typical web
application is to populate the `ThreadContext` Map with data that is
related to the entire lifespan of the request such as the user's id, the
user's IP address, the product name, etc. This can easily be done in a
servlet filter where the `ThreadContext` Map can also be cleared at the
end of the request. When an event that needs to be recorded occurs a
`StructuredDataMessage` should be created and populated. Then call
`EventLogger.logEvent(msg)` where msg is a reference to the
`StructuredDataMessage`.

[source,java]
----
import org.apache.logging.log4j.ThreadContext;
import org.apache.commons.lang.time.DateUtils;

import javax.servlet.Filter;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.FilterChain;
import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.TimeZone;

public class RequestFilter implements Filter {
    private FilterConfig filterConfig;
    private static String TZ_NAME = "timezoneOffset";

    public void init(FilterConfig filterConfig) throws ServletException {
        this.filterConfig = filterConfig;
    }

    /**
     * Sample filter that populates the MDC on every request.
     */
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain)
            throws IOException, ServletException {
        HttpServletRequest request = (HttpServletRequest)servletRequest;
        HttpServletResponse response = (HttpServletResponse)servletResponse;
        ThreadContext.put("ipAddress", request.getRemoteAddr());
        HttpSession session = request.getSession(false);
        TimeZone timeZone = null;
        if (session != null) {
            // Something should set this after authentication completes
            String loginId = (String)session.getAttribute("LoginId");
            if (loginId != null) {
                ThreadContext.put("loginId", loginId);
            }
            // This assumes there is some javascript on the user's page to create the cookie.
            if (session.getAttribute(TZ_NAME) == null) {
                if (request.getCookies() != null) {
                    for (Cookie cookie : request.getCookies()) {
                        if (TZ_NAME.equals(cookie.getName())) {
                            int tzOffsetMinutes = Integer.parseInt(cookie.getValue());
                            timeZone = TimeZone.getTimeZone("GMT");
                            timeZone.setRawOffset((int)(tzOffsetMinutes * DateUtils.MILLIS_PER_MINUTE));
                            request.getSession().setAttribute(TZ_NAME, tzOffsetMinutes);
                            cookie.setMaxAge(0);
                            response.addCookie(cookie);
                        }
                    }
                }
            }
        }
        ThreadContext.put("hostname", servletRequest.getServerName());
        ThreadContext.put("productName", filterConfig.getInitParameter("ProductName"));
        ThreadContext.put("locale", servletRequest.getLocale().getDisplayName());
        if (timeZone == null) {
            timeZone = TimeZone.getDefault();
        }
        ThreadContext.put("timezone", timeZone.getDisplayName());
        filterChain.doFilter(servletRequest, servletResponse);
        ThreadContext.clear();
    }

    public void destroy() {
    }
}
----

Sample class that uses `EventLogger`.

[source,java]
----
import org.apache.logging.log4j.StructuredDataMessage;
import org.apache.logging.log4j.EventLogger;

import java.util.Date;
import java.util.UUID;

public class MyApp {

    public String doFundsTransfer(Account toAccount, Account fromAccount, long amount) {
        toAccount.deposit(amount);
        fromAccount.withdraw(amount);
        String confirm = UUID.randomUUID().toString();
        StructuredDataMessage msg = new StructuredDataMessage(confirm, null, "transfer");
        msg.put("toAccount", toAccount);
        msg.put("fromAccount", fromAccount);
        msg.put("amount", amount);
        EventLogger.logEvent(msg);
        return confirm;
    }
}
----

The `EventLogger` class uses a `Logger` named "EventLogger". `EventLogger`
uses a logging level of OFF as the default to indicate that it cannot be
filtered. These events can be formatted for printing using the
link:../log4j-core/apidocs/org/apache/logging/log4j/core/layout/StructuredDataLayout.html[`StructuredDataLayout`].