How to use SLF4J Loggers in a JSP

Question

I'm in the process of migrating the logging of a middle-sized application from a custom solution to something more standard. I've decided on using Logback and SLF4J, and I have successfully migrated most of the Java code. However, I have quite a bit of JSPs that simply use System.out for logging. I've never worked much with JSPs, and started to wonder: how am I supposed to use proper logging in a JSP?

<%@page import="org.slf4j.Logger"%>
<%@page import="org.slf4j.LoggerFactory"%>
<%
    Logger log = LoggerFactory.getLogger(getClass());
%>
<!-- ... -->
<%
    log.info("Hello Logging!");
%>

This is what came to mind first, but it seems wrong on several points:

• way too verbose, and needs a lot of work to convert the existing JSPs
• a call is made to LoggerFactory.getLogger() every time the page is rendered (as opposed to a static logger field in a standard Java class)
• I think the name of the logger is also going to be something not really straightforward this way

Is there some kind of standard, or a best practice, or anything for logging in JSPs?

Also, IIRC, there was some kind of taglib for Log4J. Is there something similar for SLF4J (or maybe Logback)?

Answer 1

Have a look at slf4j-taglib.

Answer 2

You could try (Note the "!")

<%! org.slf4j.Logger log = org.slf4j.LoggerFactory.getLogger("JSPname"); %>

Then replace your System.out's with

<% log.info("Hello Logging!"); %>

Boris's comment should really be taken into concideration though, JSPs should not need logging in general. I would only use this technique (or anything similiar) to replace existing logging that needed to be kept.

Answer 3

public enum MyLoggerFactory {

    INSTANCE;

    @SuppressWarnings("unchecked")
    private final Map<Class, Logger> loggers = new HashMap<Class, Logger>();

    @SuppressWarnings("unchecked")
    public Logger getLogger(Class clazz) {
        if (loggers.get(clazz) == null) {
            loggers.put(clazz, LoggerFactory.getLogger(clazz));
        }
        return loggers.get(clazz);
    }
}

Then your JSP page could look like:

<%
    MyLoggerFactory.INSTANCE.getLogger(getClass()).info("Hello Logging!");
%>

Answer 4

To use logger in the jsp file, initialize the logger object in the following way:

<% Logger logger = LoggerFactory.getLogger(this.getClass()); %>

And then you could go ahead by using

logger.info(); or logger.error();, etc.

Answer 5

I think it's bad practice to trigger logging from withing the representational layer of an application. Generally, I would expect to find logging in the business logic only - a JSP cluttered with logging invocation would certainly break the separation of concerns principle.

As a temporary, yet elegant workaround, you could try and create a custom tag lib.