Log4j (SLF4J) MDC Context in Struts 2 Interceptor - filter

Question

I find that one can set the Log4j (SL4J) MDC context in a thread safe way in a general filter (Code from Adding user info to log entries in a multi-user app using Mapped Diagnostic Context)
import org.slf4j.MDC;
import javax.servlet.*;
import java.io.IOException;
public class MDCFilter implements Filter {
#Override
public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain)
throws IOException, ServletException {
User user= (User) session.getAttribute("USerSession");
MDC.put("userName", user.getUserName() );
try {
chain.doFilter(req, resp);
} finally {
MDC.remove("userName");
}
}
}
Can I do the same approach in the Struts 2 interceptor ?! What I wonder is thread safty issues.
Struts 2 interceptor and servlet filters are not both thread safe and MDC implementation is thread safe, so if above code works fine in a filter, theoretically it must be work thread safe in the interceptor.
Any comments ?!

Answer 1

Interceptors are not thread safe means that you have to write them in thread safe manner. For example below interceptor is thread safe.
public class LoggerInterceptor extends AbstractInterceptor {
#Override
public String intercept(ActionInvocation invocation) throws Exception {
HttpSession session = ServletActionContext.getRequest().getSession();
User user= (User) session.getAttribute("USerSession");
MDC.put("userName", user.getUserName() );
String result;
try {
result = invocation.invoke();
} finally {
MDC.remove("userName");
}
return result;
}
}