What's the simplest way to change from exception logging to exception handling in a Spring MVC app?

Question

My Spring MVC app is full of methods that look like this:

@RequestMapping(value = "/foo", method = RequestMethod.GET)
public final void foo(HttpServletRequest request, ModelMap modelMap){
    try{
        this.fooService.foo();
    }
    catch (Exception e){
        log.warn(e.getMessage(), e);
    }
}

Exceptions are caught and logged but not handled otherwise.

The fooService called above does the same thing, never throwing exceptions up to the controller but catching and logging them. So, actually this controller exception code will never get invoked.

What's the best and simplest approach to implement proper exception handling in my app?

Answer 1

Get rid of all catch statements if all they do is logging carelessly. catch is meant to handle the error, not hide it.

Once all these catches are removed, install one global exception resolver in Spring MVC (1, 2, 3, ...) Simply implement this trivial interface:

public interface HandlerExceptionResolver {
    ModelAndView resolveException(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex);
}

In your exception resolver you might simply log the exception once and let it go as unprocessed (return null), so that error mappings in web.xml will forward request to proper error page. Or you can handle exception yourself and render some error page. AFAIK in simplest case there is no need for register exception resolver, just define it as a Spring bean/annotate with @Service.

Remember, catch the exception only when you know what to do with. Logging is only for troubleshooting, it doesn't handle anything.

BTW this:

log.warn(e.getMessage(), e);

is not only a very poor exception handling, but it is also slightly incorrect. If your exception does not have a message, you will see mysterious null just before the stack trace. If it does, the message will appear twice (tested with Logback):

22:51:23.985 WARN [main][Foo] OMG! - this is the exception message
java.lang.IllegalStateException: OMG! - this is the exception message
    at Foo.bar(Foo.java:20) ~[test-classes/:na]

...sometimes undesirable, especially when exception message is very long.

---

UPDATE: When writing your own exception logger consider implementing both org.springframework.web.servlet.HandlerExceptionResolver and org.springframework.core.Ordered. The getOrder() should return something small (like 0) so that your handler takes precedence over built-in handlers.

It just happened to me that org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolver running prior to my handler returned HTTP 500 without logging the exception.