JBoss: Trace interceptor

Now and then I enhance methods by some log-output like the following:

public void foo() {
  logger.trace("--> foo");
  try {
    /* ... */
  } finally {
    logger.trace("<-- foo");
  }
} 

If you have multiple code-locations where you want to add trace-output like above you can do this a little bit easier by making use of interceptors (JBoss documentation).

First of all you may centralize both - the log-statements and the try-finally block - in one single class, let's call it the TraceInterceptor:

public class TraceInterceptor {
  private static final Logger logger = Logger.getLogger(TraceInterceptor.class);

  @AroundInvoke
  public Object intercept(final InvocationContext ctx) throws Exception {
    String logMsg = null;
    if (logger.isTraceEnabled()) {
      logMsg = ctx.getMethod().getDeclaringClass().getSimpleName() 
               + "." + ctx.getMethod().getName();
      logger.trace("--> " + logMsg);
    }
    try {
      return ctx.proceed();
    } finally {
      if (logger.isTraceEnabled()) {
        logger.trace("<-- " + logMsg);
      }
    }
  }
}

Two things are important: The signature of the intercept-method must be exactly like this and it must be annotated with the @AroundInvoke annotation. When using this interceptor for a method foo of class Bar, the order is as follows:

1. execute TraceInterceptor.intercept() and log "--> Bar.foo"
   
2. ctx.proceed() passes on to the next interceptor or (if no other interceptor is registered) to the actual method being called
3. after foo-method returns the finally block is executed and the interceptor logs "<-- Bar.foo"

So if we want the TraceInterceptor to intercept the method foo() of class Bar, we may annotate this at the foo() method:

public class Bar {
  @Interceptors({ TraceInterceptor.class })
  public void foo() {
    /* ... */
  }
}

Instead of annotating the method you may also place the annotation at class-level so every method would be intercepted. As you can see from the curly braces it is also possible to annotate more than only one interceptor.