How does Spring know that ThrowsAdvice.afterThrowing needs to be called? - spring

Question

How does Spring know that ThrowsAdvice.afterThrowing needs to be called?
I found documentation on the class here but I was wondering if anyone has an elegant explanation of how exactly reflection is used to look for that "afterThrowing" method specificially. I just want to see the code that does that so I can understand it better.
A link to some source code would be a sufficient answer.

Answer 1

You're looking at very old documentation (though the current one doesn't say much more).
Spring uses a ThrowsAdviceInterceptor to handle ThrowsAdvice. You can find version 4.1.4.RELEASE source code here.
Its constructor
public ThrowsAdviceInterceptor(Object throwsAdvice) {
Assert.notNull(throwsAdvice, "Advice must not be null");
this.throwsAdvice = throwsAdvice;
Method[] methods = throwsAdvice.getClass().getMethods();
for (Method method : methods) {
if (method.getName().equals(AFTER_THROWING) &&
(method.getParameterTypes().length == 1 || method.getParameterTypes().length == 4) &&
Throwable.class.isAssignableFrom(method.getParameterTypes()[method.getParameterTypes().length - 1])
) {
// Have an exception handler
this.exceptionHandlerMap.put(method.getParameterTypes()[method.getParameterTypes().length - 1], method);
if (logger.isDebugEnabled()) {
logger.debug("Found exception handler method: " + method);
}
}
}
if (this.exceptionHandlerMap.isEmpty()) {
throw new IllegalArgumentException(
"At least one handler method must be found in class [" + throwsAdvice.getClass() + "]");
}
}
scans for appropriate methods and registers them. It then wraps the target method invocation
#Override
public Object invoke(MethodInvocation mi) throws Throwable {
try {
return mi.proceed();
}
catch (Throwable ex) {
Method handlerMethod = getExceptionHandler(ex);
if (handlerMethod != null) {
invokeHandlerMethod(mi, ex, handlerMethod);
}
throw ex;
}
}
and invokes the handler if an exception is thrown.