Why is the exception null when I use onException?

If you use onException to handle exceptions, such as shown below:

.onException(Exception.class).handled(true)
  .process(new Processor() {
    @Override
    public void process(Exchange exchange) throws Exception {
      Exception cause = exchange.getException());
      // why cause exception is null ???
    }
  })
.end()

Then make notice the caused exception is no longer available from exchange.getException(), because the message is in the onException block.

Instead you can access the caused exception from a property on the exchange with the key Exchange.EXCEPTION_CAUGHT, as shown below:

.onException(Exception.class).handled(true)
  .process(new Processor() {
    @Override
    public void process(Exchange exchange) throws Exception {
      Exception cause = exchange.getProperty(Exchange.EXCEPTION_CAUGHT, Exception.class);
      // we now have the caused exception
    }
  })
.end()