Try …​ Catch …​ Finally

Camel supports the Java equivalent of try …​ catch …​ finally directly in the DSL. It aims to work like its Java sisters but with more power.

In Camel we prefix the keywords with do to avoid having same keyword as Java. So we have:

  • doTry

  • doCatch

  • doFinally

  • end to end the block in Java DSL

When using doTry …​ doCatch …​ doFinally then the regular Camel Error Handler is not in use; meaning any onException or the likes does not trigger. The reason is that doTry …​ doCatch …​ doFinally is in fact its own error handler and mimics how try/catch/finally works in Java.

Using doTry …​ doCatch …​ doFinally

In the route below we have all of them in action:

from("direct:start")
    .doTry()
        .process(new ProcessorFail())
        .to("mock:result")
    .doCatch(IOException.class, IllegalStateException.class)
        .to("mock:catch")
    .doFinally()
        .to("mock:finally")
    .end();

And in XML DSL

<route>
  <from uri="direct:start"/>
  <doTry>
    <process ref="processorFail"/>
    <to uri="mock:result"/>
    <doCatch>
      <exception>java.io.IOException</exception>
      <exception>java.lang.IllegalStateException</exception>
      <to uri="mock:catch"/>
    </doCatch>
    <doFinally>
       <to uri="mock:finally"/>
    </doFinally>
  </doTry>
</route>

Using onWhen with doCatch

You can use Predicates with doCatch to make it runtime determine if the block should be triggered or not. In our case we only want to trigger if the caused exception message contains the damn word.

from("direct:start")
    .doTry()
        .process(new ProcessorFail())
        .to("mock:result")
    .doCatch(IOException.class, IllegalStateException.class).onWhen(exceptionMessage().contains("Damn"))
        .to("mock:catch")
    .doCatch(CamelExchangeException.class)
        .to("mock:catchCamel")
    .doFinally()
        .to("mock:finally")
    .end();

And in XML DSL

<route>
  <from uri="direct:start"/>
  <doTry>
    <process ref="processorFail"/>
    <to uri="mock:result"/>
    <doCatch>
      <exception>java.io.IOException</exception>
      <exception>java.lang.IllegalStateException</exception>
      <onWhen>
        <simple>${exception.message} contains 'Damn'</simple>
      </onWhen>
      <to uri="mock:catch"/>
    </doCatch>
    <doCatch>
      <exception>org.apache.camel.CamelExchangeException</exception>
      <to uri="mock:catchCamel"/>
    </doCatch>
    <doFinally>
       <to uri="mock:finally"/>
    </doFinally>
  </doTry>
</route>

Use end() to end the block

Notice when using Java DSL we must use end() to indicate where the try …​ catch …​ finally block ends. As the example above has a finally, then the end() should be at the end of the finally block. If we are not using a finally, then the end() should be at the end of the doCatch to indicate the end there.

Instead of end() you can use endDoTry() to end and return back to the try …​ catch scope.

Using nested doTry …​ doCatch EIPs

When nesting doTry …​ doCatch from an outer doTry …​ doCatch EIP, then pay extra attention when using Java DSL as the Java programming language is not indent aware so you may write Java code that are indented in a way where you think that a catch block is associated with the other doTry but it is not.

Given the following Java DSL:

from("direct:test").routeId("myroute")
    .doTry().
        doTry().
            throwException(new IllegalArgumentException("Forced by me"))
        .doCatch(Exception.class)
            .log("docatch 1")
            .throwException(new IllegalArgumentException("Second forced by me"))
    .doCatch(Exception.class)
        .log("docatch 2")
    .end();

Then you may think that docatch2 is associated on the outer doTry because of how the code is formatted. But it is not, both docatch1 and docatch2 are in the inner doTry, and the outer doTry has no catch blocks.

So in this example the route will throw the 1st exception which is then handled in docatch1 which then throw a 2nd exception, that is not caught.

So what you must do is to end the doCatch block correct (notice how we use endDoTry() two times) as shown below:

from("direct:test").routeId("myroute")
    .doTry().
        doTry().
            throwException(new IllegalArgumentException("Forced by me"))
        .doCatch(Exception.class)
            .log("docatch 1")
            .throwException(new IllegalArgumentException("Second forced by me"))
         .endDoTry() // end this doCatch block
     .endDoTry() // end the inner doTry
    .doCatch(Exception.class)
        .log("docatch 2")
    .end();

And by using the endDoTry() we can end the block correctly, and an XML representation of the route would be as follows:

<route>
    <from uri="direct:test"/>
    <doTry>
        <doTry>
            <throwException id="throwException1"/>
            <doCatch id="doCatch1">
                <log id="log1" message="docatch 1"/>
                <throwException id="throwException2"/>
            </doCatch>
        </doTry>
        <doCatch id="doCatch2">
            <log id="log2" message="docatch 2"/>
        </doCatch>
    </doTry>
</route>
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