Try …​ Catch …​ Finally

Camel supports the Java equivalent of try .. catch and 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

Camel error handling is disabled

When using doTry .. doCatch .. doFinally then the regular Camel Error Handler does not apply. That means any onException or the likes does not trigger. The reason is that doTry .. doCatch .. doFinally is in fact its own error handler and that it aims to mimic and work like how try/catch/finally works in Java.

About doCatch and its power over Java

The doCatch in Camel is empowered over its Java sister.

First of all you can define multiple exceptions to catch in a single block.

And second of all an important aspect over the regular Java counter parts is that Camel will check in the exception hierarchy when it matches a thrown exception against the doCatch blocks. The reason is that many times the original caused exceptions is wrapped by other wrapper exceptions, typically transposing the exception from a checked to a runtime exception. Camel for instance does this by wrapping it in a RuntimeCamelException. So if the original caused exception is an java.io.IOException then Camel will still match a doCatch block defined with an java.io.IOException. And just like Java the order in which you have multiple doCatch blocks matters. Camel will iterate from the top going down and use the first doCatch that matches the exception. The reason is to keep it similar to the regular java and how it selects a catch block. This differs from the Exception Clause that has a more intelligent exception selection strategy among multiple onException definitions, where it also considers the delta in the exception hierarchy to select the best definition.

A third feature is that you can attach a onWhen predicate to signal if the catch should trigger or not at runtime.

Using try .. catch .. finally in Java DSL

In the route below we have all keywords in action. As the code is based on a unit test we route using Mock.

TryProcessorMultipleExceptionTest.java

And finally we have an example of the onWhen predicate in action. We can attach it to a doCatch block and at 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.

TryProcessorOnWhenTest.java

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

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<route xmlns="http://camel.apache.org/schema/spring">
    <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>

Using try .. catch .. finally in XML DSL

In the route below we have all keywords in action. As the code is based on a unit test we route using Mock.

SpringTryProcessorMultipleExceptionTest.xml

And finally we have an example of the onWhen predicate in action. We can attach it to a doCatch block and at 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.

SpringTryProcessorOnWhenTest.xml

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