Exception Clause

You can use the Exception Clause in the Java DSL to specify the error handling you require on a per exception type basis using the onException() method.

To get started we give quick sample before digging into how it works.
For example if you want to perform a specific piece of processing if a certain exception is raised you can do this simply via:

Here if the processing of seda:inputA or seda:inputB cause a ValidationException to be thrown (such as due to the XSD validation of the Validation component or the Relax NG Compact syntax validation of the Jing component), then the message will be sent to activemq:validationFailed queue.

You can define multiple onException clauses for different behavior

Scopes

Exception clauses is scoped as either:

  • global (for Java DSL that is per RouteBuilder instances, to reuse, see note below)
  • or route specific

Where the global are the simplest and most easy to understand. In the advanced section we dig into the route specific and even combining them. However

 

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Global scope for Java DSL is per RouteBuilder instance, so if you want to share among multiple RouteBuilder classes, then create a base abstract RouteBuilder class and put the error handling logic in its configure method. And then extend this class, and make sure to class super.configure(). We are just using the Java inheritance technique.

 

How does Camel select which clause should handle a given thrown Exception

Camel uses DefaultExceptionPolicyStrategy to determine a strategy how an exception being thrown should be handled by which onException clause. The strategy is:

  • the order in which the onException is configured takes precedence. Camel will test from first...last defined.
  • Camel will start from the bottom (nested caused by) and recursive up in the exception hierarchy to find the first matching onException clause
  • instanceof test is used for testing the given exception with the onException clause defined exception list. An exact instanceof match will always be used, otherwise the onException clause that has an exception that is the closets super of the thrown exception is selected (recurring up the exception hierarchy)

This is best illustrated with an exception:

In the sample above we have defined two exceptions in which IOException is first, so Camel will pickup this exception if there is a match. IOException that is more general is selected then.

So if an exception is thrown with this hierarchy:

Then Camel will try testing the exception in this order: FileNotFoundException, IOException, OrderFailedException and RuntimeCamelException.
As we have defined a onException(IOException.class) Camel will select this as it's the closest match.

If we add a third onException clause with the FileNotFoundException

Then with the previous example Camel will now use the last onException(FileNotFoundException.class) as its an exact match. Since this is an exact match it will override the general IOException that was used before to handle the same exception thrown.

Now a new situation if this exception was thrown instead:

Then the onException(OrderFailedException.class) will be selected - no surprise here.

And this last sample demonstrates the instanceof test aspect in which Camel will select an exception if it's an instance of the defined exception in the onException clause. Illustrated as:

Since SocketException is an instanceof IOException, Camel will select the onException(IOException.class) clause.

Configuring RedeliveryPolicy (redeliver options)

RedeliveryPolicy requires to use the Dead Letter Channel as the Error Handler. Dead Letter Channel supports attempting to redeliver the message exchange a number of times before sending it to a dead letter endpoint. See Dead Letter Channel for further information about redeliver and which redeliver options exists.

No redelivery is default for onException

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By default any Exception Clause will not redeliver! (as it sets the maximumRedeliveries option to 0).

Sometimes you want to configure the redelivery policy on a per exception type basis. By default in the top examples, if a ValidationException occurs then the message will not be redelivered; however if some other exception occurs (IOException or whatelse) the route will be retried according to the settings from the Dead Letter Channel.

However if you want to customize any methods on the RedeliveryPolicy object, you can do this via the fluent API. So lets retry in case of ValidationException up till two times.

And the spring DSL:

You can customize any of the RedeliveryPolicy so we can for instance set a different delay of 5000 millis:

Point of entry for redelivery attempts

All redelivery attempts start at the point of the failure. So the route:

Will retry from processor2 - not the complete route.

Reusing ReliveryPolicy

Available as of Camel 1.5.1 or later
You can reference a RedeliveryPolicy so you can reuse existing configurations and use standard spring bean style configuration that supports property placeholders.

Asynchronous delayed redelivery

Available as of Camel 2.4

From Camel 2.4 onwards Camel has a feature to not block while waiting for a delayed redelivery to occur. However if you use transacted routes then Camel will block as its mandated by the transaction manager to execute all the work in the same thread context. You can enable the non blocking asynchronous behavior by the asyncDelayedRedelivery option. This option can be set on the errorHandler, onException or the redelivery policies.

By default the error handler will create and use a scheduled thread pool to trigger redelivery in the future. From Camel 2.8 onwards you can configure the executorServiceRef on the Error Handler to indicate a reference to either a shared thread pool you can enlist in the registry, or a thread pool profile in case you want to be able to control pool settings.

Catching multiple exceptions

Available as of Camel 1.5

In Camel 1.5 the exception clauses has been renamed to onException and it also supports multiple exception classes:

And in Spring DSL you just add another exception element:

Using a processor as failure handler

We want to handle certain exceptions specially so we add a onException clause for that exception.

So what happens is that whenever a MyFunctionalException is thrown it is being routed to our processor MyFunctionFailureHandler. So you can say that the exchange is diverted when a MyFunctionalException is thrown during processing. It's important to distinct this as perfect valid. The default redelivery policy from the Dead Letter Channel will not kick in, so our processor receives the Exchange directly, without any redeliver attempted. In our processor we need to determine what to do. Camel regards the Exchange as failure handled. So our processor is the end of the route. So lets look the code for our processor.

Notice how we get the caused by exception using a property on the Exchange. This is where Camel stores any caught exception during processing. So you can fetch this property and check what the exception message and do what you want. In the code above we just route it to a mock endpoint using a producer template from Exchange.

Marking exceptions as being handled

Available as of Camel 1.5

Continued

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See also the section Handle and continue exceptions below

Using onException to handle known exceptions is a very powerful feature in Camel. However prior to Camel 1.5 you could not mark the exception as being handled, so the caller would still receive the caused exception as a response. In Camel 1.5 you can now change this behavior with the new handle DSL. The handle is a Predicate that is overloaded to accept three types of parameters:

  • Boolean
  • Predicate
  • Expression that will be evaluates as a Predicate using this rule set: If the expressions returns a Boolean its used directly. For any other response its regarded as true if the response is not null.

For instance to mark all ValidationException as being handled we can do this:

Example using handled

In this route below we want to do special handling of all OrderFailedException as we want to return a customized response to the caller. First we setup our routing as:

Then we have our service beans that is just plain POJO demonstrating how you can use Bean Integration in Camel to avoid being tied to the Camel API:

And finally the exception that is being thrown is just a regular exception:

So what happens?

If we sent an order that is being processed OK then the caller will receive an Exchange as reply containing Order OK as the payload and orderid=123 in a header.

If the order could not be processed and thus an OrderFailedException was thrown the caller will not receive this exception (as opposed to in Camel 1.4, where the caller received the OrderFailedException) but our customized response that we have fabricated in the orderFailed method in our OrderService. So the caller receives an Exchange with the payload Order ERROR and a orderid=failed in a header.

Using handled with Spring DSL

The same route as above in Spring DSL:

Handling and sending a fixed response back to the client

In the route above we handled the exception but routed it to a different endpoint. What if you need to alter the response and send a fixed response back to the original caller (the client). No secret here just do as you do in normal Camel routing, use transform to set the response, as shown in the sample below:

We modify the sample slightly to return the original caused exception message instead of the fixed text Sorry:

And we can use the Simple language to set a readable error message with the caused excepetion message:

Handle and continue exceptions

Available as of Camel 2.3

In Camel 2.3 we introduced a new option continued which allows you to both handle and continue routing in the original route as if the exception did not occur.

For instance to just ignore and continue if the IDontCareException was thrown we can do this:

You can maybe compare continued with a having a try ... catch block around each step and then just ignore the exception.
Using continued makes it easier in Camel as you otherwise had to use Try Catch Finally style for this kind of use case.

Example using continued

In this route below we want to do special handling of all IllegalArgumentException as we just want to continue routing.

And the same example in Spring XML:

What is the difference between handled and continued?

If handled is true, then the thrown exception will be handled and Camel will not continue routing in the original route, but break out. However you can configure a route in the onException which will be used instead. You use this route if you need to create some custom response message back to the caller, or do any other processing because that exception was thrown.

If continued is true, then Camel will catch the exception and in fact just ignore it and continue routing in the original route. However if you have a route configured in the onException it will route that route first, before it will continue routing in the original route.

Using useOriginalMessage

Available as of Camel 2.0
The option useOriginalMessage is used for routing the original input body instead of the current body that potential is modified during routing.

For instance if you have this route:

The route listen for JMS messages and validates, transforms and handle it. During this the Exchange payload is transformed/modified. So in case something goes wrong and we want to move the message to another JMS destination, then we can add an onException. But when we move the Exchange to this destination we do not know in which state the message is in. Did the error happen in before the transformOrder or after? So to be sure we want to move the original input message we received from jms:queue:order:input. So we can do this by enabling the useOriginalMessage option as shown below:

Then the messages routed to the jms:queue:order:failed is the original input. If we want to manually retry we can move the JMS message from the failed to the input queue, with no problem as the message is the same as the original we received.

useOriginalMessage with Spring DSL

The useOriginalMessage option is defined as a boolean attribute on the <onException> XML tag in Spring DSL. So the definition above would be:

Advanced Usage of Exception Clause

Using global and per route exception clauses

Camel supports quite advanced configuration of exception clauses.

You can define exception clauses either as:

  • global
  • or route specific

We start off with the sample sample that we change over time. First off we use only global exception clauses:

In the next sample we change the global exception policies to be pure route specific.

Must use .end() for route specific exception policies

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Important: This requires to end the onException route with .end() to indicate where it stops and when the regular route continues.

And now it gets complex as we combine global and route specific exception policies as we introduce a 2nd route in the sample:

Notice that we can define the same exception MyFunctionalException in both routes, but they are configured differently and thus is handled different depending on the route. You can of course also add a new onException to one of the routes so it has an additional exception policy.

And finally we top this by throwing in a nested error handler as well, as we add the 3rd route shown below:

Global exception policies and nested error handlers

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The sample above with both nested error handlers and both global and per route exception clauses is a bit advanced. It's important to get the fact straight that the global exception clauses is really global so they also applies for nested error handlers. So if a MyTechnicalException is thrown then it's the global exception policy that is selected.

Using fine grained selection using onWhen predicate

Available as of Camel 1.5.1 or later

You can attach an Expression to the exception clause to have fine grained control when a clause should be selected or not. As it's an Expression you can use any kind of code to perform the test. Here is a sample:

In the sample above we have two onException's defined. The first has an onWhen expression attached to only trigger if the message has a header with the key user that is not null. If so this clause is selected and is handling the thrown exception. The 2nd clause is a for coarse gained selection to select the same exception being thrown but when the expression is evaluated to false. Notice: this is not required, if the 2nd clause is omitted, then the default error handler will kick in.

Using onRedelivery processor

Available as of Camel 2.0

Dead Letter Channel has support for onRedelivery to allow custom processing of a Message before its being redelivered. It can be used to add some customer header or whatnot. In Camel 2.0 we have added this feature to Exception Clause as well, so you can use per exception scoped on redelivery. Camel will fallback to use the one defined on Dead Letter Channel if any, if none exists on the Exception Clause. See Dead Letter Channel for more details on onRedelivery.

In the code below we want to do some custom code before redelivering any IOException. So we configure an onException for the IOException and set the onRedelivery to use our custom processor:

And in our custom processor we set a special timeout header to the message. You can of course do anything what you like in your code.

Using onRedelivery in Spring DSL

In Spring DSL you need to use the onRedeliveryRef attribute to refer to a spring bean id that is your custom processor:

And our processor is just a regular spring bean (we use $ for the inner class as this code is based on unit testing)

Using fine grained retry using retryWhile predicate

Available as of Camel 2.0

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In Camel 2.0 to 2.3 its called retryUntil. From Camel 2.4 onwards its named retryWhile because Camel will continue doing retries while the predicate returns true.

When you need fine grained control for determining if an exchange should be retried or not you can use the retryWhile predicate. Camel will redeliver until the predicate returns false.
This is demonstrated in the sample below:

Where the bean myRetryHandler is computing if we should retry or not:

Using custom ExceptionPolicyStrategy

Available in Camel 1.4

The default ExceptionPolicyStrategy in Camel should be sufficient in nearly all use-cases (see section How does Camel select which clause should handle a given thrown Exception). However if you need to use your own this can be configued as the sample below illustrates:

Using our own strategy MyPolicy we can change the default behavior of Camel with our own code to resolve which ExceptionType from above should be handling the given thrown exception.

Using the exception clause in Spring DSL

You can use all of the above mentioned exception clause features in the Spring DSL as well. Here are a few examples:

  • Global scoped - Available in Camel 2.0

  • Route specific scoped

See also

The Error Handler for the general error handling documentation
The Dead Letter Channel for further details.
The Transactional Client for transactional behavior

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