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 exceptiononException()
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
...
:
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Here if the processing of seda:inputA
or or seda:inputB
cause a a ValidationException
to be thrown (such as due to the XSD validation of the Validator Validation component or the Relax NG Compact syntax validation of the Jing component), then the message will be sent to the activemq:validationFailed
queue.
...
.
You can define multiple onException
clauses for different behavior:
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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 theonException
clause defined exception list. An exactinstanceof
match will always be used, otherwise theonException
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:
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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:
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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:
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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.
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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 an org.apache.camel.ValidationException
occurs then the message will not be redelivered; however if some other exception occurs, e.g., IOException
or whatever, 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 org.apache.camel.ValidationException
up till two times.
Java DSL:
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Spring XML DSL:
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You can customize any of the RedeliveryPolicy so we can for instance set a different delay of 5000
millis:
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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 RedeliveryPolicy
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.
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Asynchronous Delayed Redelivery
Available as of Camel 2.4
From Camel 2.4: 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: 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:
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And in Spring DSL you just add another exception element:
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Using a Processor as a Failure Handler
We want to handle certain exceptions specially so we add a onException
clause for that exception.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.
Marking Exceptions as Handled
But the most important new feature is support for marking exceptions as being handled.
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Available as of Camel 1.5
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See also the section Handle and continue exceptions below
Using 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 behaviour 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 isnot null
.
For instance to mark all ValidationException
as being handled we can do this:
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Example
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Using Handled
In this route below we want to do special handling of all all OrderFailedException
as we want to return a customized response to the caller. First we setup our routing as:
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...
...
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:
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...
...
And finally the exception that is being thrown is just a regular exception:
...
...
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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
and as the payload and the order id 123 as orderid=123
in a header.
If the order could not be processed and thus an 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 with orderid = failed.
Overloading the RedeliveryPolicy
The default error handler used in Camel is the Dead Letter Channel which supports attempting to redeliver the message exchange a number of times before sending it to a dead letter endpoint. Sometimes you want to overload the redelivery policy on a per exception type basis. By default in the above examples, if a ValidationException occurs then the message will not be redelivered; however if some other exception occurs (such as a JDBC deadlock or remote method invocation) the route will be retried.
However if you want to customize any methods on the RedeliveryPolicy object, you can do this via the fluent API...
Code Block |
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onException(MyException.class).
maximumRedeliveries(2);
|
.
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:
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 example: to ignore and continue when the IDontCareException
was thrown we can do this:
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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.
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 example: if you have this route:
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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:
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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:
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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:
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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.
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:
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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: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 second clause is a for coarse gained selection to select the same exception being thrown but when the expression is evaluated to false.
Note: this is not required, if the second 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:
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:$
for the inner class as this code is based on unit testing):
Using onExceptionOccurred
Processor
Available as of Camel 2.17
Dead Letter Channel has support for onExceptionOccurred
to allow custom processing of a Message just after the exception was thrown. It can be used to do some custom logging or whatnot. The difference between onRedelivery
processor and onExceptionOccurred
processor, is that the former is processed just before a redelivery attempt is being performed, that means it will not happen right after an exception was thrown. For example if the error handler has been configured to perform 5 seconds delay between redelivery attempts, then the redelivery processor is invoked 5 seconds later sine the exception was thrown. On the other hand the onExceptionOccurred
processor is always invoked right after the exception was thrown, and also if redelivery has been disabled.
Note: Any new exceptions thrown from the onExceptionOccurred
processor is logged as WARN
and ignored, to not override the existing exception.
In the code below we want to do some custom logging when an exception happened. Therefore we configure an onExceptionOccurred
to use our custom processor:
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Using onRedelivery
in Spring DSL
In Spring DSL you need to use the onExceptionOccurredRef
attribute to refer to a spring bean id that is your custom processor:
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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: 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.
Example: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 configured as the sample below illustrates: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
{snippet:id=e1|lang=xml|url=camel/trunk/components/camel-spring/src/test/resources/org/apache/camel/spring/processor/onexception/deadLetterChannelHandledExampleTest.xml}
Route specific scoped
{snippet:id=e1|lang=xml|url=camel/trunk/components/camel-spring/src/test/resources/org/apache/camel/spring/processor/onexception/onExceptionSubRouteTest.xml}
See also
The Error Handler for the general error handling documentation
The Dead Letter Channel for further details.
The Transactional Client for transactional behaviorYou can mix and match these approaches; specifying a custom processor to be used after all the redeliveries fail together with customizing any aspect of the RedeliveryPolicy