Apache Camel Exception Handling | baldung

1. Overview

Apache Camel is a powerful open-source integration framework that implements several known enterprise integration patterns.

Generally when dealing with message routing using Camel, we want a way to handle errors effectively. To this end, Camel provides some strategies for handling exceptions.

In this tutorial, We will take a look at two approaches that we can use for exception handling inside our Camel application.

2. Dependency

All we need to do to get started camel-spring-boot-starter added to our pom.xml,

<dependency>
    <groupId>org.apache.camel.springboot</groupId>
    <artifactId>camel-spring-boot-starter</artifactId>
    <version>3.19.0</version>
</dependency>

3. Route making

Let’s start by defining an elementary route that intentionally throws an exception:

@Component
public class ExceptionThrowingRoute extends RouteBuilder {

    private static final Logger LOGGER = LoggerFactory.getLogger(ExceptionThrowingRoute.class);

    @Override
    public void configure() throws Exception {
        
        from("direct:start-exception")
          .routeId("exception-handling-route")
          .process(new Processor() {
              
              @Override
              public void process(Exchange exchange) throws Exception {
                  LOGGER.error("Exception Thrown");
                  throw new IllegalArgumentException("An exception happened on purpose");
                  
              }
          }).to("mock:received");
    }
}

To recap, a route is a fundamental building block in Apache Camel, typically composed of a sequence of steps, executed by Camel, which consumes and processes a message.

As we can see in our trivial example, we configure our route to consume messages directly from an endpoint called start.

Then, we throw a Illegal Argument Exception from within a new processor, Which we make inline inside our route using Java DSL.

Currently, our route doesn’t have any kind of exception handling, so when we run it, we’ll see something ugly in our application’s output:

...
10:21:57.087 [main] ERROR c.b.c.e.ExceptionThrowingRoute - Exception Thrown
10:21:57.094 [main] ERROR o.a.c.p.e.DefaultErrorHandler - Failed delivery for (MessageId: 50979CFF47E7816-0000000000000000 on ExchangeId: 50979CFF47E7816-0000000000000000). 
Exhausted after delivery attempt: 1 caught: java.lang.IllegalArgumentException: An exception happened on purpose

Message History (source location and message history is disabled)
---------------------------------------------------------------------------------------------------------------------------------------
Source                                   ID                             Processor                                          Elapsed (ms)
                                         exception-handling-route/excep from[direct://start-exception]                               11
	...
                                         exception-handling-route/proce [email protected]                                          0

Stacktrace
---------------------------------------------------------------------------------------------------------------------------------------
java.lang.IllegalArgumentException: An exception happened on purpose
...

4. a. to use try () block

Now let’s go ahead and add some exception handling to our route. in this section, We’ll take a look at the camel try () block, which we can think of as the Java equivalent finally try to catchBut directly embedded in the DSL.

But first, to help simplify our code, we’re going to define a dedicated Processor class that takes a Illegal Argument Exception – This will make our code more readable, and we can reuse our processors in other routes later:

@Component
public class IllegalArgumentExceptionThrowingProcessor implements Processor {

    private static final Logger LOGGER = LoggerFactory.getLogger(ExceptionLoggingProcessor.class);

    @Override
    public void process(Exchange exchange) throws Exception {
        LOGGER.error("Exception Thrown");
        throw new IllegalArgumentException("An exception happened on purpose");
    }
}

With our new processor, let’s use this in our first exception-handling route:

@Component
public class ExceptionHandlingWithDoTryRoute extends RouteBuilder {

    @Override
    public void configure() throws Exception {
        
        from("direct:start-handling-exception")
          .routeId("exception-handling-route")
          .doTry()
            .process(new IllegalArgumentExceptionThrowingProcessor())
            .to("mock:received")
          .doCatch(IOException.class, IllegalArgumentException.class)
            .to("mock:caught")
          .doFinally()
            .to("mock:finally")
          .end();
    }
}

As we can see, the code in our route is pretty self-explanatory. We are basically copying a regular Java finally try to catch statement using camel equivalents.

However, let’s move on to the key parts of our route:

  • First, we enclose the part of the route where we want the thrown exceptions to be caught immediately using try () method
  • Next, we close this using block and after douche way. Note that we can pass a list of different exception types we want to catch
  • Finally, we call In the end (), which defines code that always runs after try ()and any doCatch() blocks

In addition, we should note that it is important to call End() Method to denote end of block in Java DSL.

Camel also provides another powerful feature that lets us work with predicates when using doCatch() Section Matha:

...
.doCatch(IOException.class, IllegalArgumentException.class).onWhen(exceptionMessage().contains("Hello"))
   .to("mock:catch")
...

Here we add a runtime predicate to determine whether the catch block should be triggered or not. In this case, we want to trigger it only if the exception message contains the word hello, Very good!

5. Working with Exception Clauses

Unfortunately, one of the limitations of the previous approach is that it only applies to a single route.

Usually as our application grows and we add more and more routes, we probably don’t want to handle exceptions on a route-by-route basis. This will potentially lead to duplicate code, and we want a common error-handling strategy for our application.

Thankfully Camel provides an exception clause mechanism via the Java DSL to let us specify the required error handling per exception type or on a global basis:

Let’s imagine that we want to implement an exception-handling policy for our application. For our simple example, we’ll assume we only have one route:

@Component
public class ExceptionHandlingWithExceptionClauseRoute extends RouteBuilder {
    
    @Autowired
    private ExceptionLoggingProcessor exceptionLogger;
    
    @Override
    public void configure() throws Exception {
        onException(IllegalArgumentException.class).process(exceptionLogger)
          .handled(true)
          .to("mock:handled")
        
        from("direct:start-exception-clause")
          .routeId("exception-clause-route")
          .process(new IllegalArgumentExceptionThrowingProcessor())
          .to("mock:received");
    }
}

As we can see, we are using on exception method of handling when a Illegal Argument Exception Occurs and implements some specific part of processing.

For our example, we pass the processing to a custom exception logging processor class that logs only message headers. Finally, we use handled (true) The method to mark the message exchange as a handle is called before the result is sent to the mock endpoint. handled,

However, we should note that in Camel, the global scope of our code is per rootbuilder Example. So if we want to share this exception-handling code through multiple rootbuilder classes, we can use the following technique.

just create a base abstract rootbuilder put the class and error-handling logic in configure method,

After this, we can simply extend this class and make sure we call super.config() way. In short, we are just using Java inheritance technology.

6. conclusion

In this article, we learned how we can handle exceptions in our routes. First, we created a simple Camel application with several routes to learn about exceptions.

Then we learned of two concrete approaches using try () And doCatch() block syntax and, later, Exception () section.

As always, the full source code of the article is available on GitHub.

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