So in your Java code you can do
Notice in Camel 2.0 we have introduced the
There are 3 main polling methods on PollingConsumer
Waits until a message is available and then returns it; potentially blocking forever
Attempts to receive a message exchange, waiting up to the given timeout and returning null if no message exchange could be received within the time available
Attempts to receive a message exchange immediately without waiting and returning null if a message exchange is not available yet
Available as of Camel 2.0
The template supports the 3 operations above, but also including convenient methods for returning the body, etc
The example from above using ConsumerTemplate is:
Or to extract and get the body you can do:
And you can provide the body type as a parameter and have it returned as the type:
You get hold of a
ConsumerTemplate from the
CamelContext with the
Using ConsumerTemplate with Spring DSL
With the Spring DSL we can declare the consumer in the CamelContext with the consumerTemplate tag, just like the ProducerTemplate. The example below illustrates this:
Then we can get leverage Spring to inject the
ConsumerTemplate in our java class. The code below is part of an unit test but it shows how the consumer and producer can work together.
Timer based polling consumer
In this sample we use a Timer to schedule a route to be started every 5th second and invoke our bean MyCoolBean where we implement the business logic for the Polling Consumer. Here we want to consume all messages from a JMS queue, process the message and send them to the next queue.
First we setup our route as:
And then we have out logic in our bean:
Scheduled Poll Components
Quite a few inbound Camel endpoints use a scheduled poll pattern to receive messages and push them through the Camel processing routes. That is to say externally from the client the endpoint appears to use an Event Driven Consumer but internally a scheduled poll is used to monitor some kind of state or resource and then fire message exchanges.
Since this a such a common pattern, polling components can extend the ScheduledPollConsumer base class which makes it simpler to implement this pattern.
There is also the Quartz Component which provides scheduled delivery of messages using the Quartz enterprise scheduler.
For more details see:
The ScheduledPollConsumer supports the following options:
Camel 2.0: A pluggable
About error handling and scheduled polling consumers
ScheduledPollConsumer is scheduled based and its
run method is invoked periodically based on schedule settings. But errors can also occur when a poll being executed. For instance if Camel should poll a file network, and this network resource is not available then a
java.io.IOException could occur. As this error happens before any Exchange has been created and prepared for routing, then the regular Error handling in Camel does not apply. So what does the consumer do then? Well the exception is propagated back to the
run method where its handled. Camel will by default log the exception at
WARN level and then ignore it. At next schedule the error could have been resolved and thus being able to poll the endpoint successfully.
Controlling the error handling using PollingConsumerPollStrategy
Available as of Camel 2.0
org.apache.camel.PollingConsumerPollStrategy is a pluggable strategy that you can configure on the
ScheduledPollConsumer. The default implementation
org.apache.camel.impl.DefaultPollingConsumerPollStrategy will log the caused exception at
WARN level and then ignore this issue.
The strategy interface provides the following 3 methods
void begin(Consumer consumer, Endpoint endpoint)
void commit(Consumer consumer, Endpoint endpoint)
boolean rollback(Consumer consumer, Endpoint endpoint, int retryCounter, Exception e) throws Exception
The most interesting is the
rollback as it allows you do handle the caused exception and decide what to do.
For instance if we want to provide a retry feature to a scheduled consumer we can implement the
PollingConsumerPollStrategy method and put the retry logic in the
rollback method. Lets just retry up till 3 times:
Notice that we are given the
Consumer as a parameter. We could use this to restart the consumer as we can invoke stop and start:
Notice: If you implement the
begin operation make sure to avoid throwing exceptions as in such a case the
poll operation is not invoked and Camel will invoke the
Configuring an Endpoint to use
To configure an Endpoint to use a custom
PollingConsumerPollStrategy you use the option
pollStrategy. For example in the file consumer below we want to use our custom strategy defined in the Registry with the bean id
Using This Pattern
If you would like to use this EIP Pattern then please read the Getting Started, you may also find the Architecture useful particularly the description of Endpoint and URIs. Then you could try out some of the Examples first before trying this pattern out.