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Comment: Editorial cleanup

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Code Block
Endpoint endpoint = context.getEndpoint("activemq:my.queue");
PollingConsumer consumer = endpoint.createPollingConsumer();
Exchange exchange = consumer.receive();

Notice in Camel 2.0 we have introduced the ConsumerTemplateThe ConsumerTemplate (discussed below) is also available.

There are 3 main polling methods on PollingConsumer

Method name

Description

receive()

Waits until a message is available and then returns it; potentially blocking forever

receive(long)

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

receiveNoWait()

Attempts to receive a message exchange immediately without waiting and returning null if a message exchange is not available yet

ConsumerTemplate

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The ConsumerTemplate is a template much like Spring's JmsTemplate or JdbcTemplate supporting the Polling Consumer EIP. With the template you can consume Exchanges from an Endpoint.

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Option

Default

Description

pollStrategy

Camel 2.0: A pluggable org.apache.camel.PollingConsumerPollStrategy allowing you to provide your custom implementation to control error handling usually occurred during the poll operation before an Exchange have been created and being routed in Camel. In other words the error occurred while the polling was gathering information, for instance access to a file network failed so Camel cannot access it to scan for files. The default implementation will log the caused exception at WARN level and ignore it.

sendEmptyMessageWhenIdle

false

Camel 2.9: If the polling consumer did not poll any files, you can enable this option to send an empty message (no body) instead.

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ScheduledPollConsumer is scheduled based and its run method is invoked periodically based on schedule settings. But errors can also occur when a poll is 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

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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.

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