Chapter 50. Consumer Interface

Abstract

This chapter describes how to implement the Consumer interface, which is an essential step in the implementation of a Apache Camel component.

50.1. The Consumer Interface

Overview

An instance of org.apache.camel.Consumer type represents a source endpoint in a route. There are several different ways of implementing a consumer (see Section 47.1.3, “Consumer Patterns and Threading”), and this degree of flexibility is reflected in the inheritance hierarchy ( see Figure 50.1, “Consumer Inheritance Hierarchy”), which includes several different base classes for implementing a consumer.

Figure 50.1. Consumer Inheritance Hierarchy

Consumer inheritance hierarchy

Consumer parameter injection

For consumers that follow the scheduled poll pattern (see the section called “Scheduled poll pattern”), Apache Camel provides support for injecting parameters into consumer instances. For example, consider the following endpoint URI for a component identified by the custom prefix: 
custom:destination?consumer.myConsumerParam
Apache Camel provides support for automatically injecting query options of the form consumer.*. For the consumer.myConsumerParam parameter, you need to define corresponding setter and getter methods on the Consumer implementation class as follows: 
public class CustomConsumer extends ScheduledPollConsumer {
    ...
    String getMyConsumerParam() { ... }
    void setMyConsumerParam(String s) { ... }
    ...
}
Where the getter and setter methods follow the usual Java bean conventions (including capitalizing the first letter of the property name).
In addition to defining the bean methods in your Consumer implementation, you must also remember to call the configureConsumer() method in the implementation of Endpoint.createConsumer(). See the section called “Scheduled poll endpoint implementation”). Example 50.1, “FileEndpoint createConsumer() Implementation” shows an example of a createConsumer() method implementation, taken from the FileEndpoint class in the file component:

Example 50.1. FileEndpoint createConsumer() Implementation

...
public class FileEndpoint extends ScheduledPollEndpoint {
    ...
    public Consumer createConsumer(Processor processor) throws Exception {
        Consumer result = new FileConsumer(this, processor);
        configureConsumer(result);
        return result;
    }
    ...
    }
At run time, consumer parameter injection works as follows:
  1. When the endpoint is created, the default implementation of DefaultComponent.createEndpoint(String uri) parses the URI to extract the consumer parameters, and stores them in the endpoint instance by calling ScheduledPollEndpoint.configureProperties().
  2. When createConsumer() is called, the method implementation calls configureConsumer() to inject the consumer parameters (see Example 50.1, “FileEndpoint createConsumer() Implementation”).
  3. The configureConsumer() method uses Java reflection to call the setter methods whose names match the relevant options after the consumer. prefix has been stripped off.

Scheduled poll parameters

A consumer that follows the scheduled poll pattern automatically supports the consumer parameters shown in Table 50.1, “Scheduled Poll Parameters” (which can appear as query options in the endpoint URI).

Table 50.1. Scheduled Poll Parameters

NameDefaultDescription
initialDelay1000Delay, in milliseconds, before the first poll.
delay500Depends on the value of the useFixedDelay flag (time unit is milliseconds).
useFixedDelayfalse
If false, the delay parameter is interpreted as the polling period. Polls will occur at initialDelay, initialDelay+delay, initialDelay+2*delay, and so on.
If true, the delay parameter is interpreted as the time elapsed between the previous execution and the next execution. Polls will occur at initialDelay, initialDelay+[ProcessingTime]+delay, and so on. Where ProcessingTime is the time taken to process an exchange object in the current thread.

Converting between event-driven and polling consumers

Apache Camel provides two special consumer implementations which can be used to convert back and forth between an event-driven consumer and a polling consumer. The following conversion classes are provided:
  • org.apache.camel.impl.EventDrivenPollingConsumer—Converts an event-driven consumer into a polling consumer instance.
  • org.apache.camel.impl.DefaultScheduledPollConsumer—Converts a polling consumer into an event-driven consumer instance.
In practice, these classes are used to simplify the task of implementing an Endpoint type. The Endpoint interface defines the following two methods for creating a consumer instance: 
package org.apache.camel;

public interface Endpoint {
    ...
    Consumer createConsumer(Processor processor) throws Exception;
    PollingConsumer createPollingConsumer() throws Exception;
}
createConsumer() returns an event-driven consumer and createPollingConsumer() returns a polling consumer. You would only implement one these methods. For example, if you are following the event-driven pattern for your consumer, you would implement the createConsumer() method provide a method implementation for createPollingConsumer() that simply raises an exception. With the help of the conversion classes, however, Apache Camel is able to provide a more useful default implementation.
For example, if you want to implement your consumer according to the event-driven pattern, you implement the endpoint by extending DefaultEndpoint and implementing the createConsumer() method. The implementation of createPollingConsumer() is inherited from DefaultEndpoint, where it is defined as follows: 
public PollingConsumer<E> createPollingConsumer() throws Exception {
    return new EventDrivenPollingConsumer<E>(this);
}
The EventDrivenPollingConsumer constructor takes a reference to the event-driven consumer, this, effectively wrapping it and converting it into a polling consumer. To implement the conversion, the EventDrivenPollingConsumer instance buffers incoming events and makes them available on demand through the receive(), the receive(long timeout), and the receiveNoWait() methods.
Analogously, if you are implementing your consumer according to the polling pattern, you implement the endpoint by extending DefaultPollingEndpoint and implementing the createPollingConsumer() method. In this case, the implementation of the createConsumer() method is inherited from DefaultPollingEndpoint, and the default implementation returns a DefaultScheduledPollConsumer instance (which converts the polling consumer into an event-driven consumer).

ShutdownPrepared interface

Consumer classes can optionally implement the org.apache.camel.spi.ShutdownPrepared interface, which enables your custom consumer endpoint to receive shutdown notifications.
Example 50.2, “ShutdownPrepared Interface” shows the definition of the ShutdownPrepared interface.

Example 50.2. ShutdownPrepared Interface

package org.apache.camel.spi;


public interface ShutdownPrepared {

    void prepareShutdown(boolean forced);

}
The ShutdownPrepared interface defines the following methods:
prepareShutdown
Receives notifications to shut down the consumer endpoint in one or two phases, as follows:
  1. Graceful shutdown—where the forced argument has the value false. Attempt to clean up resources gracefully. For example, by stopping threads gracefully.
  2. Forced shutdown—where the forced argument has the value true. This means that the shutdown has timed out, so you must clean up resources more aggressively. This is the last chance to clean up resources before the process exits.

ShutdownAware interface

Consumer classes can optionally implement the org.apache.camel.spi.ShutdownAware interface, which interacts with the graceful shutdown mechanism, enabling a consumer to ask for extra time to shut down. This is typically needed for components such as SEDA, which can have pending exchanges stored in an internal queue. Normally, you would want to process all of the exchanges in the queue before shutting down the SEDA consumer.
Example 50.3, “ShutdownAware Interface” shows the definition of the ShutdownAware interface.

Example 50.3. ShutdownAware Interface

// Java
package org.apache.camel.spi;

import org.apache.camel.ShutdownRunningTask;

public interface ShutdownAware extends ShutdownPrepared {

    boolean deferShutdown(ShutdownRunningTask shutdownRunningTask);

    int getPendingExchangesSize();
}
The ShutdownAware interface defines the following methods:
deferShutdown
Return true from this method, if you want to delay shutdown of the consumer. The shutdownRunningTask argument is an enum which can take either of the following values:
  • ShutdownRunningTask.CompleteCurrentTaskOnly—finish processing the exchanges that are currently being processed by the consumer's thread pool, but do not attempt to process any more exchanges than that.
  • ShutdownRunningTask.CompleteAllTasks—process all of the pending exchanges. For example, in the case of the SEDA component, the consumer would process all of the exchanges from its incoming queue.
getPendingExchangesSize
Indicates how many exchanges remain to be processed by the consumer. A zero value indicates that processing is finished and the consumer can be shut down.
For an example of how to define the ShutdownAware methods, see Example 50.7, “Custom Threading Implementation”.