Exception in Spring Integration - Multiple channels defined on declaring a channel bean - spring-integration

Question

I'm a newbie to spring integration and I'm using the following code,
package services.api;
public interface GreetingService {
public void greetUsers(String userName);
}
package services.impl;
import services.api.GreetingService;
public class GreetServiceImpl implements GreetingService {
#Override
public void greetUsers(String userName) {
if (userName != null && userName.trim().length() > 0) {
System.out.println("Hello " + userName);
}
}
}
package main;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
import org.springframework.integration.Message;
import org.springframework.integration.MessageChannel;
import org.springframework.integration.support.MessageBuilder;
public class Main {
public static void main(String[] args)
{
ApplicationContext applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");
MessageChannel messageChannel = applicationContext.getBean(MessageChannel.class);
Message<String> message = MessageBuilder.withPayload("World").build();
messageChannel.send(message);
}
}
<?xml version="1.0" encoding="UTF-8"?>
<beans:beans xmlns:beans="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://springframework.org/schema/integration"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
http://springframework.org/schema/integration http://springframework.org/schema/integration/spring-integration.xsd">
<channel id="pushChannel" />
<service-activator input-channel="pushChannel" ref="service"
method="greetUsers" />
<beans:bean id="service" class="services.impl.GreetServiceImpl" />
</beans:beans>
I'm getting the following error, eventhough I've declared only one message channel
Mar 04, 2014 4:46:23 PM org.springframework.context.support.ClassPathXmlApplicationContext prepareRefresh
INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext#34d0cdd0: startup date [Tue Mar 04 16:46:23 IST 2014]; root of context hierarchy
Mar 04, 2014 4:46:23 PM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
INFO: Loading XML bean definitions from class path resource [applicationContext.xml]
Mar 04, 2014 4:46:23 PM org.springframework.beans.factory.config.PropertiesFactoryBean loadProperties
INFO: Loading properties file from URL [jar:file:/D:/Personal%20Data/Softwares/spring-framework-4.0.0.RELEASE-dist/SpringIntegration/spring-integration-3.0.0.RELEASE-dist/spring-integration-3.0.0.RELEASE/libs/spring-integration-core-3.0.0.RELEASE.jar!/META-INF/spring.integration.default.properties]
Mar 04, 2014 4:46:23 PM org.springframework.integration.config.xml.IntegrationNamespaceHandler registerHeaderChannelRegistry
INFO: No bean named 'integrationHeaderChannelRegistry' has been explicitly defined. Therefore, a default DefaultHeaderChannelRegistry will be created.
Mar 04, 2014 4:46:23 PM org.springframework.integration.config.xml.DefaultConfiguringBeanFactoryPostProcessor registerErrorChannel
INFO: No bean named 'errorChannel' has been explicitly defined. Therefore, a default PublishSubscribeChannel will be created.
Mar 04, 2014 4:46:23 PM org.springframework.integration.config.xml.DefaultConfiguringBeanFactoryPostProcessor registerTaskScheduler
INFO: No bean named 'taskScheduler' has been explicitly defined. Therefore, a default ThreadPoolTaskScheduler will be created.
Mar 04, 2014 4:46:23 PM org.springframework.beans.factory.config.PropertiesFactoryBean loadProperties
INFO: Loading properties file from URL [jar:file:/D:/Personal%20Data/Softwares/spring-framework-4.0.0.RELEASE-dist/SpringIntegration/spring-integration-3.0.0.RELEASE-dist/spring-integration-3.0.0.RELEASE/libs/spring-integration-core-3.0.0.RELEASE.jar!/META-INF/spring.integration.default.properties]
Mar 04, 2014 4:46:23 PM org.springframework.scheduling.concurrent.ThreadPoolTaskScheduler initialize
INFO: Initializing ExecutorService 'taskScheduler'
Mar 04, 2014 4:46:23 PM org.springframework.context.support.DefaultLifecycleProcessor start
INFO: Starting beans in phase -2147483648
Mar 04, 2014 4:46:23 PM org.springframework.integration.endpoint.EventDrivenConsumer logComponentSubscriptionEvent
INFO: Adding {logging-channel-adapter:_org.springframework.integration.errorLogger} as a subscriber to the 'errorChannel' channel
Mar 04, 2014 4:46:23 PM org.springframework.integration.channel.PublishSubscribeChannel adjustCounterIfNecessary
INFO: Channel 'org.springframework.context.support.ClassPathXmlApplicationContext#34d0cdd0.errorChannel' has 1 subscriber(s).
Mar 04, 2014 4:46:23 PM org.springframework.integration.endpoint.EventDrivenConsumer start
INFO: started _org.springframework.integration.errorLogger
Mar 04, 2014 4:46:23 PM org.springframework.context.support.DefaultLifecycleProcessor start
INFO: Starting beans in phase 0
Mar 04, 2014 4:46:23 PM org.springframework.integration.endpoint.EventDrivenConsumer logComponentSubscriptionEvent
INFO: Adding {service-activator} as a subscriber to the 'pushChannel' channel
Mar 04, 2014 4:46:23 PM org.springframework.integration.channel.DirectChannel adjustCounterIfNecessary
INFO: Channel 'org.springframework.context.support.ClassPathXmlApplicationContext#34d0cdd0.pushChannel' has 1 subscriber(s).
Mar 04, 2014 4:46:23 PM org.springframework.integration.endpoint.EventDrivenConsumer start
INFO: started org.springframework.integration.config.ConsumerEndpointFactoryBean#0
Exception in thread "main" org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type [org.springframework.integration.MessageChannel] is defined: expected single matching bean but found 3: pushChannel,nullChannel,errorChannel
at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBean(DefaultListableBeanFactory.java:312)
at org.springframework.context.support.AbstractApplicationContext.getBean(AbstractApplicationContext.java:985)
at main.Main.main(Main.java:17)

Answer 1

I suggest you to read more Docs:
http://docs.spring.io/spring-integration/docs/3.0.1.RELEASE/reference/html
http://www.manning.com/fisher
As you see the framework provides two explicit channels: nullChannel, errorChannel.
And they aren't the last beans which are populated by framework.
To fix your issue just provide the id of your channel to applicationContext.getBean