问题
A colleague and I are working on an application using Spring which needs to get a message from a RabbitMQ queue. The idea is to do this using (the usually excellent) spring annotation system to make the code easy to understand. We have the system working using the @RabbitListner annotation but we want to get a message on demand. The @RabbitListner annotation does not do this, it just receives messages when they are available. The demand is determined by the "readiness" of the client i.e. a client should "get" a message from te queue stop listing and process the message. Then determine if it is ready to receive a new one and reconnect to the queue.
We have been looking into doing this by hand just using the spring-amqp/spring-rabbit modules and while this is probably possible we would really like to do this using spring. After many hours of searching and going through the documentation, we have not been able to find an answer.
Here is the recieving code we currently have:
@RabbitListener(queues = "jobRequests")
public class Receiver {
@Autowired
private JobProcessor jobProcessor;
@RabbitHandler
public void receive(Job job) throws InterruptedException, IOException {
System.out.println(" [x] Received '" + job + "'");
jobProcessor.processJob(job);
}
}
Job processor:
@Service
public class JobProcessor {
@Autowired
private RabbitTemplate rabbitTemplate;
public boolean processJob(Job job) throws InterruptedException, IOException {
rabbitTemplate.convertAndSend("jobResponses", job);
System.out.println(" [x] Processing job: " + job);
rabbitTemplate.convertAndSend("processedJobs", job);
return true;
}
}
In other words, when the job is received by the Receiver it should stop listening for new jobs and wait for the job processor to be done and then start listing for new messages.
We have re-created the null pointer exception here is the code we use to send from the server side.
@Controller
public class MainController {
@Autowired
RabbitTemplate rabbitTemplate;
@Autowired
private Queue jobRequests;
@RequestMapping("/do-job")
public String doJob() {
Job job = new Job(new Application(), "henk", 42);
System.out.println(" [X] Job sent: " + job);
rabbitTemplate.convertAndSend(jobRequests.getName(), job);
return "index";
}
}
And then the receiving code on the client side
@Component
public class Receiver {
@Autowired
private JobProcessor jobProcessor;
@Autowired
private RabbitListenerEndpointRegistry rabbitListenerEndpointRegistry;
@RabbitListener(queues = "jobRequests")
public void receive(Job job) throws InterruptedException, IOException, TimeoutException {
Collection<MessageListenerContainer> messageListenerContainers = rabbitListenerEndpointRegistry.getListenerContainers();
for (MessageListenerContainer listenerContainer :messageListenerContainers) {
System.out.println(listenerContainer);
listenerContainer.stop();
}
System.out.println(" [x] Received '" + job + "'");
jobProcessor.processJob(job);
for (MessageListenerContainer listenerContainer :messageListenerContainers) {
listenerContainer.start();
}
}
}
And the updated job processor
@Service
public class JobProcessor {
public boolean processJob(Job job) throws InterruptedException, IOException {
System.out.println(" [x] Processing job: " + job);
return true;
}
}
And the stacktrace
[x] Received 'Job{application=com.olifarm.application.Application@aaa517, name='henk', id=42}'
[x] Processing job: Job{application=com.olifarm.application.Application@aaa517, name='henk', id=42}
Exception in thread "SimpleAsyncTaskExecutor-1" java.lang.NullPointerException
2015-12-18 11:17:44.494 at org.springframework.amqp.rabbit.listener.SimpleMessageListenerContainer.isActive(SimpleMessageListenerContainer.java:838)
at org.springframework.amqp.rabbit.listener.SimpleMessageListenerContainer.access$700(SimpleMessageListenerContainer.java:93)
at org.springframework.amqp.rabbit.listener.SimpleMessageListenerContainer$AsyncMessageProcessingConsumer.run(SimpleMessageListenerContainer.java:1301)
at java.lang.Thread.run(Thread.java:745)
WARN 325899 --- [cTaskExecutor-1] o.s.a.r.l.SimpleMessageListenerContainer : Consumer raised exception, processing can restart if the connection factory supports it
java.lang.NullPointerException: null
at org.springframework.amqp.rabbit.listener.SimpleMessageListenerContainer.isActive(SimpleMessageListenerContainer.java:838) ~[spring-rabbit-1.5.2.RELEASE.jar:na]
at org.springframework.amqp.rabbit.listener.SimpleMessageListenerContainer.access$700(SimpleMessageListenerContainer.java:93) ~[spring-rabbit-1.5.2.RELEASE.jar:na]
at org.springframework.amqp.rabbit.listener.SimpleMessageListenerContainer$AsyncMessageProcessingConsumer.run(SimpleMessageListenerContainer.java:1195) ~[spring-rabbit-1.5.2.RELEASE.jar:na]
at java.lang.Thread.run(Thread.java:745) [na:1.7.0_91]
The stopping of the listener works and we do receive a new job but when it try's to start it again the NPE is thrown. We checked the rabbitMQ log and found that the connection is closed for about 2 seconds and then re-opened automatically even if we put the thread in sleep in the job processor. This might be the source of the problem? The error doesn't break the program however and after it is thrown the receiver is still able to receive new jobs. Are we abusing the mechanism here or is this valid code?
回答1:
To get messages on-demand, it's generally better to use rabbitTemplate.receiveAndConvert() rather than a listener; that way you completely control when you receive messages.
Starting with version 1.5 you can configure the template to block for some period of time (or until a message arrives). Otherwise it immediately returns null if there's no message.
The listener is really designed for message-driven applications.
If you can block the thread in the listener until the job completes, no more messages will be delivered - by default the container has only one thread.
If you can't block the thread until the job completes, for some reason, you can stop()/start() the listener container by getting a reference to it from the Endpoint Registry.
It's generally better to stop the container on a separate thread.
来源：https://stackoverflow.com/questions/34332460/spring-rabbitlistner-stop-listening-to-queue-using-annotation-syntax