JMS 面试题与答案
问题 41. Why do the TopicSession.createDurableSubscriber and TopicSession.unubscribe calls raise JMSException with the message "ORA - 4020 - deadlock detected while trying to lock object"?
CreateDurableSubscriber and unsubscribe calls require exclusive access to the Topics. If there are pending JMS operations (send/publish/receive) on the same Topic before these calls are issued, the ORA - 4020 exception is raised.
There are two solutions to the problem:
1. Try to isolate the calls to createDurableSubscriber and unsubscribe at the setup or cleanup phase when there are no other JMS operations happening on the Topic. That will make sure that the required resources are not held by other JMS operational calls. Hence the error ORA - 4020 will not be raised.
2. Issue a TopicSession.commit call before calling createDurableSubscriber and unsubscribe call.
问题 42. What are the basic advantages of JMS?
JMS is asynchronous in nature. Thus not all the pieces need to be up all the time for the application to function as a whole. Even if the receiver is down the MOM will store the messages on it's behalf and will send them once it comes back up. Thus at least a part of application can still function as there is no blocking.
问题 43. Why doesn't AQ_ADMINISTRATOR_ROLE or AQ_USER_ROLE always work for AQ applications using Java/JMS API?
In addition to granting the roles, you would also need to grant execute to the user on the following packages:
* grant execute on sys.dbms_aqin to <userid>
* grant execute on sys.dbms_aqjms to <userid>
问题 44. What are the different types of messages available in the JMS API?
Message, TextMessage, BytesMessage, StreamMessage, ObjectMessage, MapMessage are the different messages available in the JMS API.
问题 45. What is the use of ObjectMessage?
ObjectMessage contains a Serializable java object as it's payload. Thus it allows exchange of Java objects between applications. This in itself mandates that both the applications be Java applications. The consumer of the message must typecast the object received to it's appropriate type. Thus the consumer should before hand know the actual type of the object sent by the sender. Wrong type casting would result in ClassCastException. Moreover the class definition of the object set in the payload should be available on both the machine, the sender as well as the consumer. If the class definition is not available in the consumer machine, an attempt to type cast would result in ClassNotFoundException. Some of the MOMs might support dynamic loading of the desired class over the network, but the JMS specification does not mandate this behavior and would be a value added service if provided by your vendor. And relying on any such vendor specific functionality would hamper the portability of your application. Most of the time the class need to be put in the classpath of both, the sender and the consumer, manually by the developer.
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