Java – the difference between a MessageListener and a Consumer in JMS

javajmsmessaging

I am new to JMS. As far as I understood Consumers are capable of picking messages from queue/topic. So why do you need a MessageListener because Consumers will know when they have picked up messages? What is the practical use of such a MessageListener?

Edit:From the Javadoc of MessageListener:

A MessageListener object is used to receive asynchronously delivered
messages.

Each session must insure that it passes messages serially to the
listener. This means that a listener assigned to one or more consumers
of the same session can assume that the onMessage method is not called
with the next message until the session has completed the last call.

So I am confused between the usage of the terms asynchronously and serially together. How do these two terms relate in describing the feature of MessageListener?

Best Answer

The difference is that MessageConsumer is used to receive messages synchronously:

MessageConsumer mc = s.createConsumer(queue);
Message msg = mc.receive();

For asynchronous delivery, we can register a MessageListener object with a message consumer:

mc.setMessageListener(new MessageListener() {
    public void onMessage(Message msg) {
        ...
    }
});

Related Solutions

Java – reflection and why is it useful

The name reflection is used to describe code which is able to inspect other code in the same system (or itself).

For example, say you have an object of an unknown type in Java, and you would like to call a 'doSomething' method on it if one exists. Java's static typing system isn't really designed to support this unless the object conforms to a known interface, but using reflection, your code can look at the object and find out if it has a method called 'doSomething' and then call it if you want to.

So, to give you a code example of this in Java (imagine the object in question is foo) :

Method method = foo.getClass().getMethod("doSomething", null);
method.invoke(foo, null);

One very common use case in Java is the usage with annotations. JUnit 4, for example, will use reflection to look through your classes for methods tagged with the @Test annotation, and will then call them when running the unit test.

There are some good reflection examples to get you started at http://docs.oracle.com/javase/tutorial/reflect/index.html

And finally, yes, the concepts are pretty much similar in other statically typed languages which support reflection (like C#). In dynamically typed languages, the use case described above is less necessary (since the compiler will allow any method to be called on any object, failing at runtime if it does not exist), but the second case of looking for methods which are marked or work in a certain way is still common.

Update from a comment:

The ability to inspect the code in the system and see object types is not reflection, but rather Type Introspection. Reflection is then the ability to make modifications at runtime by making use of introspection. The distinction is necessary here as some languages support introspection, but do not support reflection. One such example is C++

Java – What are the differences between a HashMap and a Hashtable in Java

There are several differences between HashMap and Hashtable in Java:

Hashtable is synchronized, whereas HashMap is not. This makes HashMap better for non-threaded applications, as unsynchronized Objects typically perform better than synchronized ones.
Hashtable does not allow null keys or values. HashMap allows one null key and any number of null values.
One of HashMap's subclasses is LinkedHashMap, so in the event that you'd want predictable iteration order (which is insertion order by default), you could easily swap out the HashMap for a LinkedHashMap. This wouldn't be as easy if you were using Hashtable.

Since synchronization is not an issue for you, I'd recommend HashMap. If synchronization becomes an issue, you may also look at ConcurrentHashMap.

Best Answer

Related Solutions

Java – reflection and why is it useful

Java – What are the differences between a HashMap and a Hashtable in Java

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